U.S. Department of Commerce Volume 92 Number 1 January 1994 U.S. Department of Commerce Ronald H. Brown Secretary National Oceanic and Atmospheric Administration D. James Baker Under Secretary for Oceans and Atmosphere National Marine Fisheries Service Rolland A. Schmitten Assistant Administrator for Fisheries Scientific Editor Dr. Ronald W. Hardy Northwest Fisheries Science Center National Marine Fisheries Service, NOAA 2725 Montlake Boulevard East Seattle, Washington 981 12-2097 Editorial Committee Dr. Andrew E. Dizon National Marine Fisheries Service Dr. Linda L. Jones National Marine Fisheries Service Dr. Richard D. Methot National Marine Fisheries Service Dr. Theodore W. Pietsch University of Washington Dr. Joseph E. Powers National Marine Fisheries Service Dr. Tim D. Smith National Marine Fisheries Service The Fishery Bulletin (ISSN 0090-0656) is published quarterly by the Scientific Publications Office, National Marine Fisheries Service, NOAA, 7600 Sand Point Way NE, BIN C15700, Seattle, WA 98115-0070. Second class postage is paid in Seattle, Wash., and additional offices. POSTMASTER send address changes for subscriptions to Fishery Bulletin, Super- intendent of Documents, Attn: Chief, Mail List Branch, Mail Stop SSOM, Washington, DC 20402-9373. Although the contents have not been copyrighted and may be reprinted entire- ly, reference to source is appreciated. The Secretary of Commerce has deter- mined that the publication of this period- ical is necessary in the transaction of the public business required by law of this Department. Use of funds for printing of this periodical has been approved by the Director of the Office of Management and Budget. For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. Subscrip- tion price per year: $24.00 domestic and $30.00 foreign. Cost per single issue: $12.00 domestic and $15.00 foreign. See back page for order form. Managing Editor Sharyn Matriotti National Marine Fisheries Service Scientific Publications Office 7600 Sand Point Way NE, BIN C 1 5700 Seattle, Washington 98 1 1 5-0070 The Fishery Bulletin carries original research reports and technical notes on investiga- tions in fishery science, engineering, and economics. The Bulletin of the United States Fish Commission was begun in 1881; it became the Bulletin of the Bureau of Fisheries in 1904 and the Fishery Bulletin of the Fish and Wildlife Service in 1941. Separates were issued as documents through volume 46; the last document was No. 1 103. Begin- ning with volume 47 in 1931 and continuing through volume 62 in 1963, each separate appeared as a numbered bulletin. A new system began in 1963 with volume 63 in which papers are bound together in a single issue of the bulletin. Beginning with volume 70, number 1, January 1972, the Fishery Bulletin became a periodical, issued quarterly. In this form, it is available by subscription from the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402. It is also available free in limited numbers to libraries, research institutions, State and Federal agencies, and in exchange for other scientific publications. U.S. Department of Commerce Seattle, Washington Volume 92 Number 1 January 1994 Fishery Bulletin Biological laboratory/ Je Oceancgraphic Instituuo,, Library B 2 3 1994 Contents Woods Hole, MA 02543 1 Barbieri, Luiz R., Mark E. Chittenden Jr., and Cynthia M. Jones Age, growth, and mortality of Atlantic croaker, Micropogonias undulatus, in the Chesapeake Bay region, with a discussion of apparent geographic changes in population dynamics 1 3 Bigelow, Keith A. Age and growth of the oceanic squid Onychoteuthis borealyaponica in the North Pacific 26 Burke, Vincent J., Stephen J. Morreale, and Edward A. Standora Diet of the Kemp's ridley sea turtle, Lepidochelys kempu, in New York waters 33 Ditty, James G., and Richard F. Shaw Larval development of tripletail, Lobotes sunnamensis (Pisces: Lobotidae), and their spatial and temporal distribution in the northern Gulf of Mexico 46 Ferreira, Beatrice Padovani, and Garry R. Russ Age validation and estimation of growth rate of the coral trout, Plectropomus leopardus. (Lacepede 1 802) from Lizard Island, Northern Great Barrier Reef 58 Gold, John R., and Linda R. Richardson Genetic distinctness of red drum [Sciaenops ocellatus) from Mosquito Lagoon, east-central Florida 67 Incze, Lewis S., and Terri Ainaire Distribution and abundance of copepod naupln and other small (40-300 (im) zooplankton during spring in Shelikof Strait, Alaska Fishery Bulletin 92(1), 1994 79 Jaenicke, Herbert W., and Adrian G. Celewycz Marine distribution and size of juvenile Pacific salmon in Southeast Alaska and northern British Columbia 91 Johnson, Allyn G., William A. Fable Jr., Churchill B. Grimes, Lee Trent, and Javier Vasconcelos Perez Evidence for distinct stocks of king mackerel, Scomberomorus cavalla, in the Gulf of Mexico 102 Milton, David A., Stephen J. M. Blaber, and Nicholas J. F. Rawlinson Reproductive biology and egg production of three species of Clupeidae from Kiribati, tropical central Pacific 122 Perryman, Wayne L., and Morgan S. Lynn Examination of stock and school structure of striped dolphin [Stenella coeruleoalba) in the eastern Pacific from aerial photogrammetry 132 Punsly, Richard G., Patrick K. Tomlinson, and Ashley J. Mullen Potential tuna catches in the eastern Pacific Ocean from schools not associated with dolphins 144 Sinclair, Elizabeth, Thomas Loughlin, and William Pearcy Prey selection by northern fur seals (Callorhinus ursinus) in the eastern Bering Sea 157 Stone, Heath H., and Brian M. Jessop Feeding habits of anadromous alewives, Alosa pseudoharengus, off the Atlantic Coast of Nova Scotia 171 Stoner, Allan W, and Kirsten C. Schwarte Queen conch, Strombus gigas, reproductive stocks in the central Bahamas: distribution and probable sources 180 Wilber, Dara H. The influence of Apalachicola River flows on blue crab, Callinectes sapidus. in north Florida 189 Fargo, Jeff, and Albert V. Tyler Oocyte maturation in Hecate Strait English sole (Pleuronectes vetulus) 203 Polovina, Jeffrey J., and Gary T. Mitchum Spiny lobster recruitment and sea level results of a 1990 forecast 206 List of recent NOAA Technical Reports NMFS Abstract. — Atlantic croaker, Micropogonias undulatus, col- lected from commercial catches in Chesapeake Bay and in Virginia and North Carolina coastal waters during 1988-1991 (n = l,967) were aged from transverse otolith sec- tions. Ages 1-8 were recorded, but eight-year-old fish were rare. Mar- ginal increment analysis showed that for ages 1-7, annuli are formed once a year during the pe- riod April-May. Otolith age read- ings were precise: >99% agree- ment within and between readers. Observed lengths-at-age were highly variable and growth rate decreased after the first year. De- spite the high variability in sizes- at-age, observed lengths for ages 1-7 fit the von Bertalanffy growth model (r 2 =0.99; n=753) well. No differences in growth were found between sexes. Total annual in- stantaneous mortality (Z) esti- mated from maximum age and from a catch curve of Chesapeake Bay commercial catches ranged from 0.55 to 0.63. Our results do not indicate the existence of a group of larger, older Atlantic croaker in Chesapeake Bay com- pared with more southern waters and suggest that the hypothesis of a basically different population dynamics pattern for this species north and south of Cape Hatteras, North Carolina, should be reevalu- ated. Age, growth, and mortality of Atlantic croaker, Micropogonias undulatus, in the Chesapeake Bay region, with a discussion of apparent geographic changes \n population dynamics* Luiz R. Barbieri College of William and Mary, Virginia Institute of Marine Science Gloucester Point. Virginia 23062 Present address: University of Georgia Marine Institute Sapelo Island. Georgia 31327 Mark E. Chittenden Jr. College of William and Mary, Virginia Institute of Marine Science Gloucester Point, Virginia 23062 Cynthia M. Jones Old Dominion University. Applied Marine Research LaPoratory Norfolk, Virginia 23529 Manuscript accepted 12 August 1993 Fishery Bulletin 92:1-12 (1994) The Atlantic croaker, Micropo- gonias undulatus (Linnaeus), is one of the most abundant inshore demersal fishes along the Atlantic and Gulf of Mexico coasts of the United States (Joseph, 1972). Al- though recent commercial and rec- reational catches have come prima- rily from the South Atlantic Bight and the Gulf of Mexico, Atlantic croaker still support important fisheries along the Mid-Atlantic coast, especially from Maryland to North Carolina (Wilk, 1981). In Chesapeake Bay, they are caught by commercial and recreational fishermen during late spring and early fall migrations and, to a lesser extent, during the summer. In winter, Atlantic croaker leave the Bay to overwinter off the coast of Virginia and North Carolina, where they are caught by otter trawl and gillnet fisheries (Haven, 1959). Little is known about age, growth, and mortality of Atlantic croaker in the Middle Atlantic and Chesapeake Bay regions. Studies based on length frequencies (Ha- ven, 1957; Chao and Musick, 1977) require considerable subjective in- terpretation given the extended spawning period of Atlantic croaker (Morse, 1980; Warlen, 1982; Bar- bieri et al., unpubl. ms.) and the difficulty in distinguishing modal groups at older ages (White and Chittenden, 1977; Jearld, 1983). Al- though scale-ageing has also been used (Welsh and Breder, 1923; Wallace, 1940; Ross 1988), prob- lems in applying this method to Atlantic croaker have been widely reported (Roithmayr, 1965; Joseph, 1972; Barger and Johnson, 1980; Barbieri, 1993). In this study we provide informa- tion on age, growth, and mortality of Atlantic croaker in the Chesa- * Contribution No. 1806 from the College of William and Mary, School of Marine Sci- ence/Virginia Institute of Marine Science, Gloucester Point, Virginia 23062 Fishery Bulletin 92(1). 1994 peake Bay region using a validated otolith-ageing method. We also evaluate the relationship between otolith size and fish size and age, and discuss the implications of using otoliths for ageing Atlantic croaker. Finally, based on current information on growth, and size and age compositions in Chesa- peake Bay, we discuss the hypothesis of White and Chittenden (1977) and Ross (1988) regarding the existence of a basically different population dynam- ics pattern for Atlantic croaker north and south of Cape Hatteras, North Carolina. Methods Atlantic croaker were collected between June 1988 and June 1991 from commercial pound-net, haul- seine, and gillnet fisheries which operate from early spring to early fall in Chesapeake Bay. Local fish processing houses and seafood dealers were con- tacted weekly or fortnightly, and one 22.7-kg (50-lb) box of fish of each available market grade (small, medium, or large) was purchased. Although boxes of fish were not randomly selected, Chittenden (1989) found only minor among-box differences in Atlantic croaker length compositions in pound-net and haul-seine catches. Because nearly all variation in size compositions was captured by the within-box variation, box selection did not present a problem. Since Atlantic croaker migrate from Chesapeake Bay in early fall to overwinter offshore (Haven, 1959), samples for the period November-March were obtained from commercial trawlers which op- erate in Virginia and North Carolina shelf waters. Young of the year (90-114 mm total length, TL) used to validate the first annulus on otoliths were ob- tained from the Virginia Institute of Marine Science juvenile bottom trawl survey. Fish were measured for total length (TL, ±1.0 mm), weighed for total weight (TW, ±1.0 g), sexed, and both sagittal otoliths removed and stored dry. The left otolith was transversely sectioned through the core with the diamond blade of a Buehler low- speed Isomet saw. Sections 350-500 urn thick were mounted on glass slides with Flo-texx clear mount- ing medium and read under a dissecting microscope (6-12x) with transmitted light and bright field, with the exception of samples from the period April-May, when sections were also read with reflected light and dark field to help identify the last annulus. Ages were assigned based on annulus counts; January 1 was taken as an arbitrary average birthdate when fish from one age class were as- signed to the next oldest (Jearld, 1983). Although the average spawning date (average biological birthdate) of Atlantic croaker in the Chesapeake Bay region occurs in September (Barbieri et al., unpubl. ms.), we chose, for ageing purposes, to use January 1 as the average birthdate because annuli are formed during the period April-May (see Age deter- mination below). To assess ageing precision, all otolith sections (n- 1,967) were read twice by each of two readers, and agreement between readings and readers evaluated by percent agreement. All dis- agreements were resolved by a third reading with both readers. Annuli were validated by the marginal increment method (Bagenal and Tesch, 1978). For each age, the translucent margin outside the proximal end of the last annulus was measured along the ventral side of the otolith sulcal groove (Fig. 1). Measurements (±0.02 mm) were taken with an ocular micrometer at 25x. To evaluate growth, observed lengths at ages 1-7 were fit to the von Bertalanffy model (Ricker, 1975) by using nonlinear regression (Marquardt method). Model parameters were the following: L m , the mean asymptotic length; K, the Brody growth coefficient; and t () , the hypothetical age at which a fish would have zero length (Ricker, 1975). To cor- rect for growth after the time of annulus formation, only data for September, the peak spawning and thus average biological birthdate for Atlantic croaker in the Chesapeake Bay region (Barbieri et al., unpubl. ms.), were used for growth analysis. To evaluate changes in otolith size relative to fish length and age, 30 randomly selected otoliths per age, for ages 1-7 ( 198^100 mm TL), were measured for maximum length (OL, ±0.05 mm) and maximum thickness (OT, ±0.05 mm), and weighed (OW, ± 0.001 g). After sectioning, otoliths were measured for otolith radius (OR, ±0.02 mm), defined as the dis- tance between the center of the core and the otolith outer edge along the ventral side of the sulcal groove (Fig. 1). Relationships between otolith measure- ments and fish TL were evaluated by regression analysis. The effect offish age on these relationships was evaluated by analysis of covariance (ANCOVA). Linear regression was used to determine a length- weight relationship for fish ranging from 152 to 400 mm TL (36.3 to 967.0 g TW). Difference between sexes was tested by ANCOVA. The hypothesis of isometric growth (Ricker, 1975) was tested by t-test. Instantaneous total annual mortality rates, Z, were estimated from maximum age by using Hoenig's pooled regression equation (Hoenig, 1983), by calculating a theoretical total mortality for the entire lifespan following the reasoning of Royce (1972), and by the regression method with a catch curve of combined pound-net, haul-seine, and gillnet. Barbieri et al.: Age, growth, and mortality of Micropogonias undulatus Proximal Ventra Figure 1 Transverse otolith section of an 8-year-old Atlantic croaker caught in Sep- tember 1988 in Chesapeake Bay. Arrows indicate annuli. The translu- cent zone beyond the last annulus represents additional growth after the annulus was formed during April-May. SG =sulcal groove, a = artifact of preparation. Ventral and proximal indicate axes of orientation. data for all recruited ages having five or more fish (Chapman and Robson, 1960). To avoid sampling bias associated with individual gears, we considered the age-frequency distribution obtained from data from combined gears as the best estimate of Atlan- tic croaker age composition in Chesapeake Bay (Ricker, 19751. Commercial trawl collections were not used in this analysis because they had different length compositions than the other gears and could be biased towards small fish. Because in catch curve analysis the age group represented by the apex of the catch curve may or may not be fully recruited to the gears (Everhart and Youngs, 1981), mortal- ity estimates were based on ages 3-7 only. Data from 1988 to 1991 were combined to minimize the effect of variation in year-class strength (Robson and Chapman, 1961). The right tail of the catch curve (Ricker, 1975) was tested for deviation from linear- ity by analysis of variance (ANOVA). Values of Z were converted to total annual mortality rates, A, by using the relationship A = 1 - e - z { Ricker, 1975). All statistical analyses were performed by using the Statistical Analysis System (SAS, 1988). Rejec- tion of the null hypothesis in statistical tests was based on a=0.05. F-tests in ANCOVA were based on Type III sums of squares (Freund and Littell, 1986). Assumptions of linear models were checked by re- sidual plots as described in Draper and Smith ( 1981). For the OL-TL, OW-TL, and TW-TL relation- ships, and for all ANCOVA and ANOVA analyses, data were log 10 -transformed to correct for non-lin- earity and heterogeneous variances. For the catch curve analysis, log e -transformed numbers at age were regressed on age. Unless otherwise indicated, back-transformed data and regression equations are presented in the results. Results Age determination Transverse otolith sections of Atlantic croaker show very clear, easily identified marks that can be used for ageing. Typical sections have an opaque core surrounded by a blurred opaque band composed of fine opaque and translucent zones (Fig. 1). This band represents the first annulus. The width of this annulus varies among fish, from a very narrow band that is almost continuous with the core, to a wide, well-defined band clearly separated from the core. Because of this variation in width and proximity to Fishery Bulletin 92(1). 1994 the core, the first annulus is sometimes difficult to identify. Subsequent annuli are represented by eas- ily identified, narrow, opaque bands that alternate with wider translucent bands outside the proximal margin of the first annulus (Fig. 1). Annuli are formed on otoliths once a year in the period April-May. For ages 1-7, mean monthly marginal increment plots show only one minima during the year, indicating that only one annulus is formed each year (Fig. 2). The trough starts abruptly in April, a period when there is, in general, maxi- mum variation in the mean marginal increment, suggesting that some fish have begun to form the annulus while others have not. Lowest marginal increment values occurred in May, the most inten- sive period of annulus formation. Marginal incre- E E c CD E CD CJ c ~co c en ca 18 •' ,6 ft* Age 1 jsJ Age 2 ) I! 1 9 rfj 12 rfi "1 17 ^rfl 30 IB i 14 12 6 I r 5 Age 3 12 snfin 39 12 1 9 25 1 T 4 I Age 4 7 1 T | 1 : 23 |* 10 '» 1*1 : . ,. -if II 27 a 10 JFMAMJ JASOND 3 e Age 5 21 13 7 4 * s 4 r5- Age 6 32 11 8 1 1 Age 7 nnn Age 8 H JFMAMJJASOND Months Figure 2 Mean monthly marginal increment for Atlantic croaker ages 1-8 from the Chesa- peake Bay region, 1988-91. Vertical bars are ±1 standard error. Numbers above the bars are sample sizes. Barbien et al Age, growth, and mortality of Micropogonias undulatus ment values progressively rise to a somewhat stable maximum from October through March or April, indicating a period of little or no otolith growth. Because only two age-8 fish were collected, it was not possible to validate annuli beyond age 7. To confirm our interpretation that the blurred opaque band around the otolith core represents the first annulus, (i.e., that fish hatched in the fall form a mark during their first spring), otolith sections of young of the year (94-114 mm) collected during the period March-June were examined. All those col- lected in March-April were developing fine opaque marks around the core, and all those in May-June had an opaque mark already formed (Fig. 3). Otolith age readings were very precise, both within and between readers. Percent agreement was Figure 3 Transverse otolith section of a young-of-the-year Atlan- tic croaker ( 114 mm TL) collected in June 1990 in Chesa- peake Bay. The arrow indicates the outer edge of the first annulus formed during the period April-May. SG=sulcal groove; Ve=ventral; Pr=proximal; a=artifact of preparation. 99.5% for reader 1, 99.3% for reader 2, and 99.2% between readers. In all cases of disagreement, the difference never exceeded 1 year. Only one of the 1,967 left otoliths sectioned was crystallized and could not be read. In that case, the right otolith was read. Difficulty in ageing Atlantic croaker from otolith sections did not increase with increasing age. However, proper identification of the first annulus was very important. All disagreements, independent of age, were due to problems in identifying the first annulus. Otolith size relative to fish size and age Changes in otolith size relative to fish size were not constant along all axes (Fig. 4). Otolith maximum length was the only axis that showed a linear, isometric increase with fish length. Otolith ra- dius, the axis along which annuli were read in transverse sections, showed a non-linear rela- tionship with fish length, and had the small- est r 2 of all variables (Fig. 4). The curvilinear relationship suggests that otolith growth rela- tive to fish growth slows down along this axis as fish get bigger. Despite its poor relationship with fish length, otolith radius showed a very strong linear re- lationship with fish age. An ANCOVA model showing length, age, and their interaction ex- plained 97% of the variation in otolith radius (Table 1). All factors in the model were highly significant (P<0.01). Similar models for otolith maximum length, maximum thickness, and weight were also highly significant and had high coefficients of determination (r 2 >0.85). However, significance for these models was due to fish length only, neither age nor the inter- action factor was significant. Growth Observed lengths varied greatly within ages (Fig. 5). Atlantic croaker showed a rapid in- crease in size during the first year, but annual growth rate greatly decreased during the sec- ond year, remaining comparatively low there- after (Fig. 5). On average, 64% of the cumula- tive total observed growth in length occurred in the first year and 84% was completed after two years. No differences in mean lengths at age were found between sexes (Mest at each age; P>0.05 for all ages). Mean observed total lengths for pooled sexes were 201, 263, 274, 285, 290, 307, 309, and 313 mm, for ages 1-8, respectively. Fishery Bulletin 92(1). 1994 A B 6 OR = -3 90 + 03 TL - 001 TL 2 ^10- OT = -2.73 + 0.04 TL - 0.0004 TL 2 r 2 = 43, P = 0001 £ r 2 = 65; P = 0001 Radius (mm) O CO 1.82 kg. Between 1977 and 1982, however, al- though the minimum citation weight was raised to 1.82 kg, 599 citations were issued, including 47 entries for Atlantic croaker >2.27 kg (483-610 mm TL). The largest number of citations occurred in 1979 and 1980, coinciding with Ross's (1988) sam- pling period in North Carolina. Records from the Delaware State Fishing Tournament show the same pattern as that from Virginia. The number of cita- tions was very small during the early 1970's, reached a peak in 1980, and decreased rapidly there- after. Although complete information covering their entire range is not available, state records of Atlan- tic croaker along the east coast of the United States show the same pattern. Records from Georgia to New Jersey were broken during the period 1977-82, indicating that 1) unusually large fish occurred during this period and have not occurred since; and 2) their occurrence was not limited to areas north of North Carolina. In conclusion, recent size and age composition data do not indicate the existence of a group of larger, older Atlantic croaker in the Chesapeake Bay region compared with more southern waters. His- toric information agrees well with our results and indicates that fish >400 mm TL have not repre- sented a large proportion of Atlantic croaker in this area. The abundance of unusually large fish during the period 1977-82 apparently constituted an un- usual event and may reflect passage through the fishery of a few strong year classes that seemingly disappeared after 1982. Similar episodes — the occur- rence of larger fish for a few years — have been pre- viously reported for Atlantic croaker in Chesapeake Bay (Hildebrand and Schroeder, 1928; Massmann and Pacheco, 1960), suggesting the phenomenon happens periodically. An increase in survivorship of Barbien et al.: Age, growth, and mortality of Micropogonias undulatus 1 I early spawned fish, combined with higher mortal- ity of late-spawned fish as a result of low winter temperatures in estuarine nursery areas (Mass- mann and Pacheco, 1960; Joseph, 1972; Warlen and Burke, 1991) could account for an increase in the proportion of larger fish in certain years and explain the episodic occurrence of large Atlantic croaker in this area. Our results for Chesapeake Bay, together with records of large fish south of North Carolina during 1977-82, suggest that the hypothesis of a basically different life history and population dynamics pat- tern for Atlantic croaker north and south of Cape Hatteras, North Carolina, should be reevaluated. However, sampling programs over time describing size and age compositions of Atlantic croaker throughout their range are still necessary to fully evaluate this question. Acknowledgments We would like to thank the Chesapeake Bay com- mercial fishermen and James Owens (VIMS) for helping us obtain samples. Sue Lowerre-Barbieri helped with fish processing and with otolith section- ing and reading. Claude Bain (Virginia Saltwater Fishing Tournament) and Jessie Anglin (Delaware Department of Natural Resources) provided infor- mation on Atlantic croaker recreational citation records. Ronald Hardy, Joe Loesch, Sue Lowerre- Barbieri, Jack Musick, Rogerio Teixeira, and two anonymous reviewers made helpful suggestions to improve the manuscript. Financial support was pro- vided by the College of William and Mary, Virginia Institute of Marine Science, by Old Dominion Uni- versity, Applied Marine Research Laboratory, and by a Wallop/Breaux Program Grant for Sport Fish Res- toration from the U.S. Fish and Wildlife Service through the Virginia Marine Resources Commission, Project No. F-88-R3. Luiz R. 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Chittenden Jr. 1977. Age determination, reproduction, and popu- lation dynamics of the Atlantic croaker, Micropogonias undulatus. Fish. Bull. 75:109- 123. Wilk, S. J. 1981. The fisheries for Atlantic croaker, spot, and weakfish. In H. Clepper (ed.) Proceedings of the 6th annual marine recreational fisheries sympo- sium, p. 5-27. Sport Fish. Inst., Washington. Wright, P. J. 1991. The influence of metabolic rate on otolith in- crement width in Atlantic salmon parr, Salmo salar L. J. Fish Biol. 38:929-933. Abstract. Statolith micro- structural analysis was applied to 126 specimens of the oceanic bo- real clubhook squid, Onychoteuthis borealijaponica, for estimation of age and growth rates. Specimens were captured from the western, central, and eastern North Pacific between approximately lat. 38° N and 47°N by driftnet fishing, trawling, and jigging in the sum- mers of 1990 and 1991. Results suggest that increments were de- posited at a rate of one per day. Both sexes live approximately one year; males mature at smaller sizes and younger ages than fe- males. Exponential growth models suggest that growth in length was similar for males and females (0.80% ML/day) in the central North Pacific, while growth in weight was higher for females ( 1.90% WT/day) than males (1.40% WT/day). Females in the western North Pacific exhibited faster growth rates than individuals from the central North Pacific. O. borealijaponica were estimated to have hatched year round based on back calculation of statolith incre- ments from the time of capture. Post-recruit individuals exploited in the O. borealijaponica jig fish- ery and Ommastrephes bartramii driftnet fishery typically hatched from late summer to early winter. Age and growth of the oceanic squid Onychoteuthis borealijaponica in the North Pacific Keith A. Bigelow Honolulu Laboratory, Southwest Fisheries Science Center National Marine Fisheries Service. NOAA 2570 Dole Street, Honolulu, HI 96822-2396 The oceanic boreal clubhook squid Onychoteuthis borealijaponica Okada, 1927 is common in subarc- tic waters of the North Pacific. This species ranges from the western coast of the United States and Canada to the eastern coast of Hokkaido, Japan, and the Kurile Islands, but does not occur in the Sea of Okhotsk or Bering Sea (Young, 1972; Murata et al., 1976; Naito et al., 1977a; Fiscus and Mercer, 1982; and Kubodera et al., 1983). Onychoteuthis borealijapon- ica has commercial value through- out its range. Between 1971 and 1979, commercial landings aver- aged 1,171 metric tons (t) per year from a jig fishery in oceanic waters east of Hokkaido, Japan (Okutani and Murata, 1983), and approxi- mately 254 and 2,705 t of O. bor- ealijaponica were caught in 1990 and 1991, respectively, by Japan, Korea, and Taiwan in the Ommas- trephes bartramii highseas driftnet fishery (DiNardo and Kwok, in re- view 1 ). Based on exploratory fish- ing, Fiscus and Mercer (1982) sug- gested that O. borealijaponica could be commercially exploited by a jig fishery from the Gulf of Alaska westward to the Aleutian Islands, and Murata (in Okutani, 1977) in- dicated that the potential fishery yield of O. borealijaponica may be 50,000-200,000 t in an area west of Manuscript accepted 26 July 1993 Fishery Bulletin 92:13-25 (1994) 1 DiNardo, G. T., and W. Kwok. In review. Estimates offish and cephalopod catch in the North Pacific high-seas driftnet fish- eries, 1990-91. long. 152°E and lat. 40-45°N. If a commercial fishery does develop, accurate life-history information is essential for management purposes. The general biology and feeding ecology of Onychoteuthis borealija- ponica have been investigated (Naito et al., 1977b; Okutani and Murata, 1983); however, little in- formation is available on age and growth. Average growth rates have been inferred from length-fre- quency distributions of sequential jigging samples (Murata and Ishii, 1977). This study suggested that the lifespan for boreal clubhook squid is approximately one year; females grow faster and attain a larger size (370 mm mantle length (ML)) than males (270 mm ML). Growth estimates from driftnet studies (Kubodera et al., 1983; Kubodera, 1986) were inconclusive because length-frequency modes were impossible to detect, possibly because of protracted spawning seasons or variable individual growth rates within a population. The accuracy and precision of cephalopod growth estimates have been greatly enhanced through the use of daily increments within sta- toliths (Natsukari et al., 1991). Ageing by counting statolith incre- ments allows the estimation of size at age and may provide informa- tion on individual age and growth rates. Hatchdates can be estimated by back calculation of daily incre- ments. Age and growth estimates derived from statolith analysis 13 14 Fishery Bulletin 92(1), 1994 have been obtained from a variety of neritic squid species (see review by Rodhouse and Hatfield, 1990a). The objectives of this study were to 1) estimate the age and growth of O. borealijaponica from sta- tolith microstructural analysis, 2) determine the periodicity of increment formation, 3) statistically compare appropriate growth models fit to the age- ing data, 4) determine the distribution of back-cal- culated hatching dates of O. borealijaponica and draw inferences about spawning locations, and 5) determine the relationship between age and matu- rity stages. Materials and methods Taxonomic clarifications At least five onychoteuthid species are found in the North Pacific: O. borealijaponica from subarctic waters; an undescribed species occupying the North Pacific transition zone (-29— 40"N, Bigelow, unpubl. data); and three subtropical species of the O. banksii complex (Young and Harman, 1987). Juvenile, sub- adult, and adult O. borealijaponica (69-343 mm ML) were separated from other onychoteuthid species based on the number of tentacular hooks (n =25-29) on each club. Identification of O. borealijaponica paralarvae (11.5 to 35 mm ML) was based on mantle chromatophore patterns (Bigelow, unpubl. data). Data collection Subadults, adults During July-September 1990, O. borealijaponica specimens were collected on vari- ous research cruises in the North Pacific. Most squid specimens were captured by research drift net ( mesh size=48-220 mm stretch mesh) in the western and central North Pacific, but squid jigs were also used to capture specimens from the central and eastern North Pacific (Fig. 1, Table 1). Squid samples were frozen (-20°C) upon capture and returned to the laboratory for analysis. Paralarvae, juveniles From 5 to 24 August 1991, 39 tows with a modified Cobb trawl were made along meridian 179°30'W between 36°56'N and 46°00'N, and along meridian 174°30'W between 39°00'N and 45°00'N. The trawl was dual warp, with a mouth area of approximately 140 m 2 when fish- ing and a cod-end liner constructed of 3.2 mm knotless nylon delta mesh ( Wyllie Echeverria et al., 1990; Lenarz et al., 1991). Thirty-one oblique night tows (0-150 m) and eight oblique day tows (0-750 m> were conducted. O. borealijaponica specimens from eight tows (Fig. 1, Table 1) were sorted on board and immediately frozen (-20°C, juveniles) or fixed in 95% ethyl alcohol (paralarvae). Laboratory analysis Dorsal mantle length measurements were made to the nearest millimeter (mm) on thawed specimens. Squids less than 0.5 g were blotted dry and weighed to the nearest 0.001 g, whereas larger specimens were weighed to the nearest 0.1 g. No correction was made for shrinkage of paralarvae from fixation in ethanol, because the species possesses a strong gladius and exhibited minimal shrinkage (<2%). Specimens were sexed and assigned a maturity stage (I: juvenile; II: immature; III: preparatory; IV: maturing; V: mature) based on the appearance and relative size of the gonads and accessory repro- ductive organs (Lipinski, 1979). Statoliths were dis- sected from the specimens and stored in 95% ethyl alcohol. Statolith preparation and microstructural analysis One statolith of the pair was mounted on a microscope slide in Eukitt resin (Calibrated In- struments Inc. 200 Saw Mill Rd., Hawthorne, NY 10532) with the concave side (anterior) facing up. The transparency of paralarval statoliths allowed their examination without further preparation (Fig. 2). The thickening of statoliths from larger squid (>35 mm ML) required that they be ground with fine-grained (1200-grade) carborundum paper and polished with 0.3-|am alumina-silica powder prior to microstructural examination. Increments were counted beginning at the first visible increment outside the nucleus (Fig. 3A), and continued to the margin of the dorsal dome (Fig. 3B). The diameter of the circular nucleus averaged 28.0 urn (SD=2.4 |im, n-37). The precision of increment counts was assessed by using the coefficient of varia- tion (Chang, 1982). Two nonconsecutive blind incre- ment counts were made on each statolith with trans- mitted light at a magnification of 1500x. The mean of the two increment counts was accepted if the co- efficient of variation was <7.0%, otherwise a third count was conducted. With this criteria, two incre- ment counts were acceptable for 115 statoliths, whereas three increment counts were required for 11 statoliths. Hatching dates were computed by subtracting the mean increment count from the date of capture and were pooled into monthly periods. Increment counts were assumed to represent the individuals' age in days, based on the following re- sults (periodicity of increment deposition) which provided support for the hypothesis that one incre- ment is deposited per day. Bigelow Age and growth of Onychoteuthis boreahjaponica 15 Figure 1 Location of stations in the North Pacific sampled for Onychoteuthis boreahjaponica: Western North Pacific 1990 (open circles), central North Pacific 1990 (closed circles), central North Pacific 1991 (closed triangles), and east- ern North Pacific 1990 (open squares). Periodicity of increment deposition Three sub- adult squid caught by jig or trawl in the central North Pacific were placed for two hours in 20 L of seawater containing 250 mg/L oxytetracycline hy- drochloride (OTC). After OTC exposure, squid were maintained in a 20-L tank with flowthrough sea- water under ambient photoperiod and temperature conditions. Freshly captured live saury (Cololabis saira) were introduced as prey, but no feeding was noted or observed. Squids survived up to 61.5 hours in captivity. Statoliths were prepared as above and illuminated with ultraviolet (Fig. 4) and natural light. Under fluorescent light, an ocular marker was aligned with the inner edge of the OTC band. The statolith was then examined under natural light, but increments peripheral to the band were difficult to count. Therefore, to determine the periodicity of increment deposition, statolith growth following OTC exposure was related to the average increment width prior to exposure. The distance from the in- ner edge of the OTC band to the statolith perimeter was divided by the mean width of increments prior to the OTC band. Three estimates of statolith growth after OTC exposure were made, and the average increment width calculated for 15 incre- ments prior to the OTC band. Statistical procedures Mantle length-weight relationships Mantle length-weight regressions were fit to the data by using the model WT(g) = a*ML(mm) b (D Separate ML-weight equations were developed for both sexes, and a single equation was used for squid of unknown sex (<60 mm ML). Fitting of size-at-age data Researchers have used a variety of different models to describe cephalopod growth (e.g., linear, logistic, von Bertalanffy), al- though the rationale for using a given model is usu- ally not stated. Schnute ( 1981 ) proposed a flexible four- parameter model to describe growth which includes most growth models historically used in fisheries re- search as special cases. The model takes the form Y(t): V, +(Vi v ): 1-e-""-' 1 ' -.j U,-r, ) l/i (2) 16 Fishery Bulletin 92(1), 1994 Table 1 Data on samples of Onychoteuthis borealijaponica collected for age analysis Mantle Depth Temperature length Date Lat. Long. Gear (m) CO n (mm) Western North Pacific 24 Jul. 1990 42 - 00'N 158°58'E Driftnet 0-10 14.9 5 197-316 25 Jul. 1990 43'03'N 158'59'E Driftnet 0-10 15.5 12 203-311 26 Jul. 1990 44'02'N 158 a 56E Driftnet 0-10 16.1 14 214-343 28 Jul. 1990 44'00'N 160"00'E Driftnet 0-10 16.5 15 206-339 29 Jul. 1990 43'16'N 159'58'E Driftnet 0-10 15.7 8 204-233 06 Aug. 1990 43'30'N 161"02'E Driftnet 0-10 16.0 2 275-288 20 Sep. 1990 44"45'N 160 = 03'E Driftnet 0-10 15.7 Total 1 57 182 Central North Pacific SAMPLE A 08 Jul. 1990 42'30'N 172°32'W Driftnet 0-8.5 14.8 2 165-195 04 Aug. 1990 46"30'N 152 U 30W Driftnet 0-8.5 12.1 4 147-180 10 Aug. 1990 46'30'N 157'30'W Driftnet 0-8.5 11.8 7 191-313 12 Aug. 1990 43'29'N 157'27'W Driftnet 0-8.5 14.3 1 343 SAMPLE B 06 Aug.1991 37"59'N 179'28'W Cobb 0-154 11.7-24.1 5 11.5-32 06 Aug.1991 37'55'N 179 - 26'W Cobb 0-158 11.7-24.1 H 24-35 09 Aug.1991 41°08'N 179"30'W Cobb 0-130 11.0-20.3 1 42 12 Aug.1991 43°12'N 179"30'W Cobb 0-775 3.5-16.4 1 58 12 Aug.1991 43°04'N 179"30'W Cobb 0-156 8.6-15.9 69-83 15 Aug.1991 44"59'N 179°27'W Jig 0-5 12.6 7 119-190 18 Aug.1991 45°00'N 174 31'W Cobb 0-162 6.8-13.2 4 75-82 20 Aug.1991 43'00'N 174°30'W Cobb 0-142 8.8-16.5 2 72-78 22 Aug.1991 41'14'N 174"29'W Cobb 0-730 5.6-21.1 Total 1 49 66 Eastern North Pacific 18 Aug. 1990 42°47'N 125°25'W Jig II 100 15.1 5 214-251 19 Aug. 1990 44'12'N 124"54'W Jig 0-100 15.9 2 229-236 04 Sep. 1990 44°23'N 124'44'W Jig 0-75 16.4 Total 13 20 218-312 where Y(t) is the estimated length or weight at age t, andy 1 and v., represent size at two ages t x and t.„ which are typically the youngest and oldest indi- viduals in the sample. The estimated parameters a and b describe how the model connects y ; and y 2 . Values of a and b and their 95% confidence inter- vals lead to the selection of other submodels. The Schnute model (written in Microsoft Quickbasic) was fit to the size-at-age data (Fig. 5) by nonlinear regression on an IBM-compatable mi- crocomputer. Growth modelling was restricted to individuals from the central North Pacific samples, because of inadequate age representation from the western and eastern North Pacific samples. Paralarval size-at-age estimates were included in the growth models for males and females, because size-at-age results were similar for juvenile (66-83 mm ML) males and females. Model comparison If we assume that the Schnute model exactly predicts the size of an individual, then the residual sum of squares (RSS) of this full model is an estimate of measurement error. To ascertain if a reduced model with fewer parameters (e.g., 2- parameter exponential) adequately describes the data, the RSS's from the reduced model and full model were compared using an F test statistic: ( RSS R - RSS F )/( DF h - DF F ) RSS f /DF F f- with DF R - DF F ,DF F degrees of freedom. Bigelow Age and growth of Onychoteuthis borealijaponica 17 Figure 2 Onychoteuthis borealijaponica. Light micrograph of a transverse section of a sta- tolith from a 11.5-mm mantle length paralarva. Duplicate increment counts were 61 and 63. 40 im 40 urn '" Figure 3 Onychoteuthis borealijaponica. Light micrographs of a ground statolith. (A) Increment deposition within early life history. Arrow indicates edge of nucleus. (B) Statolith microstructure within dorsal dome region. Fishery Bulletin 92(1). 1994 Figure 4 Onychoteuthis borealijaponica. UV micrograph of a ground statolith stained with tetracycline. where RSS,, is the RSS from the full (Schnute) model, RSS R is the RSS from the reduced (exponential) model, DF F is the number of degrees of freedom from the full model, and DF R is the number of degrees of freedom from the reduced model (Neter et al., 1985). Differences in the slopes of the ML- weight and size-at-age relationships by sex and geographical location were compared with analysis of covariance (ANCOVA) and F-tests (Sokal and Rolff, 1981). Data were initially In-transformed, and ANCOVA was used to test for differences in slopes of the linearized equations. Elevations of the lin- earized equations were compared with F- tests. Analyses were performed on central North Pacific male and female growth data and western North Pacific female data with the assumption that females in the western North Pacific exhibited a similar type of growth as individuals in the central North Pacific. There were too few individu- als to test for differences in growth rates 300 -i 500 -| 250 - Males 400 - Males 200 - 300 - 150 - 100 - 200 - E E 50 - 100 - X - Z 50 100 150 200 250 300 350 400 450 t 50 100 150 200 250 300 350 400 450 J o £ 400 -, 1000 n | 350 - Females Females 300 - 750 - 250 - 200 - 500 - 150 - 100 - 250 - 50 - 50 100 150 200 250 300 350 400 450 50 100 150 200 250 300 350 400 450 AGE (days) AGE (days) Figure 5 Relation between age (determined by number of increments within statoliths) and mantle length (mm) and weight (g) for male and female Onychoteuthis borealijaponica. Western North Pacific 1990 (open circles), central North Pacific 1990 (closed circles), central North Pacific 1991 (closed triangles = juveniles-subadults, open triangles = unknown sex), and eastern North Pacific 1990 (open squares). Bigelow Age and growth of Onychoteuthis borealijaponica of western North Pacific males or eastern North Pa- cific males and females. Results Statolith analysis Statolith microstructural analysis was applied to 131 squid from the western, central, and eastern North Pacific. Five statoliths (3.8%) were broken or poorly sectioned and excluded from further analy- sis. The coefficient of variation about the mean for the aged samples (n = 126) averaged 3.7% based on 2-3 increment counts for each statolith. No obvious trend existed in the coefficient of variation with the increment count or body size. Periodicity of increment formation A fluorescent OTC band was evident in the sta- toliths of the three squid exposed to oxytetracycline. While increments peripheral to the inner edge of the OTC band could not be reliably counted, the rela- tion between the growth of the statolith, rearing period, and the width of increments prior to the OTC band suggested that increments were deposited daily (Table 2). Statolith growth in the dorsal dome region ranged from 1.4 to 4.3 urn over the rearing period (26-61.5 hr). The average number of incre- ments deposited per day after oxytetracycline expo- sure was 1.30 (range 1.08-1.52) for the three squid. Mantle length-weight relationships The ML-weight relationship for paralarval O. borealijaponica from the central North Pacific is represented by the equation WT = 2.484 x 10" 5 ML 3015 ;fl 2 = 0.99(n = 36). (4) The ML-weight relationships for juvenile-adult O. borealijaponica from the western, central, and east- ern North Pacific are represented by the following equations: males: WT = 1 .873 x lO^ 4 ML 2596 ;J? 2 = 0.96(n = 43) (5) females: WT = 3.521 x \0- ML 2SX5 ;R 2 = 0.99(n = 68) (6) The slopes of the ML-weight regressions for male and female O. borealijaponica were significantly different (P<0.001). Growth A good relationship existed between the number of increments within statoliths and squid size for in- dividuals in the central North Pacific (Fig. 5). An exponential model (Table 3, Equation 7) was appro- priate to describe the ML-at-age relationship (f-1.82, F=2A9) for females (paralarvae-subadult) in the central North Pacific. A logistic model was ap- propriate to describe the ML-at-age relationship (f=1.85, f=2.93) for males (paralarvae-adult) in the central North Pacific. However, the oldest individual (394 days, 245 mm ML) was a mature male (stage V) which influenced the type of model selected. Omitting that individual resulted in the selection of an exponential model (f=2A9, F=2.55) over a logis- tic model (/"=4.73, F=2.94) to describe paralarval- subadult growth (Table 3, Equation 8). Exponential models were also fit to weight-at-age data for paralarval-subadult males and females (Table 3, Equations 9 and 10). Growth in length (% increase in length per day) was similar for males and females (0.80% ML/day) in the central North Pacific, while growth in weight was faster for females (1.90% WT/day) than males (1.40% WT/day). By using the exponential models, mantle length, weight-at-age, and absolute growth Table 2 Age validation information for Onychoteu this b orealijaponica with oxytetracycline (OTC) tech nique. Width of oxytetracycline band is the distance observed between the fluorescent band and the margin of the statolith. Mean increment width is that of the outer 15 increments formed prior to the OTC band. Width of Estimated Rearing oxytetracycline Mean increment increments No. ML Imm) period (hr) band (pm) width (|im) per day 1 162 26 1 1 1.19 1.08 2 166 61.5 4.3 1.10 1.52 3 175 44 L'K 1.16 1.32 20 Fishery Bulletin 92(1), 1994 Exponential e Pacific Ocean quati ons for growth of ma e and fe Table 3 male Onychoteuthis borea lijaponica from the central North Variable Age interval (d) n Equation r 2 Equation no. Length (F) Length (M) Weight (F) Weight (M) 62-376 62-314 62-376 62-314 36 27 36 27 mm = I8.41e 000785t mm = I7.17e 000798t g = 0.74e 00188t g = 2.19e 001381 0.97 0.89 0.92 0.82 7 8 9 10 rates (AGR, mm/day or g/day) were predicted for the initial 365 days (Table 4). The slopes of the size-at-age regression equations for females from the western North Pacific were significantly different from those for both central North Pacific males and females (Fig. 6, Table 5). Comparisons of regression slopes between central North Pacific males and females revealed no signifi- cant differences in length or weight-at-age relation- ships (P=0.424, P=0.307). Testing of elevations from the central North Pacific male and female data iden- tified a significant difference (P<0.001, Table 5); therefore, males and females in the central North Pacific grow in length and weight at a similar rate, but females display a significantly greater size at age than males (Table 4). Back-calculated hatching dates Backcalculation of hatching dates demonstrated that O. borealijaponica hatched in all months except March (Fig. 7). The distribution of hatching dates was not necessarily related to spawning intensity, as more subadult squid were available for age analy- sis than paralarvae and juveniles. Subadult and adult squid captured from July to September in the North Pacific had similar hatch dates as samples collected from the western (August-February), cen- tral (July-February), and eastern North Pacific (Au- gust-November). Paralarval and early juvenile squid captured in the central North Pacific during August 1991 were estimated to have hatched be- tween February and June, 1991. Maturity stage-age relationships Maturity stages were closely related to squid size for all three sampling areas; males, however, matured at a smaller size than females (Fig. 8). Females and males recruit to the driftnet fishery after attaining maturity stages III and IV, respectively. No mature females (stage V) were captured by any sampling Table 4 Growth of cen tral North Pacific Onychoteuth is borealijapor ica pred cted by the exponential equations based on statolith analysis Ab solute grow th rates (AGR) are given in mm or g per day. Estimated age Mi les Females Mantle length Weight Mantle length Weight (days) (mm) AGR L (g) AGR W (mm) AGR, (g) AGR W 50 25.6 0.20 4.4 0.06 27.3 0.21 1.9 0.04 75 31.2 0.25 6.1 0.09 33.2 0.26 3.0 0.06 100 38.1 0.31 8 7 0.12 40.3 0.32 4.8 0.09 125 46.5 0.37 12.2 0.17 49.1 0.39 7.7 0.15 150 56.8 0.46 17.3 0.24 59.7 0.47 12.4 0.24 175 69.4 0.56 24.3 0.34 72.7 0.57 19.8 0.38 200 84.7 0.68 34.3 0.48 88.4 0.70 31.7 0.60 225 103.4 0.83 48.5 0.67 107.5 0.85 50.8 0.96 250 126.2 1.01 68.4 0.95 130.8 1.03 81.3 1.54 275 154.1 1.23 96.5 1.34 159.2 1.25 130.0 2.47 300 188.1 151 136.1 1.89 193.7 1.53 ■J IIS II 3.95 325 229.6 1.84 192.1 2.66 235.7 1.86 332.8 6.31 350 280.3 2.24 271.0 3.75 286.7 2.26 532.5 1(1 K) 365 316.0 2.57 337.3 4.66 323.2 2.51 706.9 13.42 Bigelow: Age and growth of Onychoteuthis borealijaponica 8 " Males - Central North Pacific E , Females - Centra] North Pacific E 7 " Females - Western North Pacific I H 6- , ..-5 ' o .-- — ■" *- z S^' 3 s - ^S UJ ST J jS H «- s' Z ^0< < ^^ 2 3- ^ _c 8 - 7 - — 6 - --"'"'*"/ ' M H 5 " = 4- O nj 3 - // * :- c — i - - -1 - / ) 50 100 150 200 250 300 350 400 450 AGE (days) Figure 6 Log-lin ear growth models for male and female Ony- choteut his borealijaponica . method. There was some evidence that males (stage IV-V) and females (stage III— IV) in the western North Pacific were younger than similar stage in- dividuals from the central and eastern North Pacific. Discussion The data presented provide support for the one-in- crement-deposited-per-day hypothesis within the statoliths of Onychoteuthis borealijaponica although further work is required to rigorously test the hy- pothesis. Tetracycline was incorporated into the sta- tolith, but the animals did not feed and survival was not sufficiently long enough (2-3 days) to provide a rigorous test on the rate of increment deposition. Validation of the daily increment hypothesis has come from tetracycline labeled statolith experiments with several neritic squid species (Illex illecebrosus, Dawe et al., 1985, Alloteuthis subulata, Lipinski, 1986, Todarodes pacificus, Nakamura and Sakurai, 1991). Future statolith validation experiments with 20 - Western North Pacific suhadults/adulls 15 - 10 - 5 - ^^ Central North Pacific 0! 15 - m S io- Z 5- - 20 - ^ — subadults/adulls ■1- □- - paralarvae/j uve rules Eastern North Pacific subadulisyadulls 15 - 10 - M 5 - _^Hb JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC MONTH OF HATCHING Figure 7 Estimated hatch-date distributions by month for Ony- choteuthis borealijaponica. Table 5 Comparisons of Onychote uthis borealijaponica growth equations based on analysis of covariance. Slope Elevation (F) (F) P Length-at-age comparisons Central North Pacific, male vs. female 0.647 0.424 1002.4 <0.001 Western North Pacific female vs. central Pacific female 62.9 <0.001 Western North Pacific female vs. central Pacific male 62.2 <0.001 Weight-at-age comparisons Central North Pacific, male vs. female 1.063 0.307 1121.1 <0.001 Western North Pacific female vs. central Pacific female 65.9 <0.001 Western North Pacific female vs. central Pacific male 66.6 <0.001 22 Fishery Bulletin 92(1), 1994 V Males B IV t* 111 • II • 1 , i ' i i Males H3— -B- 100 Females 200 Mil) Females 200 did Kill 500 100 200 300 MANTLE LENGTH (mm) )(i( i 200 300 AGE (days) Figure 8 Relationship between mantle length and number of increments in sta- toliths and male and female maturity stage: western North Pacific (open circles), central North Pacific (closed circles), and eastern North Pacific (open squares). Ranges are represented by horizontal bars. active oceanic squids (e.g., Onychoteuthidae, Ommastrephidae) may require substantial mainte- nance facilities to support long-term survival. Although the rate of increment deposition derived by the statolith marking experiment should be con- sidered preliminary, indirect evidence was obtained to suggest that increments were formed daily. The hypothesis that the lifespan is 1 year (Murata and Ishii, 1977; Naito et al., 1977b) was supported by the present data where only 4 of the 126 individuals aged had more than 365 increments within the sta- tolith. In addition, back-calculated hatch dates (July-February) of post-recruit individuals exploited in the O. borealijaponica jig and Ommastrephes bartramii squid driftnet fishery were consistent with information on spawning (fall-winter) reported in the literature (Murata et al., 1976; Murata and Ishii, 1977; Naito et al., 1977b). This study suggests that spawning for O. borealijaponica occurs year round. While subadult O. borealijaponica are distributed in subarctic waters, evidence from the distribution of paralarvae, juveniles, and sexually mature females suggests that spawning may occur to the south of the subarctic boundary in the North Pacific transi- tion zone (30^42°N, terminology after Roden, 1991). In the central and eastern North Pacific, O. borealijaponica paralarvae and juveniles have been recorded from this study (38°N, 179°30'W°) and the coast of California (~33°N, Young, 1972), respectively. In the western North Pacific, spawning may oc- cur in waters of the Kuroshio Cur- rent and Kuroshio Countercurrent (Murata and Ishii, 1977; Naito et al., 1977a) or between the Kuro- shio and Oyashio fronts. Onycho- teuthid paralarvae have been cap- tured from both the Kuroshio Cur- rent and Kuroshio Countercurrent (Okutani, 1968, 1969, 1975); how- ever, distributional evidence is in- conclusive because of the taxo- nomic uncertainties of the speci- mens captured. Spawning may occur in the transitional area be- tween the Kuroshio and Oyashio fronts, as sexually mature and copulated females have been cap- tured off Hokkaido, Japan (42°30TSJ, 150°40'E and 42°15'N, 144°25'E, Murata et al., 1981). The ML-weight relationships obtained in this study for the western, central, and eastern North Pacific were similar to the values previously given for O. borealijaponica cap- tured off Japan (Murata and Ishii, 1977). Slope val- ues obtained for the ML-weight relationships (males=2.596, females=2.915) were similar to other active oceanic squids having thick muscular mantle walls. Paralarval O. borealijaponica had a higher slope value (3.015) than older males and females, consistent with previous results for loliginid squids and benthic octopods (Forsythe and Van Heukelem, 1987). There is no clear consensus on the type of model which best describes cephalopod growth, although several studies argue against the use of asymptotic models, such as Gompertz or von Bertalanffy (Forsythe and Van Heukelem, 1987; Saville, 1987). Exponential models have been typically used to describe the growth of field caught and laboratory reared paralarval squid (Yang et al., 1986; Balch et al., 1988; Forsythe and Hanlon, 1989; Bigelow, 1992, 1993). For growth estimates derived from statolith analysis, a linear model is frequently used because growth is analyzed over a short segment of the cephalopod's life history, such as post recruitment to a fishery (Rosenberg et al., 1980; Radtke, 1983; Rodhouse and Hatfield, 1990b) or habitat (Jackson and Choat, 1992). Since the Schnute model encompasses a wide range of growth models, it can be used to system- Bigelow Age and growth of Onychoteuthis borealijaponica 23 atically assess the type of growth model which best describes the data. A statistical comparison of sev- eral growth models found that growth in O. borealijaponica from the paralarval to subadult size range could be sufficiently described with an expo- nential model, though there was weak evidence that a logistic model may be sufficient to describe growth in males from the paralarval to adult size range. The most appropriate growth model (exponential or lo- gistic) for the entire life cycle of O. borealijaponica will emerge when sexually mature males and fe- males are aged. Estimated growth rates from this study were higher than estimates derived from length-fre- quency analysis of fisheries data (Murata and Ishii, 1977). Growth estimates based on length-frequency analysis with time often provide evidence of de- creased growth rate, which is usually described by an asymptotic model (Patterson, 1988). Length-fre- quency analysis may be inappropriate for estimat- ing growth in cephalopods (Jackson and Choat, 1992), either because 1) cohorts are difficult to de- tect because spawning occurs throughout the year, 2) variable individual growth rates produce Lee's phenomenon (Ricker, 1975), or 3) samples of a mi- grating population are taken at a point along the migration route, which results in overestimating growth in young squid and underestimating growth in older squid. Growth data presented for O. borealijaponica from the central North Pacific provide a useful compari- son of growth between males and females. The ex- ponential models predict that males and females grow in length at similar rates (0.80% ML/day), but females grow faster in weight (1.90% WT/day) than do males (1.40% WT/day). These rates correspond closely with the average growth rates of similar sized squids from temperate waters (e.g., Illex illecebrosus, O'Dor, 1983; /. argentinus, Rodhouse and Hatfield, 1990b). The most significant advantage of using statolith ageing techniques is the ability to produce indi- vidual rather than population statistics. Using sta- tolith analysis, spatial variations in size at age, growth parameters, and maturity stage at age were observed between O. borealijaponica individuals from the western and central North Pacific. Little is known concerning genetic variation and stock structure of O. borealijaponica in the North Pacific; however, female squid in the western North Pacific were found to grow faster than both male and fe- male squid in the central North Pacific and were younger at maturity stages III and IV than central North Pacific females. Apparent growth rate and maturity stage differences may be related to water temperatures or food availability during the paralarval stage. Forsythe and Hanlon (1989) showed that temperature had a pronounced effect on the increase in length and weight of the squid Loligo forbesi. In their laboratory study, a tempera- ture increase of 1°C increased the growth in length and weight of paralarval squid 0.5% and 2.0% per day, respectively. Subadults in the western Pacific may have hatched in the warm Kuroshio Current or in productive transition waters between the Kuroshio and Oyashio fronts. Paralarvae hatched in the western North Pacific may therefore experience higher temperatures or a greater abundance of prey species, or both, than paralarvae hatched in the central North Pacific, which could explain the ob- served spatial differences in growth. Acknowledgments I gratefully acknowledge the help of the officers and crew of the research vessels Hai Kung, and Shoyu Maru and the help of the officers, crew, and scien- tific field party of the NOAA ship Townsend Crom- well cruise 91-06. 1 would like to thank C. H. Fiscus who kindly provided the statolith samples from the eastern North Pacific and D. R. Kobayashi for assis- tance in fitting the Schnute model. This paper benefitted from comments by G. T. DiNardo, E. E. DeMartini, C. H. Fiscus, and anonymous reviewers. Literature cited Balch, N., A. Sirois, and G. V. Hurley. 1988. Growth increments in statoliths from paralarvae of the ommastrephid squid lllex (Cephalopoda: Teuthoidea). Malacologia 29:103- 112. Bigelow, K. A. 1992. Age and growth in paralarvae of the meso- pelagic squid Abralia trigonura based on daily growth increments in statoliths. Mar. Ecol. Prog. Ser. 82:31-40. 1933. Hatch dates and growth of Ommastrephes bartramii paralarvae from Hawaiian waters as determined from statolith analysis. In T. Okutani (ed.), Proceedings of the international symposium on the recent advances in cephalopod fisheries biology, p. 15-24 University Press, Tokai, Japan. Chang, W. Y. B. 1982. A statistical method for evaluating the repro- ducibility of age determination. Can. J. Fish. Aquat. Sci. 39:1208-1210. Dawe, E. G., R. K. O'Dor, P. H. Odense, and G. V. Hurley. 1985. Validation and application of an ageing tech- 24 Fishery Bulletin 92(1). 1994 nique for short-finned squid (lllex illecebrosus) J. Northwest Atl. Fish. Sci. 6:107-116. Fiscus, C. H., and R. W. Mercer. 1982. Squids taken in surface gillnets in the North Pacific Ocean by the Pacific salmon investigations program 1955-72. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-FNWC-28, 31 p. Forsythe, J. W., and W. F. Van Heukelem. 1987. Growth. In P. R. Boyle (ed.), Cephalopod life cycles, vol. II, p. 135-156. Academic Press, Lon- don. Forsythe, J. W., and R. T. Hanlon. 1989. Growth of the eastern Atlantic squid, Loligo forbesi Steenstrup (Mollusca: Cephalopoda). Aquat. Fish. Manage. 20:1-14. Jackson, G. I)., and J. H. Choat. 1992. Growth in tropical cephalopods: an analysis based on statolith microstructure. Can. J. Fish. Aquat. Sci. 49:218-228. Kubodera, T. 1986. Relationships between abundance of epipe- lagic squids and oceanographical-biological envi- ronments in the surface waters of the subarctic Pa- cific in summer. Int. North Pac. Fish. Comm. Bull. 47:215-228. Kubodera, T., W. G. Pearcy, K. Murakami, T. Kobayashi, J. Nakata, and S. Mishima. 1983. Distribution and abundance of squids caught in surface gillnets in the subarctic Pacific, 1977- 1981. Mem. Fac. Fish. Hokkaido Univ. 30(1/2): 1-49. Lenarz, W. H., R. J. Larson, and S. Ralston. 1991. Depth distributions of late larvae and pelagic juveniles of some fishes of the California current. CalCOFI Rep. 32:41-46. Lipinski, M. 1979. Universal maturity scale for the commer- cially important squids. The results of maturity classification of the lllex illecebrosus population for the years 1973-77. ICNAF Res. Doc. 79/2/38, Serial 5364, 40 p. 1986. Methods for the validation of squid age from statoliths. J. Mar. Biol. Assoc. U.K. 66:505-525. Murata, M., and M. Ishii. 1977. Some information on the ecology of the oce- anic squid, Ommastrephes bartrami (Lesueur), and Onyehoteuthis borealijaponicus Okada, in the Pacific Ocean off Northeastern Japan. Bull. Hokkaido Reg. Fish. Res. Lab 42:1-23. (In Japa- nese; English abstract.) Murata, M., T. Ishii, and H. Araya. 1976. The distribution of the oceanic squids, Ommastrephes bartramii (Lesueur), Onyehoteuthis borealijaponicus Okada, Gonatopsis borealis Sasaki and Todarodes pacificus Steenstrup in the Pacific Ocean off north-eastern Japan. Bull. Hokkaido Reg. Fish. Res. Lab 41:1-29. (In Japa- nese; English abstract.) Murata, M., M. Ishii, and M. Osako. 1981. Some information on copulation of the oce- anic squid Onyehoteuthis borealijaponica Okada. Bull. Jap. Soc. Sci. Fish. 48:351-354. (In Japanese; English abstract.) Naito, M., K. Murakami, T. Kobayashi, N. Nakayama, and J. Ogasawara. 1977a. Distribution and migration of oceanic squids (Ommastrephes bartramii, Onyehoteuthis boreali- japonicus, Berryteuthis magister, and Gonatopsis borealis) in the western subarctic Pacific region. Spec. Vol., Res. Inst. N. Pac. Fish., p. 321-337. (In Japanese; English abstract.) (Engl, transl. by W. G. Van Campen, 1991, 23 p., Trans- lation No. 144; available Honolulu Lab., Southwest Fish. Sci. Cent., Natl. Mar Fish. Serv., NOAA, 2570 Dole St., Honolulu, HI 96822-2396.) Naito, M., K. Murakami, and T. Kobayashi. 1977b. Growth and food habit of oceanic squids (Ommastrephes bartramii, Onyehoteuthis boreali- japonicus, Berryteuthis magister and Gonatopsis borealis) in the western subarctic Pacific region. Spec. Vol., Res. Inst. N. Pac. Fish., p. 339-351. 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Boyle, (ed.), Cephalopod life cycles, vol. I, p. 175-199. Aca- demic Press, London. Okutani, T. 1968. Studies on early life history of decapodan Mollusca-III. Systematics and distribution of lar- vae of decapod cephalopods collected from the sea surface on the Pacific coast of Japan 1960- 1965. Bull. Tokai Reg. Fish. Res. Lab. 55:9-57. 1969. Studies on early life history of decapodan Mollusca- IV. Squid larvae collected by oblique hauls of a larva net from the Pacific coast of east- ern Honshu, during the winter seasons. Bull. Tokai Reg. Fish. Res. Lab. 58:83-96. 1975. Studies on early life history of decapodan Mollusca-V Systematics and distribution of epipe- Bigelow Age and growth of Onychoceuthis boreahjaponica 25 lagic larvae of decapod cephalopods in the south- western waters of Japan during the summer in 1970. Bull. Tokai Reg. Fish. Res. Lab. 83:45-96. 1977. Stock assessment of cephalopod resources fished by Japan. FAO Fish. Tech. Paper 173:1-62. Okutani, T., and M. Murata. 1983. A review of the biology of the oceanic squid Onychoteuthis borealijaponica. Memoirs of the Natl. Mus. of Victoria 44:189-195. Patterson, K. R. 1988. Life history of Patagonian squid Loligo gahi and growth parameter estimates using least- squares fits to linear and von Bertalanffy models. Mar. Ecol. Prog. Ser. 47:65-74. Radtke, R. L. 1983. Chemical and structural characteristics of statoliths from the short-finned squid lllex illecebrosus. Mar. Biol. 76:47-54. Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board Can. 91:1-382. Roden, G. I. 1991. Subarctic-subtropical transition zone of the North Pacific: large-scale aspects and mesoscale structure. In J. A. Wetherall (ed. ), Biology, ocean- ography, and fisheries of the North Pacific transi- tion zone and subarctic frontal zone, p. 1- 38. NOAATech. Rep. NMFS 105, 111 p. Rodhouse, P. G., and E. M. C. Hatfield. 1990a. Age determinations in squid using statolith growth increments. Fish. Res. (Amst.) 8:323-334. 1990b. Dynamics of growth and maturation in the cephalopod lllex argentinus de Castellanos, 1960 (Teuthoidea: Ommastrephidae). Philos. Trans. R. Soc. Lond. B. Biol. Sci. 329:229-241. Rosenberg, A. A., K. F. Wiborg, and I. M. Bech. 1980. Growth of Todarodes sagittatus (Lamarck) (Cephalopoda, Ommastrephidae) from the north- east Atlantic, based on counts of statolith growth rings. Sarsia 66:53-57. Saville, A. 1987. Comparisons between cephalopods and fish of those aspects of the biology related to stock management. In P. R. Boyle (ed.), Cephalopod life cycles, vol. II, p. 277-290. Academic Press, London. Schnute, J. 1981. A versatile growth model with statistically stable parameters. Can. J. Fish. Aquat. Sci. 38:1128-1140. Sokal, R. R., and F. J. Rohlf. 1981. Biometry. W.H. Freeman, San Francisco. Wyllie Echeverria, T., W. H. Lenarz, and C. A. Reilly. 1990. Survey of the abundance and distribution of pelagic young-of-the-year rockfishes, Sebastes, off central California. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-SWFSC-147, 125 p. Yang, W. T., R. F. Hixon, P. E. Turk, P. E. Krejci, W. H. Hulet, and R. T. Hanlon. 1986. Growth, behavior, and sexual maturation of the market squid, Loligo opalescens, cultured through the life cycle. Fish. Bull. 84:771-797. Young, R. E. 1972. The systematics and areal distribution of pe- lagic cephalopods from the seas off southern California. Smithsonian Contr. Zool. 97:1-159. Young, R. E., and R. F. Harman. 1987. Descriptions of the larvae of three species of the Onychoteuthis banksii complex from Hawaiian waters. Veliger 29(31:313-321. AbStfclCt. A review of previ- ous studies on Kemp's ridley sea turtle (Lepidochelys kempii) diet was combined with information on the diet of the species in the coastal waters of New York State. Juvenile Kemp's ridleys occupy coastal Long Island, New York waters during the summer and early autumn months. Both fecal and intestinal samples collected between 1985 and 1989 were ana- lyzed to obtain information on the diet of this endangered species. Fecal and intestinal sample analy- sis, as well as information from previous studies, indicated that juvenile Kemp's ridleys primarily consume crabs. Walking crabs of the genera Libinia and Cancer appear to be the primary food sources for the species in New York waters. Diet of the Kemp's ridley sea turtle, Lepidochelys kempii, in New York waters Vincent J. Burke Savannah River Ecology Laboratory and Department of Zoology University of Georgia, Drawer E Aiken. SC 29802 Stephen J. Morreale Center for the Environment, Room 200, Rice Hall Cornell University, Ithaca, NY 14853-5601 v. Edward A. Standora Department of Biology, State University College at Buffalo 1 300 Elmwood Avenue, Buffalo, NY 1 4222 Manuscript accepted 29 September 1993 Fishery Bulletin 92:26-32 (1994) The Kemp's ridley sea turtle, Lepidochelys kempii, was placed on the United States endangered spe- cies list in December 1970 and was listed as one of the twelve most endangered species in the world by the International Union for the Conservation of Nature and Natu- ral Resources in 1986 (Federal Reg- ister, 1989; Marine Turtle Newslet- ter, 1989). Despite a recent in- crease in research on the Kemp's ridley, little attention has been fo- cused on its feeding habits. An un- derstanding of the dietary require- ments and available food resources for the Kemp's ridley is a critical com- ponent in the future management and protection of this species' habitats. While occasional glimpses into the composition of Kemp's ridley diets have been obtained, detailed quantified examinations of the spe- cies' diet have only rarely been undertaken (Table 1). In one of the earliest accounts of the diet of Kemp's ridleys, De Sola and Abrams (1933) dissected "two foot specimens" from the Georgia coast and described the main dietary component as Platyonichus ocel- latus, later renamed the spotted lady crab, Ovalipes stephensonii (Williams, 1984). Two decades later, the first pub- lished record describing the diet of the Kemp's ridley in the Gulf of Mexico was produced (Liner, 1954). In that study, gastrointestinal con- tents of eight L. kempii ranging in size from 3.2 kg to 26.6 kg were examined. All the turtles had con- sumed portunid crabs iCallinectes sp.) and occasional barnacles. Dobie et al. (1961), elaborating on the findings of Liner (1954), re- ported that small molluscs, plant parts, and mud were also contained in the gastrointestinal tracts of two of Liner's turtles. The molluscs in- cluded gastropods (Nassarius sp.) and bivalves of the genera Nuculana, Corbula, and probably Mulinia. In Virginia, Hardy (1962) dis- sected a single specimen and found that the digestive tract contained 95% Callinectes sp. and one swimmerette was identified as that of the blue crab, C. sapidus. Re- search conducted in the waters of Chesapeake Bay, Virginia, by Lutcavage (1981) indicated that three Kemp's ridley carcasses had both blue crabs and Atlantic rock 26 Burke et al.: Diet of Lepidochelys kempii 27 Table 1 Compilation of available diet stud ies of Kemp's ridley sea turtles (Lepidochelys kempii) publ shed from 1933 to 1991. The studies are ord ?red from north to south and east to west. Marquez (1973) is cited from Pritchard and Marquez (1973). Author! s) Location Diet components Life stage Hardy (1962) Chesapeake Bay Blue crabs Juvenile Lutcavage (1981) Chesapeake Bay Blue crabs Juvenile Belmund et al. (1987) Chesapeake Bay Rock and blue crabs Juvenile DeSola and Abrams (1933) Coastal Georgia Crabs [Ovalipes spp.) Not given Carr (1942) Florida Calico crabs Juvenile Liner (1954) Louisiana Blue crabs Juvenile Dobie et al. (1961) Louisiana Crabs, whelks, clams Juvenile Shaver (1991) Texas Various crab species Juvenile Marquez (1973) Tampico, Mexico Crustaceans, fish, molluscs Adult crabs (Cancer irroratus) in their digestive tracts. Recently, Shaver (1991) found that Kemp's ridleys in coastal Texas waters preyed mainly on crabs. The most commonly ingested species was the speckled crab (Arenaeus cribrarius). Many other crab species were recorded by Shaver, including purse crabs (Persephonia sp.), spider crabs (Libinia sp.), and blue crabs (Callinectes sp.). During the past decade, the role of the northeast- ern coast of the United States in the life cycle of Kemp's ridleys has received considerable attention (Carr, 1980; Morreale and Standora, 1990 '; Burke et al., 1991). The northeastern coast includes the New York area which contains over 300 km of shore- line, mainly the coastline of Long Island. Long Is- land has a variety of marine habitats, including the shallow, enclosed waters of the Peconic and south- ern bays, the deeper waters of Long Island Sound, and the Atlantic Ocean (Fig. 1). Each year Kemp's ridleys begin inhabiting the Long Island area dur- ing July (Morreale and Standora, 1989 2 ; Morreale and Standora, 1990 1 ). To date, all Kemp's ridleys encountered in Long Island have been juveniles (straight-line carapace length from 22 cm to 42 cm x=29.8 cm, SD=3.7 cm [Morreale and Standora, 1989 2 , 1990 1 ]). This size class of turtles represents a range of ages from 3 to 7 years (Zug and Kalb, 1989). Between July and early October these young Kemp's ridleys are active within the estuarine wa- ters (Long Island Sound and the Peconic Bays) and the southern bays. Kemp's ridley growth rates as 1 Morreale, S. J., and E. A. Standora. 1990. Occurrence, move- ment and behavior of Kemp's ridley and other sea turtles in New York waters. Annual report to the New York State, Dep. Environmental Conservation, April 1989-April 1990. 2 Morreale, S. J., and E. A. Standora. 1989. Occurrence move- ment and behavior of the Kemp's ridley and other sea turtles in New York waters. Annual report to the New York State, Dep. Environmental Conservation, April 1988-April 1989. high as 25% body weight per month indicate that waters around Long Island, New York, provide abundant food resources for the maintenance and growth of the juvenile turtles (Standora et al., 1989; Burke, 1990). During October the turtles begin moving out of the estuaries and into the ocean. Long distance recaptures of Kemp's ridley, green (Chelonia mydas), and loggerhead (Caretta caretta) sea turtles tagged near Long Island indicate that some turtles emigrate to the southeastern United States (Morreale and Standora, 1989 2 ; Burke, 1990; Morreale and Standora, 1990 1 ). Kemp's ridleys that do not emigrate by late November are likely to be- come cold-stunned (Burke et al., 1991). Cold-stun- ning, or severe hypothermia, occurs when ambient water temperatures fall below 10°C (Schwartz, 1978). Cold-stunning causes turtles to become tor- pid and buoyant, and eventually results in death. In Long Island, declining water temperatures usu- ally reach 10°C during early December. The cold-stunning phenomenon, other types of strandings, and live captures of sea turtles during commercial fishing operations can be utilized as sources of turtles for dietary studies. The goal of the current study is to provide a quantitative descrip- tion of the diet of Kemp's ridleys in the northeast- ern United States based on gut contents from car- casses, previously preserved dietary samples, and feces from live turtles. Materials and methods The dietary components of the Kemp's ridley were assessed by using two separate approaches. First, fecal samples were collected from live turtles and examined for their constituents. Second, complete gastrointestinal contents were removed from dead turtles and identified. Samples were obtained from 28 Fishery Bulletin 92(1), 1994 Figure 1 The waters from which Kemp's ridley sea turtles, Lepidochelys kempii, were obtained for this study can be di- vided into four habitats: Long Island Sound, where most of the stranded turtles were recovered; the Atlantic Ocean, which was the habitat of two turtles in the study; the southern bays, where one live capture and one boat-hit turtle were recovered; and the Peconic Bay system, where most of the turtles for the fecal analysis and several turtles for the digestive tract analysis wree recovered. turtles encountered in New York waters from 1985 through 1989. Nineteen fecal samples were obtained. Fourteen were collected during 1989, three during 1988, and two during 1987. Of these fecal samples, 17 were obtained from live turtles captured during warmer months (June to October) and two samples were retrieved from revived, cold-stunned turtles in late November. Captured turtles were obtained from lo- cal commercial fishermen who were asked to retain turtles caught incidentally in fishing gear (predomi- nantly pound nets). After the fishermen docked, they called a 24-hour number to reach a biologist, who generally picked up the turtle while the fishermen were still unloading their catch. All noncold-stunned Kemp's ridleys received from commercial fisheries in Long Island were alive and apparently healthy. All turtles were weighed and measured upon re- turn to the laboratory. Each turtle was then allowed to swim freely in an individual 2100-liter tank and was offered either squid or clam meat. Most Kemp's ridleys accepted the food offerings, but many fed only after the food was dangled in front of them for as long as 2-3 hours. Feeding often induced defeca- tion within a relatively short time. Tanks were checked at least three times a day for the appearance of feces. Filter intakes in the tanks were elevated and covered, except for small holes, to insure against sample loss. When feces were ob- served, they were immediately removed and placed in individual sample jars. If a turtle did not defecate within 24 hours of being placed in captivity, it was given an enema of dioctyl sodium sulfosuccinate (Disposaject brand, Pitman-Moore Inc.). If a fecal sample was still not obtained after another 24 hours, the turtle was released. The rate of food passage was examined during this study to insure that samples were not polluted with prey items eaten while the turtles were in the fishermen's nets. Gut passage rates were deter- mined for two Kemp's ridleys by feeding them declawed lobsters (Homarus americanus). Lobster was used as a tracer because it has never been re- ported as a prey item and is consumed relatively readily by the turtles. By monitoring fecal output, the amount of time between ingestion of the lobster Burke et al.: Diet of Lepidochelys kempii 29 and its first appearance in the feces was determined. All fecal samples collected for dietary analysis were immediately placed in preservative. For fecal samples obtained during 1989, animal components were preserved as described by Zinn ( 1984) and al- gae were preserved in Transeau's solution ( 10 parts formalin/30 parts ethanol/60 parts distilled H. 2 0/25 mg CuS0 4 /L). Feces obtained prior to 1989 were pre- served in 10% formalin. Analysis of the fecal samples was conducted in January 1990, after all the samples were collected. The samples were removed from the preservative and air dried for 24 hours on wire mesh in an en- closed hood. The samples were then placed in a U.S. standard number-5 mesh (4 mm) sieve and pieces smaller than 4 mm were separated out by shaking the sample in a Tyler RO-TAP testing sieve shaker for three minutes. Pieces smaller than 4 mm were not identified because of the difficulty of assigning them to a meaningful category. The amount of sample lost because of this constraint was never greater than 5% for any given sample. Each fecal sample was examined under a dissect- ing microscope and each fragment of the sample was identified to the lowest taxon possible. Fragments belonging to the same taxonomic level were grouped. A list of components (e.g., one species of crab is one component) was compiled for each sample and the data were analyzed to determine the percentage of turtles in which each component occurred. Less than 1% of the fragments could not be assigned to a taxo- nomic category. For the 1989 samples only, the relative amount of each dietary component was determined by oven drying each component from each sample for 48 hours at 60°C and weighing it. The dry weights were then used to determine the relative importance of the different dietary components in each turtle's fecal sample. Dry weight analysis was conducted by finding the percentage of each sample weight rep- resented by each component and then determining the mean for that component. This technique of analyzing dry weights as a percentage eliminated over- or under-representation of large or small fe- cal samples. A second method of determining dietary compo- nents was analysis of gastrointestinal contents from stranded, dead turtles. Stranded Kemp's ridleys died from a number of causes: cold-stunning, boat colli- sions, entanglement in a gill net, and natural and unknown causes. Whenever possible, each stranded turtle was weighed, measured (straight-line cara- pace length) and dissected. Following removal, in- testinal contents were placed in 95% ethanol (1985), 10% formalin (1986-1988), or treated in the same manner as the fecal samples (1989). Identification of intestinal tract contents was performed during 1990. All components of each sample were identified to the lowest taxon possible, generally to species. These data were used to determine the percentage of turtles in which the components occurred. Results The food passage rate analysis indicated that lob- ster was retained within the digestive tracts of the two Kemp's ridleys for seven and eight days. Be- cause fecal samples were obtained within 48 hours of receiving a turtle from a fisherman, we believe the possibility of samples having been "contami- nated" by items eaten while the turtles were in the fishermen's nets is minimal. Mean straight-line carapace length for the 19 turtles in the fecal analysis study was 32.3 cm (range=24.7 to 42.7 cm, SD=4.87). Eighteen of the 19 turtles consumed crabs (Fig. 2). Mollusc species were found in 26% of the fecal samples and algae were found in 11%> of the Kemp's ridley feces. Natu- ral and synthetic debris were present in 21% and 11% of the feces respectively. Crab species that were identified included nine- spined spider crabs, Atlantic rock crabs, and lady crabs (Ovalipes ocellatus). Further examination of only the crab portion of the feces revealed that 58% of the turtles had consumed spider crabs, 36% had eaten rock crabs, and 16% had consumed lady crabs. 100 60 w 40 O - n = 19 m '.'.'. i CRAB MOLLUSK ALGAE NATURAL SYNTHETIC DEBRIS DEBRIS Figure 2 Percent occurrence of various prey items identified in the feces of 19 Kemp's ridley sea turtles (Lepidochelys kempii) that were live-captured in Long Island waters. Each bar indicates the percent of turtles in which the prey items occurred. 30 Fishery Bulletin 92(1). 1994 a. z < CRAB MOLLUSK ALGAE NATURAL SYNTHETIC DEBRIS DEBRIS Figure 3 Mean percent of the fecal dry weight of general catergories of Kemp's ridley sea turtles (Lepidochelys kempii) prey items. Each area repre- sents the mean percent of dry weight for that com- ponent of the feces (n=14). Crabs composed the pre- dominant portion of the feces. o \- z 70 - 60 - 50 - n = 18 40 30 - 20 10 - - .-. i CRAB MOLLUSK NATURAL DEBRIS Figure 4 The percent of Kemp's ridley sea turtles (Lepidochelys kempii) from the digestive tract analy- sis that had consumed various types of ingesta. Most of the turtles had consumed crabs. Synthet- ics and algae were not present in the digestive tracts. Included in three fecal samples were crab parts from which the fragments could not be identified to genus. Mollusc species in the samples included blue mussels (Mytilus edulis) and bay scallops (Argopectin irradians). Two Kemp's ridley fecal samples contained mollusc fragments that could not be identified beyond phylum. Algal species in the samples included Sargassum natans, Fucus sp., and Ulua sp. A few turtles had small pieces of the mac- rophyte Zostera marina as well. Natural debris in- cluded such things as pebbles, small rocks, and bird feathers. Synthetic debris included only small pieces of polystyrene and latex. Analysis of fecal components with dry weights (mean of percent per sample) revealed that crabs were the predominant component of all but one of the 14 fecal samples from 1989. The mean percent of crab dry weight for the samples was 80% (Fig. 3). The mean percent dry weight for each crab species revealed that spider crabs composed 60% of the identifiable crab parts. The remainder was com- posed of 22% rock crabs and 18% lady crabs. Thus, most of the Kemp's ridleys had consumed spider crabs, which represented a large portion of the bulk. Although more turtles consumed rock crabs than lady crabs, Kemp's ridleys that consumed lady crabs had feces composed exclusively of them. For the period 1985 througn 1989, 87 dead Kemp's ridleys were recovered from Long Island's waters. Gastrointestinal tracts were removed from 40 of the 87 turtles. Eighteen of the 40 stranded Kemp's rid- leys contained identifiable diet components in the gut. All 18 turtles were juveniles. Mean straight-line carapace length for the 18 stranded turtles was 30.5 cm (range=24.8 cm to 39.7 cm, SD=3.5 cm). Thirteen of the 18 gastrointestinal tracks contained crab parts and seven contained mollusc shells (Fig. 4). The most frequently encountered crabs in the gut content samples were spider crabs and rock crabs. Spider crab fragments were found in five of the 18 samples; rock crabs were found in four of the 18 samples. Lady crabs were found in two of the samples and the blue crab (C. sapidus) was found in the digestive tract of one Kemp's ridley. Two of the turtles had crab parts in their digestive tracts that could not be assigned reliably to any genus. An additional 14 of the 40 Kemp's ridleys that were dissected had completely empty digestive tracts. All of these turtles had stranded from cold- stunning. Upon further review of necropsy data sheets from all of the Kemp's ridleys that had stranded during the study period, but from which samples were not preserved, it was noted that al- most all of the cold-stunned individuals had empty or almost empty gastrointestinal tracts. The remaining eight turtles had been collected in 1985 and 1986, and gut contents were unidentifiable because of improper preservation. These samples had been preserved for as long as five years prior to examination. Burke et al.: Diet of Lepidochelys kempn Discussion The analysis of fecal samples from live turtles and of gut contents from dead specimens strongly sug- gests that crabs are the main dietary component for Kemp's ridleys in New York waters. Crab parts were present in 18 of the 19 turtles from which fecal samples were obtained and were the predominant food item by dry weight analysis. The analysis of fecal material, however, may be biased because it examines only that material which has not been fully digested. This could cause overrepresentation of less digestible components. The gastrointestinal tract results (which are less susceptible to such bias) support the results of the fecal sample analysis. Of the 18 stranded turtles which contained identifiable food items, 13 con- tained crab parts in their guts. Gut contents can potentially be biased because of differential diges- tion. However, from our qualitative observation of the condition of the intestinal contents during dis- section, we believe the components described herein are representative of the diet. One difference between the fecal and intestinal samples was the source of the turtles. Most fecal samples were obtained from turtles captured in the Peconic Bays, but most stranded turtles were recov- ered on beaches adjacent to Long Island Sound. Presumably the dietary samples reflect feeding ac- tivities near the location of capture (or stranding). Thus, the observation of spider and rock crabs as the predominant components in the diets of both live- captured and stranded turtles emphasizes their importance as food items. The dietary components observed during the study may be related to the relative abundance of the prey species in the environment. Of the four species of crab that were identified, the spider crab was both the most frequently encountered fecal com- ponent and the predominant crab identified in the gut contents of dead turtles. During the course of our studies we have noted that the nine-spined spi- der crab was one of the most common crabs in the waters where the turtles occurred. We have observed local commercial fishermen retrieving thousands of spider crabs while hauling in their nets. The Atlan- tic rock crab was also frequently encountered in the feces and gut contents of the turtles. The rock crab is also abundant in many of the areas in which the turtles occur. Not all of the dietary make-up observed in this study can be explained by prey abundance. The green crab (Carcinus maenus) is very common in many of Long Island's estuaries but was not present in any of the turtles examined. This species usually inhabits shallower, rocky intertidal and subtidal habitats (Ropes, 1968; Williams, 1984), and our re- search on turtle behavior indicates that the Kemp's ridleys typically forage in deeper waters (Standora et al., 1990). While we have commonly encountered lady crabs in the waters where turtles forage, this species was represented in only a few samples. Also rare in the samples was the locally and commercially harvested blue crab. Both the lady crab and the blue crab are portunid crabs, capable of swimming very quickly. This characteristic differentiates the portunids from the slower walking crabs, such as the spider and rock crabs. The only molluscs consumed by turtles examined during this study included a few fragments of rela- tively thin-shelled blue mussels (Mytilus edulis) and bay scallops (Argopectin irradians), and entire shells of the small three-lined mud snail (Nassarius trivitattus). These mud snails are scavengers and can be found locally in association with dead fish and crabs (Long Island Shell Club, 1988). Their oc- currence in four turtles, all of which had been cold- stunned, may indicate that the turtles were scaveng- ing during periods of low water temperature. Because sea turtles were obtained from different sources in New York waters, it was possible to ob- tain dietary information on a larger number of Kemp's ridleys. In many of the previous studies presented in Table 1, portunid crabs were indicated as a main dietary component for Kemp's ridleys. Although this crab family was observed in some New York turtles, it was of secondary importance to the walking crabs. In terms of the overall life cycle of Kemp's ridleys, it appears that post-pelagic juveniles exploit the benthic environments of Long Island's estuaries, preying mainly on walking crabs. Data from our ongoing research indicate that sea turtles emigrat- ing from New York inshore waters travel to south- ern coastal areas. Kemp's ridleys exhibiting this behavior may join the more southerly portion of the Atlantic population. Therefore, management plans for Kemp's ridleys should consider factors that af- fect benthic fauna, especially the abundant crab populations in the northeastern region. Such im- pacts could have far-reaching effects on a critical stage in the lives of these endangered sea turtles. Acknowledgments This study was supported by a grant from the Na- tional Marine Fisheries Service under contract num- ber 40AANF902823. We thank Phil Williams for his encouragement and support. Long-term support for 32 Fishery Bulletin 92(1), 1994 sea turtle studies in New York was provided by the N.Y. State Dept. "Conservation's Return a Gift to Wildlife" program. Manuscript preparation was aided by contract DE-AC09-76SROO-819 between the University of Georgia and the U.S. Department of Energy. Workspace was provided by the State University College at Buffalo and the Okeanos Foundation. Turtle collection could not have been accomplished without the help of hundreds of vol- unteers and the commercial fishermen of Long Is- land, New York. We thank Anne Meylan for main- taining intestine samples and records from the years 1985 and 1986. For their efforts in collecting turtles, we thank C. Coogan, P. Logan, S. Sadove, and R. Yellin. The Long Island Shell Club donated mollusc voucher specimens. William Zitek graciously pro- vided necropsy facilities during 1985 and 1986, and veterinary advice during 1989 that allowed us to increase the number of fecal samples obtained. Literature cited Bellmund, S. A., J. A. Musick, R. C. Klinger, R. A. Byles, J. A. Keinath, and D. E. Barnard. 1987. Ecology of sea turtles in Virginia. Spec. Sci. Rep. No. 19, Virginia Institute of Marine Science, Coll. of William and Mary, Gloucester Point, Virginia. Burke, V. J. 1990. Seasonal ecology of Kemp's ridley (Lepidochelys kempi) and loggerhead (Caretta caretta ) sea turtles in the waters of Long Island, New York. Master's thesis, State University of New York, College at Buffalo, NY. Burke, V. J., E. A. Standora, and S. J. Morreale. 1991. Factors affecting strandings of cold-stunned Kemp's ridley and loggerhead sea turtles in Long Island, New York. Copeia 1991:1136-1138. Carr, A. 1942. Notes on sea turtles. Proc. New England Zoological Club 21:1-16. 1980. Some problems of sea turtle ecology. Am. Zool. 20:489-498. DeSola, C. R., and F. Abrams. 1933. Testudinata from southeastern Georgia, in- cluding the Okefinokee swamp. Copeia 1:10-12. Dobie, J. L., L. H. Ogren, and J. F. Fitzpartick Jr. 1961. Food notes and records of the Atlantic ridley turtle (Lepidochelys kempii) from Louisi- ana. Copeia. 1961:109-110. Federal Register. 1989. Endangered and threatened wildlife and plants. 50 CFR 17.11 and 17.12. Hardy, J. D. 1962. Comments on the Atlantic ridley turtle, Lepidochelys olivacea kempi, in the Chesapeake Bay. Chesapeake Science 3:217-220. Liner, E. A. 1954. The herpetofauna of Lafayette, Terrebonne and Vermilion Parishes, Louisiana. Proc. Louisi- ana Academy of Sciences. 17:65-85. Long Island Shell Club. 1988. Seashells of Long Island, New York. Long Island Shell Club, Inc., Long Island, NY. Lutcavage, M. 1981. The status of marine turtles in Chesapeake Bay and Virginia coastal waters. Master's thesis, College of William and Mary, VA. Marine Turtle Newsletter. 1989. IUCN resolution urges maximum size limits, protection of habitat, TED's. Mar. Turtle News- letter. 44:1-3. Pritchard, P. C. H., and R. Marquez. 1973. Kemp's ridley turtle or Atlantic ridley. International Union for the Conservation of Na- ture and Natural Resources Monograph No. 2, Marine Turtle Series. Morges, Switzerland. Ropes, J. W. 1968. The feeding habits of the green crab, Carcinus maenas (L.). Fish. Bull. 67:183-200. Schwartz, F. J. 1978. Behavioral and tolerance responses to cold water temperatures by three species of sea turtles (Reptilia, Cheloniidae) in North Carolina. Florida Marine Research Pub. 33:16-18. Shaver, D. J. 1991. Feeding ecology of wild and head-started Kemp's ridley in South Texas waters. J. Herpetol. 25:327-334. Standora, E. A, S. J. Morreale, E. Estes, R. Thomp- son, and M. Hilburger. 1989. Growth rates of Juvenile Kemp's ridleys and their movement in New York waters. Proceedings of the Ninth Annual Workshop on Sea Turtle Con- servation and Biology, p. 175-177. Standora, E. A., S. J. Morreale, R. D. Thompson, and V. J. Burke. 1990. Telemetric monitoring of diving behavior and movements of juvenile Kemp's ridleys. Pro- ceedings of the Tenth Annual Workshop on Sea Turtle Conservation and Biology, 133 p. Williams, A. B. 1984. Shrimps, lobsters, and crabs of the Atlantic coast of the eastern United States, Maine to Flori- da. Smithsonian Institution Press, Washington, D.C. Zinn, D. J. 1984. Marine mollusks of Cape Cod. Cape Cod Museum of Natural History, Brewster, Massachu- setts, 78 p. Zug, G. R., and H. J. Kalb. 1989. Skeletochronological age estimates for juve- nile Lepidochelys kempi from Atlantic coast of North America. Proceedings of the Ninth Annual Workshop on Sea Turtle. Abstract. The tripletail, Lobotes surinamensis, is the only member of the family Lobotidae in the western Atlantic Ocean, and its life history is poorly under- stood. We describe development of tripletail larvae, clarify the litera- ture on their identification, and discuss their temporal and spatial distribution in the northern Gulf of Mexico. Larval tripletail are characterized by 1) a vaulted, me- dian supraoccipital crest with spines along the leading edge; 2) precocious, heavily pigmented pel- vic fins; and 3) large preopercular spines. In addition, the surface of the frontal and supraoccipital bones have a reticulated pattern of depressions or "waffled" appear- ance. Transition to juvenile stage begins at about 9.0-9.5 mm stan- dard length. Tripletail have three supraneurals, six branchiostegal rays, 11 + 13 vertebrae, 27 dorsal rays (XII, 15), and 14-15 anal rays (III, 11-12). Overall, 75% of trip- letail larvae were found in waters >28.8°C, >30.3 ppt, and at stations >70 m deep. Larval tripletail were collected primarily from July through September and almost exclusively in surface tows. Triple- tail spawn offshore. Juveniles, al- though sporadic, are apparently not uncommon in Gulf of Mexico estuaries during summer. Larval development of tripletail, Lobotes surinamensis (Pisces: Lobotidae), and their spatial and temporal distribution in the northern Gulf of Mexico* James G. Ditty Center for Coastal, Energy, and Environmental Resources Coastal Fisheries Institute, Louisiana State University Baton Rouge, LA 70803 Richard F. Shaw Center for Coastal, Energy, and Environmental Resources Coastal Fisheries Institute, Louisiana State University Baton Rouge, LA 70803 Manuscript accepted 4 October 1993 Fishery Bulletin 92:33-45 (1994) The percoid family Lobotidae is usually considered to comprise two genera with about four species (Nelson, 1984), although Johnson ( 1984) only included Lobotes, ques- tioning the affinity of Datnioides. The tripletail, Lobotes surinamen- sis, is cosmopolitan and found in all warm seas (Fischer, 1978); one adult was recorded as far north as St. Margarets Bay, Nova Scotia (44°37'N, 64°03'W (Gilhen and McAllister, 1985). Lobotes surina- mensis is the only member of the family in the Gulf of Mexico (Gulf) (Hoese and Moore, 1977). Tripletail generally occur along the Gulf coast from April through early Oc- tober (Baughman, 1941) and mi- grate south during fall and winter (Merriner and Foster, 1974). Al- though apparently abundant no- where, adult and juvenile tripletail are not uncommon in bays, sounds, and estuaries along the north-cen- tral Gulf coast during summer (Baughman, 1941; Benson, 1982). Tripletail up to 18.6 kg and 89 cm standard length (SL) have been caught, but most average between 1 and 7 kg (Gudger, 1931; Baugh- man, 1941). Tripletail often are in- cluded as a category in Gulf fishing rodeos (Benson, 1982) because of their reputation as "a bold biter" and strong fighter (Gudger, 1931; Baughman, 1941). Tripletail enter the commercial catch on the east and west coasts of Florida and a few tons are taken annually (Fischer, 1978). The development of tripletail lar- vae and their spatial and temporal distribution is poorly understood. Hardy ( 1978) compiled information on tripletail life history. Uchida et al. (1958) and Konishi (1988) pro- vide limited information and illus- trations of tripletail larvae off Ja- pan; however, Konishi's 5.1-mm larva is misidentified. Johnson (1984) commented on cranial mor- phology. Our objectives were to de- scribe the development of tripletail larvae, to clarify the literature on their identification, and to discuss the spatial and temporal distribu- tion of larval tripletail in the north- ern Gulf of Mexico. * Louisiana State University Coastal Fish- eries Institute Contribution No. LSU- CFI-92-8. 33 34 Fishery Bulletin 92(1). 1994 Materials and methods Tripletail larvae were obtained from museum collec- tions throughout the Gulf of Mexico to determine their spatial and temporal distribution. These in- clude collections from the Southeast Area Monitor- ing and Assessment Program's (SEAMAP) ichthy- oplankton surveys of the Gulf from 1982 through 1986 (SEAMAP 1983-1987 1 ); National Marine Fish- eries Service (NMFS, Panama City, Florida) and Louisiana State University (LSU) collections from within riverine and oceanic frontal zones off the Mississippi River delta; and collections made by the Gulf Coast Research Lab (GCRL), Ocean Springs, Mississippi, and by Freeport-McMoRan Inc., New Orleans (Appendix Tables 1 and 2). SEAMAP collections from 1982 to 1986 represent the first time-interval for which a complete set of data were available. Standard ichthyoplankton survey techniques as outlined by Smith and Richardson (1977) were employed in data collection. SEAMAP stations sampled by NMFS vessels were arranged in a systematic grid of about 55-km inter- vals. NMFS vessels primarily sampled waters >10 m deep. Each cooperating state had its own sam- pling grid and primarily sampled their coastal wa- ters. Latitude 26°00'N was the southern boundary of the survey area. Hauls were continuous and made with a 60-cm bongo net (0.333-mm mesh) towed obliquely from within 5 m of the bottom or from a maximum depth of 200 m. A flowmeter was mounted in the mouth of each net to estimate volume of wa- ter filtered. Ship speed was about 0.75 m/sec; net retrieval was 20 m/min. At stations <95 m deep, tow retrieval was modified to extend a minimum of 10 minutes in clear water or 5 minutes in turbid wa- ter. Tows were made during both day and night depending on when the ship occupied the station. Overall, 1,823 bongo-net tows were collected and processed during these years. The SEAMAP effort from 1982 to 1984 also involved the collection and processing of 814 neuston samples taken with an unmetered 1x2 m net (0.947-mm mesh) towed at the surface for 10 minutes at each station. SEAMAP sampling during April and May was primarily be- yond the continental shelf, whereas that during March and from June through December was over or immediately adjacent to the shelf at stations <180 m deep. No samples were taken during January and February Additional information on the temporal and spatial coverage of SEAMAP plankton surveys 1 SEAMAP. 1983-1987. (plankton). ASCII characters. Data for 1982-1986. Fisheries-independent survey data. National Ma- rine Fisheries Service, Southeast Fisheries Center: Gulf States Marine Fish. Comm., Ocean Springs, unpubl. data. is found in Stuntz et al. ( 1985), Thompson and Bane (1986, a and b), Thompson et al. (1988), and Sand- ers et al. (1990). Collections from frontal zones off the Mississippi River delta include 311 surface-towed 1x2 m neus- ton net samples (0.333-mm mesh) made by NMFS. NMFS samples were collected during May, August, September, and December (1986 to 1989), although not all four months were sampled each year (Appen- dix Table 1). We also examined 63 surface-towed 1-m 2 Tucker trawl samples (0.363-mm mesh) taken at seven stations during July 1987, and 45 surface- towed multiple opening/closing net and environmen- tal sensing system (MOCNESS) (Wiebe et al., 1976) samples (0.363-mm mesh) collected at five stations during April 1988. These samples were from LSU collections. In addition, we examined 17 samples from stations taken by LSU inside the 100-0m isobath during October 1990. The sampling area during October 1990 extended 140 km west from Southwest Pass of the Mississippi River delta along the inner-to mid-shelf. Samples were collected with a 60— cm bongo net (0.333-mm mesh) towed ob- liquely to the surface from 5 m of the bottom or from a maximum depth of 50 m (Appendix Tables 1 and 2). Museum collections from GCRL and Freeport- McMoRan, Inc. were primarily taken off Mississippi Sound and within the Barataria Bay system of Loui- siana, respectively. Gear type and most environmen- tal data were not available from these two institu- tions (Appendix Table 2). Temperature and salinity data were from the sea surface. Hydrographic data from stations where lar- vae were taken were multiplied by the total num- ber of larvae collected at each station to derive median and mean hydrographic values. This method gives weight to distribution of larvae rather than to distribution of stations. We used percent cumulative frequency for defining the relationship between dis- tribution of larval tripletail and water temperature, salinity, and station depth. Percent frequency indi- cates the range of hydrographic conditions most of- ten associated with occurrences of tripletail larvae. Median, mean, and percent cumulative frequency statistics were calculated (SAS Institute, 1985). An examination of tripletail larvae was made to describe developmental morphology. Body measure- ments were made on 21 tripletail between 2.2 and 23.0 mm SL (Table 1) according to the methods of Hubbs and Lagler (1958) and Richardson and Laroche (1979). Measurements were made to the nearest 0.1 mm with an ocular micrometer in a dis- secting microscope. We follow Leis and Trnski's (1989) criteria for defining length of preopercular spines, body depth, head length, eye diameter, and Ditty and Shaw: Larval development and distribution of Lobotes sunnamensis 35 Table 1 Morphometries of larval triplet ail [Lobotes sunnamensis ) from the northern Gulf of Mexico. Measurements are expres sed as % standard length (SL). Preanal Head Snout Orbit Greatest Upper jaw Prepelvic SL n length lengt h length diameter body depth length distance 2.2-2.4 2 60.5-66.0 29.0-29.5 6.5-7.0 12.5-13.5 25.0-27.5 11.5-14.5 4.0-5.9 3 60.0-70.0 37.5-40.0 7.5-10.0 14.0-14.5 40.0-53.5 20.0-20.0 37.5-55.0 6.0-7.9 4 69.5-79.5 38.0-43.0 6.5-9.5 14.0-16.0 51.0-59.5 15.5-17.5 38.0-57.0 8.0-9.9 4 68.0-77.5 34.5-38.5 5.5-6.5 14.0-15.5 58.0-59.0 14.0-15.5 39.0-48.0 10.0-11.9 2 68.5-74.0 38.0-39.0 6.0-6.5 14.5-15.0 54.0-56.5 14.0-14.5 39.0-40.0 13.0-14.9 2 71.5-72.5 35.5-37.0 6.5-7.0 13.0-14.0 55.0-57.5 13.5-14.0 40.0-44.5 15.0-16.9 2 72.5-77.5 34.5-35.5 6.0-6.5 12.5-13.0 56.5-58.0 12.5-13.0 42.0-47.5 21.0-23.0 2 74.0-76.5 39.5-41.5 7.0-8.0 12.0-13.0 54.5-58.0 13.0-14.0 46.5-52.0 eye diameter/head length ratio. We consider noto- chord length in preflexion and flexion larvae synony- mous with SL in postflexion larvae and report all lengths as SL unless otherwise noted. Specimens were fixed in 10% formalin and later transferred to 70% ethyl alcohol. Representative specimens were illustrated with the aid of a camera lucida. Because of the paucity of material, only two specimens were cleared with trypsin and stained with alizarin to examine head spines. We examined the surface of the occipital and frontal bones with a scanning elec- tron microscope (SEM) after the epithelium was par- tially digested with trypsin. Soft rays of the dorsal and anal fins were counted when their pterygiophores were visible, and spines were counted when present. Results Larval morphometries and pigmentation Ninety-eight larval or juvenile tripletail were exam- ined during this study (Appendix Table 2): 7 were preflexion or flexion (<5.0 mm), 34 were postflexion (5.1 to 9.5 mm), and 57 were transforming or juve- nile (>9.5 mm). Body depth increased rapidly dur- ing preflexion and flexion with depth >50% SL by 5.0 mm. The gut was straight. Larvae had 24 myomeres which became obscured by pigment in postflexion larvae. Preanal length was 60-65% SL in preflexion larvae and increased to 70-75% SL in larvae >5.0 mm. Head length averaged 29% SL dur- ing preflexion and increased to about 40% SL in juveniles. The head became increasingly steep, and the upper profile of the forehead was concave by 20.0 mm. The eye was large and had an orbit diameter usually from 35 to 40% head length ( 12.5 to 15.0% SL) by 4.0 mm. The upper jaw reached about mid- eye. Pelvic fins were precocious, heavily pigmented, and inserted behind the pectoral fins near mid-body, usually about 40-50% SL (Table 1). The pelvic fins extended past the anus by 4.0 mm. Early preflexion larvae of 2.2-2.4 mm were sparsely pigmented; pigment was primarily re- stricted to the head and abdomen. On the head, external pigment was present on the posterior sur- face of the midbrain, posteriorly at the base of the supraoccipital crest, on the nape, and immediately anterior to the cleithral symphysis (Fig. 1). By early flexion (4.0 mm), pigment was added between the fore- and mid-brain and on the preopercle above the dorsal-most preopercular spine (Fig. 1). Pigment occurred at the tip of the upper and lower jaws and at the angle of the preopercle near the base of the angle spine by 5.0 mm. The head became heavily pigmented during postflexion. By 10.0 mm, a band of pigment extended diagonally across the head from the nape to the orbit and from below the orbit to the angle of the preopercle (Fig. 1). The eye was at the apex of this chevron-shaped band of pigment. Two parallel stripes of pigment were present between the orbits by 14.0-15.0 mm, extending from the nares to the anterior margin of the supraoccipital crest. These pigment stripes became better formed as lar- vae developed. On the abdomen, melanophores were distributed dorsally over the air bladder, and dor- sally and ventrally along the visceral mass and hindgut of early larvae (Fig. 1). By early flexion, pigment also was present on the pectoral axilla, posteriorly over the visceral mass and hindgut, and was scattered laterally over the body above the vis- ceral mass. Body pigmentation increased rapidly during early postflexion and extended posteriorly to the caudal peduncle by 6.0 mm (Fig. 1). Blotches or mottled areas of pigment formed over the body by 8.0-9.0 mm, becoming more evident as larvae devel- oped (Fig. 1). Pigment along the ventral midline between the anus and notochord tip was restricted to four to five 36 Fishery Bulletin 92(1), 1994 melanophores in early larvae. By early flexion, only one or two postanal melanophores were present along the ventral midline and these were located on the caudal peduncle and at the posterior margin of the hypural bones (Fig. 1). Pigment was also present on the devel- oping pelvic fins by early flex- ion. Melanophores were distrib- uted over the dorsal and anal spines by 6.0 mm and over the anterior-most dorsal and anal rays by 8.5-9.5 mm. Pigment covered all but the distal tips of the dorsal and anal rays by 15.0 mm. Only the base of the caudal- and pectoral-fin rays were pigmented by 13.0-14.0 mm (Fig. 1) and pigment cov- ered about 50% of the caudal fin in a 23.0-mm larva. Pigment occurred only over the proximal portion of the dorsal-most pec- toral-fin rays in the 23.0-mm larva. Head spination and fin development Tripletail larvae were charac- terized by a vaulted, median supraoccipital crest, which originated above mid-eye, and by numerous spines and ridges on the head. Larvae of 2.2-2.4 mm had five to six spines along the leading edge of the supra- occipital crest and one spine on the posterior edge (Fig. 1). Usu- ally eight spines occurred along the leading edge of the crest by 4.0 mm, giving the crest a ser- rate appearance. Length of the crest and its spines decreased as larvae grew (Fig. 1); and the entire supraoccipital crest was resorbed by 15.0-16.0 mm. The surface of the supraoccipital and frontal bones had a reticu- lated pattern of depressions or "waffled" appearance (Fig. 2). Because so few preflexion larvae were col- lected, we were unable to determine when this char- acter first appeared. A large, laterally projecting Figure 1 Larval development of tripletail iLobotes surinamensis) from the north- ern Gulf of Mexico. (A) 2.2 mm, (B) 4.0 mm, (C) 6.3 mm, (D) 8.5 mm, (E) 10.8 mm, (F) 13.7 mm. All measurements are standard length (SL). supraorbital ridge with a single spine was present above the eye of tripletail larvae by 4.0 mm. Both the supraorbital spine and ridge were resorbed by 19.0 mm. Single, simple spines were present on the Ditty and Shaw: Larval development and distribution of Lobotes surinamensis 37 posttemporal and supraclei- thrum by 4.5 mm; a low, simple ridge occurred along the pterotic at about 5.0 mm (Fig. 1). The posttemporal and supracleithral spines were par- tially covered by epithelium but both they and the pterotic ridge were visible on the larg- est specimen examined. Tripletail larvae developed two series of preopercular spines, one along the outer shelf and the other along the inner shelf. Both outer and in- ner shelves have dorsal and ventral limbs. Three spines oc- curred along the posterior mar- gin of the outer shelf of 2.2-2.4 mm larvae, the longest at its angle (Fig. 1). A fourth spine was forming but was small at 2.2 mm. Fifth and sixth spines were added by 6.0 mm; a sev- enth spine, by 7.0 mm. One to two small additional spines were added as larvae grew. By 15.5 mm, three to five spines were visible along the dorsal margin of the outer preopercular shelf, one at the angle, and usu- ally three along the ventral margin; the anterior-most spine along the ventral margin was short and blunt (Fig. 1). All spines along the outer shelf were present in the largest specimen examined (i.e., 26.0 mm). Along the inner preop- ercular shelf, one spine was present in 2.2-2.4 mm larvae and three to four spines by 5.0 mm (Fig. 1). Spines along the inner shelf were short and blunt and covered by epithe- lium. A spine occurred along the posterior margin of the subopercle by 6.0-6.5 mm, near but dorsal to the angle spine of the outer preopercular shelf. The subopercular spine was resorbed by 20.0 mm. A small, flexible spine was present dor- sally on the opercle by 10.0 mm. This spine was diffi- cult to locate on unstained larvae because it was cov- ered by integument. A continuous median finfold extended posteriorly around the body from the nape to the anus of early larvae. Pelvic fins were precocious and elongate (usually >25% SL) and had a full complement of 38 Fishery Bulletin 92(1), 1994 Figure 2 Scanning electron micrograph of the supraoccipital and frontal bones of a 6.3-mm standard length tripletail, Lobotes surinamensis, from the northern Gulf of Mexico. Magnification: 280x. elements (I, 5) by 5.0 mm (Table 2). We were unable to determine when the pelvic-fin buds formed or flexion began because of a lack of specimens between 2.4 and 4.0 mm. Development of the hypural com- plex (by 4.0 mm) coincided with that of the pterygiophores of the dorsal and anal fins. Anlagen of caudal-fin rays formed obliquely in the caudal finfold. The central-most caudal-fin rays formed first and development proceeded outward from mid-base. Notochord flexion was complete by 5.0 mm. The adult complement of 9+8 principal caudal rays were present by 7.0 mm, as were all procurrent caudal rays by 9.0-9.5 mm. All dorsal- and anal-fin ptery- giophores were present by 4.5-5.0 mm and both dorsal and anal spines developed before their rays in each fin. Dorsal and anal spines began to develop anteriorly and proceeded posteriorly to a full comple- ment of elements in each fin by 6.5 mm. Pectoral rays began to form at 5.5-6.0 mm and a full comple- ment (16 rays) was present by 7.0 mm (Table 2). A Fin ray counts of larval tripletail (Lobotes are in standard length (SL). Table surinamensis 2 i from th e northern Gulf of Mexico. Measurements Size (mm SL) n Dorsal Anal Pectoral Pelvic Caudal 4.0 1 Finbase 4.5 1 II, Anlagen 5.0 1 VII, Anlagen 6.3 1 XII. 15 7.1 1 XII, 15 10.2 1 XII, 15 Finbase I, Anlagen I. Anlagen II, 12 III, 12 III. 11 Anlagen Anlagen 13 16 16 3 I, 5 I, 5 I, 5 I. 5 I, 5 4 + 3 6 + 6 7 + 7 3-9+8-2 4-9+8-4 Ditty and Shaw: Larval development and distribution of Lobotes surinamensis 39 cleared-and-stained 10.2-mm specimen had three supraneurals, six branchiostegal rays, four upper and four lower procurrent caudal rays, 11+13 ver- tebrae, 27 dorsal rays (XII, 15), and 14-15 anal rays (III, 11-12). Scales first appeared at 9.0-9.5 mm and marked the beginning of transition to the juvenile stage. Spatial and temporal distribution Overall, 75% of tripletail larvae in this study (Ap- pendix Table 2) occurred at surface water tempera- tures >28.8°C (median=28.9°C, range=27.6-31.0°C), at salinities >30.3 ppt (median = 31.3 ppt, range=22. 0-36.0 ppt), and at stations >70 m deep (median=205 m, range=l-2707 m) (Figs. 3 and 4). Larvae <5.0 mm were collected only at stations >110 m deep. The two smallest larvae (2.2 and 2.4 mm) were taken on 28 July 1987 in a Tucker trawl sample at a station 110 m deep off Southwest Pass of the Mississippi River (Appendix Table 2). Other life stages were collected throughout the study area (Fig. 5, Appendix Table 2). Tripletail larvae were taken almost exclusively from July through September. Two specimens were collected in neuston nets outside this time period, one taken on 21 May 1983 (7.0 mm) and the other by GCRL on 9 October 1968 (10.2 mm) (Appendix Table 2). Salinity (36.5 ppt) and station depth (2,707 m) for the May specimen were the maximums re- corded for a station where larvae were collected during this study (Appendix Table 2). Larval tripletail were collected primarily near the surface. Only 2 of 528 oblique bongo-net collections between July and September yielded tripletail lar- vae («=6, 6.0-9.0 mm, 18 September 1985). Of 537 total surface net tows taken during this same time period, only 31 tows (5.8%) collected tripletail lar- 67% N = 77 28 29 30 31 TEMPERATURE (C) 22 26 27 28 29 30 31 32 33 34 35 36 SALINITY (PPT) <5 5-50 51-180 >180 DEPTH (M) Figure 3 Summary of hydrographic data from positive catch stations for larval tripletail (Lobotes surinamensis) in the northern Gulf of Mexico. Percent catch is sum of larvae by interval divided by total number of tripletail larvae collected overall. Discrepancies in n (number of larvae), among parameters, are the result of missing hydrographic data. Depth is station depth. 40 Fishery Bulletin 92(1), 1994 ' 1 MS AL \ _, f l \ GA f LA A^, 30- TX A^ -a, ^? ::|^C\V/A \ \ Yt*' ' ' ' \ tovj&i/ ° xK* \ \ wu^'° ' * * * : ' ybr* ' i^-*~~* — *"" * ' ! '. .*}?$.. o \ A . / \ LATITUDE to M V — " V + vr^ . ' A. A .fo FL \ JULY-SEPTEMBER V / 40 M 1B0 M 2-4-| — 95 i i 90 85 80 LONGITUDE Figure 4 Distribution of larval tripletail (Lobotes surinamensis) in the northern Gulf of Mexico. Plus ( + ) signs are stations sampled; open diamonds arc > positive catch stations. Data are for collections between July and September 1966-89. vae («=79) (Appendix Tables 1 and 2). Larvae from GCRL and Freeport-McMoRan collections also oc- curred primarily between July and September, but collection data are not available (e.g., total number of stations sampled and extent of sampling area). Discussion The developmental morphology of tripletail larvae from the Gulf generally agrees with limited infor- mation provided by Uchida et al. ( 1958) and Johnson (1984). Larval tripletail are characterized by Da vaulted, median supraoccipital crest with spines along the leading edge; 2) precocious, heavily pig- mented pelvic fins; and 3) large preopercular spines (Uchida et al., 1958; Johnson, 1984; this study). The supraoccipital crest is resorbed by 15.0-16.0 mm SL in Gulf specimens (this study) and by 17.5 mm TL (probably about 16.0 mm SL) off Japan (Uchida et al., 1958). Johnson (1984) described the surface of the frontal and supraoccipital bones of tripletail larvae as rugose. We would characterize these bones as having a "waffled" appearance rather than an elevated one, as implied by rugose (Fig. 2). Regard- less, this modification is found in relatively few other taxa (Johnson, 1984). Sequence of fin comple- tion in larval tripletail is Pg-Dj-Dg-A-Pj and is unlike the six patterns described by Johnson (1984). The third anal spine is the last dorsal- or anal-fin element to form. The dark band of pigment extend- ing backward from above and below the orbit in 10.0-mm larvae is present at 8.3 mm SL (10.6 mm TL) off Japan (Uchida et al., 1958) and in juveniles and adults (Gudger, 1931; Breder, 1949). We did not find the nasal spine noted by Uchida et al. (1958). The 5.1-mm TL specimen listed as L. surinamensis by Konishi (1988) lacks a supraoccipital crest and precocious pelvics, and it has a small, multi-serrate supraorbital ridge rather than the single supraor- bital spine we found. Thus, we believe that Konishi's 5.1-mm TL specimen is not L. surinamensis. Because tripletail have a cosmopolitan distribu- tion, their larvae may be confused with many taxa. Larval tripletail resemble larvae of caproids, some carangids, cepolids, drepaneids, ephippids, leiog- Ditty and Shaw. Larval development and distribution of Lobotes surinamensis 41 nathids, lethrinids, priacanthids, and Hap- alogenys sp. These taxa generally have a median supraoccipital crest, an elongate spine at the preopercular angle, and about 24 myomeres (except cepolids which have 28+ myomeres). In addition, cepolids are lightly to moderately pigmented and have fewer dorsal spines and more soft dorsal- fin rays than tripletail (Leis and Trnski, 1989). Species of other families may have a median supraoccipital crest during devel- opment, but most have pelvic fins inserted anterior to pectorals. Also, larvae of other percoid families are usually not as deep- bodied and as heavily pigmented as triple- tail by early postflexion, and few possess an elongate preopercular spine and low myomere count. Of the aforementioned taxa, only caproids, carangids, ephippids, and priacanthids occur in the Gulf of Mexico. Larvae of the caproid genus Antigonia are most similar to tripletail but have a serrate frontal crest and lower jaw, a very long and serrate preopercular angle spine, and more than 39 dorsal and 26 anal elements (Tighe and Keene, 1984; Leis and Trnski, 1989). In carangids, the two ante- rior-most anal spines are separated from the third by a distinct gap and most spe- cies have a low, median supraoccipital crest with dorsal serrations; other carangids lack a supraoccipital crest entirely. Some car- angids also have a precocious dorsal fin with elongate anterior spines or rays, or a serrated preopercular angle spine. Drepaneids have pigment on the pectoral fins and multiple barbels along the lower jaw. Both larval drepaneids and ephippids are rotund and have pelvic fins inserted anterior to the pectorals. In addition, the Gulf ephippid Chaetodipterus faber has a supraoccipital crest with a single spine dorsally rather than the vaulted, serrate supraoccipital crest found in tripletail. Atlantic spa- defish also have more anal fin elements (tripletail: A. Ill, 11-12; Atlantic spadefish: A. Ill, 17-18). Lar- val leiognathids and lethrinids have a supraoccipital crest that originates above the anterior margin of the eye and both taxa are lightly pigmented (Leis and Trnski, 1989). Also, lethrinids have higher anal fin counts and serrations along the lower jaw (Leis and Rennis, 1983), and leiognathids have a distinc- tive pattern of pigment ventrally on the tail (Leis and Trnski, 1989). Priacanthids have serrate dorsal, anal, and pelvic spines and other serrate ridges and DEPTH ZONE LENGTH (SL) <5 LZZI 5-50 KX] 51-180 rV^ Figure 5 Distribution of larval tripletail iLobotes surinamensis) in the northern Gulf of Mexico with respect to station depth (m). Length classes are combined as follows: 2 mm = 1.0-2.9 mm, 4 mm = 3.0-4.9 mm, 6 mm = 5.0-6.9 mm, etc. All measurements are standard length (SL). Numbers above bars are number of larvae in each length category. spines on the head that tripletail lack (Johnson, 1984). Hapalogenys sp. larvae are extremely simi- lar to tripletail but Hapalogenys sp. apparently lack pigmented pelvic fins, have a serrate supraorbital ridge, have a lacrimal spine, and have pterotic spines or a ridge (Johnson, 1984). Collections of early larvae (this study) and gravid females (Baughman, 1941; Merriner and Foster, 1974) suggest that tripletail spawn primarily dur- ing summer along both the U. S. Gulf and Atlantic coasts. In the Gulf, spawning begins in May, based on the collection of a 7.0-mm larva, and extends through September with peak spawning during July 42 Fishery Bulletin 92(1). 1994 and August (Appendix Table 2). These findings sup- port Baughman's (1941) observation that eggs in gravid females are largest during July and August and small or absent thereafter. Larvae are collected primarily during August and September off Japan (Uchida et al., 1958). Tripletail spawn offshore. This hypothesis of off- shore spawning is supported by the collection of all larvae <5.0 mm at stations on the outer shelf and in oceanic waters. We found no published informa- tion on larval distribution as related to water tem- perature, salinity, or station depth of capture. Larval and juvenile tripletail are collected prima- rily in surface tows (Uchida et al., 1958; this study). Juveniles are often collected with drifting sea weeds, including Sargassum, and near floating objects (Baughman, 1943; Breder, 1949; Uchida et al., 1958; Dooley, 1972; Benson, 1982) as they float on their side (Gudger, 1931; Breder, 1949). The size at which tripletail become associated with drifting sea weeds is poorly known, but Uchida et al. (1958) collected juveniles between 10.0 and 20.0 mm TL in seaweeds. Adult tripletail occur primarily in gulf waters, but enter passes, inlets, and bays near river mouths (Gudger, 1931; Baughman, 1941). The degree to which tripletail utilize estuaries during their life history is unknown. Juveniles are apparently not uncommon (although they may be sporadic) in Gulf coast estuaries during the summer. We examined eight specimens (14.5-26.0 mm) collected at the surface in waters <3 m deep (Fig. 5). Modde and Ross (1981) collected 236 juvenile tripletail (size range not given) during 1976 in the surf zone of Horn Island, Mississippi, but only one during 1975 and five during 1977. Juveniles also occur in shal- low waters (1-3 m) within the Barataria Bay sys- tem of Louisiana. 2 In contrast, juvenile and adult tripletail in the Indian River lagoon off the east coast of Florida occupy areas which average 30—31 ppt. The lagoon typically goes hypersaline, to 40 ppt, during spring when most tripletails first appear in the lagoon. Tripletail have not been observed or captured in extensive collections of oligohaline ar- eas of the St. Lucie River and Sebastian Creek. 3 Adult tripletail generally occur along the Gulf coast from April through early October (Baughman, 1941) and are caught in great numbers in Mobile Bay, Alabama, and along the Mississippi coast dur- ing summer (Baughman, 1941). Greatest concentra- tions of adults are found along the northern Gulf from St. Marks, Florida, to the St. Bernard River, 2 Leroy Kennair, Freeport-McMoRan, Inc., New Orleans, LA., pers. commun. 1993. 3 R. Grant Gilmore, Harbor Branch Oceanographic Institution, Fort Pierce, FL, pers. commun. 1993. Texas (Baughman, 1941). Seasonality of adults sug- gests that tripletail migrate south during fall and winter and return in spring (Merriner and Foster, 1974). Tripletail congregate around sea buoys, bea- cons, pilings, and other objects (Gudger, 1931) but have been collected in a wide variety of habitats including rocky and coral reef areas in deeper wa- ter (Baughman, 1941). Acknowledgments This study was supported by the Marine Fisheries Initiative (MARFIN) Program (contract numbers: NA90AA-H-MF111 and NA90AA-H-MF727). We thank SEAMAP and the Gulf States Marine Fish- eries Commission for providing specimens and en- vironmental data, and the Louisiana Board of Re- gents and the LaSER (Louisiana Stimulus for Ex- cellence in Research, contract number 86-LUM(D- 083/13) program for support during July 1987, April 1988, and October 1990 ichthyoplankton cruises. We also thank those who loaned us specimens or pro- vided data: Churchill Grimes, NMFS, Southeast Fisheries Center, Panama City, FL; Wayne Forman and Leroy Kennair, Freeport-McMoRan, New Or- leans, LA.; Stuart Poss, Gulf Coast Research Lab, Ocean Springs, MS. Thanks also to Cathy Grouchy for illustrating larvae, to Joseph S. Cope for computer assistance, and to Laura Younger for providing scan- ning electromicrographs of the frontal and occipital bones. Finally, we thank the reviewers for their com- ments in substantially improving the mansucript. Literature cited Baughman, J. L. 1941. On the occurrence in the Gulf coast waters of the United States of the triple tail, Lobotes surinamensis, with notes on its natural history. Am. Nat. 75:569-579. 1943. Additional notes on the occurrence and natu- ral history of the triple tail, Lobotes surinamensis. Am. Midi. Nat. 29(2):365-370. Benson, N. G. (ed.). 1982. Life history requirements of selected finfish and shellfish in Mississippi Sound and adjacent areas. U.S. Fish and Wildl. Serv., Office Biol. Serv., Washington, D.C., FWS/OBS-81/51, 97 p. Breder, C. M., Jr. 1949. On the behavior of young Lobotes surinam- ensis. Copeia 1949(4):237-242. Dooley, J. K. 1972. Fishes associated with the pelagic sargassum complex, with a discussion of the sargassum community. Contrib. Mar. Sci., Univ. Texas 16:1-32. Ditty and Shaw: Larval development and distribution of Lobotes surinamensis 43 Fischer, W., (ed.). 1978. FAO species identification sheets for fishery purposes. Western Central Atlantic (Fishing Area 31), Vol. 3. FAO, Rome. Gilhen, J., and D. E. McAllister. 1985. The tripletail, Lobotes surinamensis, new to the fish fauna of the Atlantic coast of Nova Scotia and Canada. Can. Field-Natur. 99(1):116-118. Gudger, E. W. 1931. The tripletail, Lobotes surinamensis, its names, occurrence on our coasts and its natural history. Am. Nat. 65: 49-69. Hardy, J. D., Jr. 1978. Development of fishes of the Mid-Atlantic Bight: an atlas of egg, larval and juvenile stages. Vol. Ill: Aphredoderidae through Rachycentridae. U.S. Fish. Wildl. Serv., Biol. Serv. Prog. FWS/OBS- 78/12. Hoese, H. D., and R. H. Moore. 1977. Fishes of the Gulf of Mexico: Texas, Louisi- ana, and adjacent waters. Texas A&M Univ. Press, College Station, 327 p. Hubbs, C. L., and K. F. Lagler. 1958. The fishes of the Great Lakes region. Univ. Mich. Press, Ann Arbor, 213 p. Johnson, G. D. 1984. Percoidei: development and relationships. In H. G. Moser, W. J. Richards, D. M. Cohen, M. P. Fahay, A. W. Kendall Jr., and S. L. Richardson (eds.). Ontogeny and systematics of fishes, p. 464— 498. Am. Soc. Ichthy. Herp., Spec. Publ. No. 1. Konishi, Y. 1988. Lobotidae. In M. Okiyama (ed.), An atlas of the early stage fishes in Japan. Tokai Univ. Press, Tokyo, 1,154 p. (In Japanese.) Leis, J. M., and D. S. Rennis. 1983. The larvae of Indo-Pacific coral reef fishes. Univ. Hawaii Press, Honolulu, 269 p. Leis, J. M., and T. Trnski. 1989. The larvae of Indo-Pacific shorefishes. Univ. Hawaii Press, Honolulu, 371 p. Merriner, J. V., and W. A. Foster. 1974. Life history aspects of the tripletail, Lobotes surinamensis (Chordata-Pisces-Lobotidae), in North Carolina waters. J. Elisha Mitchell Sci. Soc. 90<4):121-124. Modde, T., and S. T. Ross. 1981. Seasonality of fishes occupying a surf zone habitat in the northern Gulf of Mexico. Fish. Bull. 78(41:911-922. Nelson, J. S. 1984. Fishes of the World. 1984, 2nd ed. John Wiley & Sons, NY, 523 p. Richardson, S. L., and W. A. Laroche. 1979. 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A multiple opening/closing net and environ- mental sensing system for sampling zoo- plankton. J. Mar. Res. 34(3):313-326. 44 Fishery Bulletin 92(1), 1994 Appendix Table 1 Summary of total number of bongo-net/neuston-net stations examined for tripletail larvae (Loboten surina- mensis) in the Gulf of Mexico. Acronyms are as follows: SEAMAP = Southeast Area Monitoring and Assess- ment Program; NMFS = National Marine Fisheries Service, Panama City, Florida; LSU = Louisiana State University. NS means no samples. MAR APR MAY JUN JUL AUG SEP OCT NOV DEC SEAMAP 1982 77 J /0 2 69/68 71/73 102/100 26/24 NS NS 3/8 29/3 NS 1983 15/13 27/27 84/84 55/45 44/42 NS NS 39/26 NS 24/23 1984 23/0 44/0 46/0 55/54 20/26 155/162 NS 24/0 6/0 36/36 1985 29/0 NS NS 85/0 39/0 69/0 20/0 4/0 2/0 24/0 1986 NS 24/0 90/0 57/0 10/0 NS 145/0 43/0 73/0 24/0 TOTAL 144/13 164/95 291/157 354/199 139/92 224/162 165/0 113/34 110/3 108/59 NMFS 2 1986 46 1987 68 1988 55 71 36 1989 35 LSU 1987 3 63 1988'' 45 1990' 17 1 60-cm bongo net, 0.333-mm mesh, oblique-tow from depth. - 1 x 2 m neuston net, 0.947-mm mesh, 10 mill, surface-tow, unmetered. 3 lm 2 Tucker trawl, 0.947-mm mesh, 3 min. surface-tow each net, nine net collections per station, 4 lm 2 MOCNESS, nine nets of 0.333-mm mesh, 3-min. surface-tow each net, five total stations. ieven total stations. Ditty and Shaw: Larval development and distribution of Lobotes surinamensis 45 Appendix Table 2 Positive catch station data for tripletail (Lobotes surinamensis) larvae from noi •thern Gulf of Mexi co waters. Gear codes are: B=bongo net, N= :Neuston net, T=Tucker trawl, U= unknown. Station Date Gear Latitude Longitude Station depth (m) *C PPT n Length (mm SL) SEAMAP 2 1420 5-21-83 N 26*30 88*00 2707 27.6 36.5 7.0 3235 7-17-84 N 28*15 90*30 70 29.4 25.9 8.8 3238 7-17-84 N 28*30 90*30 38 29.4 25.8 7.0 3259 7-22-84 N 29*00 87*00 1251 28.9 32.8 12.3 2511 8-03-84 N 29*00 88*15 1013 27.6 32.4 7.1-18.5 2523 8-03-84 N 29*15 88*30 82 28.0 26.0 7.9 2548 8-05-84 N 29*00 88*45 249 27.6 28.7 16.8 4231 8-05-84 N 29*28 87*00 486 28.9 30.3 16 6.8-13.0 4201 8-01-85 N 28*00 84*52 205 29.6 30.8 10 10.3-15.9 4204 8-01-85 N 28*00 85*02 265 28.8 32.6 5 9.0-16.5 4210 8-02-85 N 28*21 86*00 457 28.8 32.1 4 6.8-10.0 4216 8-03-85 N 28*53 86*16 335 29.1 31.3 2 9.0 4219 8-03-85 N 28*40 86*30 457 28.9 33.6 1 9.9 4320 8-24-85 N 27*38 94*00 455 28.0 — 1 4.0 4326 8-25-85 N 27*40 93*00 265 29.7 36.0 1 7.8 4332 8-26-85 N 27*46 92*00 457 30.0 35.4 1 9.1 4484 9-18-85 B 29*07 89*44 20 27.8 29.5 2 6.0 4490 9-18-85 B 28*37 90*26 27 27.8 32.6 4 6.4-9.0 LSU 2 137 7-28-87 T 28*42 89*29 110 29.5 22.0 2 2.2-2.4 145 7-28-87 T 28*35 89*22 182 29.6 32.5 2 5.0 163 7-30-87 T 28 2 t 89*14 640 31.0 33.6 2 6.3 168 7-30-87 T 28*24 89*14 640 31.0 33.6 2 6.3 175 7-30-87 T 28*27 89*16 410 29.8 35.3 2 — 177 7-30-87 T 28*27 89*16 410 29.8 35.3 2 4.5 GCRL 5 Station 6 7-13-67 N 29*15 88*11 182 — — 1 12.5 T-108-7- -02 8-25-71 U 29*10 88*45 55 — — 1 8.7 T-108-3- 114 8-27-71 U 29*50 88*05 'J 7 — — 1 11.7 T-208-4- ■01 8-23-72 I' 29*40 88*14 38 — — 2 7.2-7.3 T- 109-6- ■02 9-21-71 V 29*20 88*21 55 — — 1 15.4 T- 109-5- 03 9-22-71 u 29*30 88*24 46 — — 1 8.6 T-209-2- ill 9-15-72 u 30*00 88*14 27 — — 2 7.7-10.7 Station 5 10-09-68 N 29*19 88*14 73 — — 1 10.2 Freeport-McMoRan'' 2 8-24-71 u 29*16 89*57 1 — — 1 14.5 3 8-10-71 u 29*22 89*48 3 — — 2 16.5-18.5 4 8-23-73 u 29*16 89*57 1 — — 1 26.0 5 8-15-66 u 29*16 89 57 3 — — 4 11.5-21.5 NMFS 5 53 8-28-88 N 29*00 88*53 149 30.3 27.5 1 10.8 58 8-29-88 N 29*07 88*49 8 J 29.5 29.0 1 13.7 5 9-03-87 N 29*12 88 43 71 29.3 32.8 1 23.0 23 9-25-86 N 28*50 89*05 195 29.4 34.0 2 7.3-13.2 32 9-06-89 N 28*49 89*16 410 29.8 35.3 1 18.7 42 9-26-86 N 29*09 88 40 77 29.3 — 1 8.6 43 9-05-87 N 28*46 89*29 104 29.2 32.1 1 7.5 ; Southeast Area Monitoring and Assessment Program. 2 Louisiana State University, Coastal Fisheries Institute, Baton Rouge. 3 Gulf Coast Research Lab. Ocean Springs, Mississippi. 4 Freeport-McMoRan. Inc., New Orleans, Louisiana. 5 National Marine Fisheries Service, Panama City Lab, Florida. Abstract. — Otoliths were used to determine the age and growth of the coral trout Plectro- pomus leopardus from Lizard Is- land area, Northern Great Barrier Reef, Australia. An alternating pattern of opaque (annulus) and translucent zones was visible in whole and sectioned otoliths. How- ever, compared to sectioned otoliths, whole readings tended to underestimate age of older fish. Otoliths of mark-recaptured fishes treated with tetracycline showed that one annulus is formed per year during the winter and spring. The oldest individual examined was 14 years of age. Schnute's growth formula was used to deter- mine the best model to describe the growth of the coral trout. The von Bertalanffy model for fork length (FL) fitted the data well and the resulting model was L t = 52.2(1 -e -0.354U + 0.766)). Line-fishing usually does not cap- ture fishes smaller than 25 cm FL, thereby excluding most 0+ and 1+ year old fish and probably the slower growing 2+ year old fish. These first three years of life rep- resent the period of fastest growth, so, if the growth curve is fitted only to the line fishing data, the growth rate of the population is underestimated. Multiple regres- sion was used to predict age from otolith weight and fish length and weight. Otolith weight was the best predictor of age in the linear model and explained as much variation in age as fish size in the von Bertalanffy model. Age validation and estimation of growth rate of the coral trout, Plectropomus leopardus, (Lacepede 1802) from Lizard Island, Northern Great Barrier Reef Beatrice Padovani Ferreira* Garry R. Russ Department of Marine Biology, James Cook University of North Queensland Townsville Q481 1, Australia *Present address: CEPENE-IBAMA. R Samuel Hardman s/n° Tamandare. Pernambuco. Cep. 55578-000. Brazil Manuscript accepted 8 September 1993 Fishery Bulletin 92:46-57 (1994) The coral trouts of the genus Plec- tropomus Oken are members of the serranid subfamily Epinephelinae, commonly known as groupers. These fishes occur in shallow tropi- cal and subtropical seas of the Indo-Pacific region (Randall and Hoese, 1986) where they usually are at the top of food chains and thus play a major role in the struc- ture of coral reef communities (Randall, 1987). Groupers typically represent an important fishery resource throughout the tropical and sub- tropical regions of the world (Ralston, 1987). On the Great Bar- rier Reef, the common coral trout Plectropomus leopardus (Lacepede 1802) is the most abundant species of the genus (Randall and Hoese, 1986) and usually the primary tar- get of recreational and commercial fishermen. The Queensland com- mercial line-fishing fleet takes a total annual catch of about 4,000 metric tons (t) of reef and pelagic species. The coral trout composes the largest single component of this catch (over 30%) with around 1200 t caught annually (Trainor, 1991). The recreational sector of this fish- ery is estimated to catch two to three times the commercial catch of reef fish (Craik, 1989 1 ). Worldwide studies on age and growth of Epinephelinae indicate that they are long lived, slow grow- ing, and have relatively low rates of natural mortality (Manooch, 1987). Fishes with these characteristics are susceptible to overfishing. Only by obtaining validated estimates of growth is it possible to determine population dynamics, estimate po- tential yield, monitor the responses of populations to fishing pressure, and properly manage the fishery. Some information on age, growth, and longevity is available for the common coral trout. On the Great Barrier Reef, Goeden (1978) estimated the growth rate of this species at Heron Island from length-frequency data. Mcpherson et al. (1988), determined age and growth of the common coral trout in the Cairns region by counts of annuli in whole otoliths. Loubens (1980) estimated age and growth for P. leopardus from New Cale- donia from counts of annuli in bro- ken and burnt otoliths. The period- icity of formation of annual rings in the latter two studies was verified through observation of marginal 1 Craik, G. J. S. 1989. Management of rec- reational fishing in the Great Barrier Reef Marine Park Tech. Memo. GBRMPA-TM- 23, 35p. 46 Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 47 increments in otoliths. Direct validation of age has not yet been attempted for P. leopardus. Fish population models usually require a general description of the growth process by means of an ap- propriate mathematical function. The von Bertalanffy (1938) growth model is the most stud- ied and the most frequently used, since its applica- tion by Beverton and Holt (1957) to the yield-per- recruit problem (Kimura, 1980; Gallucci and Quinn, 1979). Many alternative growth curves have been proposed (see Moreau, 1987) as well as the use of polynomial functions (Chen et al., 1992). In this work, Schnute's (1981) formula was used to find the model that best described the growth of P. leopardus. For several species of fishes, otolith growth has been found to continue with age, independent offish size (Boehlert, 1985; Casselman, 1990; Beckman et al., 1991). Boehlert (1985) suggested the use of otolith weight as a non-subjective, cost-effective methodology for age determination that would de- crease variability among age estimates. The aims of this study were to obtain direct vali- dation of age-at-length information and to find the model that best described the growth of the common coral trout from Lizard Island, Northern Great Bar- rier Reef, Australia. In addition, the relationship between otolith weight, body size, and age of the coral trout was studied to understand the mode of growth of the otolith and to assess the usefulness of otolith dimensions in predicting age. Materials and methods Coral trout (t?=310) were sampled in the Lizard Is- land area (lat. 14° 40' S, long. 154° 28' E) from March 1990 to February 1992. Fishes were caught by rec- reational and commercial fishermen using hook and line (77 = 184) and by recreational spearfishermen (n=94). Individuals smaller than 20-cm total length are usually not vulnerable to line fishing, so they were caught around Lizard Island by scuba divers using fence nets (77=32). Fork length (FL, cm), stan- dard length (SL, cm) and total weight (TW, g) were measured for each fish. FL is defined as the length from the front of the snout to the caudal fork, and SL is defined as the length from the front of the upper lip to the posterior end of the vertebral col- umn. A simple linear regression of the form FL= a + 6*SL was used to describe the relationship be- tween FL and SL. To describe the relationship be- tween FL and TW the variables were logarithmically transformed and the linearized version of the power function TW(g)= a*FL(cm)b was fitted to the data. In the coral trout, the sagittae are the largest of the three pairs of otoliths and were used for read- ings. Sagittae were removed, cleaned, weighed, and stored dry. Left and right sagittae, when intact, were weighed to the nearest milligram. Otoliths were prepared and read as described by Ferreira and Russ (1992). To increase contrast between bands, whole otoliths were burned lightly on a hot plate at 180°C (Christensen, 1964). Both right and left sagitta were read whole under reflected light with a dissecting microscope at 16x magnification. The otoliths, with the concave side up, were placed in a black container filled with immersion oil. Subse- quently, the left sagittae was prepared for reading by embedding in epoxy resin (Spurr, 1969) and sec- tioning transversely through the core with a Buehler Isomet low-speed saw. Sections were mounted on glass slides with Crystal Bond 509 adhesive, ground on 600- and 1200-grade sand paper, polished with 0.3— u alumina micropolish and then examined un- der a dissecting microscope at 40x magnification with reflected light and a black background (Fig. 1). Annuli were counted from the nucleus to the proxi- mal surface of the sagitta along the ventral margin of the sulcus acousticus. Terminology for otolith readings followed defini- tions of Wilson et al. (1987). Two experienced read- ers independently counted opaque zones (annuli) in each whole and sectioned otolith of a random subsample (77 = 136) to assess the precision and ac- curacy of countings obtained by the two methods. The precision of age estimates was calculated with the Index of Average Percent Error (IAPE), (Bea- mish and Fournier, 1981). Results obtained from whole and sectioned otoliths were compared by plot- ting the difference between readings obtained from whole and sectioned otoliths (Section Age- Whole age) against Section Age. The results of this com- parison indicated that whole otolith readings tended to be lower than readings from sectioned otoliths when more than six rings occurred in the otolith. Therefore, remaining otoliths were read whole first and, if the number of rings was higher than six or the whole otolith was considered unreadable, the otolith was sectioned and counts were repeated. The results were accepted and used in the analysis when the counts of the two readers agreed. If the counts differed, the readings were repeated once and if the counts still differed, the fish was excluded from the analysis. Ages were assigned based on annulus counts and knowledge of spawning season. The periodicity of annulus formation was determined with the use of tetracycline labelling. From August 1990 to Febru- ary 1992, 80 fishes were caught in a trapping pro- gram at Lizard Island fringing reeflDavis, 1992 2 ), 2 C. Davies. 1992. James Cook University, Townsville, Q4811, Australia, unpubl. data. 48 Fishery Bulletin 92(1), 1994 Figure 1 Whole (A) and sectioned (B) otolith of an 11-year-old coral trout, P. Icopardus, under reflected light with a black background showing alternating pattern of translucent and opaque bands; a = anterior, p = posterior, d = dorsal, v = ventral, di = distal, pr = proxi- mal, ds = dorsal sulcus, vs = ventral sulcus. Scale bar = 1 mm tagged with T-bar anchor tags and injected with tetracycline hydrochloride before being released. The fish were injected in the coelomic cavity under the pelvic fin with a dosage of 50 mg of tetracycline per kg offish (McFarlane and Beamish, 1987), in a con- centration of 50 mg per mL of sterile saline solution. Five fish were recaptured after periods of at least one year at large. Two of those fish were reinjected at the time of recapture and kept in captivity for periods of three to four months. To determine the time of formation of the first annulus, five young of the year were captured with Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 49 fence nets. Three of these fishes were injected with tetracycline at the time of capture, and all five fish were kept in captivity for periods of 3 to 17 months. The otoliths of the fishes treated with tetracycline were removed, sectioned, and observed under fluo- rescent light. To determine time of formation of the translucent and opaque zones, the distances be- tween events for which time of occurrence was known (i.e., between two tetracycline bands or be- tween a tetracycline band and the margin of the otolith) were measured on otolith sections and plot- ted against the corresponding time interval. The relative positions of the translucent and opaque zones to these marks were then measured and plot- ted on the same scale. While this method does not provide real distances, it standardizes the measure- ments allowing for comparison between fish of dif- ferent ages. The relation between otolith weight, fish size (length and weight), and age was analyzed. Otolith weight was plotted against FL for each age class separately. A multiple linear regression model was fitted in a step-wise manner to predict age from otolith weight and fish size and to predict otolith weight from age and fish size. The inclusion level for the independent variables was set at P=0.10. The assumptions of normality and homoscedasticity were tested by plotting the residuals from the re- gression models. The growth models were fitted to the data and their coefficients and standard errors estimated by means of standard non-linear optimization methods (Wilkinson, 1989). As the plot of the length-at-age data indicated, some form of asymptotic growth, Schnute's (1981) reformulation of the von Bert- alanffy growth equation for length in which a*0 was fitted to the data: ,-aU-t\) L,=y\ h +(y2 b -yl b ) where L f is length at age; tl and t2 are ages fixed as 1 and 14 respectively ; yl and y2 are estimated sizes at these ages; and a and b are the parameters which indicate if the appropriate growth curve lies closer to a three or two parameter sub-model. By limiting parameter values, the data were used di- rectly in selecting the appropriate sub-model, namely the generalized von Bertalanffy, Richards, Gompertz, Logistic, or Linear growth models. Sub- sequently, the original von Bertalanffy (1938) -KU -to) To evaluate the effects of gear selectivity (and consequently varying size and age composition) on the estimates of growth parameters, the von Bertalanffy growth equation was fitted first to data collected by line and spear fishing only and then to the same data combined with the fence-net sample composed of younger fish. Results Otolith reading In the coral trout, the sagittae presented a pattern of alternating translucent zones and wide opaque zones (annuli) with no sharp contrast between zones (Fig. 1). The first two annuli were notably wider and less well defined than the subsequent ones in sec- tioned otoliths. Whole sagittae were used to confirm the presence of these first annuli. In whole otoliths, annuli were clearly distinguish- able and easy to count along the dorsal side of the otolith, where up to 12 rings were counted in some otoliths. However, readings from whole otoliths tended to be lower than readings from sectioned otoliths when more than six rings were present, and this tendency increased with the mean number of rings, particularly after ten rings. (Fig. 2 ). Tetra- cycline-labelled otoliths validated the periodicity of annuli in sectioned otoliths, indicating that whole otolith readings tend to underestimate age of > 10- year-old fishes. A comparison between results of )was growth equation for length L ( = L^d-e fitted to the data. V is length at age; L x is the as- ymptotic length, K is the growth coefficient, t is age, and t o is the hypothetical age at which length is zero. £, 4 < ,1 5 3 o c 5 2 i of the variability in age of the coral trout (r^O.889, P<0.0001), with fork length accounting for 1.5% (partial r 2 = 0.015). Otolith weight was a function of age and fish size, as indi- cated by the results of the multiple regression fit- ting. The interaction between age and fork length alone accounted for 89% of the variability (r-^0.892, P<0.0001) Validation of annulus formation All fishes treated with tetracycline displayed clear fluorescent marks in their otoliths (Fig. 4). The re- sults obtained for recaptured and captive fish, rang- ing in age from one to eight years, showed that annuli are formed once per year (Fig. 5). The first annulus is formed in the otoliths of the juvenile coral trout during their first year of life (Fig. 6). The rela- tive positions of the fluorescent bands, in relation to the otolith margin and the translucent and opaque zones (annuli), indicated that the formation of the annulus occurred mainly during winter and early spring (Figs. 5 and 6). Growth model The samples obtained from line-fishing and spear- fishing were selective towards individuals larger than 25 cm FL. Consequently, the 0+ age class was not rep- resented in this sample and the age-1 year class was represented by only four individuals (Fig. 7). The sample collected with fence nets, composed of indi- viduals from the smaller size classes, consisted to- tally of individuals of the 0+ and 1+ year classes (Fig. 7). Table 2 shows the results obtained when fitting the growth model to the data including all age classes and to the data including only age >2+. Table 1 Correlation between otolith weight (mg) and fork length (cm) for each age class of the coral trout P. leopardus. Age r- P< df Age r 2 P< df 0.826 0.0001 18 8 0.481 0.0001 19 1 0.972 0.0001 10 9 0.405 0.0001 12 2 0.829 0.0001 27 10 0.120 no sig. 8 3 0.747 0.0001 19 11 0.937 0.0001 7 4 0.652 0.0001 18 12 0.526 no sig. 3 5 0.650 0.0001 30 13 0.993 0.05 2 6 0.489 0.0001 43 14 0.049 no sig. 2 7 0.514 0.0001 30 Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus Figure 4 Sectioned otolith of a recaptured individual coral trout, P. leopardus In" 0057) rescent band. Scale Bar = 0.25 mm showing fluo- When fitting Schnute's model to both sets of data, the value of the parameter b was very close to 1. In the boundary where 6 = 1, the curve was reduced to a three parameter model that corresponds to the von Bertalanffy curve for length (Schnute, 1981). The resulting growth model for all age classes, in the form of a von Bertalanffy model, was L, =52.2 (1-e- 0.354(^ + 0.766)) r = 0.895 (Fig. 8). Table 2 Von Bertalanffy growth parameters V.B. and re- spective standard errors (SE), correlation coeffi- cients (r 2 ) and degrees of freedom (df) for the growth curve fitted to all data and to the data for coral trout, P. Leopardus, >2 year old only. (SE) K (SE) (SE) df V.B. all ages V.B. age >2+ 52.20 (0.768) 0.354 (0.0241 61.29 0.132 (3.483) (0.030) -0.766 (0.097) -4.660 (1.024) 0.895 310 622 272 The results obtained when fitting the growth curve to all data and to the data for fish >2+ years old only were quite different (Table 2). From age-2 onwards, the growth rate is much slower than the one esti- mated by using all age classes, as indicated by the growth coefficient K. Consequently, the estimated L m is larger and the estimated t o is a very large, nega- tive value. The resulting growth model was L, =61.29 (l-e-0.132(f + 4.66)) r = 0.622 (Fig. 9). No systematic trend in the residuals was observed (normality test P>0.1) (Figs. 8 and 9). The relation between fork length (FL) and the standard length (SL) was SL = -0.308 + 0.852 * FL, r 2 = 0.994, and the relationship between FL and Total Weight (TW) was TW = 0.0079 *FL 3 157 0.967. Discussion While some comparisons between readings of whole and sectioned otoliths have indicated good agree- 52 Fishery Bulletin 92(1), 1994 WSSAWSSAW Tag No AUG 90 0085 r |H FEB 92 NOV 90 age = 5 | H NOV 91 NOV 90 NOV 91 3862 1 ago = 7 | | FEB 92 MAR 90 MAR 91 WW 1 age I Fluorescent | Translucent ^ Opaque YY Winter § Summer ^ Spnng /\ Autumn Figure 5 Diagrammatic representation of otoliths of mark- released-recaptured coral trout, P. leopardus, treated with tetracycline showing relative positions of the fluorescent bands, otolith margin, translucent and opaque zones. Bars represent only the distal part of the radius of the otolith section, measured from the nucleus to the proximal surface of the sagitta along the ventral margin of the sulcus acousticus. The dates on the top of the bars indi- cate time of tetracycline treatment and the dates on the end of the bars indicate time of recapture. ment (Boehlert, 1985; Maceina and Betsill, 1987), others have suggested that reading whole otoliths underestimates true age and that this problem be- comes worse with fish age (Boehlert, 1985; Hoyer et al., 1985). This is mainly due to the fact that in many species, sagittae growth is asymmetrical (Irie, 1960). Growth appears to be linear only up to a cer- tain age or size, after which additions occur mainly on the interior proximal surface, along the sulcus region (Boehlert, 1985; Brothers, 1987; Beamish and McFarlane, 1987). That seems to be the case for the coral trout, as comparison of results of whole and sectioned otoliths indicated that lateral views did not reveal many of the annual growth zones in older individuals. However, whole otoliths require much less time for analysis than sectioned ones and seem to provide more precise readings. Therefore, it is use- ful to know the limit of reliability of whole readings and to define the conditions appropriate for its use. Like the inshore coral trout Plectropomus macula tits (Ferreira and Russ, 1992), the common coral trout P. leopardus is a relatively long-lived, slow-growing species. The results on growth and longevity obtained here differ somewhat from those of previous studies. Goeden (1978), using the Petersen method, identified age cohorts up to age 5+ for P. leopardus. However, the limitations of the use of length-frequency data to estimate age of long- lived fish are well known (Manooch, 1987; Ferreira and Vooren, 1991). Mcpherson et al. (1988), using counts of annuli in whole otoliths, were able to age fish up to seven years old. Longevity was probably underestimated in their study as counts were per- formed only on whole otoliths. More recently. Brown et al. (1992) 3 analyzed whole and sectioned otoliths of coral trout from the same area as Mcpherson et al. (1988) and were able to count up to 14 rings. Loubens ( 1980) counted annuli from burnt and bro- ken otoliths and estimated a maximum longevity for 3 Brown, I. W., L. C. Squire, and L. Mikula. 1992. Effect of zon- ing changes on the fish populations of unexploited reefs. Stage 1: pre-opening assessment. Draft interim report to the Great Bar- rier Reef Marine Park Authority, Townsville, Australia, 27 p. SAWSSAWSS age = 2 | Fluorescent | Translucent ^] Opaque yry Winter ^ Sumrr ^ Spnng J\ Auturr Figure 6 Diagrammatic representation of otoliths of young- of-the year coral trout, P. leopardus, kept in captiv- ity, showing relative positions of the fluorescent bands, otolith margin, translucent, and opaque zones. Bars represent the whole radius of the otolith section, measured from the nucleus to the proximal surface of the sagitta along the ventral margin of the sulcus acousticus. The dates on the top of the bars indicate time of tetracycline treatment or cap- ture and the dates on the end of the bars indicate time of death. Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 53 so B 2+ years old coral trout, P. leopardus, and plot of residuals. Leaman, 1992). The large variability in size at a given age observed for the coral trout suggests the occurrence of individual variability in growth. The reliability of methods of growth estimation like length-frequency analysis and growth increments from marking-recapture techniques, is greatly af- fected by this kind of variation (Sainsbury, 1980), further enhancing the importance of obtaining vali- dated length-at-age estimates for exploited fish populations. The results of selective mortality are a direct effect of growth variability on the dynam- ics of abundance, and failure to consider the effects Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 55 of different growth potentials can result in gross overestimation of optimal fishing levels (Parma and Deriso, 1990). The absence of marked seasonal changes in low latitudes has led to the general belief that tropical fishes do not form annual rings in their calcified structures (Pannella, 1974). Consequently, most of the studies of age determination of tropical fishes have concentrated on examination of daily rings. This technique, however, is time consuming and lim- ited to younger ages (see Longhurst and Pauly, 1987, and Beamish and McFarlane, 1987, for review). The presence of annual marks in otoliths has been vali- dated for an increasing number of species of tropi- cal fishes (Samuel et al., 1987; Fowler, 1990; Fer- reira and Russ, 1992; Lou, 1992) showing the poten- tial of this technique to be used routinely in tropi- cal fishery management. Acknowledgments We would like to thank P. Laycock for his assistance with the otolith readings. Many thanks to Owen Roberts, who kindly gave us access to his commer- cial fishing samples. We thank M. Maida, P. Laycock, C. Davies, M. and L. Pearce, L. Vail, A. Hogget, J. St. John, and D. Zeller for help in the field and in collecting the samples. We are grateful to C. Davies for allowing us to use his trapping and mark-recapture program to validate this study. This work was supported by grants from the Brazilian Ministry of Education (CAPES), Australian Re- search Council (ARC), Fishing Industry Research and Development Council (FIRDC), and the Great Barrier Reef Marine Park Authority (Augmentative). 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Hall (eds.), Age and growth of fish, p. 81- 114. Iowa State Univ. Press, Ames. Mosegaard, H., H. Svedang, and K. Taberman. 1988. Uncoupling of somatic growth rates in Arc- tic char iSalvelinus alpinus) as an effect of differ- ences in temperature response. Can. J. Fish. Aquat. Sci. 45:1514-1524. Mugiya, Y., N. Watabe, J. Yamada, J. M. Dean, D. G. Dunkelberger, and M. Shimizu. 1981. Diurnal rhythm in otolith formation in the goldfish, Carassius auratus. Comp. Biochem. Physiol. 68A:659-662. Mulligan, T. J., and B. M. Leaman. 1992. Length-at-age analysis: can you get what you see? Can. J. Fish. Aquat. Sci. 49:632-643. Pannella, G. 1974. Otolith growth patterns: an aid in age deter- mination in temperate and tropical fishes. In T B. Bagenal (ed.), Ageing offish, p. 28-36. Unwin Bros., Surrey, England. Parma, A., and R. B. Deriso. 1990. Dynamics of age and size composition in a population subject to size-selective mortality: ef- fects of phenotypic variability in growth. Can. J. Fish. Aquat. Sci. 47:274-289. Ralston, S. 1987. Mortality rates of snappers and groupers. In J. J. Polovina and S. Ralston (eds.), Tropical snap- pers and groupers. Biology and Fisheries Manage- ment, p. 375-404. Westview Press, Inc., Boulders. Randall, J. E. 1987. A preliminary synopsis of the groupers (Perciformes: Serranidae: Epinephelinae) of the Indo-Pacific region. In J. J. Polovina and S. Ralston (eds.), Tropical snappers and groupers. Biology and Fisheries Management, p. 89- 187. Westview Press, Inc., Boulders. Randall, J. E., and D. F. Hoese. 1986. Revision of the groupers of the Indo-Pacific genus Plectropomus (Perciformes: Serranidae) (13). Bernice Pauahi Bishop Museum, Honolulu, Hawaii, 31 p. Ricker, W. E. 1969. Effects of size-selective mortality and sam- pling bias on estimates of growth, mortality, pro- duction and yield. J. Fish. Res. Board Can. 26:479-541. Sainsbury, K. J. 1980. Effect of individual variability on the von Bertalanffy growth equation. Can. J. Fish. Aquat. Sci. 37:241-247. Samuel, M., C. P. Mathews, and A. S. Baazeer. 1987. Age and validation of age from otoliths for warm water fishes from the Arabian Gulf. In R. C. Summerfelt and G. E. Hall (eds.), Age and growth of fish, p. 253-266. Iowa State Univ. Press, Ames. Schnute, J. 1981. A versatile growth model with statistically stable parameters. Can. J. Fish. Aquat. Sci. 38:1128-1140. Spurr, A. R. 1969. A low-viscosity epoxy resin embedding me- dium for electron microscopy. J. Ultrastruct. Res. (26):31-34. Trainor, N. 1991. Commercial line fishing. The Queensland Fisherman, March: 17-25. von Bertalanffy, L. 1938. A quantitative theory of organic growth. II: Inquires on growth laws. Hum. Biol. 10:181-213. Ferreira and Russ: Age-validation and growth rate of Plectropomus leopardus 57 Watabe, N., K. Tanaka, J. Yamada, and J. Dean. 1982. Scanning electron microscope observations of the organic matrix in the otoliths of the teleost fish Fundulus heteroclitus (L. land Tilapia nilotica (L.). J. Exp. Mar. Biol. Ecol. 58:127-134. Wilkinson, L. 1989. SYSTAT: the system for statistics. Evanston, IL: SYSTAT, Inc. Wilson, C. A., R. J. Beamish, E. B. Brothers, K. D. Carlander, J. M. Casselman, J. M. Dean, A. Jearld, E. D. Prince, and A. Wild. 1987. Glossary. In R. C. Summerfelt and G. E. Hall (eds.), Age and growth offish, p. 527-530. Iowa State Univ. Press, Ames, Abstract. Red drum, Sciaenops ocellatus, from Mosquito Lagoon, east-central Florida, were examined for variation in products of nine polymorphic nuclear-gene (allozyme) loci and in mitochon- drial (mt)DNA restriction sites. Genetic data from Mosquito La- goon fish were compared to simi- lar data from red drum sampled from the northeastern Gulf of Mexico (Gulf) and the Carolina coast of the southeastern United States. Significant heterogeneity among red drum from the three areas was found in the frequencies of inferred alleles at two to three allozyme loci and in the frequen- cies of six mtDNA haplotypes. Red drum from Mosquito Lagoon were as differentiated genetically from red drum in the northeastern Gulf and Carolina coast as the latter two were from each other. Genetic data are consistent with the hy- pothesis that red drum in Mos- quito Lagoon are self-contained and at least partially isolated from red drum in other U.S. waters. Genetic distinctness of red drum (Sciaenops ocellatus) from Mosquito Lagoon, east-central Florida* John R. Gold Department of Wildlife and Fisheries Science Texas A&M University, College Station. Texas 77843 Linda R. Richardson Department of Wildlife and Fisheries Science Texas A&M University. College Station, Texas 77843 Manuscript accepted 17 August 1993 Fishery Bulletin: 92:58-66 (1994) Over the past five years, our labo- ratory has carried out studies of spatial and temporal genetic varia- tion among red drum (Sciaenops ocellatus) from the northern Gulf of Mexico (Gulf) and the Carolina coast of the southeastern United States (Bohlmeyer and Gold, 1991; Gold and Richardson, 1991; Gold et al., 1993, in press). Red drum cur- rently support important recre- ational fisheries in both the north- ern Gulf and U.S. Atlantic (Mat- lock, 1984; Mercer, 1984), and both fisheries are now regulated to re- duce growth and recruitment over- fishing (Swingle et al., 1984 1 ; Goodyear, 1989 2 ). Collectively, our genetic data have indicated that red drum in U.S. waters are sub- divided with weakly differentiated subpopulations in the northern Gulf and along the Carolina coast. No genetic heterogeneity has been found among red drum from differ- ent localities within either the northern Gulf or Carolina coast (Gold et al., 1993, in press). The ge- netic data are consistent with sev- eral aspects of red drum biology and life history that suggest red drum dispersal and gene flow among contiguous bays and estuar- ies could be extensive. These in- clude 1) transport of eggs, larvae, or juveniles from spawning locali- ties near the mouths of bays or es- tuaries to adjacent bays or estuar- ies by oceanic currents (Lyczkoski- Schultz et al., 1988 3 ), 2) movement of sexually-mature adults from bay or estuarine juvenile nurseries into deeper, offshore waters prior to spawning (Matlock, 1984), and 3) formation of large, offshore schools that can migrate extensively (Overstreet, 1983; Matlock, 1984; Swingle et al., 1984 1 ). In this study, data on allozyme and mitochondrial (mt)DNA varia- tion among red drum sampled from Mosquito Lagoon on the east coast of Florida are presented and com- pared to data from previous stud- ies. The goal of the study was to * Contribution No. 24 of the Center for Bio- systematics and Biodiversity, Texas A&M University. 1 Swingle, W., T. Leary, D. Davis, V. Blomo, W. Tatum, M. Murphy, R. Taylor, G. Adkins, T Mcllwain, and G. Matlock. 1984. Fishery profile of red drum. Gulf of Mexico Fish. Mngmt. Council and Gulf States Mar. Fish. Comm., Lincoln Cntr, Suite 331, 5401 West Kennedy Blvd., Tampa, FL. 2 Goodyear, C. P. 1989. Status of red drum stocks of the Gulf of Mexico: report for 1989. Contrib. CRD 88/89-14, Southeast Fish. Cntr, Miami Lab., Coast. Res. Div., 75 Virginia Beach Drive, Miami, FL. ■' Lyczkowski-Schultz, J., J. P. Steen Jr., and B. H. Comyns. 1988. Early life history of red drum (Sciaenops ocellatus) in the northcentral Gulf of Mexico. Mississippi- Alabama Sea Grant Consortium (Project No. R/LR-12). Gulf Coast Res. Lab., P.O. Box 7000, Ocean Springs, unpubl. ms. 58 Gold and Richardson: Sciaenops ocellatus from Mosquito Lagoon 59 test the hypothesis that red drum from Mos- quito Lagoon and other U.S. waters are geneti- cally homogeneous. Red drum in Mosquito Lagoon are of particular interest because they may represent a self-contained, at least par- tially isolated subpopulation. Evidence for the latter includes documentation within the sys- tem of both post-spawning females and red drum eggs (Murphy and Taylor, 1990; Johnson and Funicelli, 1991). In addition, physical ac- cess to the Atlantic from the lagoon is limited. In brief, Mosquito Lagoon (Fig. 1) is long and narrow (54 km x 4 km) and is separated from the Atlantic by a barrier beach. The lagoon represents the northern part of the Indian River lagoonal system and has two narrow outlets: one, Ponce de Leon Inlet, is a natural pass to the Atlantic located at the northern end of the lagoon; the other, Haulover Canal, is a man-made passageway at the southern end of the lagoon that leads into the Indian River. Access to or from the Atlantic through Ponce de Leon Inlet is restricted because of a series of islands and small passageways in the north- ern part of the lagoon. Access to or from the Atlantic through Haulover Canal (completed in 1929) would only be recent, and the nearest outlet to the Atlantic south from Haulover ca- nal is roughly 90-100 km. We also were inter- ested in studying red drum from Mosquito Lagoon because our earlier work (Gold et al., 1993, in press) did not include red drum from the east coast of Florida, an area of potential importance to tests of hypotheses regarding ge- netic subdivision between red drum from the northern Gulf and the U.S. Atlantic (Gold et al., in press). Finally, adult red drum from Mos- quito Lagoon form a large part of the broodstock used by the Florida Department of Natural Resources (FDNR) to supplement and enhance the red drum fishery in Florida wa- ters. The genetic composition of Mosquito La- goon red drum is thus important to research in stocking hatchery-raised fish. Materials and methods Red drum were collected from Mosquito Lagoon during fall 1988, spring 1990, and spring 1991. Fish were captured with trammel nets. Tissues (heart, spleen, and muscle) were removed and placed in liquid nitrogen for transport to Texas A&M University where they were stored at -80°C. Ages of all but yearling (age zero) individuals (i.e., speci- mens less than 300 mm total length) were deter- Ponce de Leon Inlet 10km Figure 1 Mosquito Lagoon, east-central Florida, showing Ponce de Leon Inlet and Haulover Canal. mined from annuli on otoliths by using methods described in Bumguardner (1991). Individuals sampled in 1988 (41 total) were sur- veyed for variation at nine polymorphic allozyme 60 Fishery Bulletin 92(1), 1994 loci: ACP-2* (acid phosphatase); ADA* (adenosine deaminase); ADH* (alcohol dehydrogenase); sAAT-1* (aspartate aminotransferase); EST-1* (esterase); GPI-B* (glucose phosphate isomerase); and PEPB' , PEPD * , and PEPS' (peptidases). Techniques for ver- tical starch gel electrophoresis, details of grinding and running buffers, starch composition of gels, protein staining, and interpretation of banding pat- terns may be found in Bohlmeyer (1989) and Bohlmeyer and Gold (1991). Designation of allelic variants was based on relative mobility to the most common allele (Allele * 100). All individuals collected (109 total) were assayed for 104 mtDNA restriction sites with 13 restriction enzymes: BamKl, Bell, EcoRV, Hindlll, Ncol, Nsil, PstI, Pvull, Seal, Spel, Stul,Xbal, and Xmnl. Meth- ods used to assay mtDNAs of individual fish may be found in Gold and Richardson (1991). Homology of fragments from single digestions was tested by multiple, side-by-side comparisons. Variant patterns exhibiting only a single band of greater than 15 kb were tested for homology by using double digestions with BawHI as described in Gold and Richardson (1991). Red drum from Mosquito Lagoon were initially subdivided into year classes and tested for hetero- geneity in both allozyme and mtDNA haplotype fre- quencies. Year classes (number of individuals) were 1985 (17), 1986 (25), 1987 (11), 1988 (7), and 1989 (49). No significant heterogeneity (P>0.05) in allozyme or mtDNA haplotype frequencies was found among year classes. Subsequent data analy- ses employed three test groups: 1) red drum from Mosquito Lagoon; 2) red drum from the northeast- ern Gulf; and 3) red drum from the Carolina coast. Data for the latter two were taken from Gold et al. (1993, 1994) and represent red drum from the fol- lowing localities: northeastern Gulf — Apalachicola Bay, Riviera Bay, and Sarasota Bay (west coast of Florida); and Carolina coast — Calibogue Sound, Charleston Bay, and North Inlet (South Carolina), and the Pamlico River and Oregon Inlet (North Carolina). A map showing these localities may be found in Bohlmeyer and Gold ( 1991 ). A summary of allele frequencies at the nine polymorphic allozyme loci and the distribution of mtDNA haplotypes in each test group are given in Appendix Tables 1 and 2, respectively. For allozyme data, tests of Hardy-Weinberg equi- librium expectations and generation of Nei's (1978) unbiased genetic distance were accomplished by using BIOSYS-1 (Swofford and Selander, 1981). Deviations from Hardy-Weinberg expectations were tested by using pooled genotypes and the chi-square statistic with one degree of freedom. Significance testing of allele-frequency differences among test groups was accomplished by using 1) the G-statis- tic (Sokal and Rohlf, 1969) on contingency tables of allele counts and the BIOM-PC program (Rohlf, 1983), and 2) the ^-statistic (DeSalle et al., 1987) on arcsin, square-root transformed allele frequen- cies. For mtDNA data, significance testing of mtDNA-haplotype frequency differences was carried out by using the G- and V-statistics as described above and a Monte Carlo randomization procedure (Roff and Bentzen, 1989). Nucleon diversities and intra- and inter-populational nucleotide sequence diversities were estimated by using equations in Nei and Tajima (1981). Analysis of mtDNA data was facilitated by the Restriction Enzyme Analysis Pack- age (REAP) of McElroy et al. (1992). Significance levels for multiple tests performed simultaneously were adjusted after Cooper (1968). Results No significant deviations from Hardy Weinberg equi- librium expectations at any of the nine polymorphic allozyme loci were found following corrections for multiple tests. Two significant deviations were found in uncorrected tests: at GPI-B* (P=0.015) and PEPS' (P=0.012) in the northeastern Gulf. Both deviations appeared to be due to rare homozygotes for low fre- quency alleles. One new allele (Allele * 110 at EST- l') was found among Mosquito Lagoon fish at a fre- quency of 1.2 percent (Appendix Table 1). Estimates of allozyme variation (Table 1) indicate that red drum from Mosquito Lagoon have fewer Table 1 Allozyme variation in red drum (S iaenops ocel- latus). Mean Mean Mean number of hetero- Test sample alleles/locus zygosity/ group size/locus <± SE) locus' (±SE) Northeastern Gulf of Mexico 246 3.9 + 0.9 0.225 ± 0.076 Mosquito Lagoon, Florida 41 2.9 ± 0.6 0.206 ± 0.081 U.S. Carol id. i Coast 176 3.9 ± 0.9 0.213 ± 0.074 1 Direct-cou nt estimate. Gold and Richardson: Sciaenops ocellatus from Mosquito Lagoon 61 alleles per locus or lower estimates of mean het- erozygosity, or both, than do red drum from the northeastern Gulf and Carolina coast. The differ- ences in genetic variation, however, are non-random across loci. Heterozygosity per locus values among Mosquito Lagoon fish at loci (e.g., ACP-2* , ADA*, ADH*, sAAT-1*, and EST-1*) where alternate alleles occurred at frequencies of five percent or greater were equivalent to values among fish from the northeastern Gulf and Carolina coast (data not shown). Differences in heterozygosity per locus val- ues were observed at loci (e.g., GPI-B *, PEPB* , and PEPD* ) where alleles occurring in a frequency of one to three percent in northeastern Gulf or Carolina coast fish, or both, were not found among Mosquito Lagoon fish (Appendix Table 1). Significant heterogeneity (P<0.05) in allele fre- quencies among test groups was found by using the G-test at ADA* (G=33.92, df=22, P=0.004) and sAAT- 1* (G=13.59, df=6, P=0.036). Additional G-tests were carried out after pooling alleles whose frequency in any sample was less than 10%. Significant hetero- geneity was again found at ADA* (G=9.62, df=4, P=0.048) and also at PEPB* (G=6.86, df=2, PM3.034). Examination of allele frequencies at ADA*, sAAT-1, and PEPB* did not reveal any striking differences among test groups, suggesting that heterogeneity was due to accumulation of small differences in fre- quencies of rare alleles. At ADA*, for example, the frequency of Allele *115 was higher among Mosquito Lagoon fish and lower among Carolina coast fish; whereas the frequencies of Alleles *90 and *85 were higher among northeastern Gulf fish (Appendix Table 1). At sAAT-1* and PEPB*, slight frequency dif- ferences were apparent for Allele * 110 (higher in Mosquito Lagoon fish) and Allele 115 (higher in northeastern Gulf fish and absent from Mosquito Lagoon fish), re- spectively (Appendix Table 1). The observation that G-test heteroge- neity was due to small, cumula- tive frequency differences was cor- roborated by V-tests where no sig- nificant heterogeneity (P>0.05) in allele frequencies was found at any locus following corrections for multiple tests. MtDNA fragment patterns from single digestions with 13 restric- tion enzymes generated 36 com- posite mtDNA haplotypes among fish from Mosquito Lagoon, eleven of which (numbers 114, 134-143) have been found only in Mosquito Lagoon red drum (Appendix Table 2). Estimates of mtDNA variation (Table 2) indi- cated that nucleon diversity (the probability of any two individuals differing in mtDNA haplotype) was highest in red drum from the northeastern Gulf and lowest in red drum from the Carolina coast; whereas intrapopulational nucleotide sequence diversity (the genetic difference between any two individuals) was greatest among Mosquito Lagoon fish. These esti- mates of mtDNA variation are among the highest reported to date for a non-clupeid, marine fish spe- cies (Richardson and Gold, 1993). Highly significant heterogeneity in mtDNA- haplotype frequencies among test groups and be- tween pairwise comparisons of test groups were found in both G-tests and Monte Carlo bootstrapping (Table 3). These results indicate that all three test groups differ significantly from each other. V-tests, carried out on haplotypes found in ten or more individuals (12 haplotypes total), identified six haplotypes (Table 4) that differed significantly among test groups. Genetic distances based on allozymes and mtDNAs (Table 5) indicate that red drum from Mosquito Lagoon are at least as diver- gent genetically from red drum in the northeastern Gulf and Carolina coast as the latter two are from each other. Discussion Tests of heterogeneity clearly indicate that red drum from Mosquito Lagoon differ genetically from red drum in the northeastern Gulf and along the Caro- lina coast and that at least three subpopulations of red drum occur in U.S. waters. That the genetic differences appear more pronounced in mtDNA than Table 2 MtDNA variation in red drum (Sciaenops ocellatus). Nucleotide Number Number sequence Test of of Nucleon diversity group individuals haplotypes diversity (± SD)' Northeastern Gulf of Mexico 247 49 0.947 0.557 ± 0.298 Mosquito Lagoon, Florida 109 36 0.912 0.597 ± 0.321 U.S. Carolina Coast 174 43 0.904 0.560 ± 0.351 1 Values are in percent Standard d eviations are used instead of standard errors be- cause of the large number of pairwise comparisons used to generate mean values. 62 Fishery Bulletin 92(1), 1994 Table 3 Results of tests for heterogeneity in among red drum (Sciaenops ocellatu Mexico, Mosquito Lagoon, Florida, a mtDNA haplotype frequencies s) from the northeastern Gulf of nd the U.S. Carolina coast. Test group Results of G-tests P-value from Monte Carlo randomizations G-score P-value Northeastern Gulf vs. Mosquito Lagoon vs. Carolina Coast 159.5 <0.001' <0.001 Northeastern Gulf vs. Mosquito Lagoon 73.9 <0.001 2 <0.00T Northeastern Gulf vs. Carolina Coast 76.2 <0.001 3 <0.001 Mosquito Lagoon vs. Carolina Coast 66.2 <0.001 4 0.006 Degrees of freedom in G-tests: 48' 18 2 , 19 3 , and 27 J . Table 4 Frequency 7 of six sign ificantly he terogeneous mtDNA haplotypes of red drum {Sciaenops ocellatus) in the northeast- era Gulf of Mexico, Mosq uito Lagoon, Florida, and the U.S. Carolina coast. Northeastern Mosquito Carolina Probability Haplo- Gulf Lagoon Coast value from type (rc=247) (rc=109) (n = 174) V-test 2 8 13.3 23.8 10.3 =0.010 9 7.7 13.8 26.4 <0.001 11 9.3 1.8 7.5 =0.019 12 0.0 7.3 3.4 <0.001 21 4.4 i) i) ii 6 =0.004 29 1 () ii i) 1.7 =0.021 ' Values are in percent. 2 After DeSalle et al. (1987). in (presumed) nuclear-coding genes is not surpris- ing, given that mtDNA is expected to be at least four times more sensitive to population substructuring (Birky et al., 1983; Templeton, 1987). Because pre- vious studies (Gold et al., 1993, in press) found no evidence of genetic heterogeneity among red drum from eleven estuaries or bays in the northern Gulf or among red drum from five estuaries or bays along the Carolina coast, red drum from Mosquito Lagoon are unusual in representing a genetically distinct red drum subpopulation existing within a single bay or estuary. Campton (1992) 4 examined red drum from Mosquito Lagoon for allelic variation at several allozyme loci and found genetic homogeneity among red drum from Mosquito Lagoon, the north- ern Gulf, and the Carolina coast. He suggested that our initial study (Bohlmeyer and Gold, 1991) of allozyme variation among northern Gulf and Carolina coast red drum did not account for tem- poral variation among samples within localities. Our subsequent studies (and this one), however, have included temporal sampling of variation in both allozymes and mtDNA and have demonstrated that weak (but significant) genetic heterogeneity exists (Gold et al., 1993, in press). Sampling error associated with specimen procure- ment in varying time and space may account for the different results ob- tained in Campton's (1992) 4 study and this one. However, in Campton's (1992) 4 study, the total G-statistic, obtained by summing individual G-values and their associated degrees of freedom, was signifi- cant at the 0.01 level. This suggests the existence of spatial or temporal genetic heterogeneity, or both, among the locali- ties sampled. Genetic differentiation of red drum in Mosquito Lagoon is consistent with the hypothesis that red drum in Mosquito Lagoon represent a self-contained, at least partially isolated subpopulation. Three lines of evidence support this hy- pothesis. First, genetic differences be- tween red drum from Mosquito Lagoon and red drum sampled elsewhere involve frequencies of alleles at two or three pu- tative nuclear-gene loci and frequencies of at least six mtDNA haplotypes. Differentiation of several, presumably independent and selectively-neutral, genetic markers suggests a genome-wide effect re- lated to at least partial isolation and reduced gene flow (Wright, 1978; Hartl and Clark, 1989). Second, inferred nuclear-gene alleles present in low fre- quency in red drum sampled outside of Mosquito Campton, D. E. 1992. Gene flow estimation and population struc- ture of red drum iSaaenops ocellatus) in Florida. Final Rep. Coop. Agrmt. No. 14-16-009-1522, U.S. Fish & Wildl. Serv, Natl. Fish. Res. Cntr., 7920 N.W. 71st St., Gainesville, FL. Gold and Richardson: Sciaenops ocellatus from Mosquito Lagoon 63 Table 5 Matrix of Nei's (1978) unbiased genetic distance based on allozymes (upper diagonal) and Nei and Tajima's (1981) corrected interpopulational nucle- otide sequence divergence based on mtDNAs (lower diagonal) among red drum (Sciaenops ocellatus) from the northeastern Gulf of Mexico, Mosquito Lagoon, Florida, and the U.S. Carolina coast. Interpopulational nucleotide sequence di- vergence values are in percent. Northeastern Mosquito Carolina Gulf Lagoon Coast Northeastern Gulf 0.000 0.001 Mosquito Lagoon 0.006 0.002 Carolina Coast 0.006 0.009 Lagoon were not found in red drum from Mosquito Lagoon; whereas one inferred allele and eleven mtDNA haplotypes were unique to red drum from Mosquito Lagoon. The distribution of low frequency nuclear-gene alleles and mtDNA haplotypes is con- sistent with reduced gene flow concomitant with allele-frequency drift expected in isolated subpopu- lations. Finally, both females with ovaries contain- ing postovulatory follicles and spawned red drum eggs have been documented in Mosquito Lagoon (Murphy and Taylor, 1990; Johnson and Funicelli, 1991), clearly indicating that red drum spawn within the system. Assuming red drum in Mosquito Lagoon represent a partially isolated, self-contained subpopulation, one question of interest is how long the subpopula- tion has been semi-isolated. Geological evidence (Mehta and Brooks, 1973, cited from Johnson and Funicelli, 1991) indicates that several tidal inlets once connected Mosquito Lagoon to the Atlantic, the last of which is estimated to have closed about 1,500 years ago. Assuming some variation in the geologi- cal estimate, this date does not differ substantially from an estimate of 2,900 ± 1,550 (SD) years based on 1) a corrected interpopulational nucleotide se- quence divergence (between red drum in Mosquito Lagoon and red drum elsewhere) of 0.0058 ± 0.0031 (SD) percent, and 2) an evolutionary rate for verte- brate mtDNA of 0.01 substitutions/bp/lineage/Myr (Brown et al., 1979; Wilson et al., 1985). Given on- going debates about molecular clocks, the correspon- dence between the two temporal estimates is note- worthy. Because the genetic distinctness of Mosquito La- goon red drum appears to stem largely from physi- cal isolation, the biological reasons for subdivision between red drum in the northern Gulf and those along the Carolina coast remain unknown. Possible reasons for this subdivision could include 1) current patterns between the Gulf and U.S. Atlantic, 2) absence of suitable near-shore habitats along the southeastern coast of Florida, or 3) differences in biogeographic provinces (Gold et al., 1993, in press). Similar genetic discontinuities between U.S. Atlan- tic and Gulf coast fauna have been described by Avise and co-workers (reviewed in Avise, 1992). Their hypothesis is that the concordant phylogeographic patterns provide evidence of simi- lar vicariant histories that are tentatively related to episodic changes in environmental conditions dur- ing the Pleistocene (Avise, 1992). The relative inac- cessibility of Mosquito Lagoon suggests that sam- pling red drum from north or south of Mosquito Lagoon may be more informative for testing hypoth- eses regarding phylogeographic subdivision between the northern Gulf and the U.S. Atlantic. A last point to consider is the use of Mosquito Lagoon red drum as broodstock for stock enhance- ment programs. It could be argued that red drum from Mosquito Lagoon differ genetically from red drum sampled elsewhere (e.g., the northeastern Gulf) and should be used only for stock enhancement at localities where no genetic differences exist. Al- ternatively, it could be argued that the genetic dis- tinctiveness of red drum in Mosquito Lagoon is rela- tively small and possibly inconsequential. This fol- lows from the observation that the documented ge- netic difference between red drum in Mosquito La- goon and red drum sampled elsewhere is consider- ably less than that, on average, among races of man (Cann et al., 1987). One other consideration might be to cross red drum from Mosquito Lagoon with red drum from elsewhere (e.g., the northeastern Gulf) in order to increase performance from potential heterotic effects. Acknowledgments Assistance in procuring red drum specimens from Mosquito Lagoon was provided by J. Burch, J. Camper, B. Denis, C. Furman, M. Murphy, G. Ramos, and D. Roberts. Their assistance is grate- fully acknowledged. Special thanks are extended to C. Amemiya and D. Roberts for providing no-cost lodging during field trips. We also thank B. Colura and B. Bumguardner for carrying out age determi- nations from otoliths, D. Bohlmeyer and C. Furman for assistance in the laboratory, R. Taylor for pro- viding historical information on the construction of Haulover Canal, and M. Murphy for providing criti- 64 Fishery Bulletin 92(1). 1994 cal comments on a draft of the manuscript. Work was supported by the Texas A&M University Sea Grant College Program (grants NA85AA-D-SG128 and NA89AA-D-SG139), by the MARFIN Program of the U.S. Department of Commerce (grants NA89- WC-H-MF025 and NA90AA-H-MF107), and by the Texas Agricultural Experiment Station (Project H- 6703). This paper represents number XI in the se- ries "Genetic Studies in Marine Fishes." Literature cited Avise, J. C. 1992. Molecular population structure and the bio- geographic history of a regional fauna: a case his- tory with lessons for conservation biology. Oikos 63:62-76. Birky Jr., C. W., T. Maruyama, and P. Fuerst. 1983. Mitochondrial DNAs and phylogenetic relationships. In S. K. Dutta (ed.), DNA system- atics, p. 107-137. CRC Press, Boca Raton, FL. Bohlmeyer, D. A. 1989. A protein electrophoretic analysis of popula- tion structure in the red drum (Sciaenops ocellatus). M.S. thesis, Texas A&M University, College Station, TX. Bohlmeyer, D. A., and J. R. Gold. 1991. Genetic studies in marine fishes. II: A pro- tein electrophoretic analysis of population struc- ture in the red drum Sciaenops ocellatus. Mar. Biol. 108:197-206. Brown, W. M., M. George Jr., and A. C. Wilson. 1979. Rapid evolution of animal mitochondrial DNA. Proc. Natl. Academy Sci. (USA) 76:1967- 1971. Bumguardner, B. W. 1991. Marking subadult red drums with oxytetracycline. Trans. Am. Fish. Soc. 120:537-540. Cann, R. L., M. Stoneking, and A. C. Wilson. 1987. Mitochondrial DNA and human evolution. Nature 325:31-36. Cooper, D. W. 1968. The significance level in multiple tests made simultaneously. Heredity 23:614-617. DeSalle, R., A. Templeton, I. Mori, S. Pletscher, and J. S. Johnston. 1987. Temporal and spatial heterogeneity of mtDNA polymorphisms in natural populations of Drosophila mercatorum. Genetics 116:215-233. Gold, J. R., and L. R. Richardson. 1991. Genetic studies in marine fishes. IV: An analysis of population structure in the red drum (Sciaenops ocellatus) using mitochondrial DNA. Fish. Res. 12:213-241. Gold, J. R., L. R. Richardson, C. Furman, and T. L. King. 1993. Mitochondrial DNA differentiation and popu- lation structure in red drum (Sciaenops ocellatus) from the Gulf of Mexico and Atlantic Ocean. Mar. Biol. (In press.) Gold, J. R., T. L. King, L. R. Richardson, D. A. Bohlmeyer, and G. C. Matlock. In press. Genetic studies in marine fishes. VII: Allozyme differentiation within and between red drum (Sciaenops ocellatus) from the Gulf of Mexico and Atlantic Ocean. J. Fish Biol. 116:175-185. Hartl, D. L., and A. G. Clark. 1989. Principles of population genetics, 2nd ed. Sinauer Assoc, Inc., Sunderland, MA. Johnson, D. R., and N. A. Funicelli. 1991. Spawning of the red drum in Mosquito La- goon, east-central Florida. Estuaries 14:74-79. Matlock, G. C. 1984. A basis for the development of a management plan for red drum in Texas. Ph.D. diss., Texas A&M University, College Station, TX. McElroy, D., P. Moran, E. Bermingham, and I. Kornfield. 1992. REAP-The Restriction Enzyme Analysis Package. J. Hered. 83:157-158. Mehta, A. J., and H. K. Brooks. 1973. Mosquito Lagoon barrier beach study. Shore and Beach 41:27-34. Mercer, L. 1984. A biological and fisheries profile of red drum, Sciaenops ocellatus. Spec. Sci. Rep. 41, North Carolina Dep. Nat. Resour. Community Dev, Div. Mar. Fish., Raleigh, NC. Murphy, M. D., and R. G. Taylor. 1990. Reproduction, growth, and mortality of red drum, Sciaenops ocellatus, in Florida. Fish. Bull. 88:531-542. Nei, M. 1978. Estimation of average heterozygosity and ge- netic distance from a small number of indi- viduals. Genetics 89:583-590. Nei, M., and F. Tajima. 1981. DNA polymorphism detectable by restriction endonucleases. Genetics 97:145-163. Overstreet, R. M. 1983. Aspects of the biology of the red drum, Sciaenops ocellatus, in Mississippi. Gulf Res. Rep. (Suppl.) 1:45-68 Richardson, L. R., and J. R. Gold. 1993. Mitochondrial DNA variation in red grouper (Epinephelus morio) and greater amberjack (Seriola dumerili) from the Gulf of Mexico. ICES J. Mar. Sci. 50:53-62. Roff, D. A., and P. Bentzen. 1989. The statistical analysis of mitochondrial poly- morphisms: chi-square and the problem of small samples. Mol. Biol. Evol. 6:539-545. Rohlf, F. J. 1983. BIOM-PC: a package of statistical programs to accompany the text BIOMETRY. W. H. Free- man & Co., San Francisco, CA. Sokal, R. R., and F. J. Rohlf. 1969. Biometry. The principles and practice of sta- Gold and Richardson: Saaenops ocellatus from Mosquito Lagoon 65 tistics in biological research. W. H. Freeman & Co., San Francisco, CA. Swofford, D. L., and R. B. Selander. 1981. BIOSYS-1: a FORTRAN program for the comprehensive analysis of electrophoretic data in population genetics and systematics. J. Hered. 72:281-283. Templeton, A. R. 1987. Genetic systems and evolutionary rates. In K F. S. Campbell and M. F. Day (eds.), Rates of evolu- tion, p. 218-234. Australian Acad. Sci., Canberra. Wilson, A. C, R. L. Cann, S. M. Carr, M. George Jr., U. B. Gyllensten, K. M. Helm-Bychowski, R. G. Higuchi, S. R. Palumbi, E. M. Prager, R. D. Sage, and M. Stoneking. 1985. Mitochondrial DNA and two perspectives on evolutionary genetics. Biol. J. Linnaean Soc. 26:375-400. Wright, S. 1978. Evolution and the genetics of popu- lations. Univ. Chicago Press, Chicago, IL. Appendix Table 1 Allele frequencies at nine polymorphic loci among red drum iSciaenops ocellatus) from the northeastern Gulf of Mexico, Mosquito Lagoon, Florida, and the U.S. Carolina coast. Northeastern Mosquito U.S. Northeastern Mosquito U.S. Locus Gulf of Lagoon, Carolina Locus Gulf of Lagoon, Carolina allele Mexico' Florida coast 7 allele Mexico' Florida coast' ACP-2' '125 0.002 0.012 0.000 EST-f '115 0.087 0.073 0.063 '110 0.000 0.012 0.000 '100 0.911 0.915 0.937 '100 0.911 0.915 0.898 (n) (246) (41) (175) "95 0.089 0.073 0.102 in) (246) (41) (176) ADA' '150 0.000 0.012 0.003 GPI-B' '130 0.036 0.024 0.028 '-110 0.004 0.000 0.003 '125 0.315 0.354 0.372 '-100 0.976 1.000 0.971 '118 0.006 0.000 0.003 '-50 0.020 0.000 0.026 '115 0.081 0.122 0.028 in) (247) (41) (176) '113 0.002 0.000 0.003 '110 0.061 0.012 0.060 PEPB' '100 0.443 0.452 0.469 '115 0.022 0.000 0.006 '90 0.010 0.000 0.003 '100 0.974 1.000 0.991 '85 0.024 0.000 0.003 '85 0.004 0.000 0.003 '78 0.000 0.000 0.000 M (247) (41) (176) '75 0.018 0.024 0.028 '65 0.004 0.000 0.000 PEPD' in) (247) (41) (176) '115 0.002 0.012 0.009 '100 0.968 0.988 0.968 ADH' '85 0.030 0.000 0.020 '-100 0.508 0.451 0.566 '75 0.000 0.000 0.003 '-75 0.458 0.525 0.391 in) (247) (41) (176) '-50 0.028 0.012 0.020 '-20 0.006 0.012 0.023 PEPS' in) (246) (41) (175) '105 0.040 0.024 0.023 '100 0.958 0.976 0.977 sAAT-1' '95 0.002 0.000 0.000 '120 '110 0.000 0.134 0.012 0.171 0.017 0.120 in) (247) (41) (176) '100 0.856 0.817 0.854 ' Data are from Gold et al. (in press). '90 0.010 0.000 0.009 in) (242) (41) (175) 66 Fishery Bulletin 92(1), 1994 Appendix Table 2 Distribution of mtDNA haplotypes among red drum iSciaenops ocellatus) from the northeastern Gulf of Mexico, Mosquito Lagoon, Florida, and the U.S. Carolina coast. Composite North- Composite North- mtDNA eastern Mosquito U.S. mtDNA eastern Mosquito U.S. Haplo digestion Gulf of Lagoon, Carolina Haplo- digestion Gulf of Lagoon, Carolina type pattern' Mexico- Florida coast- type pattern' Mexico 1 ' Florida coast- 1 ABAAAAAAAAAAA Ill 4 10 56 AGAAAAAAAAAAA 1 2 ABCCAAAAAAAAA in 6 3 57 AAAAAABAAAEAA — — 1 3 ABBACAAAAAAAA 11 1 10 58 BBAAAFAAAAAAA 3 — — 4 EAAAAABAAAAAA 1 — — 60 FBBAAAAAAACAA — — 1 5 BAAAACBAAAAAA 1 — — 61 AAAAAAAADAAAA — — 1 6 CBAAAAAAAAAAA 2 1 1 62 BBBAAAAAAAAAA — — 1 7 AAABAAAAAAAAA 7 1 — 64 AAAEAABAAAAAA 5 — — 8 AAAAAABAAAAAA 33 26 18 66 BBADAAAAAAAAA — — 1 9 BAAAAAAAAAAAA 19 15 46 68 BBAEAAAAAAAAA 1 — — 10 BBAAAAAAAAAAA 9 2 4 69 AFAAAABAAAAAA 4 — — 11 AAAAAAAAAAAAA 23 2 13 70 ACAAAAAACAAAA 1 — — 12 CBAAAABAAAAAA — 8 6 76 BAAAAAAAABAAA 2 — — 13 ABCAAACAAAAAA 1 — 4 77 AB AAAG FAAAAAA 1 — — 14 BBFAAAAAAABAB — — 4 82 ABAAAAFAAAAAA 4 — — 15 AAAAAABACAAAA — 1 2 89 BIAAAAAAAAAAA — — 1 16 ACAAAAAAAAAAA 6 2 4 90 BAAAAAGAEAAAA — 1 1 18 ABAACAAAAAAAA 5 2 1 91 AAAAAAABAAAAA — — 1 L9 BBAAADAAAAAAA — 2 5 92 ABBAFAAAAACAA — — 1 20 ABBAAAAAAACAA — 3 2 93 AAAFGAAAEAAAA — — 21 BABAAAAAAAAAA 1 1 — 1 94 AAAAAABAAAADA — — 22 BAAAAABAAAAAA 4 1 2 95 BAAAAHAAAAAAC 2 — — 23 AAAABAAAAAAAA 17 6 8 96 BCAAAAAAAAAAA — — 24 AAAAAAAAAAAAC 5 2 1 97 H B AAAAAAAAAAA — — 25 ADCCAAAAAAAAA 2 — 1 98 BAAABAAAAAAAA — — 26 BABABAAAAAAAA 3 1 1 99 B B B AAAAAAAFAA — — 27 AACCAAAAAAAAA — 5 1 100 AAIAAABAAAAAA — — 28 ABAADAAAAAAAA — 2 2 101 ABCCAAFAAAAAA — — 29 AAAAABABAAAAA 10 — 3 106 AAAAIAAAAAAAC — — 31 DBCAAAAAAAAAA 1 — — 107 BAAAABABAAAAA 2 — — 35 AB B AAAAAAAAAA — 2 4 114 AC B AAAAAAAAAA — 1 — 36 ABADAAAAAAAAA 1 — 1 121 ABADAAAADAAAA 1 — — 45 BABAAABAAAAAA 2 — — 134 AAAAGABAAAAAA — — 46 ABEAAAAAAAAAA 1 1 — 135 BBJAADAAAAAAA — — 47 BBAAAFAAEAAAA 2 — — 136 BBADAAABAAAAA — — 48 AAEAAAAAAAAAA 1 — — 137 BBAAAAACAABAA — — VJ CBBAAAAAAAAAA 3 — — 138 BBAAAAAAAABAB — — 50 BBHAAAAAAABAB — — 139 AAACAAAAAAAAA — — 51 ABCAAAAAAAAAA — 1 140 ABACAAAAAAAAA — — 52 BBAAAAACAABAB — — 141 AACABAAAAAAAA — — 53 ABBAAAAAAAFAA 2 — 142 AACAAABAAAAAA — — 54 B.AAEAAAAAAAAA — — 143 ABAACAAAAAAAA — — 55 AHCCAAAAAAAAA — — ' Letters (from left to right! are digestion patterns for: Nco\, Sc/I, Seal, Pvull, Spel, Xbal, Xmnl, HindlU, Stul, BamHl, EcoRV, Pstl, and Nsil Details regarding fragment sizes of individual digestion patterns are available upon request. 2 Data are from Gold et al (1993). AbStfclCt. Microzooplankton retained by a 41-um mesh was sampled along a 50-km transect in the Shelikof Strait between Kodiak Island and the Alaska Pen- insula. We sampled once each year during spring (April-May) 1985- 1989 using Niskin bottles closed at 10-m depth intervals. Sampling was conducted near the time and place of peak hatching of walleye pollock (Theragra chalcogramma) larvae. We examined horizontal and vertical patterns of abundance of potential prey organisms, espe- cially copepod nauplii, and de- scribed these patterns with respect to the oceanography of the Strait. Hydrography, nutrients, chloro- phyll-a and net zooplankton data also were collected and were used to help interpret the microzoo- plankton patterns. Copepod nau- plii composed from 46 to 82% of all organisms in the formalin-pre- served samples. Eggs (3-35%), ro- tifers (up to 14%) and loricate tintinnids (up to 11%) were the next most abundant taxa. The abundance of microzooplankton varied greatly across the Strait and, for copepod nauplii, had maxima associated with the Alaska Coastal Current. A meso- scale feature in the coastal current appeared to influence the distribu- tion of microzooplankton and may affect feeding conditions for larval walleye pollock. Significant differ- ences in abundance of copepod eggs and nauplii were detected between some transects. The inte- grated, 0-60 m depth, across-strait average abundance of copepod nauplii varied from a low of 5.8 x 10 3 nr 2 (sampled in 1985) to a high of 17.6 x 10 3 nr 2 (1987). The maximum concentration found in these same transects varied from 18 to 144 L' 1 , respectively. Be- tween 60 and 70% of the nauplii sampled were of a size (>125 urn total length) composing approxi- mately 98% of the naupliar diet of larval walleye pollock in spring. Distribution and abundance of copepod nauplii and other small (40-300 jim) zooplankton during spring \n Shelikof Strait, Alaska* Lewis S. Incze Bigelow Laboratory for Ocean Sciences West Boothbay Harbor. ME 04575 Terri Ainaire Bigelow Laboratory for Ocean Sciences West Boothbay Harbor, ME 04575 Manuscript accepted 17 September 1993 Fishery Bulletin 92:67-78 (1994) The high mortality rate of marine fish larvae is attributed to high rates of predation (Moller, 1984; Bailey and Houde, 1989), sensitiv- ity to feeding conditions (Thei- lacker and Watanabe, 1989) and interactions between these factors (Houde, 1987; Purcell and Grover, 1990). The larvae of temperate fishes often occur during spring, when planktonic production is in early stages of its annual cycle and is easily disrupted or delayed by adverse conditions. Also, larvae have small search volumes and generally small energy reserves (Bailey and Houde, 1989). Thus, a spatial or temporal "match" or "mismatch" between the demand for larval food and its availability seems intuitively likely and has been the subject of much research (e.g., Lasker, 1981; Buckley and Lough, 1987; Cushing, 1990). The quest to quantify feeding relation- ships has led to continuing efforts to reduce container effects in ex- perimental studies (Gamble and Fuiman, 1987; McKenzie et al., 1990), to improve the sensitivity of physiological measurements (e.g., Buckley et al., 1990), to understand the small-scale distribution of prey in the field (Owen, 1989), and to understand the role of mixing in enhancing or retarding interactions between predator and prey (Rothschild and Osborne, 1988; Davis et al., 1991). In the ocean, feeding takes place in a complex spatial array of biological and physical conditions. Any study of rate-influencing processes that af- fect larvae must take into account the distribution of these conditions in order to understand effects at the population level. In this paper we examine the springtime community of small zooplankton, primarily copepod nauplii, that may be prey for larval walleye pollock, Theragra chal- cogramma, in Shelikof Strait, Alaska (Fig. 1), and we report on the distribution and abundance of these organisms with respect to oceanographic conditions. A large population of walleye pollock spawns in the Strait in late March and early April, forming dense ag- gregations of planktonic eggs in the deepest part of the sea valley be- tween Kodiak Island and the Alaska Peninsula. Hatching occurs from middle or late April through early May (Kendall et al., 1987; Incze et al., 1989; Yoklavitch and Bailey, 1990). While the eggs re- main mostly below 150 m, larvae * Bigelow Laboratory Contribution No. 93- 006. Fisheries Oceanography Coordinated Investigations Contribution No. 0186. 6 7 6 3 Fishery Bulletin 92(1). 1994 160° 150° 140° . i -60 N 156° 154° 152 c 156° 154° Figure 1 Top panel shows location of the study area and a generalized scheme of the surface circulation. Middle and bottom panels show Shelikof Strait and the sampling transect. Stations are numbered con- secutively beginning with 55 near the Kodiak Island shore; only the end and middle stations are labeled. are found primarily in the upper 50 m (Kendall et al., 1993 1 ) and have been shown to prey heavily on copepod nauplii during the first several weeks of development (Dagg et al., 1984; Kendall et al., 1987; Canino et al., 1991). The upper water column of Shelikof Strait con- sists of at least three distinct water types (Reed and 1 A. W. Kendall Jr., L. S. Incze, P. B. Ortner. S. R. Cummings, and P. K. Brown. 1993. The vertical distribution of eggs and larvae of walleye pollock in Shelikof Strait, Gulf of Alaska. Sub- mitted to Fish. Bull. Schumacher, 1989). A cold, slightly freshened, tur- bid coastal water band of narrow width (<10 km) remains near the Alaska Peninsula (northern) side of the Strait. This water receives its signature from glacial melt-waters draining into Cook Inlet at the northern end of the Strait and thus varies season- ally in volume. A second water type is encompassed in the Alaska Coastal Current (ACC), part of a baroclinic current running more or less continuously along 1000 km of the Alaskan south coast. The ACC flows from northeast to southwest in a band approxi- mately 20 km wide through the middle portion of the Strait, but it has a highly variable current struc- ture marked by numerous baroclinic instabilities (Mysak et al., 1981; Vastano et al., 1992). In the vertical, the southward flow of the ACC induces an opposite bottom flow of more saline, nutrient rich water that enters the sea valley at the shelf edge south of the study area (Fig. 1; see Reed et al., 1987). A third water type is made up of waters from a mixture of sources, including outer shelf and oceanic intrusions. Most of this water enters from the north and flows the length of the Strait along Kodiak Is- land, but current meter measurements and satellite imagery show that water sometimes enters from the south (Schumacher, 199 1 2 ). The work reported here was undertaken as part of a multi-disciplinary program (Fisheries Oceanog- raphy Coordinated Investigations: FOCI) aimed at understanding the influence of environmental fac- tors on the early life history of walleye pollock spawned in the Strait (Schumacher and Kendall, 1991). An extensive grid of sampling stations occu- pied in early May 1985, the first year of the pro- gram, showed that the spring bloom of large diatoms did not occur homogeneously throughout the Strait. Rather, in that year, large diatoms bloomed first in a band which occupied the longitudinal mid-portion of the Strait (Incze, unpubl. observ.). Hydrographic data show that this feature was in the ACC, which had at that time a shallower upper mixed layer than elsewhere in the Strait. It seemed likely, therefore, that conditions affecting the feeding and growth of larval walleye pollock would be subject to dynam- ics of the ACC and would differ across the Strait as well as through time. As part of the research pro- gram, a standard across-strait transect was estab- lished near the southern end of the Strait proper (about halfway up the sea valley: Fig. 1). This transect has been sampled with a CTD (Conductiv- ity, Temperature, Depth) as often as ship and re- search schedules have permitted. Biological sam- 2 J. Schumacher. 1991. Pacific Marine Environmental Labora- tory, Seattle, WA, unpubl. data. Incze and Ainaire: Distribution and abundance of copepod naupln 69 pling begins along this transect near the time of larval hatching each spring and proceeds down-cur- rent (westward) over time. In this paper we report on across-shelf patterns of abundance and vertical distribution of copepod nauplii and other small zoop- lankton from 1985 through 1989 and relate these patterns to hydrographic conditions, chlorophyll concentrations, and distributions of selected taxa of adult female copepods. Materials and methods For convenience, we use the term microzooplankton to refer to small zooplankton captured and pre- served by methods described below. Hydrography, nutrients, and microzooplankton were sampled with a CTD and rosette sampler along a transect of sta- tions across Shelikof Strait, Alaska, during spring from 1985 through 1989 (Fig. 1) (sampling dates are listed in Table 2). Hydrographic (CTD) data were obtained near bottom at 7 stations at 7-km inter- vals and were processed to give 1-m averaged data of salinity, temperature and density. Nutrients were sampled at five or more stations on the transect by removing water samples from 10-L Niskin bottles tripped at standard depths of 10, 20, 30, 50, 75, and 100 m; below this depth we sampled with lower reso- lution, generally at 50-m intervals, plus a sample near bottom. Nutrient concentrations were deter- mined after the cruise by using standard autoanalyzer techniques on frozen samples (Whitledge et al., 1981 3 ). Chlorophyll data were obtained from nutrient sampling depths in the up- per 100 m in 1988 and 1989. Analyses were con- ducted on board the vessel following methods of Yentsch and Menzel (1963) as modified by Phinney and Yentsch (1985) with 0.45-|im Millipore HA ac- etate filters. Microzooplankton was sampled from Niskin bottles were tripped at 10-m intervals from to 60 m in 1985 and from 10 to 60 m in other years. We used the same bottles as for nutrient and chloro- phyll samples for those depths which were common to all. The number of stations sampled varied over the years, beginning in 1985 with stations 55, 58, and 61. In 1986 and 1987 we included station 60. In 1988 we sampled all seven stations along the transect, and in 1989 we sampled all except station 57. Niskin bottles were sampled for nutrients and chlorophyll when called for; the remaining contents of the bottles were filtered through small (6 x 18 cm) 3 Whitledge, T. E., S. C. Molloy, C. J. Patton, and C. D. Wirick. 1981. Automated nutrient analyses in seawater. Tech Rep. No. BNL-51398, Brookhaven Natl. Lab., Upton, NY. conical nets made of 41-um mesh nylon netting. Material retained on the netting was flushed into 4— ounce (120 mL) glass jars by using 0.45-um fil- tered seawater and was preserved in a final solu- tion of 5% formalin:seawater. Larger zooplankton was sampled at all seven stations by using 60-cm diameter bongo samplers equipped with 333-um mesh nets and towed in double-oblique fashion from the surface to about 10 m off bottom. From 1986 onward, a 20-cm bongo sampler with 150-um mesh nets was attached to the towing wire 1 m above the larger sampler to try to improve on the sampling of smaller copepods. Properties of each tow were moni- tored by time, wire angle from the towing block, mechanical flowmeters mounted across the mouth of each net, and a bathykymograph attached to the bridle of the large bongo. In the laboratory, each microzooplankton sample was filtered onto a 41-um mesh sieve, stained over- night in Rose Bengal, transferred to a 10-mL scin- tillation vial and examined in approximately 2-mL aliquots. Microzooplankton was analyzed by using a stereo dissecting microscope equipped with an image analysis system consisting of a high-resolu- tion video camera and computer software to make measurements and record data (Incze et al., 1990). The microscopist made identifications, placing each organism into one of thirteen categories (Table 1), and directed the orientation of measurements. Cope- pod nauplii were measured for total length (TL) and maximum width. Total length was the carapace length ("prosome"), plus the abdomen ("urosome") when present. The latter section often was curled beneath the carapace, necessitating measurement along a curved line. We measured the diameter of eggs and only the total body length of all other or- ganisms. In most cases the entire sample was ana- lyzed, but 25% of the original sample sometimes pro- vided adequate counts, which we established as at least 50 nauplii per sample. Subsampling was done by increasing the stored sample volume to 200 mL, dividing as necessary, then recondensing the mate- rial for examination. Subsampling was checked for accuracy by completely analyzing both half-portions from 30 samples. Final counts of microzooplankton were corrected for the subsampling fraction and for differences in the original volume of water filtered and are presented as number of organisms per li- ter. Integrated abundances (No. m~ 2 ) were estimated for the upper 60 m of the water column by using a trapezoidal algorithm. Vertical and horizontal patterns of micro- zooplankton distribution were plotted by using an inverse distance gridding technique ("Surfer", Golden Software, Inc., Golden, CO) with a grid size 70 Fishery Bulletin 92(1). 1994 Table 1 (A) Composition of microzooplankton in Shelikof Strait during spring, expressed as a percent of total organisms counted. Hyphens indicate values greater than zero but less than 2%; non-zero val- ues shown are rounded to nearest whole number. Shed ovisacs are from Oithona spp.; "Other" in- cludes infrequent and unidentified organisms. (B) Vertically integrated abundances of organisms are averaged across Shelikof Strait for each year; "All other" refers here to all categories from (A) com- bined except for those specifically listed. A Percent composition ( 'ategory 1985 1986 1987 1988 1989 Copepod nauplii Other nauplii Invertebrate eggs Ovisacs Copepods Euphausiids Rotifers Tinitinnids Larvaceans Polychaetes Echinoderms Foraminifera Other 50 46 54 82 76 25 3 9 35 2 7 L3 14 11 B Average integrated abundance (1000s m -2 ) from 0-60 m Copepod nauplii Invertebrate eggs All other Total 5.8 13.9 17.6 9.4 9.6 3.0 10.4 4.6 5.7 13.3 30.0 3.6 0.4 0.6 8.6 1.9 2.6 29.8 11.8 12.8 set at 25 units in both the X and Y directions. The same technique was used for contouring CTD and nutrient data. A subset of contours from all three data types was compared by inspection to the origi- nal input data to look for artifacts caused by the contouring software. Integrated abundances of nau- plii across the Strait were compared for the four years which had late April-early May sampling (1985, '86, '88, '89). Data were taken from those sta- tions (#55, 58, 61) sampled every year in the series and were compared by using a non-parametric two- way analysis of variance (ANOVA) on ranks (also referred to as the Quade test: Conover, 1971). A multiple comparison based on ranks (Conover, 1971) was applied when the ANOVA showed statistically significant differences. We used the estimated abundances of adult fe- male copepods (No. m" 2 ) from the oblique bongo tows to consider possible sources of planktonic eggs and nauplii sampled in our study. Data are from a da- tabase being used to describe spatial and interannual patterns of major zooplankton taxa (FOCI Database, National Marine Fisheries Service, Seattle); subsampling and counting followed stan- dard procedures and are detailed in a series of five reports (e.g., Siefert and Incze, 1991 4 ). The relative contribution of each taxon to the standing stock of planktonic copepod eggs and early nauplii was esti- mated by using egg production rates reported in the literature or from unpublished data. This is simplis- tic, because it ignores changes in egg and naupliar concentrations as a function of birth rate, develop- ment time, and mortality, all of which may vary considerably. However, the calculations provide a rough evaluation of potential sources of nauplii in Shelikof Strait. Sizes of eggs and early nauplii (e.g., Nauplius I [NI]) were used when reports were found. We used the following information: Calanus marshallae (eggs 175-185 |im, fecundity 12 eggs d : [Runge, 1990 5 1; Calanus pacificus (eggs ca. 160 urn, fecundity 38 eggs d" 1 [Runge, 19841; NI ca. 220 Urn CL [Fulton 19721); Metridia pacifica (eggs 150 urn [Runge, 1990 6 1; fecundity 2.5 eggs d" 1 [Batchelder and Miller, 1989)); Pseudocalanus spp. (eggs ca. 110-130 urn retained in ovisacs [Frost, 1987]; fecundity 4 eggs d" 1 [Dagg et al., 1984; Paul et al., 1990]; NI ca. 180 pirn CL [Fulton, 1972]). Jeffry Napp 7 and Kenric Osgood 8 both have found that Metridia pacifica held in the laboratory may produce eggs at higher rates, and they suggest that the population average at times may be several times greater than the rate given above. Results In this section we designate different transects by the year in which they were sampled but do not mean to imply that the differences necessarily were interannual. We address this distinction in the discussion section. Nitrate concentrations in bottom waters were highest in 1985, 1988, and 1989 (>25 ug-at L" 1 com- 4 Siefert, D. L. W., and L. S. Incze. 1991. Zooplankton of Shelikof Strait, Alaska, April and May 1989: data from Fisheries Ocean- ography Coordinated Investigations (FOCI) cruises. Alaska Fish. Sci. Center, NOAA, Seattle, WA, 119 p. 5 J. Runge. 1990. Insti. Maurice Lamontagne, Mont-Joli, Que- bec, Canada, pers. commun. 1990. 6 J. Runge. 1993. Inst. Maurice Lamontagne, Mont-Joli, Quebec, Canada, unpubl. data. 7 Jeffry Napp. Nat. Mar. Fish. Serv., Alaska Fishereis Science Center, Seattle, WA, pers. commun. 1993. 8 Kenric Osgood, Dep. Oceanography, Univ. Washington, Seattle, WA, pers. commun. 1993. Incze and Ainaire: Distribution and abundance of copepod nauplii 71 pared to <20 ug-at L : in the other years); in sur- face waters they were lowest in 1987 (mostly <2 ug- at L" 1 ), followed by 1986 (<4 ug-at L M and 1989 (<5 Ug-at L _1 ) (Fig. 2). Surface nitrate distributions gen- erally reflected density structure. Isopleths of den- sity (Fig. 2), salinity, and temperature show larger '£■ en CD on >- < 2 IN 3 01 (uj) H)dao IO CN 11 •Q O a «I 1 g 05 Ed Tl J r U IO e X OS a ^ E k .r a c C/J r ft 05 >- bC J3 C CO C -n 9 C -n a cd i- '<: X -5 o - £ « CD 1- 1^ -c 172 r/l H en o c 0) > a s a 01 •- a a en -j 01 r !* cd +^ >i — *J en C -a o> S ^ T-l CO C S 7- ho 1 c i-l '5 w CO - -r =L a is a 0. rn ^H ~- 11 ^-* P O CL 3 c Hi Cii — 1 -C d H c tj -/. od iC x - tn CL, .2 72 Fishery Bulletin 92(1). 1994 volumes of high density (high salinity) bottom wa- ter in 1985, 1986, and 1989 compared with other years. The upper mixed layer generally was deep- est on the northern end of the transect, near the Alaska Peninsula, with a steeply sloping density gradient near the middle. The exception, in 1988, is discussed later. Averaged across the Strait, the up- per mixed layer was deepest in 1985 and shallow- est in 1986 and 1987. Observations of phytoplankton clogging sampling nets during the cruises showed that the spring bloom of large diatoms occurred latest in 1985. By this approximation, what probably was the major spring bloom in the Strait began after the first week of May in 1985, whereas it already was well under- way when we began sampling in early May 1986 and 1989 and late April 1988. A grid of sampling stations that extended to the northern end of the Strait in 1985 showed that the bloom in that year formed first in a band along the middle of the Strait for virtually its full length of 300 km. Our grid in- terval was not sufficiently fine to resolve the width of the bloom feature, but our findings are consistent with a diameter <25 km. Our samples were dominated numerically by cope- pod nauplii, which composed from 46 to 827c of all organisms sampled along the transect over the five- year period (Table 1), followed in most years by cope- pods eggs, from 3.5 to 35 f /r. Of the remaining taxo- nomic categories, only a few ever contributed more than 57c of the total organism count: small copep- ods (including copepodid stages), tintinnids, rotifers, Nauplii Late April - Early May 1.8 x 10 6 o 1.2 x 10 6 90 urn long and all nauplii >128 urn. Our data showed a steep decline in frequency of nauplii with length <110 um, between the above esti- mates, and width <50 um, corresponding to the relationship 110/2.2 = 50. Most of the nauplii did not have urosomal segments, so total length and maximum width are equiva- lent to prosome length and width for most of our data. The abundance and size distribution of eggs differed substantially between years (Fig. 8). The greatest number (and smallest median size [ca. 75-um diameter]) of planktonic eggs was present in 1986; the fewest eggs occurred in 1988, when median size was the largest (ca. 165 urn). Abundances of potentially significant contributors to the standing stocks of copepod eggs and nauplii are given in Table 2. Among the taxa of interest, Calanus pacificus had low adult female numbers because most individuals were in copepodid stage 5 (C5) during spring. Other adult female copepods Chl-a (mg m 2 ) 20 177 10 142 7 20- (5 n E i io- I A ^ o £ N> |X t — • Distance (km) 20 30 Figure 4 Mean number of nauplii and total microzooplankton per li- ter in the upper 60 m across the study transect in April 1988 (top panel), viewed looking westward. Numbers at the top of the panel show integrated (0-100 m) chlorophyll-a con- centrations (mg m -2 ). Temperature (°C) and salinity (g kg -1 ) are shown in the middle and bottom panels, respectively. Data can be compared with nutrient distributions (Fig. 2), dynamic topography (Fig. 5), and depth distributions of nauplii (Fig. 6). were broadly distributed across the Strait, but the maximum concentration of each taxon occurred in the northern half (among stations 58-61) in all but one instance. The across-Strait patterns of low and high abundances within species were similar from year to year and statistically significant (Spearman rank correlation test, P<0.05). The shift in mesh sizes for Pseudocalanus spp. collections limits the between-transect comparisons that can be made. (Note that there are interspecific differences within 74 Fishery Bulletin 92[ I). 1994 57°30 ;-»T llll Alaska Peninsula 155°30' 154 D 30 Longitude ( W ) Figure 5 Contours of 0-150 m dynamic height in western Shelikof Strait during April 1988. Solid circles show locations of CTD stations. The study transect is the farthest northeast sec- tion. Open circles denote those transect stations with the highest microzooplankton standing stocks (cf. Figs. 4, 6). A dynamic high (H) and low (L) are labelled; arrows show inferred flow. the genus that prohibit any simple correction for different mesh collections: see Frost, 1987.) Within these limitations, data for 1985 and 1986 (333 pm) were statistically different (Wilcoxon signed rank test, P=0.076), whereas the multi-year comparison for early spring samplings (1986, 1988, 1989: 150 pm mesh) showed no statistically significant differ- ences (Quade test, a= 0.05). Among early spring values, there were no statistically significant differ- ences in abundance of Metridia spp.. Discussion The method of sampling and preservation used in this study under-represented smaller components of the microzooplankton (James, 1991) but was ad- equate to capture the majority of prey items of lar- val walleye pollock based on prey sizes reported from earlier studies of Clarke (1984: Bering Sea), Nishiyama and Hirano (1983, 1985: Bering Sea), Dagg et al. (1984: Bering Sea); and Kendall et al. (1987: Shelikof Strait). For small larvae of 5-10 mm standard length (SL) in those studies, copepod nau- plii composed the majority of items found in larval stomachs. They also made up the bulk of estimated volume or carbon content of prey when these values were calculated (Incze et al., 1984; Nishiyama and Hirano, 1983). The 10-m vertical resolution of our sampling method almost certainly failed to detect the highest concentrations of prey available to larval walleye pollock under some conditions, such as in small patches (Owen, 1989), but prob- ably reflects adequately the average abun- dances found at different depths in the wa- ter column, in different sections across the Strait and in different transects. Size-frequency distributions of sampled nauplii and dimensions of the sampling mesh suggest that there was virtually com- plete retention of nauplii with total length > 125 pm. In most cases these measure- ments were carapace ("prosome") lengths. Unpublished data from stomach content studies (Canino, 1992 9 ) show that ca. 98% of the nauplii consumed by larval walleye pollock collected during our cruise in May 1989 had carapace length > 125 pm. Be- tween 60 and 70% of the nauplii in our samples were of this size (Fig. 8). Concentrations and integrated abun- dances of nauplii differed across Shelikof Strait in patterns that appear to be related to circulation features. Our data indicated that standing stocks and maximum concen- trations of copepod nauplii in spring were greatest in the ACC, which is also where greatest chloro- phyll-o concentrations occurred (latter data for 1988, 1989; cf. Figs. 4, 6, 7). The lowest naupliar concen- trations of the early spring samplings occurred in 1985, which had the weakest stratification. In gen- eral, nauplii were most abundant at 20-m depth except in 1988, when maximum concentrations oc- curred at 30-m depth in the deeper mixed perimeter of the anticyclonic feature. The lowest standing stock of nauplii coincided with the latest apparent phytoplankton bloom in 1985, but we cannot deter- mine if lower individual copepod egg production rates or lower standing stocks of copepods were re- sponsible because we lack adequate collections ( 150— pm mesh) of Pseudocalanus spp. in 1985. Alterna- tively, the low naupliar standing stocks could have been due to higher predation, but our data show that springtime populations of predators were gen- erally low and were similar among years. Our data suggest that the distribution of copepod nauplii and some other microzooplankton across 9 M. Canino. 1992. Natl. Mar. Fish. Serv., Alaska Fisheries Sci- ence Center, Seattle, WA, unpubl. data. Incze and Ainaire: Distribution and abundance of copepod nauplii 75 naupui (So. r 1 ) 1985 (R=0-18;CI=2) Distance (km) 1986 (R=1-56;CI=4) 3 y \0 JO ' 1987 (R=1-144;CI=10) 1988 (R=0-26; Cl=2) 10 » Figure 6 Contour plots of naupliar concentrations (no. Lr 1 ) across Shelikof Strait during spring. Numbers in parentheses after the year (upper left of each plot) show the range (R) of data and the contour inter- val (CI) used in plotting. Transects are viewed look- ing westward. Shelikof Strait were subject to the influence of baroclinic instabilities. The timing and rotational sense of these instabilities therefore may have a large influence not only on the distribution of wall- eye pollock larvae themselves (Reed et al., 1989; Incze et al., 1990; Vastano et al., 1992), but also on the feeding conditions they experience. For example, the feature sampled in 1988 covered a substantial Chlorophyll - a (ug I 1 ) Distance (km) 20 JO T" Figure 7 Chlorophyll-a distributions across Shelikof Strait, May 1989, looking westward (data may be compared with nutrient and hydrographic structure in Fig. 2 and naupliar concentrations in Fig. 6). portion of the main spawning and hatching area. Although we do not have extended observations of this feature, Vastano et al. (1992) showed that eddy- like features may remain near the hatching area for as long as two weeks, a substantial portion of the hatching period (Yoklavitch and Bailey, 1990). If walleye pollock larvae migrate vertically into the center of a dynamic high after hatching, then the amount of time that passes before they are advected into better feeding conditions (in this case at the periphery of the high ) may be important to early larval feeding and growth. The average integrated abundance of copepod nauplii across the Strait was different for the vari- ous transects. The maximum values that were seen in 1987 probably can be attributed to the compara- tively late sampling of that year. However, among the four years with similar timing of transect sam- pling, there remained statistically significant differ- ences that may have been important to hatching walleye pollock larvae (see Canino et al., 1991, for feeding conditions and larval RNA/DNA ratios). Since hatching takes place over a relatively short time period (Yoklavitch and Bailey, 1990), the phas- ing of hatching and upper layer conditions may play an important role in establishing the larval year class. Unfortunately, we do not know how long the observed conditions persisted in each year relative to the population hatching time or to other require- ments of the early feeding period in larval develop- ment. Advection (Incze et al., 1989) and short-term fluctuations in mesoscale circulation (Vastano et al., 1992) may cause conditions in the Strait to change quickly, requiring more frequent sampling and im- proved techniques to rapidly assess prey distributions. Nauplii that were most abundant in the diet of larval pollock must have come from copepods large enough to be retained by mesh sizes used on the 76 Fishery Bulletin 92(1), 1994 1986 nauplii 500— >> U g 400-| 3 ST 300 1*1 200— 100— 1986 rn-i-! — -^ 50 100 150 200 260 300 Size ((im) C.D.F. 1.0 1985 0.8 y£^\% 0.6 // nauplii 0.4 - /'" 0.2 - 0.0 1 y s i i i i l 14- 13- 12 11 10- 5 4 3 2 1 50 100 160 200 260 300 1988 eggs JL 60 100 160 200 260 300 Size (um) Figure 8 Size-frequency distribution of nauplii and eggs. Graph in upper left shows size frequency of nauplii from 1985. Graph in upper right shows the full range of size distributions of nauplii by comparing the cumulative distribution functions (CDF) for the two extremes, 1985 and 1986. Size distributions of eggs are shown in the two lower graphs for years with the smallest (1986) and largest (1988) eggs. Note changes in frequencies shown on the various ordinates. bongo samplers (Table 2). Based on the average abundance and fecundity (see Methods) of adult fe- male copepods, the approximate contribution of each species to the daily production of NI would be: Pseudocalanus spp., >75% ; Metridia pacifica, 18%; Calanus marshallae, 49r; and Calanus pacificus, <1%. These percentages are useful only for the rela- tive scaling they permit; many factors may influence copepod reproduction rates, and rates of develop- ment and mortality will influence further the total standing stock of nauplii contributed by each spe- cies. These results agree with those of Dagg et al. (1984) with respect to the importance of Pseudo- calanus spp. naupliar production for larval walleye pollock feeding. Our results differ in the greater inferred role of Metridia spp., probably because of the deep waters of the Shelikof sea valley compared with the Bering Sea shelf where Dagg and his co- authors worked. The numerous small nauplii <120 (im that we sampled are from unknown sources. The abundance and fecundity of M. pacifica suggest that they were significant contributors to populations of planktonic eggs and that Calanus marshallae plays a lesser role. A large number of small planktonic eggs <150-um diameter are not accounted for by the adult female copepods retained by our nets. Acknowledgments This research was supported by the U.S. National Oceanic and Atmospheric Administration through the FOCI program. We thank J. Schumacher for providing CTD data, M. Canino for sharing unpub- lished data on larval walleye pollock diet, K. Incze and Ainaire: Distribution and abundance of copepod nauplii 77 Table 2 Abundance (no. m~ 2 ) of adult female copepods on a transect across western Shelikof Strait during spring. Data are listed vertically showing mean, (standard deviation) and range. Metridia pacifica is Metridia pacifical M. lucens; unidentified Metridia spp. are not included in this tally. Hyphens indicate absence of data. Taxon and mesh size Year and day 1985 (3 May) 1986 (3 May) 1987 (19 May) 1988 (27 Apr) 1989 (10 May) Pseudocalanus spp. 150 |im — 14,183 (6,523) 6,758-18,994 41,058 (25,527) 6,108-78,976 13,634 (4,128) 7,846-20,316 8,450 (4,026) 2,870-12,563 Pseudocalanus spp. 333 pm 9,119 (4,767) 2,509-16,110 16,232 (8,295) 7,848-30,573 33,098 (19,398) 6,273-51,729 Calanus marshallae 333 pm 130 (146) 0-431 82 (72) 0-211 610 (532) 0-1,343 125 (93) 0-238 618* (786) 0-2,196 Metridia pacifica 333 pm 5,082 (4,128) 68-11,899 3,168 (1,956) 24-6,340 9,537 (5,570) 288-5,715 3,211 (1,626) 288-5,715 2,713 (2,549) 0-6,945 Calanus pacificus 333 pm 15 (27) 0-73 2 (4) 0-9 28 (61) 0-164 133 (228) 0-521 McCauley for early work with microzooplankton sorting, D. Siefert for processing net zooplankton samples and our many sea-going colleagues for their help in the field. Our work benefitted from discus- sions with A. Kendall, K. Bailey, J. Schumacher, and J. Runge, and our manuscript from comments by M. Mullin, J. 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Hatching period, growth and survival of young walleye pollock (Theragra chalcogramma) as determined from otolith analysis. Mar. Ecol. Progr. Ser. 64: 13-23. Abstract. Distribution and size during their first summer at sea were determined for juvenile salmon (Oncorhynchus spp.) caught in oceanic waters off north- ern British Columbia and South- east Alaska, and in marine waters within the Alexander Archipelago of Southeast Alaska. More than 10,000 juvenile salmon were caught in 252 purse-seine sets during August 1983, July 1984, and August 1984. Distribution was patchy; juvenile salmon were highly aggregated, rather than dispersed randomly. Distribution and size of pink salmon (O. gorbuscha), sockeye salmon (O. nerha), and chum salmon (O. keta) were similar but differed from coho salmon (O. kisutch). Chinook salmon (O. tshawytscha) were ex- cluded from most analyses because few were caught. Sizes were con- sistent with the concept that juve- nile salmon in more northern and seaward locations had been at sea longer than those in more south- ern and inshore locations. Juvenile salmon migration up the Pacific coast did not peak in abundance off Southeast Alaska until August; movement from inside to outside waters was not complete by the end of August. The migration band of juvenile salmon in outside wa- ters of Southeast Alaska extended beyond the continental shelf to at least 74 km offshore, twice the dis- tance previously reported. Marine distribution and size of juvenile Pacific salmon in Southeast Alaska and northern British Columbia Herbert W. Jaenicke Adrian G. Celewycz Auke Bay Laboratory, Alaska Fisheries Science Center National Marine Fisheries Service. NOAA 1 1305 Glacier Highway. Juneau. Alaska 99801-8626 Manuscript accepted 28 September 1993 Fishery Bulletin 92:79-90 (1994) The general migratory movements of Pacific salmon (Oncorhynchus spp.) during their first year at sea have been described (Hartt and Dell, 1986), but little information is available on the seaward migration of juvenile salmon from the inside waters of Southeast Alaska into the Gulf of Alaska. Salmon moving sea- ward from streams inside South- east Alaska pass first through the complex waterways of the Alexander Archipelago, the "inside waters" of Southeast Alaska. Upon entering the Gulf, these salmon either occupy outer coast inlets or move into exposed outside waters. Salmon entering exposed outside waters either migrate north along the coast or move progressively far- ther offshore (Hartt and Dell, 1986). Determining when and at what size juvenile salmon from Southeast Alaska utilize different habitats during their seaward mi- gration to the Gulf may facilitate understanding the high mortality during their first few months at sea (Parker, 1968; Bax, 1983; Furnell and Brett, 1986). Our goal was to ascertain the distribution and migration of juve- nile Pacific salmon during their first summer at sea after they leave nearshore estuarine habitats. Specific objectives were 1) to deter- mine relative distribution, abun- dance, and size of juvenile salmon in exposed outside waters, in protected waters adjacent to the outer coast, and in the inside waters of Southeast Alaska, and 2) to compare abun- dance and size of juvenile salmon in outside waters of Southeast Alaska and northern British Columbia. Methods Study area and time The study area extended from Lituya Bay, Southeast Alaska, to the northern end of Vancouver Is- land, British Columbia (Fig. 1). Three major habitats were sampled: 1) outside waters (the North Pacific Ocean and Gulf of Alaska adjacent to the outer coast of Southeast Alaska and British Columbia); 2) outer coast inlets (protected waters along the outer coast of Southeast Alaska); and 3) inside waters (marine waters within the Alexander Archipelago). Southeast Alaska was further di- vided at lat. 56°N into a northern and southern region for some analyses. Fishing effort was con- centrated in the northern region of Southeast Alaska (Fig. 1). 79 80 Fishery Bulletin 92(1), 1994 Figure 1 Locations seined in Southeast Alaska and British Columbia in 1983 and 1984. The delineation between northern and southern Southeast Alaska is indicated by the dotted line (running along 56°N lat.). We sampled in Southeast Alaska during three periods: 6 August-3 September 1983 (hereafter des- ignated August 1983), 9-24 July 1984, and 1-30 August 1984. Sampling in British Columbia was conducted 1-6 July 1984. Survey stations in outside waters were located along transects perpendicular to shore (Fig. 1). The nearshore station of each transect was as close to land as net depth and safety permitted. Stations were usually sampled progressively offshore at 5.6 km (3 nautical miles [nmi]) intervals in 1983 and at 9.3 km (5 nmi) intervals in 1984. Sampling gen- erally did not extend beyond 37 km offshore except in Southeast Alaska in August 1984, when transects extended as far as 74 km offshore. Distances are rounded to the nearest 1 km in the text. In large passages in the inside waters, sets were often made along transects near the en- trance to outside waters (Fig. 1). Multiple sets were also made in clusters in the larger inlets. Gear Stations were sampled with table and drum seines as described by Browning (1980). The 28-m NOAA RV John N. Cobb fished a table seine in August 1983 and August 1984; the 24-m FV Bering Sea fished a drum seine in July 1984. Sets were made at predetermined locations without reference to visual or instrument sightings of fish. All sets were round hauls: the net was set in a semi-circle, held open 3—5 minutes, closed, pursed, and retrieved by means of a hydraulic power block (table seine) or a hydraulic roller (drum seine). Only catches from effective seine sets are listed (Table 1). Although the seines differed in size, mesh, and area enclosed, the two nets were assumed to be comparable in their ability to capture juvenile salmon. The table seine was 455 m long; depth tapered from 37 m in the wing to 11 m in the bunt; web sizes (stretch mesh) were 89 mm and 57 mm in the wing, and 25 mm in the bunt. The drum seine was 503 m long, 46 m deep, and had 32-mm mesh in the wing, and 25 mm in the bunt. Depths fished were assumed to be adequate for sampling juvenile pink (O. gorbuscha), chum (O. keta), sockeye (O. nerka), and coho (O. kisutch) salmon, which usually occupy the upper 10 m of the wa- ter (Manzer, 1964; Godfrey et al., 1975; Hartt, 1975). To compensate for the larger surface area enclosed by the drum seine (20,150 m 2 ) compared to the table seine (16,467 m 2 ), drum seine catches (July 1984) were reduced during analyses by 18.3% to standardize the catch per unit of effort (CPUE). This standardization caused the July 1984 catches reported to be sometimes less than the number of fish measured for size that period. Catch processing and analysis The catch was processed aboard ship and in the Auke Bay Laboratory. The number of juvenile salmon captured in each set was counted if the catch was small (i.e., <100 fish) or estimated gravimetri- cally if the catch was large. Up to 100 salmon from each set were preserved in 10% formalin in seawater Jaenicke and Celewycz: Marine distribution and size of juvenile Pacific salmon Table 1 Number of uvenile salmonid s caught by species , period, and hg bitat. All seining occurred in Southeast Alaska (SE AK) except in July 1984 when the outside waters of Briti sh Colum bia (B.C.) were alsc sampl ed. Number of fish caught dumber Period Habitat of sets Pink' Churn 1 ' Sockeye 1 ' Coho J Chinook 5 All species August 1983 Inside waters 54 2,011 385 178 201 3 2,778 Outer coast inlet 27 680 85 23 1 789 Outside waters 8 20 2 9 27 58 Subtotal 89 2,711 472 187 251 4 3,625 July 1984 Inside waters 18 91 16 17 197 19 340 Outer coast inlet 14 10 2 24 36 Outside waters B.C. 21 573 189 581 33 5 1,381 SE AK 33 181 34 109 28 1 353 Subtotal 86 855 241 707 282 25 2,110 August 1984 Inside waters 37 1,850 163 23 375 23 2,434 Outer coast inlet 4 12 3 i) 15 Outside waters <37 km seaward 26 866 152 171 128 5 1,322 >37 km seaward l(i 522 63 119 26 730 Subtotal 77 3,238 390 313 532 28 4,501 All Inside waters 109 3,952 564 218 77.3 45 5,552 Outer coast inlet 45 690 99 50 1 840 Outside waters 98 2,162 440 989 242 11 3,844 Total 252 6,804 1,103 1,207 1,065 57 10,236 ; Oncorhynchus gorbuscha. 2 0. kela. 3 0. nerka. 4 0. kisutch. 5 0. tshawytsc ha for later species identification and size measure- ments (fork length [FL] to nearest mm). If more than 100 juvenile salmon were captured in a set, the excess fish were released alive. Graphs (Chambers et al., 1983) and exploratory data analysis (Tukey, 1977) were used to present catch data because the data had a nonnormal dis- tribution with values clumped at zero (many seine sets did not capture juvenile salmon). Transforma- tions of catch data were ineffective in making the distribution more symmetrical. Quantile plots (Chambers et al., 1983), which show individual catches from smallest to largest, were used to de- scribe the statistical distribution of catches of each species. Chinook salmon (O. tshawytscha) were ex- cluded from the remaining analyses because few were caught. Morisita's Index of Aggregation (Morisita, 1959; Poole, 1974) was used to test whether each salmon species was randomly dis- persed or aggregated in marine waters of Southeast Alaska. Morisita's index is defined as N £«,(«,-!> i l rc(n-l) N, where N is the number of samples, n { is the num- ber of individuals in the z'th sample, and n is the total number of individuals in all samples. The sig- nificance of I & is tested with the Ftest described by Poole (1974). Spearman's rho (p) correlation test (Daniel, 1978) was used to measure association be- tween each possible pairing of the four main species caught (pink, chum, sockeye, and coho salmon). For comparisons, catch data were split into cells by 1) species, 2) habitat (outside waters, outer coast inlets, and inside waters), 3) region (northern South- east Alaska, southern Southeast Alaska, and Brit- ish Columbia), and 4) time period (August 1983, July 1984, and August 1984). CPUE was used as an index of abundance; frequency of occurrence (FO) 82 Fishery Bulletin 92(1), 1994 was used as a measure of presence of juvenile salmon. Five null hypotheses were tested during fish length analyses of the four species. The first four hypotheses stated that size of a species did not dif- fer for fish from 1) outside and inside waters, 2) outside waters >37 km offshore and <37 km offshore, 3) northern and southern waters, and 4) July and August of 1984. The alternate hypotheses stated that fish were larger in 1) outside than inside wa- ters, 2) outside waters >37 km offshore than outside waters <37 km offshore, 3) northern than southern waters, and 4) August than July of 1984. The fifth hypothesis stated that length did not differ among species caught within each period. A number of one-tailed, two-sample ^-tests were conducted under null hypotheses 1-4. Only cells that varied in one dimension were directly com- pared. (For example, under the hypothesis that mean sizes of fish from northern and southern wa- ters did not differ, the mean lengths of pink salmon in the inside waters of northern and southern South- east Alaska in August 1983 could be compared be- cause the difference between these two cells was in only one dimension — north versus south. ) Each pos- sible pairwise comparison under one of the hypoth- eses was treated as a separate, single, and indepen- dent test, and all comparisons were equally weighted. No ^-tests could be conducted if one cell had only one fish length. For the overall probability statement, the following statistic was used (Winer, 1971): 22>" where it, lnP. Under the hypothesis that the observed probabili- ties were a random sample from a population of probabilities having a mean of 0.50, the % 2 statistic has a sampling distribution which is approximated by the x 2 distribution having 2k degrees of freedom, where k is the number of comparisons (Winer, 1971). For size hypothesis 5 (no difference in mean fork length among salmon species), ANOVA was applied by pooling observations for each species from all habitats and regions. In effect, the pooled species length distribution is a weighted sum of the compo- nent distributions represented by the individual samples. Mean lengths of different species were compared separately for each period. If the overall F-test was significant, all possible species compari- sons within a period were tested with two-tailed t- tests. Experimentwise error was controlled at a = 0.05 by adjusting the critical value for each t-test to a = 0.0085, by using the Dunn-Sidak method (Sokal and Rohlf, 1981). Results Total catch Over 10,000 juvenile Pacific salmon were captured in 252 seine sets during the three sampling periods (Table 1). The catch consisted of 66% pink salmon, 11% chum salmon, 12%' sockeye salmon, 10% coho salmon, and 1%> chinook salmon. Pink salmon were the most abundant species (CPUE=27), with 6,804 caught. Chinook salmon were the least abundant species (CPUE=0.23), with only 57 caught. Statistical distribution of catch Catch distribution of juvenile salmon was extremely patchy. None were caught in 22% of the sets; more than half were captured in 5% of the sets. Plotting catch abundance against quantiles illustrated that the underlying statistical distribution for each spe- cies was clustered around zero (Fig. 2). Chinook salmon had the lowest FO in catches ( 12%), followed by sockeye salmon (32%), chum salmon (397/ ), pink salmon (45%), and coho salmon (54%). Coho salmon (median catch=l) was the only species with a me- dian catch >0. Juvenile salmon had highly aggregated distribu- tions. Morisita's Index of Aggregation (I & ) was sig- nificantly (P<0.001) greater than 1, indicating all species had aggregated distributions in each habi- tat and for all habitats pooled (Table 2). Species associations Pink, chum, and sockeye salmon catches were closely associated with each other. Catches of pink, chum, and sockeye salmon were positively and sig- nificantly (P<0.05) correlated (Table 3). In contrast, coho salmon abundance was not correlated with that of other salmon (Table 3). Abundance By habitat In Southeast Alaska and British Co- lumbia combined, pink salmon were the most abun- dant species in each habitat (Table 1). The total pink salmon catch exceeded the catch of each of the other species by six times or more. In Southeast Alaska, the CPUE of juvenile pink, chum, coho, and chinook salmon was greater in in- side waters than in outside waters (Fig. 3), whereas sockeye salmon were more abundant in outside waters than inside waters (Fig. 3). For each species, the lowest CPUE and FO were in the outer coast inlets; sockeye salmon were never captured in an outer coast inlet (Fig. 3). The FO of pink, chum, and sockeye salmon was higher in outside than inside waters; the opposite was true for coho salmon (Fig. 3). Jaenicke and Celewycz: Marine distribution and size of juvenile Pacific salmon 83 1500 - ,.i 1000 - Pink talmon 500 - ..---"'" , _J 100 - Chum talmon ...-■"""' ~ 50- £ B O 400 - | . — r^ fe Sockeya talmon a E 200- 3 c ...-•'""* £ O J n ioo - 1 _, J .A Coho talmon 50 - ,- '■ , _J 10 - 5 - Chinook salmon , • . J ° 6 0.2 0.4 I 0.6 08 1 median Fraction of ordered data Figure 2 Quantile plots of abundance of the five species of juvenile Pacific salmon (pink, Oncorhynchus gorbuscha; chum, O. keta; sockeye, 0. nerka; coho, 0. kisutch; chinook, 0. tshawytscha) caught in 252 purse-seine sets in Southeast Alaska in 1983 and 1984 and in British Columbia in 1984. The ranked catches are from the smallest (0) to largest (1) on the X axis. A theoretical normal distribution is in- dicated by the dotted lines. By distance offshore in outside waters Dis- tribution of juvenile salmon varied by distance off- shore. Substantial numbers offish were captured up to the maximum distance fished offshore (74 km, Fig. 4A). At intervals offshore, abundance and pres- ence of each species is shown by the 3RSSH smoothed (Tukey, 1977) natural logarithms (In) of CPUE (Fig. 4B) and smoothed FO (Fig. 4C) respec- tively. Highest In CPUE of pink and chum salmon was near the center of the distance fished offshore (Fig. 4B). The transformed CPUE of sockeye salmon, the least abundant species nearshore (Fig. 4B), was greatest 37-74 km offshore, indicating they may have been abundant beyond 74 km. The In CPUE of coho salmon suggests it was the least abundant species beyond 56 km (Fig. 4B). Table 2 Morisita's Index of Aggregation (J 8 ) and the asso- ciated F-value for seine catches of juvenile pink, chum, sockeye, and coho salmon taken in indi- vidual habitats (inside waters, outer coast inlets, outside waters I and all these habitats pooled in Southeast Alaska in August 1983, July and Au- gust 1984. Dashes indicate no fish captured. Salmon species Habitat h F Pink' Inside waters 20.0 695.7* Outer coast inlet 10.7 153.0* Outside waters 3.6 54.9* All habitats pooled 18.5 474.5* Chum 2 Inside waters 13.6 66.6' Outer coast inlet 9.0 18.7* Outside waters 5.4 15.3* All habitats pooled 12.7 47.6* Sockeye 3 Inside waters Outer coast inlets Coho-' 11.8 22.7* Outside waters 15.1 23.2 All habitats pooled 9.5 24.2 Inside waters 4.4 25.1 Outer coast inlets 2.9 3.1 Outside waters 7.8 19.5 All habitats pooled 6.2 24.3 F-value is significant for P < 0.001. 7 Oncorhynchus gorbuscha. 2 0. keta 3 O. nerka. 4 O. kisutch. Table 3 Spearman's rank cc rrelati on coefficient (p) test of pair rankings of juvenile salmon species catches taken during 252 separate sets in Southeast Alaska and British Columbia Comparison of Correlation between species of species pair rankings salmon (pi Pink'/Chum 2 +0.75* Pink/Sockeye' +0.68* Pink/Coho 4 +0.14 Chum/Sockeye +0.55* Chum/Coho +0.13 Sockeye/Coho +0.11 Significant association at P < 0.05 . with rejection criteria adjusted for mu tiple comparisons. 1 Oncorhynchus gorbusc ha. - keta. 7 0. nerka. ' O kisutch 84 Fishery Bulletin 92(1). 1994 40 30 Outside waters Outer coast inlets Inside waters Pink Coho Chum Sockeye Salmon species Figure 3 Catch per unit of effort (CPUE) and frequency of occurrence of juvenile salmonids (pink, Oncorhynchus gorbuscha; chum, O. keta; sockeye, O. nerka; coho, O. kisutch; in outside waters (77 sets), outer coast inlets (45 sets), and inside waters ( 109 sets) in South- east Alaska in 1983 and 1984 combined. Pink and chum salmon FO was lowest nearshore, then increased and stabilized mid-distance offshore, around 37 km (Fig. 4C). Pink salmon were caught in all sets beyond 37 km and had the highest FO of all species; sockeye salmon FO remained constant 2-74 km offshore. Coho salmon FO was the highest nearshore (2 km) of all species, then the FO stabi- lized at 37 km and beyond (Fig. 4C). By sampling period Abundance of juvenile salmon in Southeast Alaska increased from July (CPUE=11) to August (CPUE=58) 1984 for all species. Summed over all habitats, pink, chum, sockeye, and coho salmon had higher FO's and abundance in August than in July. In outside waters, CPUE of each spe- cies increased two to seven times from July to Au- gust 1984, with juvenile pink salmon showing the largest increase (Fig. 5). In inside wa- ters, CPUE of pink and chum salmon increased 10 and 5 times respectively from July to August, whereas CPUE's of sockeye and coho salmon remained constant (Fig. 5). For all four species, FO increased in outside waters but de- creased in inside waters from July to August 1984 (Fig. 5). The low number of sets (four) made in outer coast inlets of Southeast Alaska in August 1984 precluded seasonal comparisons of CPUE or FO for this habitat. Size Juvenile salmon were larger in outside waters than in inside waters. Thirteen matched pairs of size samples could be compared under the hypothesis that size did not vary between outside and inside waters; the fish were larger in the outside water in all comparisons (Table 4, x 2 =133.66, df=26, P<0.005) and the null hypothesis was rejected. Juvenile salmon in outside waters were larger farther seaward. Of the eight possible matched pairs of samples compared under the hypothesis that size was not different between outside waters >37 km offshore and <37 km offshore, the juvenile salmon were larger >37 km seaward in all compari- sons (Table 4, x 2 = 67.44, df=16, P<0.005). Juvenile salmon in northern waters were larger than those in southern waters. The fish were larger in the northward locations than southward locations in 18 of 23 possible paired size comparisons (Table 4, X 2 =214.76, df=46, P<0.005). Juvenile salmon were larger in August than in July. Of the matched size samples compared under the hypothesis that size was not different between August and July of 1984, fish in August were larger than in July in 10 of 12 comparisons (Table 4, X 2 =145.36, df=24, P<0.005). The sizes between the different species of juvenile Pacific salmon differed significantly (P<0.05) (Table 5). Coho salmon juveniles were significantly larger than other species in each sampling period; mean length of coho salmon was always at least 40% greater than in other species, whereas pink, chum, and sockeye salmon were within 9% of each other. Juvenile sockeye salmon were significantly larger Jaenicke and Celewycz: Marine distribution and size of juvenile Pacific salmon than pink salmon in each sampling period and were significantly larger than chum salmon in 1984. In both July and August 1984, pink and chum salmon did not differ in size, and in August 1983 chum and sockeye salmon did not differ in size. Discussion Fish distribution Each species of juvenile salmon was highly aggregated rather than dis- persed randomly. In contrast to our results, Hartt and Dell (1986) seldom observed zero catches and therefore concluded that juvenile salmon in the ocean were evenly dispersed. Several differences between our study and theirs may explain the differing conclu- sions. Seines used by Hartt and Dell were longer than ours and were held open for 30 minutes instead of 3-5 minutes. Our catches may be more of a point estimate or instantaneous pic- ture of fish abundance, whereas their seines were more likely to intercept at least part of a juvenile salmon school. More importantly, Hartt and Dell did not separate juvenile salmon by species when considering their distribution. Species associations Juvenile pink, chum (4) (3) (5) (4) (2) L I 1 1 1 , I i 65 74 and sockeye salmon were generally closely associ- ated with each other in their distribu- tion. The distribution of these species, however, differed from the distribution of coho salmon, a result consistent with the conclu- sions of Hartt and Dell (1986) and Waddell et al. (1989). In the inside waters and outer coast inlets, we found that pink, chum, and sockeye salmon had a lower FO than coho salmon, indicating that those species were more highly aggregated and sparsely distributed than coho salmon. Paszkowski and Olla ( 1985) found that behavior patterns of juvenile coho salmon promoted dispersion, not aggregation. The utilization of similar areas in this study by juvenile pink, chum, and sockeye salmon correlates with the high degree of diet overlap observed between these species; in contrast, juvenile coho salmon showed 28 37 46 Distance offshore (km) Figure 4 Abundance of juvenile salmonids (pink, Oncorhynchus gorbuscha; chum, O. keta; sockeye, O. nerka; coho, O. kisutch ) by distance off- shore in the outside waters of Southeast Alaska in August 1984 (36 sets). Abundance is shown in terms of (A) catch per unit of effort (CPUE), (B) the smoothed natural logarithm of CPUE. and (C) the smoothed frequency of occurrence of the catches; number of sets is in parentheses. All distances are rounded to the nearest kilometer. Actual distance between intervals (except the first) is 9.3 km. little diet overlap with the other species. 1 Healey ( 1991) reported that juvenile pink, chum, and sockeye salmon in British Columbia were also aggregated. Migration The migration of juvenile salmon off Southeast Alaska (Hartt and Dell, 1986) consists of two components: 1) fish migrating north from the Pacific Northwest and British Columbia, and 2) fish from Southeast Alaska migrating from inside to outside waters. J. H. Landingham, Auke Bay Laboratory. 11305 Glacier High- way, Juneau, AK 99801-8626, pens, commun. Jan. 1992. 86 Fishery Bulletin 92(1), 1994 1 Outside I waters X ?■ Outer coast inlets Inside waters July 1984 August 1984 UJ Z> 0. o o C ID i_ i- 3 O O O >- o c ID 3 O" 0> 100 Pink Chum Sockeye Coho Pink Chum Sockeye Coho Salmon species Figure 5 Catch per unit of effort (CPUE) and frequency of occurrence of juvenile salmonids (pink, Oncorhynchus gorbuscha; chum, O. keta; sockeye, O. nerka; coho, O. kisutch) in the outside waters (69 sets), outer coast inlets (18 sets), and inside waters (55 sets) in South- east Alaska in July and August 1984. Note change in scale of CPUE from July to August 1984. Juvenile salmon migrations along the Pacific coast in 1984 did not peak off Southeast Alaska until, at earliest, August. In July, CPUE's were much higher in the outside waters of British Columbia than in Southeast Alaska. By August, CPUE of juvenile salmon in outside waters of Southeast Alaska had increased fivefold, and FO had increased for each species. Hartt and Dell (1986) observed that juve- nile salmon abundance peaked in August in outside waters of Southeast Alaska. In Southeast Alaska, juvenile sockeye salmon probably begin their ocean migration to the Gulf of Alaska before juvenile pink and chum salmon, based on two observations from our study. First, the sock- eye salmon did not occur in protected waters along the outer coast of Southeast Alaska like the other species: no sockeye salmon were captured in an outer coast inlet. Second, sockeye salmon was the only species with a higher CPUE in outside waters than in inside waters. This higher abundance outside, coupled with low abundance in inside waters in July and August, is consistent with the conclusion that sockeye salmon commence their ocean migration before pink or chum salmon (Straty, 1981; Healey, 1982). The migration of pink salmon from the inside waters of Southeast Alaska lasts until at least September. Martin (1966) concluded that late July and early August were the peak periods of juvenile pink salmon migration from the inside waters. However, our data show that pink salmon abundance in inside waters increased from July to August and that pink salmon were more abundant in inside waters than outside waters in August, thus indicat- ing that migration out of the inside waters was not complete in August. The seasonal migration of juvenile chum salmon out of Southeast Alaska could not be determined from the abun- dance data of this study. The migration of juvenile pink, chum, and sockeye salmon out of the inside waters in Sep- tember and later has not been studied. The offshore migration of coho salmon in Southeast Alaska is more complex. CPUE and FO of coho salmon in inside waters remained relatively constant for July and August. Coho salmon was the only species with both a higher CPUE and FO in inside wa- ters than in outside waters in August. These data suggest extensive residency in inside waters for a substantial portion of coho salmon juveniles in Southeast Alaska. Other researchers have found that some juvenile coho salmon remain in the east- ern Pacific Ocean inside waters until late fall (Healey, 1984; Hartt and Dell, 1986; Orsi et al., 1987). Winter residency of juvenile coho in inside waters of Southeast Alaska is apparently rare. 2 Hartt and Dell ( 1986) and Pearcy and Fisher ( 1990) also found coho salmon offshore as early as May or June; Hartt and Dell ( 1986) noted that juvenile coho salmon migrated seaward earlier than the other salmon species, presumably because of their larger 2 J. A. Orsi, Auke Bay Laboratory, L1305 Glacier Highway, Ju- neau, AK 99801-8628. pers. commun. Jan. 1992. Jaenicke and Celewycz: Marine distribution and size of juvenile Pacific salmon 87 Table 4 Fork length (FL) of juvenile salmonids sampled by period, habitat, north (N) or south (S) region, and dis- tances offshore in outside waters of Southeast Alaska in 1983 and 1984 and outside waters of British Colum- bia (B.C.) in 1984. Values are mean ± standard error, with number of samples in parenthesis. In brackets under the values are the specific paired size comparisons used in the null hypothesis testing of sizes by: northern vs. southern waters (Al, A2, ..., A23); outside vs. inside waters (Bl, B2, ..., B13); August vs. July 1984 (CI, C2, ..., C12); and outside waters >37 km offshore vs. outside waters <37 km offshore (Dl, D2, ..., D8). Dashes indicate no fish caught. Period Habitat (region) FL of salmon (mm) Pink' Chum 2 Sockeye- 3 Coho" Aug 83 Inside (N) 169 ± 0.8 (890) [All 180 ± 1.8 (199) [A2] 163 ± 2.7 (74) 233 ± 1.8 (136) [A3] Inside (S) 121 ± 1.9 (10) [Al, Bl] 139 ± 4.6 (18) [A2, B2] — 227 ± 11.9 (5) [A3, B3] Outer coast inlet (N) — 166 ± 4.9 (4) [A4] — 221 ± 6.3 (11) [A5] Outer coast inlet (S) 124 ± 0.5 (404) 133 ± 1.5 (76) [A4] — 217 ± 7.9 (11) [A5] Outside (S) 153 ± 3.6 (19) [Bl] 141 ± 13.5 (2) [B2] 152 ± 2.6 (9) 234 ± 3.6 (25) [B3] July 84 Inside (N) 121 ± 1.7 (94) [A6, B4, CI] 112 ± 5.2 (19) [B5, C2] 136 ± 5.9 (20) [B6, C3] 193 ± 2.0 (206) [A7, B7, C4] Inside (S) 132 ± 1.2 (3) [A6, B8] 135 ± (1) — 202 ± 7.8 (3) [A7, B9] Outer coast inlet (N) 105 ± 10.9 (4) 139 + (1) — 177 ± 3.6 (27) Outside (N) 135 ± 0.8 (207) [A8, A9, B4, C5] 133 ± 2.3 (38) [A10, All, B5, C6] 151 ± 2.1 (111) [A12, A13, B6, C7] 220 ± 4.5 (26) [A14, A15, B7, C8] Outside (S) 134 ± 4.6 (10) [A8, A16, B8, C9] 161 ± 18.5 (2) [A10, A17, C10] 157 ± 2.6 (19) [A12, A18, Cll] 224 ± 7.5 (8) [A14, A19, B9, C12] Outside (B.C.) 128 ± 1.0 (126) [A9, A16] 132 ± 1.5 (46) [All, A17] 128 ± 0.9 (197) [A13, A18] 129 ± 10.3 (7) [A15, A19] Aug 84 Inside (N) 143 ± 1.0 (358) [B10, CI] 125 ± 1.2 (118) [Bll, C2] 157 ± 2.1 (18) [B12, C3] 234 ± 1.9 (168) [B13, C4] Outer coast inlet (S) — 132 ± 6.1 (12) — 246 ± 12.2 (3) Outside (N) 144 ± 0.6 (730) [A20, B10, C5] 160 ± 2.0 (93) [A21, Bll, C6] 159 ± 1.5 (75) [A22, B12, C7] 267 ± 5.6 (33) [A23, B13, C8] <37 km 143 ± 0.8 (457) [Dl] 157 ± 2.2 (73) [D2] 156 ± 1.7 (52) [D3] 266 ± 6.5 (28) [D4] >37 km 146 ± 1.0 (273) [Dl] 169 ± 4.1 (20) [D2] 165 ± 2.8 (23) [D3] 274 ± 2.3 (5) [D4] Outside (S) 139 ± 1.0 (373) [A20, C9] 144 ± 2.1 (66) [A21, C10] 149 + 0.9 (141) [A22, Cll] 265 ± 3.3 (37) [A23, C12] <37 km 135 + 1.2 (243) [D5] 144 ± 2.7 (38) [D6] 148 ± 1.0 (103) [D7] 263 ± 3.3 (35) [D8] >37 km 144 ± 1.4 (130) [D5] 145 ± 3.5 (28) [D6] 152 ± 1.5 (38) [D7] 291 ± 15.0 (2) [D8] ; Oncorhynchus gorbuscha. 2 0. keta. 3 O. nerka, 4 O. kisulch. 88 Fishery Bulletin 92(1). 1994 size. An early component of coho salmon juveniles could have moved offshore in June, prior to our sam- pling effort. More extensive sampling from late spring through fall is required to define the timing of migrations of coho salmon in the waters of South- east Alaska. The sizes of juvenile salmon we captured support the findings of Hartt and Dell (1986) that fish in more northern locations have been at sea longer than those in southern locations. Hartt and Dell (1986) observed a general increase in mean length of juvenile salmon from south to north in the out- side waters from Washington to Southeast Alaska. In the coastal waters off Oregon and Washington, larger, presumably older, juvenile coho salmon were found farther north (Pearcy and Fisher, 1988). As- suming they were similar in size on entering the sea, the smaller fish in the southerly locations are recent arrivals from nearby production areas, whereas the larger fish in the northerly locations have been at sea longer and probably migrated from more south- erly production areas (Hartt and Dell, 1986). Our studies also reveal juvenile salmon in Southeast Alaska were larger in the outside waters than in- side waters and farther offshore in the outside wa- ters than closer to shore. The progression of juve- nile salmon migrations over a season may be size- dependent (Healey, 1982, 1984), and certain phases of migration may depend on fish reaching a thresh- old size. According to Hartt and Dell ( 1986), the off- shore migration into the Gulf of Alaska of juvenile Table 5 Comparison of mean fork lengths (FL) of juvenile salmonids caught in the marine waters (all habitats pooled) of Southeast Alaska and north- ern British Columbia in 1983-84. Sample size = n; standard deviation of the size in mm = s. The hypothesis was that there were no size dif- ferences between species during the same period. The rejection crite- ria were adjusted for multiple comparisons so that experimental error did not exceed a = 0.05. Species having the same letter in a column were not significantly different by size. August 1983 July 1984 Salmon species mean FL Immi n mean FL (mm) n Pink' 1,323 Chum 2 299 Sockeye'* 83 Coho'' 188 155 c 165 6 162'' 232° 29 31 23 22 444 108 347 277 130' 129 r 138'' 193 1 1 17 20 30 1,461 289 234 241 Oncorhynchus gorbuscha. b O. keta. c O. nerka. d O. kisutch. pink, chum, and sockeye salmon does not begin until September or October when fish are 180-230 mm or greater in mean FL. However, our findings show that these species are found offshore earlier (in August) and at a much smaller size (145-170 mm mean FL). Width of migration band Juvenile Pacific salmon typically migrate in nearshore waters during their first few months at sea (Straty, 1981); however, the width of this migra- tion band varies regionally (Straty and Jaenicke, 1984; Hartt and Dell, 1986). Juvenile salmon con- centrated within 37 km of shore along the broad continental shelf (<183 m deep) off Oregon and Washington (Miller et al., 1983; Pearcy and Fisher, 1990). Hartt and Dell (1986) concluded that the band of juvenile salmon was within 37 km of shore off Southeast Alaska where the continental shelf is narrow, but that the band widened in the northern Gulf of Alaska where the shelf is wider. Our results indicate that the coastal band of mi- grating juveniles can be much wider than 37 km and that the offshore migration beyond 37 km may be- gin as early as August. Catches of juvenile salmon 74 km offshore — the maximum distance we fished offshore — and the catch distributions indicate that some juvenile salmon (pink, chum, and sockeye) may have been abundant even farther seaward. Two- thirds of the juvenile salmon captured in outside wa- ters in August 1984 were beyond the continental shelf. The width of the migration band is probably in- fluenced by the Alaska Coastal Current — a dominant feature in the circulation of Gulf of Alaska coastal waters. This freshwater-driven current be- gins along the British Colum- bia coast and flows north then west within 20 km of shore into the Bering Sea (Royer, 1984). The strength of this current is affected by local precipitation, wind, air temperature, and other meteorological condi- tions. Millions of juvenile salmon migrate through the cur- rent every year en route to more oceanic waters. Cooney (1984) theorized that the cur- rent represents a critical early- feeding habitat in the summer and early fall. In modeling the early-ocean limitations of Pa- cific salmon production, Wal- August 1984 mean FL (mmi 142'' 141 r 153'' 253° 18 22 12 20 Jaenicke and Celewycz: Marine distribution and size of juvenile Pacific salmon 89 ters et al. (1978) noted that production predictions were critically sensitive to the width of the coastal band within which salmon migrate during their first summer at sea. We recommend additional sampling be conducted from June through September to bet- ter document 1) the width of the coastal band of ju- venile salmon migrations through the summer and 2) the timing of offshore migrations beyond 37 km from the outer coast. Acknowledgments We thank the biologists and technicians who helped in the field and laboratory. We also thank the crew on the NOAA RV John N. Cobb and FV Bering Sea for their cooperation during seining operations. The FV Bering Sea cruise was part of a cooperative coastwide survey from California to Southeast Alaska with W Pearcy, Oregon State University. We especially acknowledge the review of the manuscript by A. Wertheimer. Literature cited Bax, N. J. 1983. Early marine mortality of marked juvenile chum salmon (Oncorhynchus keta) released into Hood Canal, Puget Sound, Washington, in 1980. Can. J. Fish. Aquat. Sci. 40:426-435. Browning, R. J. 1980. Fisheries of the North Pacific: history, spe- cies, gear and processes. Alaska Northwest Publ., Anchorage, 423 p. Chambers, J. M., W. S. Cleveland, B. Kleiner, and P. A. Tukey. 1983. Graphical methods for data analysis. Duxbury Press, Boston, 395 p. Cooney, R. T. 1984. Some thoughts on the Alaska Coastal Cur- rent as a feeding habitat for juvenile salmon. //; W. G. Pearcy (ed.), The influence of ocean condi- tions in the production of salmonids in the North Pacific, p. 256-268. Oregon State Univ. Sea Grant College Program Rep. ORESU-W-83-001. Daniel, W. W. 1978. Applied nonparametric statistics. Houghton Mifflin, Boston, 503 p. Furnell, D. J., and J. R. Brett. 1986. Model of monthly marine growth and natu- ral mortality for Babine Lake sockeye salmon (Oncorhynchus nerka). Can. J. Fish. Aquat. Sci. 43:999-1004. Godfrey, H., K. A. Henry, and S. Machidori. 1975. Distribution and abundance of coho salmon in offshore waters of the North Pacific Ocean. Int. North Pac. Fish. Comm. Bull. 31, 80 p. Hartt, A. C. 1975. Problems in sampling Pacific salmon at sea. In Symposium on evaluation of methods of estimating the abundance and biological attributes of salmon on the high seas, p. 165-231. Int. North Pac. Fish. Comm. Bull. 32. Hartt, A. C, and M. B. Dell. 1986. Early oceanic migrations and growth of ju- venile Pacific salmon and steelhead trout. Int. North Pac. Fish. Comm. Bull. 46:1-105. Healey, M. C. 1982. The distribution and residency of juvenile Pa- cific salmon in the Strait of Georgia, British Colum- bia, in relation to foraging success. In B. R. Melteff and R. A. Neve (eds.), Proceedings of the North Pa- cific aquaculture symposium, p. 61-69. Alaska Sea Grant Rep. 82-2. Univ. Alaska, Fairbanks. 1984. The ecology of juvenile salmon in Georgia Strait, British Columbia. In W. J. McNeil and D. C. Himsworth (eds.), Salmonid ecosystems of the North Pacific, p. 203-229. Oregon State Univ. Press, Corvallis. 1991. Diets and feeding rates of juvenile pink, chum, and sockeye salmon in Hecate Strait, Brit- ish Columbia. Trans. Am. Fish. Soc. 120:303-318. Manzer, J. I. 1964. Preliminary observations on the vertical dis- tribution of Pacific salmon (Genus Oncorhynchus) in the Gulf of Alaska. J. Fish. Res. Board Can. 21:891-903. Martin, J. W. 1966. Early sea life of pink salmon. In W. L. Sheridan (ed.). Proceedings of the 1966 Northeast Pacific pink salmon workshop, p. 111-125. Alaska Dep. Fish Game, Info. Leafl. 87, Juneau. Miller, D. R., J. G. Williams, and C. W. Sims. 1983. Distribution, abundance and growth of juve- nile salmonids off the coast of Oregon and Wash- ington, summer 1980. Fish. Res. 2:1-17. Morisita, M. 1959. Measuring the dispersion of individuals and analysis of the distributional patterns. Mem. Fac. Sci. Kyushu Univ. Ser. E. (Biol) 2:215-235. Orsi, J. A., A. G. Celewycz, D. G. Mortensen, and K. A. Herndon. 1987. Sampling juvenile chinook salmon (Oncorhynchus tshawytscha) and coho salmon (O. kisutch) by small trolling gear in the northern and central regions of southeastern Alaska, 1985. U.S. Dep. Commer., NOAA Tech. Memo. NMFS F/NWC-115, 47 p. Parker, R. R. 1968. Marine mortality schedules of pink salmon of the Bella Coola River, central British Columbia. J. Fish. Res. Board Can. 25:757-794. Paszkowski, C.A., and B. L. Olla. 1985. Social interactions of coho salmon (Oncorhynchus kisutch ) smolts in seawater. Can. J. Zool. 63:2401-2407. Pearcy, W. G., and J. P. Fisher. 1988. Migrations of coho salmon, Oncorhynchus kisutch, during their first summer in the ocean. Fish. Bull. 86:173-195. 90 Fishery Bulletin 92(1), 1994 1990. Distribution and abundance of juvenile salmonids off Oregon and Washington, 1981- 1985. U.S. Dep. Commer., NOAA Tech. Rep. NMFS 93, 83 p. Poole, R. W. 1974. An introduction to quantitative ecology. McGraw-Hill, NY, 532 p. Royer, T. C. 1984. Annual and interannual variability of tem- perature and salinity in the Gulf of Alaska with emphasis on the coastal waters. In W. G. Pearcy (ed.), The influence of ocean conditions in the pro- duction of salmonids in the North Pacific, p. 244- 255. Oregon State Univ. Sea Grant College Pro- gram Rep. ORESU-W-83-001. Sokal, R. R., and F. J. Rohlf. 1981. Biometry, the principles and practices of sta- tistics in biological research. W. H. Freeman & Co., NY, 859 p. Straty, R. R. 1981. Trans-shelf movements of Pacific salmon. In D. W. Hood and J. A. Calder (eds.), The eastern Bering Sea shelf: oceanography and resources 1:575-595. U.S. Dep. Commer., NOAA, Off. Mar. Pollut. Assess., Juneau, AK. Straty, R. R., and H. W. Jaenicke. 1984. Estuarine influence of salinity, temperature, and food on the behavior, growth, and dynamics of Bristol Bay sockeye salmon. In W. J. McNeil and D. C. Himsworth (eds.), Salmonid ecosystems of the North Pacific, p. 247-265. Oregon State Univ. Press, Corvallis. Tukey, J. W. 1977. Exploratory data analysis. Addison-Wesley Publishing, Reading, MA, 506 p. Waddell, B. J., M. C. Healey, and J. F. T. Morris. 1989. Data analysis of 1986 and 1987 Hecate Strait juvenile salmon surveys. Can. Tech. Rep. Fish. Aquat. Sci. 1719, 76 p. Walters, C. J., R. Hilborn, R. M. Peterman, and M. J. Staley. 1978. Model for examining early ocean limitation of Pacific salmon production. J. Fish. Res. Board Can. 35:1303-1315. Winer, B. J. 1971. Statistical principles in experimental design. McGraw-Hill, NY, 907 p. Abstract. Evidence support- ing a two stock hypothesis for king mackerel, Scomberomorus cavalla, in the Gulf of Mexico was devel- oped principally from the results of electrophoretic patterns of one polymorphic dipeptidase locus and supporting evidence from mark- recapture, charterboat catch, and spawning studies. There are two identifiable stocks of king mackerel in the Gulf of Mexico: a western stock and an eastern stock. The western stock migrates northward along the Mexico-Texas coast during the spring and early summer from its winter grounds in Mexico (Yucatan Peninsula). This stock has a high frequency of the dipeptidase PEPA-2*a allele. The eastern stock migrates at the same time north- ward along the eastern coast of the Gulf of Mexico from its winter grounds in south Florida (Gulf of Mexico and Atlantic coast). This stock has a high frequency of the dipeptidase PEPA-2*b allele. Both stocks migrate simultaneously into the northern Gulf of Mexico and mix at varying degrees in the northern summering grounds (Texas to northwest Florida). Evidence for distinct stocks of king mackerel, Scomberomorus cavalla, in the Gulf of Mexico Allyn G. Johnson William A. Fable Jr. Churchill B. Grimes Lee Trent Southeast Fisheries Science Center National Marine Fisheries Service. NOAA 3500 Delwood Beach Road Panama City. Florida 32408 Javier Vasconcelos Perez Instituto Nacional de la Pesca Mexico City. Mexico Manuscript accepted 17 August 1993 Fishery Bulletin 92:91-101 (1994) The king mackerel, Scomber- omorus cavalla, is a widely distrib- uted, coastal pelagic species in the western Atlantic Ocean. This scombrid is found from the Gulf of Maine to Rio de Janiero, Brazil, in- cluding the Gulf of Mexico and Caribbean Sea (Rivas, 1951; Collette and Nauen, 1983). It is a valuable resource that supports fisheries throughout most of its range (Manooch et al., 1978). The U.S. and Mexico have been major exploiters of king mackerel resources. U.S. commercial land- ings have been reported since 1888. Landings have ranged from 2,213 metric tons (t) (1972) to 4,746 t (1974). U.S. recreational catches are estimated to be two to ten times larger than the commercial catches (Deuel and Clark, 1968; Deuel, 1973; Manooch, 1979; U.S. Dep. Commer., 1984, 1986, 1987). In Mexican waters, commercial land- ings for king mackerel from 1968 to 1988 have ranged from 784 t ( 1968) to 6,133 t (Collins and Trent, 1982 1 ). Because king mackerel are pres- ently managed in the southeastern U.S. (represented by more than eight states and two regional fish- ery management council jurisdic- tions) and support both recre- ational and mixed gear commercial fisheries, the identities of compo- nent stocks are important. Current management of king mackerel fish- eries assumes two migratory stocks with overlapping ranges, one in the U.S. Atlantic Ocean and one in the Gulf of Mexico (Gulf of Mexico and South Atlantic Fishery Manage- ment Councils, 1985). This separa- tion is based on mark-recapture results (Sutherland and Fable, 1980; Williams and Godcharles, 1984 2 ; Sutter et al., 1991). The concept of a stock is one of the most fundamental to fishery management. A stock is variously defined, ranging from the strict definition of a single interbreeding population to a unit capable of in- 1 L. A. Collins and L. Trent, Natl. Mar. Fish. Serv., Panama City, FL, pers. commun. 1992. 2 Williams. R. O., and M. F. Godcharles. 1984. Completion report, king mackerel tagging and stock assessment. Project 2- 341-R. Fla. Dep. Natl. Resour. Unpubl. Rep., 45 p. 9 1 92 Fishery Bulletin 92(1). 1994 dependent exploitation or management and contain- ing as much of an interbreeding unit or as few re- productively isolated units as possible (Royce, 1972). An additional term that has been used to define the stock concept used in fishery management is "unit stock" which was referred to by Kutkuhn (1981) as "one consisting of randomly interbreeding members whose genetic integrity persists whether they re- main spatially and temporally isolated as a group, or whether they alternately segregate for breeding and otherwise mix freely with members of other unit stocks of the same species." This term is more func- tional for application to many marine resources which have identifiable components but for which reproductive isolation has not been demonstrated. We consider stock and unit stock to be identical with regard to king mackerel resources at the present time. Using Kutkuhn's (1981) definition, this report presents evidence of two stocks of king mackerel existing in the Gulf of Mexico (the Gulf), an east- ern and a western stock which winter off south Florida and off the Yucatan peninsula (Mexico), re- spectively. In the spring these fish migrate along their respective coasts to summer areas in the northern Gulf. The concept of two Gulf of Mexico stocks was first presented by Baughman ( 1941). He based his hypothesis on observations by fishermen of simultaneous migrations along the eastern and western sides of the Gulf. More recently, May (1983) 3 reported electrophoretic differences in king mackerel between the eastern and western Gulf. Using more recent tagging data and electrophoretic information, Grimes et al. (1987) reintroduced the hypothesis. Additional evidence for a two-stock hypothesis is the following: 1 Fish movements along the coast, as indicated by mark-recapture studies (Fable et al., 1990 4 ). 2 The simultaneous migration along the eastern and western coasts of the Gulf in spring and early summer as detected by analysis of charterboat CPU data (Trent et al., 1987b). 3 The difference in spawning times of king mack- erel in the northern and southern areas of the Gulf (Grimes et al., 1990). 3 May, B. 1983. Genetic variation in king mackerel (Scomberomorus cavalla). Final Rep. Fla. Dep. Natl. Resour. Contract C-14-34, 20 p. 4 Fable, Jr., W.A., J. Vasconcelos P., K. M. Burns, H. R. Osburn, L. Schultz R., and S. Sanchez G. (1990). King mackerel, Scomberomorus cavalla, movements and migrations in the Gulf of Mexico. Natl. Mar. Fish. Serv., Panama City Lab., Panama City, FL (unpubl. ms.l. We report the results from electrophoretic inves- tigations and summarize current information from tagging, migration, and spawning time studies. We also propose a possible mechanism to explain the observed results with regard to the water circula- tion of the area. Methods and materials Samples of muscle tissue, along with fork length (mm) and sex, were collected during 1985 through 1990 from fish obtained in recreational and commer- cial fisheries from North Carolina to Yucatan (Table 1). The samples were frozen as soon as pos- sible in the field and then shipped frozen to the Na- tional Marine Fisheries Service's Panama City Labo- ratory. Muscle tissue (about 10 grams) was excised from each sample and stored in a freezer (in 1985 at -5° to -10°C and from 1986 through 1990 at -100°C). Tissue extracts were prepared by mixing equal volumes of muscle tissue and distilled water and grinding with glass rods to uniform pastes. Extracts were centrifuged at 3,400 rpm (1,000 x G) for five minutes, then supernatants were drawn onto 4 mm x 8 mm filter paper inserts (Whatman 1). Starch gel electrophoretic separation of the ex- tracts was performed following the methods of Kristjansson (1963). Electrophoretic buffers were those of A) Markert and Faulhaber (1965), and B) N-(3-aminopropyl)-morpholine-citrate (pH 6.1) buffer of Clayton and Tretiak (1972). The gel con- sisted of 35 g of starch (Sigma Chemical Co. lots 123F-0591, 35K-0383, and 94F-0536) plus 250 mL of buffer. Amperage during electrophoresis was kept below 50 MA, and voltage varied between 100 and 400 V, depending on the buffer. Temperature was maintained at 2°C by using a refrigerated cooling system (see Aebersold et al., 1987, for description). After electrophoresis, the gels were sliced into four horizontal sections and stained for dipeptidase (EN 3.4.-.-). In 1985 (1,223 fish) and 1988 (879 fish), 27 additional enzymes were examined. Methods fol- lowed May (1983) 3 and Aebersold et al. (1987). We conducted statistical analyses using Biosys-1 (Swofford and Selander, 1981) to test for conform- ance to Hardy-Weinberg expectations and spatially related differences in allele frequencies compared to distance and physical feature subdivisions. The Kolmogorov-Smirnov goodness-of-fit test was used for comparing allele distributions by size of fish (100-mm-FL intervals), while the chi-square contin- gency test was used for comparing allele distribu- tions by sex (see Sokol and Rohlf (1981) for proce- dures i. Johnson et al.: Evidence for distinct stocks of Scomberomorus cavalla 93 -o c C m .Q n -r in CO in tr- co as oo CO CO CO oo co c- m o o o co o o co o o o o o o o o o O O O O CO CM t"- in .—• in cm .— i oo CO CO CO CO CO CO CD CTi ^ — in ^ CO CO CM CO r— C i oo ^- -r II en en OO CO CO on CO m oo on c C7S _■ CO — i O CO — O o CO o •3' CM CM © CO „ f h n °£ -rf CO CO ^ CO O CO L r in cm co co co" co co" CM cc -r ■■T. , — i in r- on CO CM CO CO CO CO r— GO -1 £- t- CO r i CO oo O CO O Tf o en t- o o CT) O O co t- co en co oo oo ao oo en en en en en en — co r- oo en co fl oo oo oo oo en .zi en en en en en co t- oo en ao oo co oo en en en en cu co t— co en co m m ^ o — oo oo oo co en — ao co oo cncncncncncn & en en en 3 ,2 94 Fishery Bulletin 92(1). 1994 ■D dj 3 C C o u .15 c o § HO CM C— ■** CO •— < — " t— .— CM ao -CO 00 t-QN CO CM •— < t— ♦-« i— < ■^ to co m O) t- r-( rH t-H Tf — CO — — r- co cm o co co cm •— 1 CO i— < --H CO — — CM CO C- C- -*!• CO Oi M 1 rH rH O) (N t^ Oi - o n ^ o m .—I ,-1 .—I .— I lO W O O O O O rH o o o o o o i— I CO CM CO CM O O CM OHHH dodo CO OlOl t CD CM — I CO CO CO © o i-j o o cd cd © © © © ^ •— • uO CO ■^ CO CO o «-< O O O rH O ©' © cd o © r- <— < CO CTJ .— CO © ■— i — , ,-. 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J* > 'St Johnson et al.: Evidence for distinct stocks of Scomberomorus cavalla 95 Results Of the 50 loci surveyed in 1985, 30% were variable. In 1988, the 50 loci were again surveyed (879 fish from 10 locations) and 24% of the loci were found to have variants. Variations other than dipeptidase (EN 3.4.-.-) PEPA-2 were found in low frequency (uncommon allele 0.000 to 0.063) in 18 polymorphic systems. Occurrence of these variants differed be- tween locations and years. Electrophoretic variants were found for loci including aspartate aminotrans- ferase (EN 2.6.1.1) sAAT ", acid phosphatase (EN 3.1.3.2) ACP-2*, adenosine deaminase (EN 3.5.4.4) ADA , adenylate kinase (EN 2.7.4.3) AK-1* and AK- 2' , alanine aminotransferase (EN 2.6.1.2) ALAT-1 and ALAT-2', esterase-D (EN 3.1.-.-) ESTD-2' and ESTD-3 , fructose-bis-phosphate aldolase (EN 4.1.2.13) FBALD-2 \ glucose-6-phosphate isomerase (EN 5.3.1.9) GPI-1* and GPI-2\ isocitrate dehydro- genase (NADP + ) (EN 1.1.1.42) sIDHP , malic en- zyme (NADP + ) (EN 1.1.1.38) ME-2' , mannose-6- phosphate isomerase (EN 5.3.1.8) MPf, dipeptidase (EN 3.4.-.-) PEPA-1 , phosphogluconate dehydroge- nase (EN 1.1.1.44) PGDH*, and phosphoglucomutase (EN5.4.2. 2) PGM-2*. Use of very low-frequency variations for stock identification of king mackerel was impractical, be- cause sufficient sample sizes (numbers of fish) for detection during short time periods (one month or less) were unavailable. Tagging studies (Fable et al., 1990 4 ) indicated that discrete geographic population units were not available during the time intervals required to obtain sufficient samples. Only dipepti- dase (glycyl-leucine substrate) 5 consistently varied between locations. In 1985 (1,223 fish), 1986 (1,537 fish), 1987 (2,120 fish), 1988 (1,631 fish), 1989(1,502 fish), and 1990 (963 fish), muscle tissues were ex- amined for the dipeptidase variation. This enzyme developed on electropherograms as two zones of activity, and showed the pattern of a two allele ("a and b) polymorphism in the most anodal zone {PEPA-2\ in most collections, as described by May [1983]). We refer to May's 1 and 2 alleles (electro- morphs) as a and *b, respectively (Fig. 1). A third allele (*c) which is anodal of the a allele was found in 1988 and 1989 collections from Veracruz, Mexico to Alabama. 6 Only one homozygote (*c*c) and 20 heterozygotes Cc'a) were found from 3,487 fish. 5 Enzyme is also active with valyl-leucine and leucyl-tyrosine as substrates. 6 The genetic nomenclature for this polymorphic system accord- ing to the recommendations of Shaklee, et al. (1990), is dipep- tidase 3A.-APEPA-2') with three variant alleles '//(). '105, and '100. These alleles are represented in this report as *c, *a, and 'b, respectively. PEPA-2 PEPA-1 ■Tit ••!••.— IV'Ii T (D 'a'a a'b Vb (2) I • ^(3) Figure I King mackerel iScomberomorus cavalla) dipeptidase (PEPA-P and PEPA-2*): (1) schematic of gel with 25 samples (PEPA-2* a is 0.700), (2) schematic of en- largement of section of PEPA-2 on gel showing three phenotypes ( a a, ab, b 6), and (3) photo- graph of actual gel section used for schematic (2). Because of the rareness of this allele Cc), it was combined with allele a for analysis. Allele frequencies and phenotypic distributions varied extensively within and between areas from 1985 to 1990 (Table 1). The majority of monthly collections conformed to the Hardy-Weinberg expec- tation; however, many of the yearly collections did not conform. In general, higher *a allele frequencies were found west of Florida than in Florida and along the Atlantic coast. The phenotypic distributions of the dipeptidase polymorphism were not significantly correlated with body length, with few exceptions. When the pheno- typic distribution was compared by 100-mm-FL size intervals for five geographic locations (Atlantic coast, Alabama-Mississippi, Louisiana, east Texas, and south Texas) by year, only seven of the 78 com- parisons were significantly different (Kolmogorov- Smirnov goodness-of-fit test, P<0.05). Four of these 96 Fishery Bulletin 92(1). 1994 deviant collections occurred in the northern Gulf (east Texas and Alabama-Mississippi). The other three (1988— *a*a phenotype on Atlantic coast; 1989- *b*b, and 1990-*a*a phenotypes in northwest Florida) are believed to have resulted from sampling inadequacies (in 1988, only 9 *a*a were collected on the Atlantic coast, and in 1989 northwest Florida had 136 of the 275 *b*b in the <600-mm-FL cell, which represented 167 of the 344 fish; and in 1990, northwest Florida had 12 *a*a of the 17 *a*a in the 900, 1,000, and >1,100 mm cells). When allele distributions were compared by sex at seven locations for each year in which sufficient data were available, eight of the 23 allele compari- sons deviated significantly (chi-square contingency test, P<0.05). Six deviant collections occurred in the northern Gulf (Texas-Mississippi 1985-1989) and were from collections that did not conform to Hardy- Weinberg expectations with regard to their pheno- typic distributions. Two others occurred in Veracruz, Mexico (1988 and 1990). The total allele-sex (1985- 90) comparisons for the seven locations did not de- viate significantly, except for Veracruz, Mexico. Veracruz collections were dominated by small fish (<600 mm FL) of which sex determination was dif- ficult, especially early in the year (Jan. -July) be- cause of undeveloped gonads. Sex could only be de- termined for 68% of the fish tested from this area. The geographic pattern of dipeptidase (PEPA-2*) (1985-90) indicated that western Gulf differed from eastern Gulf and Atlantic coast king mackerel. In all years except 1985, comparison of allele counts (Table 1) of the various geographic groupings of the Gulf varied significantly (P<0.05) both within the Gulf and between the Gulf and the Atlantic coast. On the Atlantic coast (north of Florida vs. Florida), the variation was found not significant (except in 1990). The trend in these comparisons was for ex- cess *a allele in the western Gulf and for excess *b allele in the eastern Gulf and the Atlantic coast. Discussion Comparisons of subdivisions (Table 2) show a con- sistently higher level of PEPA-2*a in western Gulf king mackerel and a deficit of this allele in king mackerel in the eastern Gulf and along the Atlan- tic coast. Electrophoretic data (ours and that of May ( 1983 ) 3 indicating high dipeptidase PEPA-2* a frequency in the western Gulf and low *a frequency in the east- ern Gulf and along the Atlantic coast supports a two stock hypothesis for king mackerel in the Gulf. Sup- porting information can be obtained from other in- vestigations: mark-recapture (Fable et al., 1990 4 ), charterboat catches (Trent et al., 1987b) and spawn- ing date analysis (Grimes et al., 1990). Fish move- ments indicated by mark-recapture are consistent with the two stock hypothesis. The charterboat in- formation provides evidence of simultaneous north- ward migration on both sides of the Gulf, while the spawning date information offers evidence for repro- ductive isolation. The king mackerel dipeptidase (PEPA-2") varia- tion found in 1985-90 was similar to the variation first reported by May (1983) 3 . His data showed higher dipeptidase *a allele frequencies for Louisiana (0.618) and Texas (0.736) than were found eastward. Temporal variations in the PEPA-2* allele frequen- cies are difficult to interpret without taking into consideration the migratory behavior. The variation was extreme at some locations, giving the impres- sion that the samples were collected from different or mixed schools from different origins. For example, in east Texas (Galveston-Freeport area) (1986), five discrete collections (5 July-28 August) of 27 to 56 fish each (204 total) were sampled. The PEPA-2* a frequencies were 0.933, 0.769, 0.202, 0.839, and 0.037 (in collection order). In other collection peri- ods, variations in frequencies indicated that we had sampled the same school of fish. For example, in Louisiana (1987) three collections 7 days apart (21 Aug.^1 Sept.) were obtained. Their PEPA-2* a fre- quencies were 0.590 (50 fish), 0.580 (50 fish), and 0.594 (48 fish). In view of the extreme variability of PEPA-2* frequencies, numerous deviations from Hardy- Weinberg expectations, and sampling difficul- ties (one or more schools per collection), proper spa- tial subdivision and grouping of collections for test- ing specific hypotheses is arduous. The expanse of the sampling area (Virginia to Yucatan) can be di- vided into various subdivisions representing dis- tance or physical features (Table 2). Examples of subdivisions by distance are the following: Missis- sippi westward vs. Alabama eastward, Alabama to Florida Keys, Florida vs. Atlantic coast, and Florida east vs. Georgia northward. Examples of physical subdivisions are the following: Florida peninsula (Florida east coast versus Florida west coast), east- ern Gulf and Atlantic coast (Alabama to Florida Keys versus Atlantic coast), and northern and west- ern Gulf (Louisiana-Mississippi versus Texas versus Mexican sector of the Gulf) (See also Collard and Ogren, 1990). Caution should be applied to interpreting electro- phoretic results in which variation has not been proven to be of genetic origin by the use of breed- ing analysis (i.e., crossing of phenotypes and analy- Johnson et al.: Evidence for distinct stocks of Scomberomorus cavalla 97 sis of offspring). Deviation from Hardy- Weinberg expectations can result from stock mixing, natural selection, or drift in small populations (Smith, 1990). While we favor the inter- pretation that these king mack- erel data suggest stock mixing, consideration should be given to natural selection as the ul- timate maintenance factor of PEPA-2* frequencies as sug- gested for dipeptidase (PEPA- LT*) and other variations found in Menidia beryllina (Johnson, 1974). Electrophoretic data suggest that two stocks of king mack- erel occur in the Gulf, a west- ern stock with high frequency of the *a allele and an eastern stock with a low frequency of the *a allele. The northern Gulf appears to be a zone of mixing of these two stocks during the summer. Our electrophoretic information does not distin- guish the eastern Gulf fish from those along the Atlantic coast. Historical tagging data showed migration between south Florida and the north and northwest Gulf. Williams and Godcharles (1984) 2 (and Sutter et al.'s later analysis (1991) of Williams and Godcharles' data) can be exam- ined in light of the two stock hypothesis. Williams and Godcharles tagged approxi- mately 12,000 king mackerel off south and southeast Florida, primarily in winter months. Forty-nine tags were recovered in the northeast Gulf and another 49 tags were re- turned from the northwest Gulf. Almost all tagged fish were recaptured in the warmer months of the year, supporting the hypothesis of migration from wintering grounds in southeast Florida waters to northern Gulf of Mexico waters Table 2 Comparisions o f geographic groupings of a llele counts of dipeptidase (PEPA- 2*) in king mac terel. {Scomberomorus cava lla), 1985- -90. Location' Year Alleles X 2 df P Remarks MS westward vs. AL eastward (distance) 2 1985 1,620 297.3417 <0.001 Deficient *b in MS westward 1986 1,676 340.9499 <0.001 Devidient *6 in MS westward 1986 3,976 283.7311 <0.001 Deficient *b in MS westward 1988 2,468 812.6335 <0.001 Excess *b east of Al Deficient *a east of AL 1990 1,926 793.5280 <0.001 Excess *b east of AL Deficient *a east of AL Key West, FL westward vs. Atlantic coast (physical) 1985 2,630 329.0983 <0.001 Excess *a in Gulf 1986 2,662 879.2843 <0.001 Excess *a in Gulf 1987 3,865 271.3356 <0.001 Excess *a in Gulf 1988 3,084 643.4390 <0.001 Excess *b in Atl. coast Deficient *a in Gulf 1989 3,004 657.913 <0.000 Excess *b in Atl. Coast Deficient *a in Atl. Coast 1990 1,926 339.2062 <0.001 Excess *b in Atl. coast Deficient *a in Atl. coast AL to Key West, FL vs. Atlantic coast (distance) 1985 1.518 0.0040 >0.90 1986 1,258 33.1770 <0.001 Excess *a in Gulf 1987 1,550 64.6325 <0.001 Deficient *a in Atl. coast 1988 1,022 10.4639 <0.001 Excess *a in Atl. coast Deficient *a in Gulf 1989 1,406 6,2033 >0.01 Excess *a in Gulf Deficient *a in Atl. Coast 1990 864 22.0855 <0.001 Excess *a in AL to Key West, FL Deficient *a in Atl. coast Within northerr and western Gulf (LA-MS, TX, MX) (physical) 1985 1,110 7.9835 2 >0.01 1986 1,410 135.5281 3 <0.001 Excess *b in LA-MS Excess *a in MX 1987 2,416 71.5602 2 <0.001 Excess *b in LA-MS Excess *a in MX 1988 2.062 40.1994 2 <0.001 Excess *b in LA-MS Deficient *b in TX 1989 1,598 70.2421 2 <0.001 Excess *b in LA-MS Deficient *a in LA-MS 1990 1,062 120.9159 2 <0.001 Excess *b in LA-MS Deficient in *a in LA-MS Deficient in *b in MS Within Atlantic coast (N of FL vs. FL) (distance) 1985 1,008 0.0738 1 >0.70 1986 992 1.8493 1 >0.10 1987 336 0.1133 1 >0.70 1988 616 0.9336 1 >0.30 1990 388 6.0278 1 >0.01 Excess *a in FL ' Abbreviations are used for states: AL=Alabama; Fl ^Florida, LA=Louisiana; MS=Mississippi. TX=Texas; MX=M >XlCO : ' In parentheses { ) general ci assification of range subc lvisions. See text 98 Fishery Bulletin 92(1), 1994 in the summer. These authors also tagged fish off North and South Carolina, but none were recovered in the Gulf. According to Fable et al. (1990), 4 king mackerel tagged in northwest Florida have been recovered in south Florida. Typically, these are the smallest and youngest tagged in the southeast United States. Sutherland and Fable ( 1980) showed that northeast Gulf fish migrated to south Florida. However, addi- tional tagging (Fable et al., 1990 4 ) showed that northeast Gulf fish eventually moved westward to Louisiana, Texas, and Mexico waters when they had been free for a sufficient time and grown to a larger size. Tagging off Louisiana from 1983 to 1985 (Fable et al., 1987) indicated that the northwest Gulf may have year round residental large king mackerel that mix in the warm months with smaller migrants from south Florida and Mexico. Recent tagging data (Fable et al., 1990 4 ) from this region have provided additional recoveries from both south Florida and Mexico, strengthening this interpretation. Addi- tional support is provided by the occurrence in Loui- siana of a year-round king mackerel fishery, whereas elsewhere the fishery is seasonal. In contrast to historical reports, recent tagging (Fable et al., 1990 4 ) showed movements between Texas and Mexico. Fish tagged in Texas waters mi- grate to both Florida and Mexico. Additionally, fish movements between Texas and eastward (as far as Panama City, FL) were documented. Mark-recapture data (Fable et al., 1990 4 ) from tagging in Mexican waters suggest that the states of Campeche and Yucatan are wintering areas for king mackerel in the western Gulf. Fish tagged in warmer months (April-July) in Texas, Tamaulipas, and Veracruz were found in Campeche and Yucatan in the winter. Tagging efforts (Fable et al., 1990 4 ) in Veracruz have provided evidence of northward mi- grations to Tamaulipas and Texas in spring and sum- mer, and movement to the Yucatan peninsula in winter. Additional evidence supporting two Gulf stocks can be found in catch-effort data of king mackerel. Although the data are complicated by different fish- ing strategies depending on the type of fishery (rec- reational or commercial) and regulatory closures, detailed analysis of catch data from the southeast- ern United States charterboat fishery indicated that in spring and early summer some stocks of fish si- multaneously migrated northward along the west- ern and eastern coasts of the Gulf (Trent et al., 1987b). They also developed the ". . . idea that part of the population of large fish remains in the Loui- siana area year-round and that the abundance of these fish is greatest during cold months." The fishery for king mackerel in Louisiana is unique among the fisheries in the northern Gulf of Mexico in that it is year-round; elsewhere it takes place mainly from late spring to late fall. The win- ter fishery (commercial hook-and-line) in Louisiana began in 1981-82. Distinctive differences character- ized winter and spring-fall seasons: 1) the smallest fish (both males and females) were caught April to October whereas the largest fish were caught be- tween November and March; 2) females were more abundant in the winter fishery than at other times of the year (Trent et al, 1987a). For two or more populations to maintain separate identities they must be isolated, either physically or reproductively (Hartl, 1980). In the case of Gulf king mackerel, there is evidence for reproductive isola- tion. Grimes et al. (1990) presented a detailed ex- amination of the distribution and occurrence of lar- val and juvenile king mackerel in the Gulf (based on published reports, neuston sampling, and Mexi- can trap net and trawl collections). The spawning season in the northern Gulf (U.S. waters), as indicated by the seasonal occurrence of larvae, is May to Octo- ber. Larval collections off Mexico were sparse and of- fered little information on spawning seasonality. The summer spawning period in the northern Gulf was also indicated by seasonal gonadal devel- opment of king mackerel (Finucane et al., 1986). They reported that reproductive activity occurred from May through September; a few fish were re- productively active as early as April and as late as October. However, spawning dates of January through August for Mexican juveniles estimated from otolith data showed a bimodal distribution, which suggests that spawning seasons in Mexican waters are different from those in the northern Gulf (Grimes et al., 1990). Two of the four collections of juvenile king mack- erel in Mexico used by Grimes et al. ( 1990) had tis- sue samples (Tampico, July 1986, and Playa Norte, Sept. 1986), and we analyzed these samples for PEPA-2* variation. Spawning dates of fish in the Tampico collection ranged from mid-February to mid-April and PEPA-2' a frequency was 0.896. The Playa Norte collection's spawning dates ranged from mid-April to mid-July and PEPA-2* a frequency was 0.600 (Table 1). Water circulation data for the Gulf of Mexico (Salsman and Tolbert, 1963 7 ) and information from Trent et al. (1987b), Grimes et al. (1990), Fable et al. 1990, 4 along with our data on king mackerel, sug- 7 Salsman, G. G., and W. H. Tolbert. 1963. Surface currents in the northeastern Gulf of Mexico. U.S. Navy Mine Defense Laboratory, Panama City, FL, Res. and Dev. Rep. 209, 43 p. Johnson et al.: Evidence for distinct stocks of Scomberomorus cavalla 99 gest one plausible scenario with regard to king mackerel stocks in the Gulf of Mexico. A western population exists that winters and spawns in the Gulf of Campeche. The Mexican Current serves as an entrainment system for its young. As these young become older and larger, they are able to cross the region of offshore advection and utilize the north- ern Gulf area (Texas to Florida) for summer feed- ing. This stock of fish has a high PEPA-2*a fre- quency and spawns earlier in the year than fish in the northern and eastern Gulf of Mexico. No infor- mation (tagging, electrophoretic, or reproductive) is available on fish of the Yucatan Straits area and the Caribbean Sea to evaluate their relation to the west- ern Gulf of Mexico fish. An eastern population of king mackerel uses the eastern and northern Gulf of Mexico area as entrainment systems for its young and the northern Gulf (Florida-Texas) as summer feeding grounds. The spawning area extends from Texas to northwest Florida between April and Oc- tober; the majority of spawning probably occurs in the northwest Florida-Louisiana area. Tagging stud- ies suggest that this stock uses south Florida and the southeast coast of Florida as its wintering grounds. The Louisiana area is somewhat of an enigma. Tagging studies indicate that the area is used by fish from both sides of the Gulf, fish are in the area year- round, PEPA-2'a frequencies are between the ex- tremes of the east and west Gulf, and tag recover- ies from winter tagging in Louisiana have been from Louisiana and westward, whereas recoveries from summer tagging were both east and west of Louisi- ana. Additionally, Finucane et al. (1986) suggested an earlier distinct peak in gonadal development (May) for Louisiana-Mississippi than in northwest Florida (August) and in Texas (August). The ques- tion still remains: Does the Louisiana area have an independent spawning population that utilizes the northern Gulf currents for its life cycle? The exist- ing evidence (especially tagging) suggests the area is not independent; however, information comes from larger fish. Thus, the area may be occupied by individuals from both sides of the Gulf which may or may not reproduce in the area. Further investi- gation especially on the younger life stages using other methods of analyses may answer this question. Another group (stock) of king mackerel that im- pinges upon the Gulf of Mexico resources (officially recognized by Fishery Management Councils) is the Atlantic Migratory Group. This group has a vary- ing range from Virginia to southwest Florida de- pending on the time of the year (Gulf of Mexico and South Atlantic Fishery Management Councils, 1985). The stock is considered to winter in South Florida and ranges along the Atlantic coast to North Carolina and South Carolina during the summer. The fish probably spawn from May to October with a peak in July (Finucane et al., 1986). These fish are currently regulated as a group with seasonal south- ern boundaries of lat. 25°48'N (the Collier/Monroe County line, FL) from 1 April to 31 October and lat. 29° 25'N (the Volusia/Flagler County line, FL) from 1 November to 31 March. Tagging information sup- ports this separation (Gulf of Mexico and South Atlantic Fishery Management Councils, 1985). PEPA-2' a allele frequencies are generally low (0.00-0.10) along the Atlantic coast as in the east- ern Gulf of Mexico. The higher PEPA-2*a values (>0.10) occasionally encountered may be the result of fish entrapped in water masses coming up the coast from outside the east coast of Florida. This possibility is suggested by the recovery along this coast of drift bottles that were released in the Yucatan Straits area (Salsman and Tolbert, 1963 7 ). All these stocks need to be further investigated in order to be elevated to the status of genetic stocks (i.e., completely isolated reproductive populations of the same species). Conclusion Four lines of evidence for a two stock hypothesis for the Gulf of Mexico king mackerel have been pre- sented. The two stock hypothesis states that the Gulf contains a western stock of king mackerel, which winters in Mexico and migrates in spring and early summer to the northern Gulf (Texas-Alabama), and an eastern Gulf stock which winters in south Florida and migrates in spring and early summer to the northern Gulf. The two stocks mix in the northern Gulf during the summer. The four lines of evidence are the following: 1 Dipeptidase (PEPA-2' ) data showing western Gulf fish high in *a allele and eastern fish low in *a allele. 2 Mark-recapture data showing movement along both sides of the Gulf from south to north. 3 Catch data indicating simultaneous migrations northward on each side of the Gulf in early spring and summer. 4 Estimates of spawning dates suggesting pos- sible temporal and spatial differences between the northern and southern Gulf. Acknowledgments Especially helpful in collecting specimens and data were staff members of the following organizations: 100 Fishery Bulletin 92(1). 1994 Florida Department of Natural Resources (Tallahas- see, FL); Gulf Coast Research Laboratory (Ocean Springs, MS); Institute Nacional de la Pesca (Mexico City, Mexico); Louisiana State University (Baton Rouge, LA); Mote Marine Laboratory (Sarasota, FL); North Carolina Division of Marine Fisheries (Morehead City, NO; Savannah State College (Sa- vannah, GA); Texas Parks and Wildlife (Austin, TX); Virginia Institute of Marine Sciences (Gloucester Point, VA); and the various laboratories of the Na- tional Marine Fisheries Service, Southeast Fisher- ies Center (Miami, FL). Special thanks go to B. May, Cornell University (Ithaca, NY) for sharing his ex- perience with king mackerel with us, to K. M. Burns, Mote Marine Laboratory (Sarasota, FL) for coordinating field work and obtaining specimens in Mexico, and to P. 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Abstract. — The spawning seasonality, fecundity, and daily egg production of three species of short-lived clupeids, the sardine Amblygaster sirm, the herring Herklotsichthys quadrimaculatus, and the sprat Spratelloides delicatulus were examined in Kiribati to assess whether vari- able recruitment was related to egg production. All species were multiple spawners, reproducing throughout the year. Periods of increased spawning activity were not related to seasonal changes in the physical environment. Spawn- ing activity and fish fecundity were related to available energy reserves and, hence, food supply. The batch fecundity of A. sirm and S. delicatulus also varied inversely with hydrated oocyte weight. The maximum reproductive life span of each species was less than nine months and averaged two to three months. Each species had a similar spawning frequency of three to five days, but this varied more in A. sirm and S. delica- tulus. Amblygaster sirm had the highest fecundity and potential lifetime egg production, but the number of eggs produced per ki- logram of fish was highest in the small sprat S. delicatulus. Monthly estimates of the daily egg production of each species varied with the proportion of the population that was spawning. Estimates of egg production showed little similarity to the fre- quency distribution of birthdates back-calculated from length-fre- quency samples. The distribution of back-calculated birthdates con- firmed that fish spawned in all months, but the proportion born each month varied widely from species to species and year to year. The reproductive strategy of these species ensures that successful spawning is likely, and so the level of recruitment is more de- pendent on post-hatching survival rates than on egg production. Reproductive biology and egg production of three species of Clupeidae from Kiribati, tropical central Pacific David A. Milton Stephen J. M. Blaber Nicholas J. F. Rawlinson CSIRO Division of Fisheries. Marine Laboratories, RO. Box 1 20, Cleveland, Queensland 4 1 63, Australia Manuscript accepted 24 September 1993 Fishery Bulletin 92:102-121 (1994) The sprat Spratelloides delicatulus, the herring Herklotsichthys quadri- maculatus, and the sardine Ambly- gaster sirm are the dominant tuna baitfish species in the Republic of Kiribati (Rawlinson et al., 1992). All three species inhabit coral reef lagoons and adjacent waters. Sprats school in shallow water around reefs and adjacent seagrass during the day. Herring also form dense schools in shallow water along the shoreline and among reefs during the day (Williams and Clarke, 1983). Unlike the other species, sardines school near the bottom of the lagoon during the day (Conand, 1988). All species disperse into the mid and upper waters of the lagoon during the night to feed and become available to the com- mercial fishery. A major source of lost fishing time by pole-and-line vessels in Kiribati has been irregular baitfish catches (Maclnnes, 1990). These important tuna baitfish species have shown large seasonal and interannual fluctuations in abun- dance since they were first re- corded during the 1940s (McCar- thy, 1985 1 ; Rawlinson et al., 1992). Both A. sirm and H. quadrima- culatus disappear from baitfish catches for variable periods and can be absent for months or years (Kiribati Fisheries Division, 1989 2 ). Changes in abundance may be related to variable or irregular re- cruitment, because many clupeoids (especially clupeids and engraulids) have little capacity to compensate for environmental variation during the period of peak spawning and egg production (Cushing, 1967, 1971). Most clupeids, including some tropical species, are multiple spawners (Alheit, 1989). Multiple spawning should be advantageous for short-lived species because it enables them to maintain rela- tively stable population sizes in unpredictable environments (Armstrong and Shelton, 1990). Multiple spawning has been estab- lished for few tropical clupeids (e.g., Sardinella brasiliensis; Isaac- Nahum et al., 1988). Of the three major baitfish species in Kiribati, only S. delicatulus has been shown to be a multiple-spawner (Milton and Blaber, 1991). All three species are subject to high natural mortal- ity in Kiribati (Rawlinson et al., 1992), thus lifetime egg production 1 McCarthy, D. 1985. Fishery dynamics and biology of the major wild baitfish species particulary Spratelloides delicatulus, from Tarawa, Kiribati. Kiribati Fisheries Div., Tarawa, Kiribati, 53 p. 2 Kiribati Fisheries Division. 1989. Fisher- ies Division 1989 Annual Rep., Ministry of Natural Resources Development, Tarawa, Kiribati, 38 p. 102 Milton et al.: Reproductive biology and egg production of three species of Clupeidae 103 may be increased if they spawned multiple batches of eggs. Egg production of multiple spawning species de- pends on reproductive life span, the time between spawnings, and the age structure of the population (Parrish et al., 1986). Batch fecundity of S. delica- tulus varies widely between sites, both within and between countries (Milton et al., 1990). In a short- lived species such as S. delicatulus (<5 months; Milton et al., 1991), reproductive life span may have an important influence on potential lifetime egg production. Batch fecundity of H. quadrimaculatus does not appear to vary throughout its distribution, and ranges from 4,000 to 10,000 eggs (Marichamy, 1971; Hida and Uchiyama, 1977; Williams and Clarke, 1983; Moussac and Poupon, 1986; Conand, 1988). Fish mature at about 90 mm in length at six months of age (Williams and Clarke, 1983), and they sur- vive for at least one year (Milton et al., 1993). Little is known of fecundity and egg production of A. sirm. Fecundity of the species is related to length and weight, with a mean of 20,000 eggs per batch, and individuals probably spawn more than one batch of eggs (Conand, 1988). Temperate clupeids vary widely in life-history parameters (e.g., Clupea spp., Jennings and Beverton, 1991). Food availability and environmen- tal conditions affect the size and number of eggs of Pacific herring (Clupea pallasi) (Hay and Brett, 1988). Results of studies of temperate clupeoids suggest that they do not spawn during periods of high food abundance, but store energy as fat for later reproductive activity (Hunter and Leong, 1981; lies, 1984). There are no similar studies of tropical clupeids. Encrasicholina heterolobus, a tropical engraulid, does not deplete energy reserves in the liver or soma during spawning (Wright, 1990). Fish with higher condition factor (K) also had higher fecundity. Stored energy or fish condition that may influence both spawning frequency and batch fecundity have a marked influence on egg production and, hence, affect subsequent recruitment (Ricker, 1954; Beverton and Holt, 1957). Adult reproductive varia- tion should strongly influence recruitment in short- lived tropical species that have short larval phases and rapid growth. An example is S. delicatulus which, in the Solomon Islands, live a maximum of five months and mature at about two months of age (Milton and Blaber, 1991; Milton et al., 1991). Amblygaster sirm and H. quadrimaculatus live less than two years (Milton et al., 1993) and mature in 6-12 months (Williams and Clarke, 1983; Conand, 1988). In this study, we examined the variability in re- productive biology of the three major baitfishes in Kiribati to determine the influence of adult repro- ductive variability on subsequent recruitment. Our objective was to test the hypothesis that reproduc- tive biology of short-lived clupeids is adapted to maintaining relatively stable population sizes. We determined potential life-time egg production and whether estimated egg production is related to the frequency distribution of back-calculated birthdates. Methods and materials Study areas The Republic of Kiribati covers an area of 3 x 10 6 km 2 in the central Pacific ocean and comprises three main island groups (Gilbert, Phoenix, and Line Is- lands) (see Inset Fig. 1). The Gilbert Island group is the most populated, consisting of 16 coral reef islands. All islands in the group have a typical ocean platform coral reef structure and have been built up by scleractinian corals and coralline algae on a sub- merged mountain (Gilmour and Colman, 1990 3 ). Most atolls consist of small islets lying on the east- ern side of a lagoon with an open western side due to the prevailing easterly winds. Most typically have passages between the islets through which water is exchanged. The four study sites (Abaiang, Butaritari, Tarawa, and Abemema) were typical of islands in the Gilbert Island group; all had narrow islets on their south- ern and eastern sides, except Abaiang (Fig. 1). La- goons were mainly shallow (20-30 m deep), often with large areas of intertidal seagrass or sand on their eastern sides. Bottom topography of the deeper parts of the lagoon was generally smooth, with some coral outcrops. Our study sites were similar to those described by Hobson and Chess (1978) in the Marshall Islands. Environmental parameters On each sampling occasion, we measured the time of collection, sea surface temperature (°C), cloud cover (okters), wind direction and speed, and moon phase because these factors may be related to spawning or recruitment (Dalzell, 1985, 1987; Peterman and Bradford, 1987; Milton and Blaber, 1991). For each site, monthly rainfall data for 1989 Gilmour, A. J., and R. Colman. 1990. Report on a consultancy on a pilot environmental study of the outer island development program. Republic of Kiribati. Graduate School of the Environ- ment, Macquarie Univ., Australia, 151 p. 104 Fishery Bulletin 92(1), 1994 173°E I 175°E 177°E Butaritari Abaiang > Tarawa Gilbert Is 2°N- (D .Abemama $><0 o°- <%> <^ 100 kms 1S0°E Gilbe rt Is ,-, < *.r ^Solomon Australia N ^Tuvalu Fiii % Figure 1 Map of Gilbert Islands, Kiribati showing the four study sites (Butaritari, Abaiang, Tarawa, and Abemama). Inset shows the ter- ritorial boundary of Kiribati, the Gilbert Islands, and their posi- tion in the Pacific. and 1990 were obtained from the Kiribati Govern- ment Meteorological Division. Sampling Fifty to 1,000 Amblygaster sirm, Herklotsichthys quadrimaculatus, and Spratelloides delicatulus were collected monthly at one or more of four sites in Kiribati (Butaritari, Abaiang, Tarawa, and Abemama; Fig. 1) between August 1989 and May 1991. Additional samples of A. sirm and H. quadrimaculatus were collected in November 1988 and January 1989 from Tarawa. Fish were caught by several methods at each site. Most samples were collected from the commercial tuna baitfish catches each month at each site. Supplementary samples were ob- tained by beach-seining (H. quadrimaculatus and S. delicatulus), cast-netting (H. quadrimaculatus) in shallow water during the day, or gill- netting (25- and 38-mm stretched mesh) at night near baitfishing opera- tions. All fish were preserved in 70% ethanol. Reproductive biology Laboratory studies All fish collected from commercial baitfish sampling were measured (standard length in millimetres), and a subsample of 20 to 60 specimens weighed (±0.005 g). Go- nads, otoliths, liver, and viscera were removed and the amount of visible fat subjectively estimated. Both ovaries from the first 20 females of each spe- cies at each site for each month were dried of surface moisture, weighed (±0.001 g) and stored in 4% formalin- seawater for histology. Testes, ovaries of other fish, liver, and the soma were dried at 60°C to a constant weight. Otoliths were used to estimate the age (in days) of each fish by methods outlined in Milton et al. ( 1993). Addi- tional samples offish caught by other methods were treated separately, but in a similar way. We report only on results of studies offish collected from commercial samples unless otherwise stated. For histological preparations, go- nads were embedded in paraffin, sec- tioned at 9 mm, and stained with Ehrlich's haemotoxylin and eosin (McManus and Mowry, 1964). Gonad maturation stages were defined follow- ing Cyrus and Blaber (1984) and Hunter and Goldberg (1980), and were similar to those of Moussac and Poupon (1986) for H. quadrimaculatus from the Seychelles. We staged each gonad accord- ing to the relative numbers of cells at each develop- mental stage (Young et al., 1987; Table 1), and the presence of any post-ovulatory follicles was noted. The percentage of each histological section that cor- responded to each developmental stage was subjec- 2°S- Milton et al.: Reproductive biology and egg production of three species of Clupeidae 105 Table 1 Criteria used for staging female gonads of tropi- cal clupeids stained with haematoxylin and eosin. Stage Histology (1) Immature Chromatin nucleolar stage — prefollicle cells surround each oocyte (2) Developing/resting Perinucleolar stage — uniform staining cytoplasm (3) Maturing Yolk vesicle formation; some non-staining yolk (lipid) (4) Ripe Vitellogenic stage — red- staining yolk; developed chorion (5) Running ripe Globular red-staining yolk; (spawning) oocytes hydrated; develop- ment complete (6) Spent Presence of post-ovulatory follicles; cortical alveoli present and/or atresian of remaining ripe oocytes tively estimated. Post-ovulatory follicles were aged according to stages found in other multiple-spawning clupeoids (Hunter and Goldberg, 1980; Goldberg et al., 1984; Isaac-Nahum et al., 1988). Gonosomatic indices (GSI) were calculated as the ratio of wet gonad weight to somatic weight (total weight minus gonad weight), expressed as a percentage. Similarly, we calculated a hepatosomatic index (HSI) as the ratio of liver dry weight to somatic dry weight (total weight minus en- tire viscera), expressed as a percentage. Length and age at sexual maturity were defined as the minimum size and age at which fish had ripe oocytes (Stage 4), determined by histological exami- nation. Fish that had running-ripe oocytes (Stage 5) were recorded as in spawning condition. We defined the length and age at first spawning as the small- est size where the proportion of running-ripe oocytes in the section exceeded 85% for more than 50% of the fish of that length or age. We chose this crite- rion after examining large numbers of histological sections with running-ripe oocytes. In these sections they always represented more than 85% of the sec- tion area. Our results were similar to that found in other tropical clupeoids (Milton and Blaber, 1991). The reproductive life span of the population of each species at each site each month was determined from the oldest fish (Milton et al., 1993) in each sample minus the age at first spawning. We estimated batch fecundity for each species from fish that had been examined histologically and had oocytes that were starting to hydrate ( ripe-early running ripe; Stages 4-5; Table 1), but we did not examine the fecundity of fish with any empty fol- licles. An advanced modal size group of oocytes could be distinguished in ripe fish. We separated a subsample of between half (A. sirm) and all (S. delicatulus) of the ovary and weighed it. The num- ber of eggs in the advanced mode was counted and the fecundity was estimated by multiplying the number of eggs in the subsample by the ratio of total gonad weight to subsample weight. Fecundity esti- mates were made within three to four days after the ovary was removed from the fish to minimize the potential bias of differential absorption of fixative by oocytes and surrounding somatic tissue. We used hydrated oocytes from fish caught be- tween 2000 and 2330 hours to estimate egg weight. Oocyte weights were estimated from hydrated oo- cytes in ovaries that were almost ready to spawn (late Stage 5; Table 1). We measured oocyte dry weight by counting 10 samples of 10 oocytes from each ovary, drying the oocytes at 50° C to a constant mass and weighing each subsample separately. We scored visceral fat on a five-point scale. If a fish had less than 25% of the intestine covered in fat deposits, it was scored as (1); 25-50%, (2); 50- 75%, (3); and 75-100%, (4). A fish scored (5) when all intestine was covered with fat and deposits were also present around the stomach (Nikolsky, 1963). The proportion of females examined histologically each month that had post-ovulatory follicles (POF; Stage 6) was used to evaluate reproductive season- ality. We determined that these fish had spawned within the previous 15-48 hours, because these structures decompose and cannot be recognised af- ter that time (Hunter and Goldberg, 1980; Clarke, 1989). In samples where no fish had POF's, we used the proportion of fish in the histological subsample whose sections had greater than 85% running-ripe oocytes (Milton and Blaber, 1991). We used this proportion to calculate monthly estimates of mean daily oocyte production and the number of batches of oocytes spawned each month (Parrish et al., 1986). We estimated daily oocyte production (n/kg of adults; egg production index) for samples collected from commercial baitfishing, because these samples were assumed to be most representative of the popu- lation. Our methods were similar to those of Parker (1980, 1985), which have been used to estimate the spawning biomass of a number of multiple spawn- ers (Armstrong et al., 1988; Pauly and Palomeres, 1989; Somerton, 1990). However, our methods dif- fered because we used commercial catch per unit of effort (CPUE) as an index of adult abundance. Egg production index ^(fiPF.SR^^WiYcPUE (1) 106 Fishery Bulletin 92(1). 1994 where f- is the proportion of females in the ith length class, p is the proportion of the sample spawning, F is the fecundity of a fish of that length taken from the fecundity-length regression, SR t is the sex-ratio of the ith length class and W i is the total weight of fish in the ith sample. CPUE was estimated from the monthly catch returns of the commercial fleet. We chose this method of estimat- ing egg production because S. delicatulus have de- mersal eggs (Leis and Trnski, 1989) and the eggs of A. sirm and H. quadrimaculatus are difficult to sample adequately in the large areas of suitable habitat in each lagoon. For comparison with adult spawning data, we back-calculated the distribution of birthdates offish collected in each length-frequency sample by using the growth equations of Milton et al. (1993). Fre- quencies in each age class were adjusted for mor- tality by using the estimates of Rawlinson et al. (1992). The distribution of birthdates was also back- calculated for H. quadrimaculatus and S. delica- tulus length-frequency samples from previous stud- ies at one site (Tarawa) January 1976 to February 1977 (R. Cross, 1978 4 ) and May 1983 to April 1984 (McCarthy, 1985 1 ). We used age distribution in these earlier studies and those of the present study to ex- amine seasonal, annual, and site-related differences in the reproductive life span of each species. Statistical analyses Inter- and intra-specific differ- ences in fat index, HSI and K were examined with Fisher's r-tests to account for unequal sample sizes. Seasonal and site-related differences in fecundity (expressed as oocytes per gram) were examined by analysis of covariance with weight as the covariate. Hydrated oocyte weight and reproductive life span were examined by one-way analysis of variance. We examined the relative influence of exogenous and endogenous factors on the fecundity of each species at each site by stepwise regression (Sokal and Rohlf, 1981). We included the following: length, weight, age, sea-surface temperature (°C), wind speed (in knots), moon phase (expressed by fitting a sin/cosin curve to the number of days since the last full moon before the sample was taken divided by the number of days in a lunar month (29.5) (Milton and Blaber, 1991), fish condition (K: weight/length 3 ), fat, and HSIC7r ). We retained only those variables that significantly improved the fit of the model (P<0.05). Because several of these variables were correlated, we did a partial-correlation analysis be- tween these variables and fecundity, and the results 4 Cross, R. 1978. Fisheries research notes. Fisheries Division, Ministry of Commerce and Inductry, Tarawa, Kiribati, 58 p. of the two approaches were compared. If the variable most related to fecundity in the stepwise regression was not the one most related to fecundity in the par- tial-correlation analysis, the stepwise regression model was discarded and no relationship was assumed. In order to estimate egg production (Eq. 1), we estimated the proportion of females in each 5-mm length class from the total sample of each species. The variance of these estimates was calculated by using the normal approximation to the binomial dis- tribution (Walpole, 1974). We assessed whether the monthly percentage of annual egg production was related to the proportion of annual recruitment in the same month by rank-correlations (Conover, 1980). The average age of the potential spawning popu- lation in each sample was compared by a nested analysis of variance with month of sampling nested within year. Significant differences between treat- ments were identified from comparison of the least- squares means of each treatment, as sample sizes differed between cells (Sokal and Rohlf, 1981). Results Environmental parameters Sea-surface temperature in Kiribati varied little throughout the year. During the study period, tem- peratures varied between 29°C and 32°C (Table 2). Rainfall varied along the Gilbert Island group; rain- fall was higher in Butaritari than at the other sites. Some rain fell throughout the study period but was more intense during 1990 at all sites. Rainfall dur- ing 1989 was below the long-term average at all sites and was 16-50% that of 1990. The highest rainfall fell during the north-east monsoon (Decem- ber-April) at all sites. Winds were mostly light, and varied in direction seasonally, blowing from the east during the monsoon, but from the south-south-west for the rest of the year (Table 2). Reproductive biology Maturation The length and age at first maturity of A. sirm varied between sites (Table 3). Ambly- gaster sirm matured younger and smaller in Kiribati than elsewhere. Length and age at first spawning were much greater than the length or age when fish reached sexual maturity, but this size was similar to that of fish from northern Australia (Table 3). Herklotsichthys quadrimaculatus matured and were capable of spawning at 70 mm length and 4 months of age (Table 3). The relative size and age at which fish matured (as a proportion of maximum size and Milton et al.: Reproductive biology and egg production of three species of Clupeidae 107 age) did not differ among fish from the four sites. In Kiribati, S. delicatulus become sexually mature at 40 mm and two months of age and spawn shortly afterwards. Compared to the other species, the length and age at maturity and first spawning varied less among sites (Table 3). The three spe- cies differed in the length and age at sexual maturity and first spawning. However, as a proportion of their maxima, the three species were similiar Cu- test; P>0.1). All matured and spawned at about 70% of maxi- mum size and 50% of maxi- mum age (Table 3). Timing of spawning We iden- tified recent spawning by the presence of post-ovu- latory follicles in the ovaries. In A. sirm, follicles were detected in samples collected between 0100 to 1630 hours, and new post-ovulatory follicles (iden- tified as day-0 [<24 hr]; Hunter and Goldberg, 1980; Goldberg et al., 1984) were observed in fish collected between 0100 and 0510 hours. Female H. quadrimaculatus with post-ovulatory follicles were collected between 2130 and 1630 hours and day-0 follicles were found in samples collected between 2130 to 0300 hours. In female H. quadrimaculatus caught after 0300 hours, follicles could not be dis- tinguished from day-1 type POF's, as the follicles de- generated rapidly. Similarly, we detected post-ovu- latory follicles in female S. delicatulus collected from 2210 to 1930 hours, and follicles of all females col- lected earlier than 0845 hours were identified as day-0. Those in females of the single sample col- lected later in the day ( 1930) were assigned as day-1. Spawning season There was protracted spawning in A. sirm with periods of intense spawning activ- ity (Fig. 2). During both 1989 and 1990, fish spawned August to October and also during May- June in 1990. Condition, fat index, and HSI were less during spawning periods and reached a peak in March-April 1990, i.e., before spawning (Fig. 2). We found less fat deposits in spent fish and the fish were in poorer condition than fish with gonads in other stages of development (P<0.05; Table 4). We noted no significant differences in HSI among fish with gonads at the same stage of development. Herklotsichthys quadrimaculatus spawned throughout the study period: 20 to 50%> of the popu- Table 2 Mean water temperature (°C) , wind speed (kn), clou d cover, and monthly rainfall (mm) at four sites in Kiribati from November 1988 to May 1991. Parameter Butaritari Abaiang Tarawa Abemama Water temperature (°C) 30.2 ± 0.3 30.2 ± 0.4 29.5 ± 0.1 29.9 ± 0.2 Range 28-32 27-33 29-30 29-31 Wind speed (kn) 2.2 ± 0.6 4.2 ± 0.9 5.4 ± 1.2 2.2 ± 0.2 Range 0-7 0-10 1-15 1-5 Prevailing direction East East East East Cloud cover (okters) 2 ± 0.6 5 ± 0.6 3 ± 0.5 1 ± 0.4 Range 0-6 1-7 0-7 0-4 Monthly rainfall (mm) (1945- -88 263 ± 35 181 + 35 165 ± 35 128 ± 33 Range 7-908 0-761 0-824 0-728 Monthly rainfall 1989 (mm) 184 ± 29 42 ± 10 77 ± 23 36 ± 10 Range 51-351 0-108 6-235 3-102 Monthly rainfall 1990 (mm) 404 ± 37 - 298 ± 51 202 ± 31 Range 195-614 - 19-643 93-402 Months sampled 14 12 18 13 lation spawned each month (Fig. 3). Female condi- tion, fat index, and HSI all followed a similar pat- tern during the study but did not appear to be di- rectly related to spawning activity. Fish in spawn- ing condition had the highest HSI, fat, and condi- tion values, but these were only significantly greater than those of spent fish (P<0.05; Table 4). Spratelloides delicatulus spawned almost continu- ously throughout the study period but spawning varied in intensity (Fig. 4). Peak spawning occurred during different periods in each of the years sampled. Female HSI and fat index showed a simi- lar pattern during the study but monthly changes in these parameters or fish condition did not follow the spawning cycle. We found no significant differ- ences in HSI or fat index for females with ovaries in different stages of development (P>0.1; Table 4). Fish condition was lower among spent fish than in ripe or spawning fish (P<0.05; Table 4). Females with ripe ovaries had higher mean HSI, fat, and condition than those in other stages of development, but these differences were not significant (Table 4). Fecundity The relative fecundity of A. sirm and H. quadrimaculatus did not differ among sites or sea- sonally within sites in Kiribati (ANCOVA with weight as covariate; overall P>0.07; Table 5). How- ever, the relative fecundity of H. quadrimaculatus was significantly different between fish from Tarawa and Abemama (<-test; P<0.05). Batch fecundity of both species did not differ among sites in Kiribati. Within their respective species groups, both species had simi- lar batch fecundities to the other species listed, al- though their relative fecundities were lower (Table 5). 108 Fishery Bulletin 92(1), 1994 Table 3 Length and age at sexual maturity and first spawning of Amblygaster sirm, Herklotsichthys quadrimaculatus, and Spratelloides delicatulus from various populations throughout their range. (L t = length at maturity, L & . = length at first spawning, L max = maximum size, T mal = age at maturity, T f = age at first spawning, maximum age, K = Kiribati, I = India, SI = Solomon Islands). 2 Length at Length at first Age at first maturity (mm) spawning (mm) Age at maturity(d) spawning (d) Species Site «w/*w> a-fip/LmaJ (T IT mat 1 max) ^ fsp IT max ) Source' A. sirm Kiribati 110 (0.50) 180 (0.80) 150 (0.29) 330 (0.65) (1) New Caledonia 132 (0.72) — 295 (0.40) — (2) N. Australia 174 (0.79) 193 (0.88) — — (3) Sri Lanka 166 (0.88) — -330 (0.80) — (4) Mean 146 (0.72) — — H. quadrimaculatu 8 Hawaii 80 (0.63) 90 (0.70) 160 (0.53) 190 (0.63) (5) Marshall Is. 90 (0.82) — 190 (0.72) — (6) Fiji 95 (0.78) 98 (0.80) 275 (— ) 294 (— ) (7), (8) Butaritari (K) 65 (0.68) 70 (0.74) 125 (0.50) 135 (0.53) (1) Abaiang (K) 70 (0.74) 70 (0.74) 125 (0.37) 125 (0.37) (1) Tarawa (K) 69 (0.72) 70 (0.73) 138 (0.45) 150 (0.48) (1) Abemama (K) 70 (0.64) 72 (0.65) 140 (0.34) 150 (0.36) (1) New Caledonia 91 (0.64) — 244 ( — ) — (9) Andaman Is. (I) 99 (0.81) 104 (0.85) — — (10) Seychelles 97 (0.71) — 150 (0.30) — (11) Mean 83 (0.72) 82 (0.74) 172 (0.46) 174 (0.47) S. delicatulus Fiji 35 (0.56) 39 (0.63) 52 (0.43) 61 (0.51) (7). (8) Butaritari (K) 40 (0.68) 40 (0.68) 65 (0.51) 68 (0.54) (1) Abaiang (K) 45 (0.75) 53 (0.88) 62 (0.51) 80 (0.64) (1) Tarawa (K) 45 (0.68) 50 (0.76) 77 (0.50) 90 (0.57) (1) Munda (SI) 37 (0.58) 37 (0.58) 72 (0.47) 78 (0.51) (12), (13) Vona Vona (SI) 37 (0.66) 37 (0.66) 68 (0.53) 72 (0.56) (12), (13) Tulagi (SI) 38 (0.60) 38 (0.60) 73 (0.55) 75 (0.57) (12), (13) Maldives 38 (0.69) 40 (0.73) 90 (0.60) 97 (0.65) (13), (14) India 42 (0.71) — — — (15) Mean 40 (0.66) 42 (0.69) 70 (0.51) 78 (0.57) ' Sources: (1) present study. (2) Conand (1991). (3) Okera (1982), (4) Dayaratne and Gjosaeter (1986). (5) Williams and Clarke (1983), (6) Hida and Uchiyama (1977), (7) Lewis et al. (1983), (8) Dalzell et al. (1987), (9) Conand (1988), (10) Marichamy (1971), (11) Moussac anf Poupon (1986), (12) Milton and Blaber 11991), (13) Milton et al. (1991), (14) Milton et al. (1990), (15) Mohan and Kunhikoya (1986). Using stepwise linear regression, we found that fecundity was related to weight in all species (Table 6; Fig. 5). Fecundity of A. sirm was significantly correlated with HSI and fish condition. Fish condi- tion, HSI, and fat index were all correlated with fecundity in H. quadrimaculatus (Table 6). Fecun- dity was significantly correlated with weight and condition at two of the four sites. Although, when data from all sites were combined, weight and fat index were the only significant correlates. Fecundity of S. delicatulus varied widely among sites, both within Kiribati and among countries (Table 5). In Kiribati, relative fecundity was higher at Butaritari than at Abaiang (P<0.05), but differed less than among sites in the Solomon Islands. Fe- cundity did not vary seasonally at any site. Relative fecundity of S. delicatulus was highest in New Caledonia — significantly higher than at all other sites except Butaritari in Kiribati (Table 5). How- ever, the relative fecundity of S. delicatulus was lower than its congeners, S. gracilis and S. lewisi, at sites where they co-occurred (Table 5). We found that the fecundity of S. delicatulus cor- related strongly with fish weight (Fig. 5). The only other factor related to fecundity in S. delicatulus was HSI. There was a significant relationship be- tween fecundity and HSI at Butaritari and Tarawa and when all data were combined. Spawning fish had a higher HSI at Butaritari than at other sites (2.24 ± 0.13 vs. 1.41 ± 0.08; P<0.001). The HSI of male S. delicatulus that had a GSI similar to that of spawning females (>5%) was also Milton et al.: Reproductive biology and egg production of three species of Clupeidae 109 o o 5 4 (5 3 2 1 i~+i ~i — i — i — i — r B J -\ — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i i i 1.5 -, j'f'm'a'm'j'j'a's'o'n'd 1989 j'F m a'm'j'j'a's'o' 1990 Figure 2 Monthly variation (±95% confidence limits) in (A) condition, (B) visceral fat index, (C) hepatosomatic index and (D) proportion spawning of female Amblygaster sirm from Kiribati between January 1989 and October 1990. 5-i £ 4 o 3 H Ny*** V****V 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 4 -i 3 CO O - 1 B &*k K/V^V I I I I 1 1 I I I I 1 I I 1 I I I 1 I I I rn n I r~\ r~ * 2 -\ £ H #-****^ i i 1 1 i i i i i i i i i i 1 1 i i i i i i i i i i 1 1 i i 100 Figure 3 Monthly variation (±95% confidence limits) in (A) condition, (B) visceral fat index, (C) hepatosomatic index and (D) proportion spawning of female Herklotsichthys quadrimaculatus from Kiribati be- tween November 1988 and April 1991. higher at Butaritari (1.41 ± 0.06; N=57) than at other sites (Abaiang HSI=1.07 ± 0.11; N=7; Tarawa HSI=0.81 ± 0.09; A/=14). The proportion of male S. delicatulus that had GSI greater than 5% was also higher at Butaritari (36%) than at other sites (Abaiang 17.5%; Tarawa 20%). Oocyte weights of A. sirm and S. delicatulus dif- fered significantly from site to site (Table 7). In S. delicatulus, we found the greatest oocyte weight at Abemama and Abaiang — significantly higher than at Butaritari and Tarawa (P<0.01). Oocyte weights in A. sirm were also higher at Abaiang (P<0.001; Table 7). We found no significant differences among sites for oocyte weights of H. quadrimaculatus. Sex ratio The sex-ratio of A. sirm, H. quadrima- culatus, and S. delicatulus changed as fish grew but only among the largest length classes of each spe- cies were there significant deviations from a ratio of 1:1. In all three species, females dominate the largest length classes (Fig. 6). In our samples, we found significantly more female A. sirm and S. delicatulus among fish larger than the length at first spawning (180 and 45 mm respectively). With H. 10 Fishery Bulletin 92(1), 1994 Table 4 Mean hepatosomatic index (HSI: %), visceral fat index (Fat) and condi- tion (K: dry weight/length 3 ) of Amblygaster sirm, Herklotsichthys uadrimaculatus and Spratelloides delicatulus at different stages of gonadal development (SE = standard error ± N = number of females examined). Species Stage HSI ± SE Fat ± SE tf(xlO-6)+SE N A. sirm maturing 0.38 ± 0.06 ripe 0.43 ± 0.04 spawning 0.39 ± 0.06 spent 0.42 ± 0.05 H. quadrimaculatus maturing ripe spawning spent 0.87 ± 0.08 0.96 ± 0.06 1.04 ± 0.07 0.69 ± 0.04 S. delicatulus maturing 1.41 ± 0.19 ripe 1.98 + 0.10 spawning 1.84 ± 0.15 spent 1.46 ± 0.10 3.4 ± 0.6 3.2 ± 0.3 2.6 ± 0.4 1.7 ± 0.2 1.4 ± 0.1 1.6 ± 0.1 1.8 ± 0.1 1.7 ± 0.1 1.3 ± 0.2 1.6 ± 0.1 1 3 • n 1 1.2 ± 0.1 4.05 ± 0.25 4.14 ± 0.07 4.03 ± 0.13 2.77 ± 0.13 ;!ii 16 6 3.89 ± 0.05 45 3.81 + 0.05 127 3.91 ± 0.05 95 3.53 ± 0.06 40 2.36 ± 0.06 2.51 ± 0.04 2.46 ± 0.04 2.28 ± 0.04 15 41 35 55 higher lifetime egg production at all sites than did co-occur- ring S. delicatulus. The num- ber of days between successive spawnings influenced esti- mates of lifetime egg produc- tion. Although longer in A. sirm, the difference was not significant (Table 8). quadrimaculatus, females dominated among fish over 80 mm (Fig. 6). Egg production The number of spawnings per month and the daily egg production of all species generally followed the pattern of the proportion spawning (Fig. 7). We found lower daily egg produc- tion in A. sirm than in the other species. During the period of maximum spawning activity, A. sirm and H. quadrimaculatus spawned up to 20 times per month (Fig. 7), and S. delicatulus spawned daily. Reproductive life span The reproductive life span of A. sirm was significantly longer in Tarawa (60.1 + 15.4 days) than at the other sites during 1989-90 (P<0.01; Table 8). Similarly, we found H. quadrimaculatus had a longer reproductive life span at Abemama (141.8 ± 30.9 days) than at other sites during 1989-91 (P<0.01; Table 8). During the same period, the reproductive life span of S. delicatulus was similar at all sites (57.5 ± 4.6 days). However, the reproductive life span of S. delicatulus at Tarawa varied significantly between years; fish caught during 1990-91 were not as old as those in previous j'ears (P<0.05; Table 8). No corresponding pattern was observed in H. quadrimaculatus from Tarawa. Herklotsichthys quadrimaculatus and S. delicatulus lived significantly longer after maturity than A. sirm (P<0.01). Our estimates of maximum lifetime egg produc- tion of A. sirm were similar at the two sites (Abaiang and Tarawa). Herklotsichthys quadrimaculatus had Recruitment Amblygaster sirm recruited from a single protracted period in Kiribati during 1989 (March to October; Fig. 8). We found a greater pro- portion of survivors had been born between March and July than in all other months except September (P<0.05). There were insufficient data to com- pare monthly egg production with recruitment, but the pe- riod of highest recruitment corresponded with the times of greatest spawning activity. However, this did not appear to be directly related to the absolute number of oocytes produced (Fig. 7). The proportion of H. quadrimaculatus born each month differed over the four years (P<0.05; Fig. 9). In 1976, the greater proportion were born from November to March, while in 1983 over 40% were born during July. Fish caught during 1989-90 showed a different pattern. The highest proportion in 1989 were born in May, whereas in 1990 the high- est proportion were born in January. Over all 4 years' data, December ( 15.4% ) and July ( 13.7% ) had the greatest mean proportion of births (P<0.05), but the July value may be biased by the large value in 1983 (Fig. 10). Where data were comparable, we found no relationship between proportion of annual recruitment and monthly egg production (r.=0.70, P<0.10, N=6 in 1989; r s =-0.15, P>0.5, N=U in 1990). The proportion of S. delicatulus born each month varied considerably among the four years examined (Fig. 10). December had the highest proportion of births in 1976. In 1983, most fish were born between May and August, and a similar pattern was found in 1989. By comparison, the distribution of birthdates was more evenly spread in 1990 (Fig. 10). The months with the largest mean proportion across the four years were May (11.2%), June (14.9%), July (15.8%), and December (11.9%). We found a nega- tive relationship between the proportion of births and egg production in 1990 (r s =-0.58; P<0.05, 7V=10). Milton et al.: Reproductive biology and egg production of three species of Clupeidae I 1 *. 35 | 3.0 o 2.5 o 2.0 i i i i i i i i i i i i i i i % ^vy/** 2- B v fi^^A 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3-j i 1 V 1 1 1 1 1 1 1 "I - 1 — n — r~rn — i — i — l 100 -| 80- r 60 40 20 D n'dIjfMaMj'j'a's'o'n'dIj'fWaMj'j'a's'o'n'dIj'fWaM 1991 1989 1990 Figure 4 Monthly variation (±95% confidence limits) in (A) condition, (B) visceral fat index, (C) hepatosomatic index and (D) proportion spawning of female Spratelloides delicatulus from Kiribati between November 1988 and May 1991. 30000 - A 20000 - F= 180 5W 10 r 2 =0.48 N=32 sb o 10000- ° s' s^° 8000-. 50 100 150 3000 - c F = = 699.9W r 2 =074 1 2000- o° o ?y^^ N=87 O Ocf <4/"o o 1000 - „ooe o - Figure 5 The relationship between batch fecundity and fish weight for (A) Amblygaster sirm, (B) Herklotsichthys quadrimaculatus and (C) Spratelloides delicatulus from Kiribati. Discussion The reproductive cycles of A. sirm, H. quadrima- culatus, and S. delicatulus in Kiribati are similar to that reported for temperate multiple-spawning clupeoids (Hunter and Goldberg, 1980; Gil and Lee, 1986; Shelton, 1987; Alheit, 1989). Most studies on multiple spawning clupeoids have been on engraulids; these species spawn many batches of eggs each year and have variable batch fecundity (Alheit, 1989). Our results for H. quadrimaculatus and S. delicatulus from Kiribati agree with previ- ous reproductive studies of these species in tropical areas (McCarthy, 1985 1 ; Moussac and Poupon, 1986; Milton and Blaber, 1991). In the tropics, both spe- cies spawn throughout the year, but have periods when spawning activity is greater. In more temper- ate parts of their range, the reproductive season of both H. quadrimaculatus and S. delicatulus is shorter and coincides with increases in water tem- perature in early summer (Williams and Clarke, 1983; Lewis et al., 1983; Conand, 1988). 1 12 Fishery Bulletin 92|l), 1994 Table 5 Mean length (mm), age (y ears), fecundity, relative fecundity (eggs g ') of Amblyg aster sirm, Herklot sichthys quadrimaculatus, and Spratelloides delicatulus and other tropical and subtropical clupeids ( sardines, her- rings, and sprats) (K = Ki ribati, SI = Solomon Island 3, I = India, P.N.G. = Papua New Guinea UK = = United Kingdom, SU = Soviet Un ion, G = Germany ). Length Age Fecundity Rel. fecundity Species Site ± SE ± SE ± SE ± SE N Source Sardines Amblygaster sirm Abaiang (Kl 189 ± 5 0.97 ± 0.03 18789 ± 2757 187.1 ± 25.3 7 (1) Tarawa (K) 194 ± 1 1.04 ± 0.03 20327 ± 1391 192.0 ± 12.0 25 (1) New Caledonia 139-177 0.90-2.2 8000-27780 300.0 ± 16.9 24 (2), (3) Sardinella brasiliensis Brazil 162 ± 2 — 23318 ± 2065 356 ± 37 23 (4) S. marquesensis Marquesas Is. 109 ± 6 — 4150 + 1000 — 6 (5) S. zunasi Korea 75-142 1-3 8800-58800 — 31 (6) Herrings Herklotsich thys uadnmaculatus Hawaii 80-121 — 1155-6296 160-311 46 (7) Marshall Is. 100 + 2 0.59 ± 0.02 4755 ± 380 — 7 (8) Butaritari (K) 75 ± 1 0.45 ± 0.01 1844 ± 108 295.5 ± 12.1 44 (1) Abaiang (K) 75 ± 1 0.45 ± 0.02 1975 ± 133 317.4 ± 19.3 27 (1) Tarawa (K) 76 ± 1 0.44 ± 0.02 2353 ± 110 344.1 ± 10.2 63 (1) Abemama (K) 84 ± 2 0.61 ± 0.04 3008 ± 207 319.1 ± 22.7 33 (1) Andaman Is. (I) 95-115 — 8353 ± - — 19 (9) Seychelles 88-127 — 4500-8000 — 24 (10) Opisthonema libertate Mexico 142 ± 1 — 57125 + 1850 553 ± 14 115 (11) Sprats Spratelloides dehcatul us Butaritari ( K) 52 ± 2 0.27 ± 0.01 1359 ± 143 867 ± 55 1') (1) Abaiang (K) 52 ± 1 0.21 ± 0.02 973 ± 43 667 + 35 7 (1) Tarawa (K) 54 ± 1 0.29 ± 0.01 1255 ± 54 735 ± 25 49 (1) Abemama (K) 41 ± 1 0.20 ± 0.02 524 + 95 702 ± 75 12 (1) Munda (SI) 48 ± 1 0.26 ± 0.01 799 ± 45 554 ± 25 57 (12) Vona Vona (SI) 49 ± 1 0.26 ± 0.01 925 ± 102 717 ± 45 28 (12) Tulagi (SI) 46 ± 1 0.21 ± 0.01 926 ± 93 567 ± 49 28 (12) New Caledonia 45 — 710 883 ± 14 20 (2) India 40 ± 3 — 608 ± 54 — 15 (13) S. gracilis Munda (SI) 50 0.19 514 504 1 (14) Vona Vona (SI) 37 ± 1 0.15 + 0.01 505 ± 51 882 ± 68 13 (12) P.N.G. 53 ± 2 — 2592 ± 313 1690 ± 96 18 (15) Maldives 59 ± 1 0.29 ± 0.02 1998 ± 137 1073 ± 54 33 (12) India 40 + 5 — 790 ± 71 962 ± 53 If. (13) S. lewisi Munda (SI) 44 ± 1 0.18 + 0.01 887 + 20 925 ± 16 219 (14) Vona Vona (SI) 42 ± 1 0.14 ± 0.01 930 ± 51 1032 + 36 62 (14) Tulagi (SI) 49 ± 1 0.28 ± 0.02 1290 ± 84 1230 ± 69 29 (14) Sprattus sprattus Scotland (UK) 108 3 2729 187 64 (16) Baltic Sea (SU) 121 1.9 2174 232 46 (17) North Sea (G) — 2 — 413 — (17) Sources: (li present study, (21 Conand (1988), (3) Conar d (1991), (4i Isaac-Nahum et al. (1988). (5) Nakamura and Wilson (19701, (6) Gil and Lee (19861. (7) Williams and Clarke (1983), (8) Hid a and Uchiyama (1977), (9) Manchamy (1971), (10) Moussac ar d Poupon (1986), (11) Torres-Villegas and Perezgomez (1988), (121 Miltor et al. (1990). (13i Mohan and Kunhikoya (1986), (14) Milton unpubl. data. 1 15i Dalzell (19851, (16) De Silva (1973), (17i Alheit (1988). Although we found A. sirm also had an extended spawning season in Kiribati, the species may not spawn throughout the year. Our result differs from previous studies that found the spawning season lasted two to five months during early summer (Conand, 1991) or the monsoon period (Rosa and Laevastu, 1960; Dayaratne and Gjosaeter, 1986). Neither temperature nor rainfall appear to be the Milton et al.: Reproductive biology and egg production of three species of Clupeidae I 13 proximate stimuli for spawning of A. sirm in Kiribati. Tempera- ture was constant throughout the year and rainfall was higher at all sites in Kiribati between December and April, when spawning activity was lowest. Most spawning activity in this species occurred during the second half of the year when the prevailing wind di- rection changed from east to west, associated with the north- west monsoon that starts at this time (Burgess, 1987 5 ). Our limited wind and rainfall data did not indicate that increased spawning activity in A. sirm was related to the shift in weather pattern. Gonad maturation and spawning were also linked to changes in fish liver-weight (HSI), visceral fat, and condi- tion of each species. Either HSI or fat index and condition were all significantly reduced in postspawning fish. Amblygas- ter sirm stores energy in the viscera rather than in the liver. Other multiple-spawning clup- eoids also transfer energy from stored fat to reproductive tis- sue (Dahlberg, 1969; Okera, 1974; Hunter and Leong, 1981). In contrast, spent H. quadrimaculatus and S. delieatulus had reduced HSI, which suggests that the liver is the energy store utilized during repro- duction (Diana and MacKay, 1979; Smith et al., 1990). Energy stored in this organ would be readily available for rapid assimilation; hence, fish could spawn multiple batches of eggs rapidly. Studies of temperate herring, Clupea harengus, have shown that gonad maturation is linked to food availability and fat storage (Linko et al., 1985; Henderson and Almatar, 1989; Rajasilta, 1992). Ovaries of all three species in Kiribati and of S. delieatulus in the Solomon Islands (Milton and Blaber, 1991) vary in a similar way to herring. Milton and Blaber (1991) did not find a direct rela- tion between spawning and prey availability. This suggests that while gonad maturation in these clu- 5 Burgess, S. M. 1987. The climate of western Kiribati. New Zealand Meterological Service, Wellington, NZ. Miscellaneous publ. 188, part 7. Table 6 Stepwise regression of the re lationship between various endogenous factors and fish fecundity from sites in Kiribati. 200 Length (mm) 100 <30 40 50 60 70 80 90 >90 Length (mm) 100 BO 60 - 40 20 - C <30 35 40 45 50 55 60 >60 Length (mm) Figure 6 Ontogenetic change in the proportion female of (A) Amblygaster sirm , (B) Herklotsichthys quad- rimaculatus and (C) Spratelloides delicatulus (±95"7r confidence limits) from Kiribati. peids is probably linked to cycles in prey abundance, fat storage may reduce the effects of short-term fluc- tuations in prey abundance on reproduction. Diel timing of spawning events was similar for all species. We found new post-ovulatory follicles (day- 0) in females collected from 2130 hours onwards with the greatest proportion detected after 0100. This indicates that these species spawn during the early part of the night, probably prior to midnight. Our results are consistent with previous studies that found high densities of A. sirm eggs in the plank- ton after midnight (Delsman, 1926; Lazarus, 1987). Studies of other sardines (Goldberg et al., 1984; Isaac-Nahum et al., 1988; Re et al., 1988) and tropi- . TO Q j'f'm'a'm'j'j'a's'o'n'd 1989 Avr j'f'm'a'm'jjVs'o' 1990 Time o n c i ND J FMAMJ J ASOND J FMAMJ J ASONCJj FMAM 1989 1990 1991 Time t"| i i i i i i i i i i i i n i i i i i n i I i i i ii ND J FMAMJ J ASOND J FMAMJ J ASOND J FMAM 1989 1990 1991 Time Figure 7 Monthly estimates of daily egg production of (A) Amblygaster sirm, (B) Herklotsichthys quadrima- culatus and (C) Spratelloides delicatulus from Kiribati during the study period. cal clupeoids (Clarke, 1987) also showed that spawn- ing peaked before midnight. Length and age at sexual maturity of A. sirm and H. quadrimaculatus in Kiribati differed from those Milton et al.: Reproductive biology and egg production of three species of Clupeidae 1 15 Tab e 8 Mean reproductive life spa n (in days) and days between spawning of Amblygaster sirm, Herklotsichthys quadrimaculatus, and Spra telloides delicatulus from four sites in Kiribati (TV = number of length- frequency samples; No. = number of months examined). Days Max. lifetime Reproductive between egg production Species Site Year life span ± SE Range N spawning Range No. (> 10 4 l A. sirm Abaiang 1989-1990 19.0 + 6.4 0-66 L2 20.0 Tarawa 1989-1990 60.1 ± 15.4 0-127 7 41.6 Abemama 1989-1990 3.2 + 3.1 0-19 6 Overall 1989-1990 26.7 ± 6.8 0-127 25 6.2 ± 2 3 1.5-25.9 10 38/ H. quadrimaculatus Butaritari 1989-1991 47.3 + 15.0 0-201 14 11.9 Abaiang 1989-1991 73.9 + 15.6 0-201 17 12.8 Tarawa 1976/83/89-91 84.1 + 7.8 0-254 64 19.3 Abemama 1989-1991 141.8 + 30.9 0-286 12 27.7 Overall 1989-1991 80.6 ± 8.6 0-286 74 3.1 ± 0.3 1.3-4.7 15 21.1 S. delicatulus Butaritari 1989-1991 53.6 + 4.6 24-74 15 1.9 Abaiang 1989-1991 49.2 ± 5.1 21-80 11 1.5 Tarawa 1989-1991 66.9 + 10.6 0-144 16 3.5 Tarawa 1976 76.6 ± 10.5 45-129 7 3.1 Tarawa 1983/84 84.3 ± 9.5 34-152 16 3.7 all 1989 90.0 + 13.4 53-144 7 3.2 all 1990/91 51.0 ± 4.1 0-109 35 2.5 Overall 1989-1991 57.5 ± 4.6 0-144 42 5.2 ± 1.8 1-30 16 3.2 20 1989 (n = 717) III (%) c Recruitme o I III l.llllllll.l J FMAMJ JASOND Time Figure 8 The proportion of Amblygaster sirm (±95% confidence limits 1 sampled between August 1989 and July 1990 born each month in 1989, backcalculated from length-frequency samples. in other parts of their range (Table 3). We found few differences within Kiribati, but both species became sexually mature and spawned at much shorter body lengths than at other locations. Herklotsichthys quadrimaculatus did not grow as large in Kiribati as elsewhere (Milton et al., 1993). but the propor- tion of maximum size at which this species matured was similar throughout its range. Milton and Blaber ( 1991) found regional differences in length at sexual maturity in other small tropical clupeoids; they sug- gested these differences were consistent with the hypothesis of Longhurst and Pauly (1987) that fish of any species living in cooler water will grow to and mature at a larger size through the interaction of oxygen supply and demand. Our data on H. quadrimaculatus is consistent with this hypothesis — the other studies were all at sites at higher lati- tudes than Kiribati, where the water temperature is lower. Also, the proportion of maximum size at which fish matured was similar at all locations, despite the absolute differences in size at maturity in Kiribati. By comparison, A. sirm matured at a smaller size and grew to a larger size in Kiribati than at other locations (Milton et al., 1993). The proportion of maximum size at which fish matured was also lower than found in previous studies and was less than the proportion common to a wide range of clupeoids (70%; Beverton, 1963). In response to severe fishing pressure, the size and age at sexual maturity of several sardine species have been found to decline (Murphy, 1977). Presumably, this is because any density-dependent effects are reduced during early growth (Beverton and Holt. 1957; Ware, 1980). Amblygaster sirm can have high or variable adult mortality in Kiribati (Rawlinson et al., 1992), favouring early maturation (Stearns and Crandall, 1984). Length at first spawning was a similar proportion of maximum size for the three species and was con- ] 16 Fishery Bulletin 92(1), 1994 50 1976 40 (n = 13131) 30 20 -| 10 ■■■■-■■..■II 50 | 1983 40 (n= 1705) 30 ? 20 b 10 1 ° ll_«j 5 50 1989 o £40 (n = 4745) 30 20 m 10 ___ll...llll 50 1990 40 (n = 2747) 30 20 : laHlll-l. J FMAMJ JASOND Time Figure 9 The proportion of Herklotsichthys quad- rimaculatus born each month in 1976, 1983, 1989, and 1990, back-calculated from length- frequency samples (95'/ confidence limits of all proportions are all less than 1.5%). 30 1976 • (n = 3002) 20 10 ..lllll — ll 30 | 1983 (n = 9198) 20 I- lent (%) o o l...lll. .1 fc 5 30 _ 1989 (n = 2014) 01 20 l| 10 III ■■lllll— 30 -i 1990 (n = 16419) 20 10 o- .llllllllll. J FMAMJ JASOND Time Figure 10 The proportion of Spratelloides delicatulus born each month in 1976, 1983, 1989, and 1990, back-calculated from length-frequency samples I95'i confidence limits of all propor- tions are all less than 1.59f ). sistent with the close relation with maximum size found by Blaxter and Hunter (1982) for other clupeoids. These authors also noted a latitudinal effect; fish from lower latitudes spawned at a smaller proportion of maximum size. Temperate clupeids (especially herrings, Clupea spp.) show a great plasticity in the number and size of eggs produced; many species show seasonal, and inter-annual, as well as geographic, variation in their reproductive outputs (Alheit, 1989; Jennings and Beverton, 1991) reflecting energetic resources and environmental conditions (Hay and Brett, 1988; Henderson and Almatar, 1989). By comparison, the tropical herring, H. quadrimaculatus, spawned throughout the year and showed negligible tempo- ral or spatial variation in fecundity, egg weight, or inter-spawning interval. This indicates that egg production was almost constant throughout the study period and suggests that adult food resources and larval survival are predictable or relatively con- stant (Sibly and Calow, 1983). In comparison to other species, S. delicatulus had a higher relative fecundity that was also correlated with HSI. Females in spawning condition also had a higher HSI at Butaritari. Commercial CPUE was highest at this site (Rawlinson et al., 1992) and S. delicatulus spawned more, smaller eggs than at other sites where relative fecundity was lower. These data suggest that the fecundity of S. delicatulus may be influenced by the amount of energy stored in the liver. This energy store would be important in a small multiple-spawning species; it would enable the fish to continue spawning dur- ing short periods of reduced food supply (Hay and Brett, 1988). The length of the inter-spawning in- terval has been shown experimentally to be related Milton et al.: Reproductive biology and egg production of three species of Clupeidae I / to food supply in other fish species (Townshend and Wootton, 1984). Fish at Butaritari may experience a more predictable environment that enables them to produce more eggs of smaller size than fish in more variable environments. In contrast, A. sirm delayed spawning beyond the size and age at sexual maturity and did not spawn until one year old. As fecundity was related to weight, delayed spawning enabled A. sirm to grow faster than the other species (Milton et al., 1993) and have a higher batch fecundity when spawning started. Murphy (1968) hypothesized that delayed spawning and longer reproductive life span would evolve in response to variable reproductive success. However, Armstrong and Shelton (1990) demon- strated that, even with a short reproductive life span, multiple spawners had a high probability of successful reproduction when subject to random environmental fluctuations over time. Thus, delay- ing spawning would be of adaptive advantage if mortality was low (Roff, 1984) because batch fecun- dity and lifetime egg production would be increased. Our estimates of the reproductive lifespan of A. sirm indicate that this species spawns fewer times in their lifetime than other species and thus would also have less chance of successful spawning than other species. Given that this is the longest-lived of the species examined, our estimate of overall mean lifespan may be biased by the small number of months sampled. Large fish may be under-repre- sented in small catches and may contribute to un- derestimating the reproductive potential of A. sirm. Herklotsichthys quadrimaculatus had a longer reproductive life span and spawned more frequently than did the other species. Reproductive life span varied little among sites (except Abemama) and there was no significant temporal variation, which suggests that survival rates of large adult H. quadrimaculatus are fairly constant in Kiribati. This is reflected in their life-history parameters, which varied little among sites or over time. In con- trast, the frequency distribution of back-calculated birthdates indicated that overall survival was vari- able both between and within years, and was not related to monthly egg production. We have no es- timates of adult abundance during the study period, and so population egg production could not be as- sessed. However, the annual CPUE and abundance of H. quadrimaculatus in the baitfishery were simi- lar in the three years for which both data sets were available (Rawlinson et al., 1992). This suggests that population size was relatively constant during this period. If so, then variation in post-hatching survival probably has an important influence on recruitment in this species (Smith, 1985). The reproductive life span of the smallest species, S. delicatulus, was intermediate between the other species and varied little among sites during 1989 and 1990. Unlike H. quadrimaculatus, the reproduc- tive life span of S. delicatulus varied between years, which suggests that survival rates are not as con- stant or as predictable as those of H. quadrimaculatus. Potential lifetime egg production of each female was only one tenth that of other spe- cies, but, because of the larger number of females, monthly estimates of daily egg production were higher. The distribution of back-calculated birthdates varied between years, but a greater pro- portion of births fell in May-August, irrespective of che pattern of egg production. Annual CPUE of S. delicatulus (Rawlinson et al., 1992) was similar in 1989 and 1990, which suggests that fishing mortal- ity had not contributed to the increased mortality that reduced the reproductive life span in 1990. The reproduction and abundance of S. delicatulus may be more directly influenced by its environment than are the other species. Adult survival is vari- able and low (Tiroba et al., 1990); egg production varies, probably in response to food supply, and sur- vival to recruitment is unpredictable. Yet the poten- tial for successful reproduction with this strategy may still be relatively high (Armstrong and Shelton, 1990). In contrast, H. quadrimaculatus appears to be able to offset environmental variability to produce a relatively constant supply of eggs. The distribution pattern of back-calculated birthdates of each species was not consistent among species. Months when a higher proportion survived differed for each species during all years; months with highest mean survival were not the same for any species. This suggests that the effects of envi- ronmental conditions such as seasonal food avail- ability or favorable physical conditions are not the same for each species. Alternatively, other factors such as predation (Rawlinson et al., 1992) may have greater influence on survival to recruitment. Egg production by S. delicatulus was positively corre- lated to survival rates in 1989 and negatively cor- related in 1990. This seems unrelated to fish abun- dance as catch rates were higher in 1989 than in 1990 (Rawlinson et al., 1992). Large variations in recruitment, reflected in catch rates of the main baitfishes do not appear to be di- rectly linked with variations in egg production. All spawn in the lagoon for most of the year, and dis- tribution of birthdates indicated recruitment in most months. Although the absolute level of recruitment varied throughout the year, multiple spawning re- duces fluctuations in population size due to environ- mental variability and should ensure that relatively 18 Fishery Bulletin 92(1), 1994 stable population sizes are maintained. Earlier stud- ies of A. sirm and H. quadrimaculatus in Tarawa lagoon suggested that these species spend at least part of their life outside the lagoon (R. Cross, 1978 4 ; McCarthy, 1985 1 ). If this is the case, fluctuations in the relative abundances of these species may be re- lated to migrations; a better understanding of the fac- tors causing large-scale movements is necessary before predicting the potential yield of this fishery. Acknowledgments We thank staff of the Kiribati Fisheries Division for assistance with fieldwork during the project. 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J. 1987. Reproductive biology of three species of 1990. The reproductive strategy of Stolephorus midwater fishes associated with the continental heterolobus in the south Java Sea. In S. J. M. slope of eastern Tasmania, Australia. Mar. Biol. Blaber and J. W. Copland (eds.), Tuna baitfish in 95:323-332. Abstract. Determination of stock structure for striped dol- phins (Stenella coeruleoalba) in the eastern Pacific has been prob- lematic, because very few speci- mens have been available for study. We compared length data obtained from vertical aerial pho- tographs of 28 schools of striped dolphins from the northern and southern regions of the eastern tropical Pacific and found no sig- nificant differences in average length for adult animals (> 180cm) or for adult females, defined here as dolphins closely accompanied by a calf. Analyses of back-pro- jected birth dates for dolphins >155cm revealed a broad pulse in reproduction extending from the fall through the spring; however, sample size was inadequate to compare timing of reproduction between the two areas. Striped dolphins measured from aerial photographs were longer on aver- age than those killed incidentally in fishing operations. We found a pattern of segregation by size be- tween schools that is analogous to the separate schools of juveniles and adults that are found in the western Pacific. We hypothesized that the specimen data base may be biased because tuna purse- seine fishermen in the eastern tropical Pacific may selectively set on schools composed of younger, smaller dolphins. Examination of stock and school structure of striped dolphin (Stenella coeruleoalba) in the eastern Pacific from aerial photogrammetry Wayne L. Perryman Morgan S. Lynn Southwest Fisheries Science Center National Marine Fisheries Service. NOAA 8604 La Jolla Shores Drive. La Jolla. Calif 92037 Manuscript accepted 20 September 1993 Fishery Bulletin 92:122-131 (1994) Because striped dolphins, Stenella coeruleoalba, are killed incidentally in purse-seine fishing for yellowfin tuna in the eastern tropical Pacific (ETP), the National Marine Fisher- ies Service (NMFS) is required by the Marine Mammal Protection Act (as amended in 1988) to monitor trends in their abundance (Holt and Sexton, 1989; Wade and Gerrodette, in press). To satisfy this congressional mandate, infor- mation on stock structure is re- quired. The determination of stock structure for striped dolphins in the ETP has been particularly dif- ficult because of the small number of animals killed in the tuna fish- ery and, therefore, small number of specimens available for study (DeMaster et al., 1992). In the ab- sence of morphological, life history, or genetic data to provide evidence of reproductive isolation, stocks of striped dolphins have been identi- fied provisionally based on discontinuities in distribution. With more sighting data from ob- servers aboard fishing vessels and research cruises, the number of proposed stocks has decreased from five or six (Smith, 1979 1 ; Holt and Powers, 1982) to one (Dizon et al., in press) pending availability of additional data. For this report, we examined length data to help clarify the issue of stock structure. These data were extracted from vertical aerial pho- tographs collected during line transect surveys and are thus pre- sumably free of any "sampling" bi- ases associated with the fishery. Here, we compare length samples from aerial photographs of animals from the northern and southern stock regions proposed by Perrin et al. (1985) for evidence of differences in average length or timing of re- production. Data were then com- pared with measurements avail- able from specimens killed inciden- tally in purse-seine fishing. We also examined the frequency distribu- tion of lengths within individual schools. These data were used to test for size-age segregation, as reported for dolphins taken in the drive fishery on the Pacific coast of Japan (Miyazaki, 1977; Miyazaki and Nishiwaki, 1978). Methods Length measurements were made on vertical aerial photographs of 28 schools of striped dolphins (Fig. 1). We photographed the schools with a KA-45A military reconnaissance 1 Smith, T. D. (ed). 1979. Report of the sta- tus of porpoise stocks workshop; 27-31 August, La Jolla, California. U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Southwest Fish. Sci. Cent, P.O. Box 271, La Jolla, CA 92038. Admin Rep., L.l-79- 41, 120 p. 122 Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 123 30' 20' 10' 10' 20' 160° W 150° 120° 1 10" 90» 80° W Figure 1 Distribution of schools of striped dolphin, Stenella coeruleoalba, (dark circles), from which data were taken for this report. Boundaries for northern and southern stocks were taken from Perrin et al. 1985. camera mounted below the fuselage of a Hughes 500D helicopter that was launched from the NOAA Ship David Starr Jordan. This photographic sam- pling was part of a long-term research effort con- ducted by NMFS to monitor trends in abundance of dolphin populations in the ETP (Holt and Sexton, 1989; Wade and Gerrodette, in press). The reconnaissance camera was equipped with a very fast, medium focal length lens (152 mm) and a forward image motion compensation system that eliminated the blur normally found in images taken from a low altitude, high-speed platform. We used Kodak Plus-X Aerecon II (thin-base) film, exposed through a medium yellow filter, throughout the experi- ment. This filter significantly reduced the amount of blue light reaching the film, thus enhancing both the contrast and resolution of our photographs. The observer sitting in the right front seat of the helicopter triggered the camera, controlled cycle rate and shutter speed, and adjusted the forward motion compensation system. As each firing pulse was sent to the camera, a data acquisition system recorded the time that the image was captured and an alti- tude reading from the helicopter's radar altimeter. To check for accuracy in our recorded altitude data (A J, we photographed calibration target arrays and compared altitude calculated from measurements of these known distances with recorded altitude (see Perryman and Lynn, 1993). We found a consistent bias in A r and used the lin- ear regression equation shown below to calculate a corrected altitude (AJ for each photograph used in this report. A c = (A r ) 1.013 - 33.755 (r 2 = 0.993) . Length determination We reviewed the images of 88 schools of striped dolphins photographed from 1987 through 1990 and selected the images of 28 schools that provided the best combination of image clarity and water pen- etration. From this sample, we selected the photo- graphic pass over each school that captured the larg- est number of dolphins swimming parallel to and 124 Fishery Bulletin 92(1), 1994 very near the surface. Dolphins were not measured if either the rostrum or tail flukes were not clearly visible or if they were surfacing, diving, or jumping, which would make them appear shorter when viewed from above. Because there was from 80 to 90% overlap between adjacent photographs, the same dolphin could often be measured in two to four photographs. If more than one length was available for a dolphin, the largest length was selected, as- suming it was the best determination of true length. This helped to minimize the reduction in apparent length caused by the normal swimming movements of the dolphins (Scott and Perryman, 1991; Perryman and Lynn, 1993). We measured each dolphin from the tip of the rostrum to the trailing edge of the tail flukes (Fig. 2). These points were selected because the fluke notch that is used to determine standard length (Norris, 1961) was very difficult to see in most of the images. For adult specimens, this measurement should exceed standard length by 2-2.5 cm (Chivers, 1993 2 ). The measurements were made on sections of the original black and white negatives that we captured with a high-resolution video camera and transferred to a Macintosh Ilci computer. Image enhancement and length measurements were made with the aid of the digital image processing and analysis program, Image (version 1.37), which was developed by the National Institute of Health (W. Rasband, Research Services, Bethesda, Maryland). The length of each dolphin was determined by mul- tiplying its length on the image by the scale of the photograph ( scale= A/lens focal length ). Data analysis Perrin et al. (1985) compared the mean lengths of physiologically adult male and female dolphins from 2 S. Chivers. 1993. Southwest Fisheries Science Center, La Jolla, California 92037. unpubl. data. ■Photo Length- putative geographic stocks of several species to pro- vide supporting morphological evidence for repro- ductive isolation. For our analyses, we used length as the criteria for eliminating the youngest dolphins from our sample. Based on the length data for adult striped dolphins in Perrin et al. (1985) and a review of our length sample, we estimated that the mini- mum length for adult female striped dolphins in the eastern Pacific is about 180 cm. We used this length as our first cut-off point, and tested for differences U-test) between the means of our length samples (<180 cm) from the northern and southern regions (Fig. 1). Since the selection of this value was some- what arbitrary, we repeated the tests on data sets with minimum values of 185 and 190 cm. Based on behavioral arguments reviewed in Per- ryman and Lynn ( 1993), we assumed that the larger dolphin swimming closely alongside a calf was an adult female. Since this determination was based on behavior and not on examination of sexual charac- ters, we qualify the term in quotation marks, "adult female," whenever we are referring to a length sample based on this assumption. A <-test was used to compare the mean lengths of "adult females" from the northern and southern regions. We also per- formed a power analysis to determine what range of differences between means we could expect to detect (probability of type II error < 0.10) for this analysis and the ones described in the paragraph above. Calf birth dates We examined the length data from striped dolphins estimated to be one year old or less for evidence of pulses in reproduction (see Barlow 1 1984], for spot- ted and spinner dolphins; Perryman and Lynn (1993], for common dolphins). Ninety centimeters was used as the best estimate of average length at birth and 155 cm for average length at one year for striped dolphins in the eastern Pacific (Gurevich and Stewart, 1979 ;i ). We as- sumed postnatal growth was linear during the first year and back-projected the birth dates for all dolphins <155 cm in length. Our goal here was not to determine the ex- Photo Length ■Standard Length- Figure 2 Illustration of the difference between points used to determine standard length and length as measured from our vertical photographs. 1 Gurevich, V. S., and B. S. Stewart. 1979. A study of growth and re- production of the striped dolphin iStenella coeruleoalba). U.S. Dep. Commer., NOAA, Natl. Mar. Fish. Serv., Southwest Fish. Sci. Cent., P.O. Box 271, La Jolla, CA 92038. Final Rep to NOAA, SWFC Con- tract 03-78-D27-1079, 29 p. Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 125 act date of birth for each dolphin but rather to exam- ine the distribution of birth dates, based on the same assumptions, from the two regions. We used Kupier's modification of Kolmogorov's test for com- parisons of circular distributions (Batschelet, 1965) to compare the calculated distribution of birth dates with a uniform distribution. Comparisons with specimen data We conducted four tests to compare the sample of photogrammetric lengths with data collected from striped dolphins killed incidentally in purse-seine fishing in the ETP (Perrin et al., 1976). The data from specimens included the information published by Perrin et al. ( 1985) and a small set of data from dolphins killed since 1985. T-tests were used to com- pare the mean length of "adult females" with the mean length of adult female specimens and with the mean length of lactating adult female specimens. We also compared the mean (f-test) and shape (Kolmogorov-Smirnov test) of the photogrammet- rically determined length distribution of striped dolphins > 180 cm with data from specimens > 180 cm in length. School structure Examination of the structure of schools of striped dolphins captured in the drive fishery in Japan has revealed a distinct pattern of segregation based on sex, maturity, and length (Miyazaki, 1977, 1984; Miyazaki and Nishiwaki, 1978). Researchers have categorized these schools as adult, juvenile, or mixed depending on the proportion of juvenile dolphins (excluding calves) captured. In these studies, length (<174 cm) or age (<1.5 years) was used as the crite- rion for eliminating nursing calves from the sample; the remainder of the dolphins was determined to be juvenile or adult by direct examination of the go- nads. We examined the length distributions for the pho- tographed schools to see if an analogous pattern of segregation in schools from the eastern Pacific was detectable. We divided our samples into two length categories which we labeled juvenile or adult. The minimum length for the juvenile category was set at 165 cm to eliminate nursing calves as described above. We selected this minimum value because 1) length at birth for striped dolphins from the ETP is apparently about 10 cm shorter than that reported from the western Pacific (Miyazaki, 1977; Gurevich and Stewart, 1979 3 ), and we assumed that the dif- ference in the average length at weaning was ap- proximately the same; 2) dolphins larger than 165- 170 cm in length were very rarely found swimming in the characteristic cow/calf configuration we see in our photographs. We selected 195 cm as the upper bound for the juvenile category because this appears to be about the minimum size for adult male striped dolphins that have been killed in the ETP tuna purse-seine fishery (Perrin et al., 1985). This value was keyed to male length data because the studies of school structure from Japan indicated that a disproportion- ate number of the dolphins captured in juvenile schools were males (Miyazaki and Nishiwaki, 1978). Thus dolphins in each school were categorized as juvenile if they were between 165 and 195 cm in length and as adult if they were > 195 cm in length. The goal in this classification scheme was to create one category that would be composed of mostly ju- venile and young adult dolphins and another that would include mostly adult animals. We used chi-square analysis to test the hypoth- esis that the number of dolphins in the two catego- ries in our schools was independent of school. For this analysis, we eliminated schools from which we had measured less than 20% of the school or fewer than 17 dolphins. The second criterion was estab- lished to minimize the number of predicted values in the chi-square analysis that were less than five. Application of these criteria reduced our sample to 21 schools for this test. Because the selection of 195 cm for the cut-off between the two size categories probably includes more adult females in the juve- nile category than males, we decreased the limit to 190 cm and repeated the chi-square test. We also conducted a regression analysis to determine whether the proportion of the measured sample in the juvenile category was related to school size. With the exception of the power analyses and birth date comparison which were done by hand, all tests presented in this report were performed with the program StatView developed by Abacus Con- cepts (Berkeley, CA). Unless noted otherwise, tests were considered significant for P values < 0.05. Results Regional comparisons We compared the average length of striped dolphins from the northern and southern regions and found no significant differences between the samples (Table 1; Fig. 3). In tests for differences in mean lengths of "adult females" (Fig. 4), no differences were found between the regions. Although none of the differences was significant, means of the 126 Fishery Bulletin 92(1), 1994 samples from the northern region were generally a few centimeters smaller than those from the south, a pattern reported by Perrin et al. (1985). This level of difference was less than we could detect given the available sample and the variability of our data Table 1 Results of r-tests for differences between means of length samples from striped dolhin, Stenella coerueoalba, from the northern (Nor) and south- ern (So) regions. Sub-sample (cm) n Nor/So mean (cm) Nor/So P (2-tailed) >180 >185 >190 "Adult females" 160/251 154/484 140/450 19/63 205.1/205.9 206.07207. 7 207.9/209.2 200.2/204.0 0.476 0.138 0.230 0.201 30 25 • 20 • S 15 H o 10 - Northern Region n = 202 I I i I I M tk 80 100 120 140 160 180 200 220 240 260 Length (cm) 80 70 60 50 40 30 20 10 o Southern Region n = 616 ': m n p-i r-f ^=^ 1 40 160 180 200 Length (cm) Figure 3 Distribution of lengths of striped dolphins, Stella coeruleoalba, measured from the northern and southern regions. (Table 2). With this length sample, it appears that we can expect to detect differences between means that differ by at least 4 cm. Table 2 Minimum detectable diffe rences between means for ^-tests for samples from striped dolphins, Stenella coerureoalba, from the northern (Nor) and Southern (So) regions. Beta error set at 0.10. Minimum Variance detectable Sub-sample (cm) Nor/So lvalue difference (cm) >180 164.99 190.11 1.963 4.01 >185 148.23 162.59 1.964 3.82 >190 122.21 141.94 1.964 3.72 "Adult females" 53.61 147.57 1.292 9.63 8 "Adult Females" - n = 19 — Northern Region 7 - b 5 - 4 - 3 - 2 1 n 140 150 160 180 190 200 Length (cm) 210 220 230 240 "Adult Females" n = 63 - Southern Region 140 150 160 170 180 190 200 210 220 230 240 Length (cm) Figure 4 Distribution of lengths of "adult females", defined here as stroped dolphins, Stenella coeruleoalba, closely associated with a calf, measured from the northern and southern regions. Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 127 The sample from the northern region was too small to test for a seasonal pat- tern in reproduction, but the distribution of back-projected births from the south- ern region differed significantly from the uniform distribution (P<0.01; Figs. 5 and 6). Reproduction for striped dolphins from the southern region appears to be broadly pulsed in the fall through spring period. Photogrammetric and specimen data Since significant differences between length samples from the northern and southern regions could not be detected, we pooled length data from the two re- gions in the tests that follow. We found that "adult females" were significantly longer (4.8 cm) on average than adult females from the specimen data base. When the test was repeated by using length data for lactating females from the specimen data base, the two samples no longer differed significantly (Table 3). Striped dolphins > 180 cm in length from the photogrammetric sample were sig- nificantly longer on average than the sample based on the same length crite- ria from specimen data. We also per- formed a Kolmogorov-Smironov test to compare the two distributions (Fig. 7) and found that they differed signifi- cantly (P<0.01). Northern Region 6 - 5 R . R fl . H , . R fl Southern Region 1 m i i! lis. o =1 < Northern and Southern Region ,fl, .B RTOlfc i Figure 5 Distribution of back-projected birth dates for striped dolphins, Stenella coeruleoalba, from the northern and southern regions and for the two regions combined. Table 3 Results of comparisons between means of length data for striped dolphins, Stenella coerueoalba, taken from specimens (spec) and aerial photo- graphs (photo) (f-tests), and the distribution of lengths >180cm (Kolmogorov-Smirnov \k and si test) from these two sources. n Mean (cml P Comparison spec/photo spec/photo (2-tailed) Adult females specimen/photo 50/82 198.2/203.0 0.007 Lactating specimens/ "adult females" 23/82 199.8.203.0 0.202 > 180cm f-test 256/681 199.19/205.73 0.0001 >180cm h and s 256/681 Z=3.378 0.0007 School size and structure We performed a chi-square test to determine whe- ther the number of dolphins in our two size catego- ries were distributed randomly between schools (Fig. 8) and the hypothesis was significantly rejected when the maximum length for the juvenile category was 195 or 190 cm (P<0.001). With a maximum value of 190 cm, four expected values generated by the test were lower than five. When these schools were deleted from the test or lumped with adjacent schools to eliminate these low expected values, the test results remained highly significant. When school size was regressed against propor- tion in the juvenile category, the slope of the regres- sion was not significantly different from zero. Thus, in our sample, the proportion of small dolphins in a school was not related to school size. 128 Fishery Bulletin 92(1). 1994 100% r 90% 80% 70% \ qj 13 ST 60% \ LL 50% 1 > TO 40% 3 3 30% O 20% 10% - 0% ? Birth Months Figure 6 Cumulative distribution of back-projected striped dolphin, Stenella coeruleoalba, birth dates (solid squares) and those predicted by a uni- form distribution of births (open squares). D □ Length (cm) Figure 7 Length-frequency distributions for specimens of striped dolphin Stenuella coeruleoalba, (> 180 cm) taken incidentally in purse-seine fishing in the eastern tropical Pacific and striped dolphins sampled photogrammetrically that are > 180 cm. Samples from northern and southern regions are combined in this figure. Discussion We found no significant differences in our length samples of striped dolphins from the northern and southern regions to support a recommendation that they be managed as separate stocks. This must be tempered by the fact that length differences of a scale not detectable in our sample, i.e. < 4 cm, could exist. The case for two stocks is also weakened by the distribution of sightings of this species from re- cent research vessel surveys (Wade and Gerrodette, in press). These data indicate that, al- though a hiatus in striped dolphin distribution exists in the typically tropical (high temperature, low salinity) inshore habitat centered around lat. 15° N, there appears to be a broad avenue for movement between the northern and southern regions in the upwelling modified habitat east of long. 110° West (Au and Perryman, 1985; Reilly, 1990). When we compared our sample of lengths for "adult females" and dolphins > 180 cm with data from specimens killed incidentally in purse-seine fishing, we found that the means from the photogram- metric sample were significantly larger (by about 3-6 cm). This does not seem unreasonable at first glance because our measure- ments to the trailing edge of the flukes rather than to the fluke notch introduces a positive bias in the photogrammetric data of about 2-2.5 cm. Also, the "adult female" category probably in- cludes only those females who have carried and given birth to a live calf, thus eliminating the younger, presumably smaller, fe- males who are physiologically adult but have not yet had a suc- cessful pregnancy. However, these results for adult females are con- trary to previous comparisons of photographic and specimen data for northern and central common dolphins (Perryman and Lynn, 1993) and eastern spinner dol- phins (Perryman, unpubl. data). Since the photogrammetric data for all of these taxa were collected in the same manner, it seems likely that the difference between the two striped dolphin samples reflects some form of selectivity in either or both sampling systems. The schools of striped dolphins that we photo- graphed showed a pattern of segregation by length Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 129 > O z LU o in rr 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 JO 25 20 15 - 10 5 35 30 25 20 15 10 5 35 30 25 20 ' 15 10 5 SCHOOL 1 School Size - 79 No. Measured - 26 J3- M, " i XL SCHOOL 2 School Size - 222 No. Measured = 46 -E3- SCHOOL 3 School Size - 73 No Measured - 30 n SCHOOL 4 School Size - 1 73 No Measured - 16 1 l , P. . -^ SCHOOL 5 School Size - 87 No. Measured = 33 rrm n i IT SCHOOL 6 School Size = 1 S1 No. Measured ■= 42 m* SCHOOL 7 School Size - 25 No Measured = 9 1 " n 100 120 140 160 18 200 220 240 SCHOOL 8 School Size - 56 No Measured - 29 SCHOOL 9 School Size - 86 No. Measured - 54 . rv r^V t Length cated le LENGTH (CM) Figure frequencies for each school of striped dolphi ngths of dolphins that were included in the SCHOOL 10 School Size = 23 No Measured = 1 7 SCHOOL 11 School Size = 100 No. Measured - 10 SCHOOL 12 School Size - 54 No Measured = 29 la SCHOOL 14 School Size - 46 No Measured « 30 ra , n , Jfl W SCHOOL 13 School Size- 124 fh .*.m im» & m_ 100 120 140 180 200 220 240 LENGTH (CM) 8 ns, Stenella coeruleoalba. Shaded bars indi juvenile categeory. that is very similar to that reported from the west- ern Pacific (Miyazaki, 1977; Miyazaki and Nishiwaki, 1978). It also appears that the propor- tion of smaller dolphins in our sample of schools is not related to school size. Possibly this segregation is the explanation for differences between specimen and photogrammetric data sets. Tuna fishermen select dolphin schools for encircle- ment based mainly on the amount of tuna associ- ated with the school. Schools of younger/smaller 130 Fishery Bulletin 92(1), 1994 > o z LU ZD o UJ rr LL 35 30 25 20 15 10 5 15 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 SCHOOL 15 School Size- 125 No Measured 60 SCHOOL 16 School Size - 59 No. Measured = 1 4 ll .■■ I'll SCHOOL 17 School Size. 100 No Measured. 51 P I (71 fT-n MyPI^H 1 In SCHOOL 18 School Size - 95 No Measured - 25 ■itti 1 SCHOOL 19 School Size - 88 No Measuied - 55 it SCHOOL 20 School Size = 10 No Measured • 5 SCHOOL 21 School Size = 30 No Measured - 9 140 160 180 200 220 240 LENGTH (CM) 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 35 JO 25 20 15 10 5 35 30 25 20 15 10 5 35 30 25 20 15 10 5 SCHOOL 22 School Size - 40 No Measured - 10 SCHOOL 23 School Size- 175 No Measured - 66 t>.cwx SCHOOL 24 School Size - 58 No Measured - 16 Jl l j SCHOOL 25 School Size - 76 R No. Measured = 36 mi m~ ~ h SCHOOL 26 School Size - 48 No Measured - 30 XI J3L im SCHOOL 27 School Size - 23 No Measured . 9 S i HI SCHOOL 28 School Size - 44 No Measured = 21 JGL 100 120 140 160 180 200 220 240 LENGTH (CM) Figure 8 (Continued) striped dolphins might carry more tuna and be cap- tured more frequently than schools composed of adult animals. If the bond between yellowfin tuna and dolphins is related to size and hydrodynamics as suggested by Edwards i 1992) then it may be that the smaller striped dolphins are hydrodynamically more suitable for this association. Juvenile schools of striped dolphins are made up of animals that are about the same length as schools of spotted or spin- ner dolphins for which the tuna-dolphin association appears to be the strongest. Perryman and Lynn: Stock and school structure of Stenella coeruleoalba 131 Acknowledgments A. E. Dizon, D. P. DeMaster, W. F. Perrin, and two anonymous reviewers read the manuscript and pro- vided very useful suggestions. Valuable assistance and specimen data were provided by S. Chivers. Sev- eral of the photographs for this analysis were taken by J. Gilpatrick and R. Westlake. This work would have not been possible without the field support of the Officers and Crew of the NOAA Ship David Starr Jordan and the pilots and mechanics of NOAA's Aircraft Operations Center. Literature cited Au, D. W. K., and W. L. Perryman. 1985. Dolphin habitats in the eastern tropical Pacific. Fish. Bull. 83:623-643. Batschelet, E. 1965. Statistical methods for the analysis of prob- lems in animal orientation and certain biological rhythms. Am. Inst. Biol. Sci. Monograph, 57 p. Barlow, J. 1984. Reproductive seasonality in pelagic dolphins (Stenella spp.): implications for measuring rates. Rep. Int. Whaling Comm. Spec. Issue 6:191-198. DeMaster, D. P., E. F. Edwards, P. Wade, and J. E. Sisson. 1992. Status of dolphin stocks in the eastern tropi- cal Pacific. In D. R. McCullough and R. H. Barrett (eds.). Wildlife 2001: populations, p. 1038-1050. Elsevier Science Pubis., New York. Dizon, A. E., W. F. Perrin, and P. A. Akin. In press. Stocks of dolphins (Stenella spp. and Del- phinus delphis) in the eastern tropical Pacific: a phylogeographic classification. NOAA Tech. Rep. Edwards, E. F. 1992. Energetics of associated tunas and dolphins in the eastern tropical Pacific Ocean: a basis for the bond. Fish. Bull. 90:678-690. Holt, R. S., and J. E. Powers. 1982. Abundance estimation of dolphin stocks in the eastern tropical Pacific yellowfin tuna fishery deter- mined from aerial and ship surveys to 1979. NOAA Tech. Mem. NMFS SWFSC 23, 95 p. Holt, R. S., and S. N. Sexton. 1989. Monitoring trends in dolphin abundance in the eastern tropical Pacific using research vessels over a long sampling period: analyses of 1986 data, the first year. Fish. Bull. 88:105-111. Miyazaki, N. 1977. School structure of Stenella coeruleoalba. Rep. Int. Whaling Comm. 27:498- 499. 1984. Further analysis of reproduction in the striped dolphin, Stenella coerueoalba, off the Pa- cific coast of Japan. Rep. Int. Whaling Comm. Spec. Issue 6:343-353. Miyazaki, N., and M. Nishiwaki. 1978. School structure of the striped dolphin off the Pacific Coast of Japan. Scientific Reports of the Whales Research Institute. 30:65-115. Norris K. S. (ed.) 1961. Standardized methods for measuring and re- cording data on smaller cetaceans. J. Mammal. 42:471-476. Perrin, W. F., J. M. Coe, and J. R. Zweifel. 1976. Growth and reproduction of the spotted por- poise, Stenella attenuata, in the offshore eastern tropical Pacific. Fish. Bull. 74:229-269. Perrin, W. F., M. D. Scott, G. J. Walker, and V. L. Cass. 1985. Review of the geographical stocks of tropical dolphins (Stenella spp. and Delphinus delphis) in the eastern Pacific. NOAA Tech. Rep. NMFS 28. 28 p. Perryman, W. L, and M. Lynn. 1993. Identification of geographic forms of common dolphin (Delphinus delphis) from aerial photogrammetry. Mar. Mammal Sci. 9:119-137. Reilly, S. B. 1990. Seasonal changes in distribution and habitat differences among dolphins in the eastern tropi- cal Pacific. Mar. Ecol. Prog. Ser. 66:1-11. Scott, M. D., and W. L. Perryman. 1991. Using aerial photogrammetry to study dolphin school structure. In K. Pryor and K. S. Norris, (eds.), Dolphin societies-discoveries and puzzles, p. 227-241. Univ. California Press, Berkeley. Wade, P. R., and T. Gerrodette. In press. Estimates of cetacean abundance and distribution in the eastern tropical Pacific. Rep. Int. Whaling Comm. 43. Abstract. — The eastern Pa- cific purse-seine tuna fishery has historically been very productive, yielding up to 400,000 metric tons (t) per year of primarily yellowfin, Thunnus albacares, and skipjack, Katsuwonus pelamis. However, ef- forts to minimize dolphin (prima- rily spotted dolphin, Stenella attenuata, spinner dolphin, S. longirostris, and common dolphin, Delphinus delphis) mortality inci- dental to tuna seining in the east- ern Pacific ocean have been in- creasing. Therefore, predictions of what the tuna catches will be in the future, if there is a ban or moratorium on catching dolphin- associated tuna, are useful. Based on recruitment levels, age-specific catchability coefficients for yellow- fin tuna caught without dolphins, and average fishing effort ob- served during 1980-88, we pre- dicted that yellowfin catches would be reduced by an average of about 25%. These results were verified by Monte Carlo simula- tions, by using average effort and randomly selected yellowfin re- cruitment and catchability coeffi- cients from 1980 to 1988, which predicted a mean annual decrease of 55,563 t or 24.7% of yellowfin catch. The actual reduction in yel- lowfin catch might be greater be- cause 1) fishing effort will prob- ably decline, 2) the range of the fishery might be reduced to the traditional inshore non-dolphin regions, and 3) yellowfin recruit- ment could be reduced by the change in age structure and popu- lation size likely to result from a moratorium. Because skipjack sel- dom associate with dolphins, redi- rection of fishing effort to schools of tuna not associated with dol- phins would probably result in in- creased skipjack catch rates. How- ever, the magnitude of the in- crease is difficult to estimate, be- cause the population dynamics of skipjack are poorly understood. Finally, this study predicted that the catches in the first years after a moratorium on dolphin sets would not necessarily reflect long- term catches. Potential tuna catches in the eastern Pacific Ocean from schools not associated with dolphins Richard G. Punsly Patrick K. Tomlinson Ashley J. Mullen Inter-American Tropical Tuna Commission 8604 La Jolla Shores Dr. La Jolla. CA 92037 Manuscript accepted 22 July 1993 Fishery Bulletin 92:132-143 (1994) Since the late 1950's, purse-seine fishermen in the eastern Pacific Ocean (EPO), knowing that schools of yellowfin tuna (Thunnus alba- cares) often associate with dolphins (primarily spotted dolphins, Sten- ella attenuata, spinner dolphins, S. longirostris, and common dolphins, Delphinus delphis), have used the dolphins to help locate and capture yellowfin. Dolphins are relatively easy to detect, being larger and closer to the surface than yellowfin. In fact, the most efficient means of catching the 2- and 3-year-old yel- lowfin, which comprise the largest component of the tuna catch in the EPO, is purse-seine fishing for dol- phin associated schools (Punsly and Deriso, 1991). Yellowfin remain associated with dolphins while the net is being set around the dolphin herds. The fishermen attempt to release all of the dolphins from the net; however, incidental mortality sometimes occurs through entang- lement. As a result of increasing public pressure to prevent mortality of dolphins incidental to tuna purse seining, elimination of setting on dolphin-associated tunas is being considered. Therefore, fishermen, biologists, and managers need to know the extent to which tuna catch in the EPO might be reduced by the elimination of sets on dol- phin-associated fish. The objective of this study was to estimate this potential reduction in the catch. No such estimates have been pub- lished previously. Tuna catches could be affected by a ban or moratorium on dolphin sets in six ways: 1 The overall catchability of yel- lowfin by purse seiners could be reduced. 2 The yield per recruit of yellow- fin could decline because non- dolphin-associated yellowfin caught by purse seiners are mostly composed of fish younger than the optimum age of entry (Calkins, 1965; Allen, 1981). 3 The average age of yellowfin and mean biomass may be reduced by fishing on younger age groups. This might not only re- duce the catch in weight, but also reduce the spawning poten- tial and possibly the resulting recruitment. 4 Since the offshore EPO purse- seine fishery is directed prima- rily at dolphin-associated fish (Fig. 1, A and B), a moratorium on setting on dolphin herds could result in a contraction of the range of the fishery into in- shore regions. The number of fish recruited to this new smaller area might be lower than the number recruited to the entire area. Lower effective re- cruitment would also result in lower catches. 5 If a moratorium on catching dol- phin-associated tuna occurs, 132 Punsly et al.: Potential non-dolphin-associated tuna catches in the eastern Pacific Ocean 133 some purse-seine fishermen may decide to move to other oceans or retire, which would reduce total fishing effort and hence the catch. 6 Since skipjack tuna (Katsuwo- nus pelamis), the only other primary target species in the fishery, seldom associate with dolphins, their catch may in- crease if effort remains at 1980-88 levels and is directed only toward tuna schools not associated with dolphins. Because no relation between spawners and recruitment of yel- lowfin has been established (Bayliff, 1992, p. 62), the possible effects of reduced recruitment were not addressed in this study. Also, since the authors cannot predict how many seiners would leave the EPO, or how much the fishery would contract, these two factors were not considered. In other words, this study only at- tempted to estimate how much tuna catches might change due to changes in yellowfin catchability, yield per recruit, total biomass, and age structure. To measure the possible effects of changing the mode of fishing from being directed toward prima- rily dolphin-associated schools of tuna ("dolphin sets," Allen, 1981) to one directed at exclusively free- swimming schools ("school sets") and floating-object-associated schools ("log sets," Greenblatt, 1979), we first estimated what the tuna catches would have been in previous years if dolphin sets had been replaced by non-dolphin sets. Then the estimates were compared with actual catches. Our method used non-dolphin-set catchability coefficients and total effort to estimate what the catches would have been during 1980-88 if there had been a mora- torium on dolphin sets beginning in 1980. Other works in which catches were estimated for alter- Figure 1 (A) Geographic distribution of average yellowfin tuna (Thunnus albacares) catch by purse seiners, during 1980-88, from schools associ- ated with dolphins (Delphinidae). Catches are expressed in metric tons by 2.5-degree quadrangles. (B) Geographic distribution of average yel- lowfin catch by purse seiners, during 1980-88, from schools not associ- ated with dolphins. Catches are expressed in metric tons by 2.5-degree quadrangles. 134 Fishery Bulletin 92(1), 1994 1 -10 Boat-Days CD 11 -50 Boat-Days J\ 51 - 100 Boat-Days 101 -200 Boat-Days 201 - 500 Boat-Days Greater than 500 Boat-Days Figure 2 (A) Geographic distribution of total purse-seiner fishing effort during 1980-88 which lead to dolphin (Delphinidae) sets. Effort levels are ex- pressed in boat-days of fishing by 2.5-degree quadrangles. (B) Geo- graphic distribution of total purse-seiner fishing effort during 1980-88 which lead to non-dolphin sets. Effort levels are expressed in boat-days of fishing by 2.5-degree quadrangles. native catchability coefficients include Holt ( 1958), Jones ( 196 1 ), and Bartoo and Coan (1978). Materials and methods Data The Inter-American Tropical Tuna Commission's (IATTC) logbook and length-frequency data bases were used in this study. The log- book data base, described in Or- ange and Calkins (1981), Punsly (1983; with emphasis on set types), and Punsly (1987; with emphasis on yellowfin catch rates), contains information on the fishing activities of about 90^ of the purse seiners in the EPO. Total catches were estimated by multiplying the logbook catches by the ratio of the sum of the un- loading weights to the sum of the logbook catches. Geographic dis- tributions of the logbook data on catch and effort, during 1980-88, for both dolphin-associated and unassociated schools are shown in Figures 1 and 2. The length-fre- quency data base, described by Hennemuth (1957). Punsly and Deriso (1991), and Tomlinson et al. (1992), has information from samples of about 12-15^ of the catch. Age-specific yellowfin abun- dances from cohort analysis (Pope, 1972; also called sequential computation of stock size in Ricker, 1975; and virtual popula- tion analysis in Gulland. 1965) were taken from Bayliff ( 1990). Data from 1980 to 19S8 were used in this study. Data before 1980 were not used because of the difficulty in modeling the closed seasons for yellowfin (Cole, 1980). Data after 1988 were not used be- cause cohort analysis cannot pro- duce accurate abundance esti- mates for cohorts which have not been in the fishery for a sufficient period of time. Semi-annual age groups used in this study were described in detail Punsly et al.: Potential non-dolphm-associated tuna catches in the eastern Pacific Ocean 135 in Bayliff (1992, p. 52). Monthly age compositions were estimated by combining 1-cm length-interval data into semi-annual age groups by fitting multinormal distributions to the data with the aid of the computer program NORMSEP, (Abramson, 1971), and constraining the fit to the growth param- eters of Wild (1986). "X" and "Y" cohorts were de- fined as those fish reaching 30 cm, which correspond to the approximate age of first recruitment, during the fourth and second quarters of the year, respec- tively. Age groups in our study, 0.5 to 5.5 in 0.5 year increments, correspond to the Y0, XI, Yl ... Y5 co- horts, respectively, in Table 21 of Bayliff (1992). Estimates of fishing effort The total monthly effort by purse seiners was esti- mated as E = f Y l\ om J om om I .' om ' where o, refers to the observed mixture of set types, Y is the yellowfin catch unloaded by purse sein- ers in month (m),y om is the yellowfin catch reported in the IATTC logbooks and f is the effort, in boat- days of fishing, reported in the logbooks. Effort on non- dolphin sets for all purse seiners was estimated by ^nm / , / .lnmcs^omcs/y v„ where f nmcs is the fishing effort which lead to non- dolphin (n) sets by monitored vessels of size (s) from country (c), Y omcs is the total catch of yellowfin from unloadings by size (s) vessels from country (c), and y is the total yellowfin catch by monitored ves- J omcs J "* sels. These estimates were stratified by country and size of vessel because the proportion of dolphin sets is affected by these two factors. Estimation of yellowfin catches if all effort were non-dolphin This method used age-specific, monthly catchability coefficients by fishing mode and allowed the future population structure to be affected by previous catches. First, age-specific catchability coefficients for non- dolphin sets in) in each month (m) were estimated for each semi-annual age group (/'): Qnmj ~ ^nmj y^nm^ mj J i where C are the monthly, total, non-dolphin purse-seine catches (in numbers of fish) of semi- annual age group (j) and N mj are the age-specific, monthly, average abundances estimated by the co- hort analysis (Bayliff, 1990). Beginning with the population structure in January 1980, obtained from cohort analysis, we estimated what the catch in each month of each semi-annual age group would have been without dolphin sets; i.e., pmj (N mj q nmj E om )/(q nm] E om + Mj ) where Mj is the age-specific, instantaneous, monthly natural mortality (Bayliff, 1992, p. 52). Yield in weight was estimated by Y =W (i)C , 1 pmj m y J ' pmj ' where W(j) is the estimated mean weight of age (j) yellowfin in month m caught during 1980-88. The subsequent month's abundance of semi-annual age group (j) was estimated to be Estimates of skipjack catches if all effort were non-dolphin Skipjack are suspected to be mostly transient in the EPO (Joseph and Calkins, 1969), so we assumed that depletion is probably unimportant. Thus, the ratio of the total effort to the non-dolphin effort was used to estimate skipjack catches: Y pm (SJ): Y nm (SJ)E om /E nn where Y m (SJ) is the potential (p) non-dolphin, skip- jack catch and Y nm (SJ) is the actual non-dolphin-set, skipjack catch. In essence, skipjack catches were estimated to be linear extrapolations of catch rates to higher levels of effort. £*m+Xj mj -iq nm .E om+ M,) except for the months of recruitment (May and January), when N JAN2 and N MA y, 3 were set e Q ual to the historical recruitment previously estimated for that time period by cohort analysis. Yellowfin form the first semi-annual age group (those fish hatched in the middle of the current year) were not included in the analysis because they were not recruited until the next year, when they became semi-annual age group 3. Each January, the semi-annual age groups were graduated as follows: N JANJ+2 =N DEC. J iaDEi ' +M, 136 Fishery Bulletin 92(1). 1994 Monte Carlo simulation The age-structure method produced catches specific to the observed time-series of recruitment and age- specific catchability coefficients during 1980- 88. Additional information can be gained by estimating what the trend in catches would be if the recruit- ment and catchability trends were different. In or- der to explore the range of resulting catches which might have occurred under various conditions, a Monte Carlo simulation was used. Paired simula- tions were performed for both the observed mixed- mode fishery and a fishery in which all effort was directed toward non-dolphin-associated tuna. Fre- quency distributions of differences between catches from the two simulated fisheries provide a more comprehensive estimate of future expectations. The simulations used quarterly time steps and 1,000 replicates. At each quarter of each year in each replicate, a year between 1980 and 1988 was ran- domly selected with replacement (i.e., each year could be selected more than once). Pairs of quarterly catchability coefficients (one from the observed mix- ture of fishing modes and one for the non-dolphin sets only) estimated for the corresponding year, were used in the calculations during the time steps. Quar- terly coefficients were calculated with the same equation as that for the monthly coefficients with months replaced by quarters. Quarterly fishing ef- forts were set to the 1980-88 averages. The same average total effort was applied to both the observed and non-dolphin fishing-mode models. Recruitment was simulated to occur in the second and fourth quarter. For each year in each simula- tion, a randomly selected year was chosen. Recruit- ment pairs (X and Y) from the randomly selected year were used for both fishery models. Initial popu- lation sizes and age structures were also set to the 1980-88 averages. One thousand differences between the simulated catches for the mixed- mode and non-dolphin only scenarios were generated for a time series of nine years. The 95% confidence intervals corresponded to the 50th and 950th highest differences from the 1,000 simulations. Because yellowfin usually live for less than 5 years (Fig. 3), results for the last (9th) year were unaffected by the initial age structure. Results Deterministic approach If trends in total effort, recruitment, and non-dol- phin-set catchability coefficients had been the same as during 1980-88, with all effort directed at non- dolphin sets, yellowfin catches (Table 1, column Table 1 Estimated annual tuna (Scombridae) catches by purse seiners in the eastern Pacific ocean, in thousands of metric tons. Year OYF NYF QYF OSJ NSJ OT QT NT 1980 170 129 158 131 155 301 313 284 1981 190 152 146 120 151 310 297 303 1982 134 111 120 99 129 233 249 240 1983 104 96 98 58 73 162 171 169 1984 155 103 125 HI 90 215 215 193 1985 227 132 169 49 99 276 268 231 1986 286 193 168 64 113 350 281 305 1987 285 243 195 62 120 347 314 363 1988 303 266 229 85 123 388 352 389 Mean 206 158 156 81 117 286 274 275 OYF NYF yellowfin tuna iThunnus albacares) - observed mixture of set types. yellowfin tuna - all effort directed at non-dolphin ( Delphimdae) sets, using the observed monthly catchability coefficients for non-dolphin sets. QYF = yellowfin tuna - all effort directed at non-dolphin sets, using the average, observed, quarterly catchability co- efficients for non-dolphin sets. OSJ = skipjack tuna {Katsuwonus pelamis) - observed mixture of set types. skipjack tuna - all effort directed at non-dolphin sets, yellowfin plus skipjack tuna - observed mixture of set types. yellowfin plus skipjack tuna - effort directed at non-dol- phin sets, using quarterly average catchability coeffi- cients. yellowfin plus skipjack tuna - all effort directed at non- dolphin sets, using monthly catchability coefficients. NSJ = OT = QT = NT NYF) were estimated to have averaged 77% of the observed catch (Table 1, column OYF). The range was from 58% in 1985 when dolphin-associated tuna fishing was good to 93% in 1983 when dolphin-as- sociated tuna fishing was poor. The reasons why the ratio of estimated catch without dolphin sets to the observed catch varied annually can be seen in Fig- ures 3-7. For example, the high estimated bio- masses of 1.5-year-old yellowfin in 1988 (Fig. 4), coupled with their high non-dolphin-set catchability coefficients (Fig. 5), produced an estimated catch of 266,000 t for all effort directed at non-dolphin sets, which was almost as high as the 303,000 t catch estimated from the catchability coefficients for the observed mixture of set types (Fig. 6). Catchabilities could have increased in 1988 for a variety of reasons, including the use of deeper nets, the use of "bird radar" (relatively new radar used for detecting birds which commonly have tuna beneath them) or envi- ronmental factors, such as a shoaling of the ther- mocline (Green, 1967). For a given level of effort, catches depended on the age-specific abundances (Figs. 3 and 4) and catchability coefficients (Figs. 5 and 6). Consequently, the estimated catches if all effort were directed at non-dolphin sets approached Punsly et al.: Potential non-dolphin-associated tuna catches in the eastern Pacific Ocean 137 1 - y Vi < Z 21 1 — i — r - 1 — i — i — i t i i i '■rff Rtk \aL ~K_ ; '■■All TK : : -^z TK '■ '■ H~ Ht^ r-| -n-n-^ : -r^TTTTT^_ 1982 1981 1980 Figure 3 Estimated average annual biomasses (t) of yellowfin tuna (Thunnus albacares) by semi-annual age group for the observed mixture of set types. In the left panel, biomasses are summarized by age within year. In the right panel, biomasses are summarized by year within age group. Age refers to the age in years at the middle of the year. -nSl - V) -T. < z rrrihTh-w JH r-TTfl ITr-^- r^TTTl^. 1988 1986 1984 1983 1982 1981 1980 VI < Z 3 -1 1 T 1 1 - : rrr^r4~m : rr>^rn HI r^m nrrrrrj I I I I 1 I I - AGE 5.5 AGE 5 . C AGE 4 . 5 AGE 4 . C AGE 3.5 AGE 1 . AGE 0.5 Figure 4 Estimated average annual biomasses (t) of yellowfin tuna (Thunnus albacares) by semi-annual age group for all effort directed at non-dolphin (Delphinidae) sets. In the left panel, biomasses are summarized by age within year. In the right panel, biomasses are summarized by year within age group. Age refers to the age in years at the middle of the year. Fishery Bulletin 92(1), 1994 J 0. JTw-lhTr - 1987 ^-TTH rTfTH^ : r^Vn-rr-ITln ^TH^rrT~r-^l ' rT^-r^-fT^l 1980 _□=. rm-vj rThmT> TTT 1:JJlHJJ1l. rT-mll-r n,^ .□L AGE 5 . 5 AGE 5 . AGE 4 . 5 AGE 4 . AGE 3 . 5 Figure 5 Average annual non-dolphin-set yellowfin tuna (Thunnus albacares) catchability coefficients (q in boat-days - ^) by semi-annual age group. In the left panel, coefficients are summarized by age within year. In the right panel, coefficients are summarized by year within age group. Annual catchability coefficients are estimated as the mean of the monthly coefficients. Age re- fers to the age in years at the middle of the year. =r£ ^^fn-u -mW^ ■j-ri~H—i 1983 1982 1981 1980 JZL oiEt£^d rffl FT-fTM I rh : -■■i '' I ' I AGE 5 . 5 AGE 5 . C AGE 3 . AGE 2 . 5 AGE 2 . AGE 1 . 5 AGE 1 . AGE . 5 Figure 6 Average annual observed yellowfin tuna (Thunnus albacares) catchability coef- ficients ( rfff Ti n i ^-T -1 — ' — ' h - >•— 1 — i — i ' l ' l ' l ' J l Ctbi . 1988 1987 1986 1985 1984 1983 1982 1981 1980 — i — i — i — i — r- IZI B- I ! T I n^Tr-r^ o. Oil □£l TI - AGE 4 . r^ OZL TTT-rL J~ r— r ~1 1 1 I~~I 1 1 ! T- AGE 5.5 AGE 5.0 AGE 3.5 AGE 3 . AGE 2.5 AGE 2 . AGE 1 . 5 AGE 1.0 AGE . 5 Figure 7 Average annual differences between the observed and non-dolphin (Delphinidae) catchability coefficients (boat-days -1 ). In the left panel, differences are summa- rized by age within year. In the right panel, differences are summarized by year within age group. Age refers to the age in years at the middle of the year. Nega- tive values (those pointing down) indicate that those non-dolphin-set catchability coefficients were greater than the observed coefficients. the observed levels when the non-dolphin-set catchability coefficients were greater than or equal to the observed overall catchability coefficients (Fig. 7, negative values) for the age groups of the greatest biomass (Figs. 3 and 4). Estimated total yel- lowfin plus skipjack catches, if all effort were di- rected at non-dolphin sets, ranged from 84% during 1985 to 104% in 1983. Estimates (Table 1, column QYF) of what the catches would have been without dolphin sets, us- ing the quarterly average (over years) non-dolphin- set catchability coefficients for 1980-88, indicate that yellowfin catchabilities on non-dolphin sets increased in the late 1980's. Average quarterly catchability coefficients produced noticeably higher catches than the observed non-dolphin-set monthly coefficients in 1983-85 when the observed coeffi- cients on small fish were low. On the other hand, average quarterly catchability coefficients produced lower catches during 1986-88, when the observed non-dolphin-set coefficients were high. Monte Carlo simulation The Monte Carlo simulations (Table 2) predicted that, if total effort, recruitment, and non-dolphin-set catchability coefficients had varied randomly throughout their 1980-88 distributions, and current levels of effort and recruitment had been main- tained, changing to a fishery with all effort directed toward non-dolphin sets would have resulted in an average reduction of 55,563 t (24.7%) of yellowfin catch per year. The 95% confidence interval, based on the 50th and 950th highest simulated differences was 24,000 to 91,000 t (10%-42%). The entire fre- quency distribution of the differences between the two fishing-mode models in the 9th year is shown in Figure 8. Simulated recruitment estimates were selected from the observed values during 1980-88. Thus, average recruitment used in the simulations was higher than the mean actual recruitment to the initial 1980 population structure, which was partly a result of the poor recruitment during 1978 and 1979. Consequently, simulated catches were higher for both the observed mixed mode fishery (229,000 t per year) and the non-dolphin-set only fishery (175,000 t). Yield per recruit Estimated yellowfin catches from both the determin- istic approach (Table 1) and the Monte Carlo simu- 140 Fishery Bulletin 92(1), 1994 ■j. a. Oj IX o o o r-i E c >> u c OJ 3 CT U L- 250 200 150 100 50 50 60 70 80 90 100 110 Yellowfin Catch Retained (%) Figure 8 Frequency distribution of the percent of the yel- lowfin tuna (Thunnus albacares) catch in weight retained in the ninth year of a ban on dolphin (Delphinidae) sets, from 1,000 Monte Carlo simu- lated replicates. The percent of catch retained is calculated as 100x (the catch if all effort were directed at non-dolphin sets) / (the catch from the observed mixture of set types). Table 2 Monte-Carlo simulated annual yellowfin tu na (Th / n n u s albacares) catch es, in thousands of tons, from 1980- -88, quarter y, average catchability coefficien ts. YEAR OYFM OYFL OYFU NYFM NYFL NYFU PCM PCL PCU 1 202 184 229 150 121 186 72 ill S3 2 215 183 247 164 127 205 76 62 89 3 227 183 276 170 129 217 76 59 91 4 229 183 283 175 131 223 71) 68 ss 5 236 188 2H6 181 139 230 76 r,!i 91 6 245 195 302 187 138 241 76 ill 91 7 237 ISO 294 180 138 229 75 59 90 8 231 182 281 176 134 228 76 58 91 9 229 1S1 279 174 131 222 75 58 90 OYFM = mean veilowfin tuna catch *or the observed mix ;ure of se t tvpes. OYFL = OYFM Iowei 959! confidence interval. OYFU = OYFM upper 959! confidence interval NYFM = mean yellowfin tuna catch using total effort and non-dolphin catchability coeffi- cients. NYFL = NYFM lower 95% confidence interval NYFU = NYFM upper 959c confidence interval PCM = mean percent of c atch retained 1 100 x NYFM/OYF: PCX = PCM 1 awer 95% confidence interval. PCU = PCM l pper 95% confidence interval. lations (Table 2) were heavily influenced by the re- cruitment and fishing effort levels used. Recruit- ment in the future may be different from that of past, because of changes in population size, age structure, and environmental factors. Therefore, actual future catches could be different from what we estimated. For these reasons, results in terms of reduction in yield per recruit are of interest. We estimated that the change to non-dolphin sets only would result in the reduction of the yield per recruit of yellowfin from the observed value of 2.8 kg per recruit to 2.1 kg as shown in Figure 9. In addition, effort levels could change in the future, perhaps as a reaction to the moratorium. Therefore, estimates of yield per recruit for various levels of effort might be useful. If effort levels change in the future, the multipliers on the X-axis in Fig. 9 could be used to estimate the potential yellowfin catch. Discussion In order to predict what the tuna catches might be in the future if there were a moratorium on dolphin sets, we estimated what the tuna catches would have been during 1980-88, had there been a mora- torium on dolphin sets beginning in 1980. Using these estimates to predict future catches required the following assumptions: 1 Age-specific, non-dolphin catchability coefficients will be the same in the future as during 1980-88. 2 Fishing effort will remain at 1980-88 levels. 3 The geographic distribution of effort will be the same as during 1980-1988 (Fig. 2, A and B combined). 4 Recruitment will be at 1980-88 levels. 5 Natural mortality will not change in the future. 6 Skipjack abundance will not significantly change. Significant deviations from these assumptions could make our estimates less valid. There- fore, the potential ramifications of deviations from the assump- tions are discussed in detail below. Major changes in the vulner- ability of non-dolphin-associ- Punsly et al.: Potential non-dolphin-associated tuna catches in the eastern Pacific Ocean 141 *: 8 OC 1.5 S a. ■o O 1.0 ated yellowfin to purse seiners could result in significantly different catches than we estimated. Allen and Punsly (1984) showed that both environmental and vessel efficiency factors affect the catchability of yellowfin by purse sein- ers in the EPO. Improvements in vessel efficiency could increase future catchability coefficients; whereas, envi- ronmental factors could produce either higher or lower catchability coefficients than those observed during 1980-88. Environmental factors affecting catchability could conceivably mask the effects of a moratorium on dolphin sets for several years. For example, if a moratorium on dolphin sets had been imposed at the beginning of 1983, the low catch in 1983 would have made it appear that the decline resulted from the moratorium. However, we predicted that a moratorium would have had the smallest effect in 1983 (Table 1). Fish- ermen, biologists, and managers should be aware that catches during the first year after a moratorium starts may not be indicative of long- term averages. However, since 9 years of data were used, our long-term average estimates should only be affected by long-term changes in catchability. An assumption that effort will be lower in the future may be more realistic than our assumption that effort will remain at 1980-88 levels. However, we could not predict the extent to which effort might be reduced because it is affected by ex-vessel tuna prices at canneries all over the world, the prices of other foods, and the cost of fuel. Nevertheless, if we could estimate what the effort reductions would be in the future, the effort multipliers in the the yield- per-recruit estimates in Fig. 9 could still be used. If the fishery contracted into the traditional in- shore school- and log-set areas after a moratorium on dolphin sets, then catches may be lower than we estimated them to be. For example, if the area fished were smaller, and mixing between the fish inside and outside the area were incomplete, then the new fishing area would encompass fewer fish than the total area. Therefore, all of the population sizes of yellowfin used in the equations in the methods sec- tion would be overestimated. Recruitment estimates, which are estimates of the number of 30-cm yellow- fin, would also be overestimated. In addition, if fish- ing effort remained high, but the range contracted, then a gear- competition effect might lower the catch of both yellowfin and skipjack. However, since effort levels are expected to decline after a moratorium, j I l I I I i Observed Mixture of Fishing Modes No Dolphin Sets 1 I I I L 0.2 0.4 O.e 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 Effort Multiplier Figure 9 Yield per recruit of yellowfin tuna (Thunnus albacares) (kg) for the observed 1980-88 fishery (solid line) and a fishery with all effort directed at non-dolphin (Delphinidae) sets (dashed line). An effort multiplier of 1.0 refers to the 1980-88 average effort. localized depletion of tuna due to a contracted fish- ery is unlikely. We assumed that yellowfin recruitment in the future would not be affected by the changes in popu- lation size and age structure which might result from re-directing effort toward smaller fish, because a relationship between yellowfin spawning biomass and recruitment has not yet been demonstrated. However, a spawner-recruit relationship for yellow- fin may be discovered in the future, because better estimates of yellowfin fecundity by size offish, sea- son, and area are currently being developed at IATTC. When this work is completed we may be able to predict recruitment levels and their resulting catches more accurately in the future. If future re- cruitment levels could be estimated, the future catches could be derived by multiplying the recruitment esti- mates by the yield per recruit shown in Figure 9. Environmental factors have long been suspected of having significant effects on yellowfin recruit- ment. For example, favorable conditions in the late 1980's may have contributed to the large number of recruits (Bayliff, 1992). In 1987, the number of re- cruits was so large that the effect of a moratorium in 1988 would have been masked by a high catch of 1.5 year old yellowfin, first recruited during 1987. In 1988, the high abundance of 1.5 year old fish (Fig. 4) coupled with their high catchability for non-dol- phin sets (Fig. 5) caused the estimated yellowfin catch if all effort were directed at non-dolphin sets to be almost as high as the estimated actual catch. 142 Fishery Bulletin 92(1). 1994 In order to predict future recruitment, the IATTC is currently studying the relationship between the environment and yellowfin recruitment. If they are successful the yield-per-recruit estimates in Figure 9 could be multiplied by the recruitment estimates to better predict future yellowfin catches. Little is known about the rate of natural mortal- ity of yellowfin. However, there is no reason to be- lieve this rate will change. But, if it does change, a reasonable assumption would be that if natural mortality goes up, catch will go down and vice versa. Little is known about skipjack population dynam- ics. We assumed that local depletion is negligible for skipjack. However, since skipjack are primarily caught in association with floating objects, if the amount of effort per floating object increases as a result of effort being re-directed from dolphin-asso- ciated tunas to floating objects, then the chances of depletion is certainly possible. If this occurs, our estimates of skipjack catch rates will be too high. This effect could be compounded during years in which floating objects are scarce, because the num- ber of sets per floating object would increase. Since the skipjack catches have been increasing in the west- ern Pacific Ocean, their abundance and catch in the eastern Pacific could be lower than our estimates. A moratorium on dolphin sets is likely to result in reduced catchability, yield per recruit, average age, and total biomass of yellowfin. The catch of yellowfin, based on these factors only, was predicted to decline by approximately 55,600 t (25%). On the other hand, skipjack catches could increase, making the reduction in total tuna catches much smaller (4%). The effects of reductions in fishing effort, the range of the fishery, and recruitment were not ana- lyzed in this study because they are currently un- predictable; however, all three would result in an additional decrease in total tuna catches. If better predictions of effort levels and yellowfin recruitment are made, the yield-per-recruit estimates in Figure 9 could be used in conjunction with them to better predict yellowfin catches. The results of our analy- sis indicate that catches in the first years after a moratorium begins may not be indicative of the long- term catches. Fishermen, biologists, and managers should not consider these first-year catches as indices of future catches, because recruitment and catchability vary annually. On the other hand, our estimates of future average catches should be useful unless there are long-term changes in catchability or recruitment. Acknowledgments We would like to thank James Joseph, director of investigations of the Inter- American Tropical Tuna Commission for suggesting the need for this re- search, Richard B. Deriso for his many methodologi- cal suggestions, Alejandro Anganuzzi for his reviews and for sharing his knowledge about about dolphins, and William H. Bayliff for his extensive editorial re- views of this manuscript. Literature cited Abramson, N. J. 1971. Computer programs for fish stock assessment. F.A.O. Fish. Tech. Pap. 101, 154 p. Allen, R. L. 1981. Dolphins and the purse-seine fishery for yel- lowfin tuna. I.A.T.T.C. Int. Rep. 16, 23 p. Allen, R. L., and R. G. Punsly. 1984. Catch rates as indices of abundance of yellow- fin tuna, Thunnus albacares, in the eastern Pacific Ocean. I.A.T.T.C. Bull. 18(4):301-379. Bartoo, N. W., and A. L. Coan. 1978. 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Sea CM. 1965(3), 9 p. Hennemuth, R. C. 1957. Analysis of methods of sampling to determine the size composition of commercial landings of yel- lowfin tuna (Neothunnus rnacropterus) and skip- jack (Katsuwonus pelamis). I.A.T.T.C. Bull. 2(5):174-243. Punsly et al.. Potential non-dolphin-associated tuna catches in the eastern Pacific Ocean 143 Holt, S. J. 1958. A note on the simple assessment of a pro- posal for mesh regultaion. I. C.N. A. F. Annual Proc. 8(4):82-83. Jones, R. 1961. The assesment of the long term effects of changes in gear selectivity and fishing effort. Mar. Res. Scot. 1961(2), 19 p. Joseph, J., and T. P. Calkins. 1969. Population dynamics of the skipjack tuna (Katsuwonis pelamis) of the eastern Pacific Ocean. I.A.T.T.C. Bull. 13(1), 273 p. Orange, C. J., and T. P. Calkins. 1981. Geographical distribution of yellowfin and skipjack tuna catches in the eastern Pacific Ocean, and fleet and total catch statistics 1975- 1978. I.A.T.T.C. Bull. 18(1):1-120. Pope, J. G. 1972. An investigation of the accuracy of virtual population analysis using cohort analysis. Int. Comm. Northwest Atl. Fish. Res., Bull. 9:65-74. Punsly, R. G. 1983. Estimation of the number of purse-seiner sets on tuna associated with dolphins in the eastern Pacific Ocean during 1959-1980. 18(3):227-299. I.A.T.T.C. Bull. 1987. Estimation of the relative annual abundance of yellowfin tuna, Thunnus albacares, in the east- ern Pacific Ocean during 1970-1985. I.A.T.T.C. Bull. 19(31:263-306. Punsly, R. G., and R. B. Deriso. 1991. Estimation of the abundance of yellowfin tuna, Thunnus albacares, by age groups and re- gions within the eastern Pacific Ocean. I.A.T.T.C. Bull. 20(2):98-131. Ricker, W. E. 1975. Computation and interpretation of biological statistics of fish populations. Fish. Res. Board Can., Bull. 191, 382 p. Tomlinson, P. K., S. Tsuji, and T. P. Calkins. 1992. Length frequency estimation for yellowfin tuna. I.A.T.T.C. Bull. 20(6):357-398. Wild, A. 1986. Growth of yellowfin tuna, Thunnus albacares, in the eastern Pacific Ocean based on otolith increments. I.A.T.T.C. Bull. 18(61:423^182. Abstract. Gastrointestinal tract contents were evaluated from 73 female and juvenile male north- ern fur seals (Callorhinus ursinus) for analysis of their diet in the Bering Sea. Fur seals were col- lected from August to October of 1981, 1982, and 1985. Juvenile walleye pollock (Theragra chalco- gramma) and gonatid squid were the primary prey. Pacific herring (Clupea pallasi) and capelin (Mallotus villosus), considered im- portant fur seal prey in previous reports, were absent from the diet. Prey species and size varied among years and between near- shore and pelagic sample loca- tions. Interannual variation in the importance of pollock in the diet of fur seals was positively related to year-class strength of pollock. Midwater (n=23) and bottom (rc=116) trawls were conducted at the location of fur seal collections to determine availability of fish and squid relative to prey species eaten by fur seals. The species and size composition of prey taken by fur seals was similar to midwater trawl collections, but differed from bottom trawl catches. Contrary to earlier conclusions that northern fur seals are opportunistic in their feeding habits, we conclude that fur seals are size-selective mid- water feeders during the summer and fall in the eastern Bering Sea. Prey selection by northern fur seals (Callorhinus ursinus) in the eastern Bering Sea Elizabeth Sinclair Thomas Loughlin National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA 7600 Sand Point Way, N.E., Seattle, Washington 98 11 5 William Pearcy College of Oceanography, Oregon State University Oceanography Administration Building 1 04 Corvallis, Oregon 97331 Manuscript accepted 19 August 1993 Fishery Bulletin: 92:144-156 (1994) The Pribilof Island population (St. George and St. Paul Islands) of northern fur seals (Callorhinus ursinus) represents approximately 75% of the total species breeding population. Between 1975 and 1981, the Pribilof Island population declined from 1.2 million to an es- timated 800,000 animals (York and Hartley, 1981; Fowler, 1985). Abun- dance levels on St. Paul Island ap- pear to have stabilized (York and Kozloff, 1987) at a level 60-70% below estimates of the 1940s and 1950's, and at one-half the esti- mated carrying capacity (Fowler and Siniff, 1992). The number of animals continues to decline on St. George Island (York, 1990). The objectives of this study were to determine the species and size of prey eaten by northern fur seals in the eastern Bering Sea, to compare the seals' present diet with that prior to the population decline, and to examine the seals' consumption of prey relative to prey availability. Previous studies on the feeding habits of northern fur seals in the eastern Bering Sea (Scheffer, 1950a; Wilke and Kenyon, 1952; Wilke and Kenyon, 1957; North Pacific Fur Seal Commission Re- ports 1962, ' 1975, 2 and 1980 3 ; Fiscus et al., 1964; Fiscus et al., 1965; Fiscus and Kajimura, 1965) were conducted prior to the 1975- 81 population decline and prior to the 1970s development of a com- mercial walleye pollock (Theragra chalcogramma) fishery in the Bering Sea. Neither the size of fur seal prey, nor fur seal selection of prey relative to real-time availabil- ity have been previously examined in detail. Methods Northern fur seals were collected from 17 to 28 October 1981; from 24 September through 6 October 1 North Pacific Fur Seal Commission Re- port on Investigations from 1958 to 1961: Presented to the North Pacific Fur Seal Commission by the Standing Scientific Committee on 26 November 1962, 183 p. Available: Alaska Fish. Sci. Cent., NOAA, NMFS, 7600 Sand Point Way NE., BinC15700, Seattle, WA 98115-0070. - North Pacific Fur Seal Commission Re- port on Investigations from 1967 through 1972: Issued from the headquarters of the Commission, Washington, D.C., June 1975, 212 p. Available: Alaska Fish. Sci. Cent., NOAA, NMFS, 7600 Sand Point Way NE., BinC15700, Seattle, WA 98115-0070. 3 North Pacific Fur Seal Commission Re- port on Investigations during 1973-76: Issued from the headquarters of the Com- mission, Washington, D.C., February 1980, 197 p. Available: Alaska Fish. Sci. Cent., NOAA, NMFS, 7600 Sand Point Way NE., BinC 15700, Seattle, WA 98115-0070. 144 Sinclair et al.: Prey selection by Callorhinus ursmus 45 59° N • Seal samples (all years) D Marinovich midwater trawls A Diamond midwater trawls Middle Shelf Domain Figure 1 The study area with midwater trawl locations and northern fur seal collection positions. All midwater trawls were conducted in 1985. Seal numbers 1-17 were collected in 1981, 18-40 were collected in 1982, and 41-83 were collected in 1985. 1982; and from 6 to 16 August 1985. Collections were made within 185 km of the Pribilof Islands over the continental shelf, continental slope, and oceanic domain of the eastern Bering Sea (Fig. 1). Seals were shot from a small craft and returned to the NOAA ship Miller Freeman (65-m stern trawler) for examination within 1.5 hours of collec- tion. The esophagus of each seal was checked for food as an indication of regurgitation, and the gas- trointestinal (GI) tract was removed and frozen. Gastrointestinal tract contents were later thawed and gently rinsed through a series of graded sieves (0.71, 1.00 or 1.40, and 4.75 mm in 1981 and 1982; 0.50, 1.00, 1.40, and 4.75 mm in 1985). Fleshy re- mains were preserved in 10% formalin. Fish otoliths and bones were stored dry. Cephalopod rostra and statoliths were preserved in 70% isopropyl alcohol. Prey identification was based on all remains, in- cluding otoliths. Otoliths were not used for fish iden- tification in earlier fur seal diet studies because stomach samples were stored in formalin, which dissolves otoliths. Techniques and references for the identification of prey based on otoliths include Fitch and Brownell (1968), Morrow (1979), Frost and Lowry (1981), and otolith reference collections (see Acknowledgments). References for cephalopod beak and statolith identification include Clarke (1962), Young (1972), Roper and Young (1975), Clarke (1986), and beak and statolith reference collections (see Acknowledgments). A tooth was collected from each fur seal that was shot and ages were derived from direct readings of canine tooth sections follow- 146 Fishery Bulletin 92(1), 1994 ing Scheffer (1950b). In the analysis of data, males and females of all ages were treated as one group because of small sample sizes. The highest number of either upper or lower cephalopod beaks and left or right otoliths was re- corded as the maximum number of each species present. If deterioration made some left and right otoliths of a species indistinguishable, they were counted and the total was divided by 2. The fre- quency of occurrence and number of individuals from each prey taxon was calculated for each seal. The fork length (FL) of pollock and dorsal mantle length (DML) of squid was measured directly when whole prey were present in the stomachs. In the absence of whole prey, body size was estimated by measurement of otoliths and beaks. The maximum length of pollock otoliths and lower rostral length (LRL) of gonatid squid beaks were measured to the nearest 0.05 mm with vernier calipers. Squid DML's were estimated by comparison of LRL measure- ments to the LRL/DML relationship of 51 gonatid squid caught in trawls conducted in the vicinity of seal collections. Walleye pollock fork lengths were estimated by regression against otolith length (Frost and Lowry, 1981). For otoliths measuring: > 10.0mm,(FL) Y = 3. 175X - 9.770 ( R = 0.968) < 10.0mm, (FL) Y = 2.246X- 0.5 10 (/? = 0.981). Walleye pollock ages were estimated from these lengths based on length-age relation described by Smith (1981) and Walline (1983) for walleye pollock from the Bering Sea. Otoliths may dissolve or erode to varying degrees depending on their size and duration in fur seal stomachs. We evaluated the bias introduced in FL estimates due to eroded otoliths by assigning otoliths to four condition categories (excellent, good, fair, and poor) based on amount of wear. After qual- ity categorization, the maximum lengths of otoliths (except those in "poor" condition) were measured for estimation of body length by regression, and length frequencies of each category were determined inde- pendently. Cephalopod beaks are more resistant to digestion than otoliths and were typically identifiable. Beaks with chipped, worn, or broken rostra were rare and were not measured. Cephalopod beaks were identi- fied to species when possible, but most were catego- rized into two groups referred to as Gonatopsis bo- realis-Berryteuthis magister or Gonatus madokai- Gonatus middendorffi . The two individual species within each group can be separated based on their external morphology and statolith structure, but cannot presently be separated based on beak struc- ture alone (Clarke, 1986). Trawl collections of potential seal prey Trawls were conducted throughout the study area from the Miller Freeman between 1900 and 0600 hours within the vicinity of seal collections (Fig. 1). Both bottom and midwater trawls were conducted to provide a relative measure of the availability and size of potential fur seal prey species. Bottom trawls were made at 52-498 m (.v=139 m) depths with an 83/112 Eastern bottom trawl (17-m width, 2.3-m height mouth opening; 3.2-cm codend liner mesh; 360-mesh circumference; 200-mesh depth; 30-m bridle). Thirty-nine bottom trawls were conducted in 1981 (14 October-4 November), 51 in 1982 (24 September-8 October), and 26 in 1985 (5 August- 22 August). Seven 1985 trawls were made beyond maximum recorded dive depths of adult female seals (257 m; Ponganis et al., 1992). They were included in analyses because the species and size offish and squid caught were consistent with those caught by bottom trawl within seal dive depths. Collection and sorting methods and calculation of bottom trawl catch per unit of effort (CPUE) values followed Smith and Bakkala (1982). The total bot- tom trawl catch was randomly split into a sample of about 2500 kg. Individual species of fishes were identified and weighed (wet) and CPUE (no./ha) was estimated based on distance trawled. In 1981 and 1982, cephalopods were classified as squid or octo- pus and discarded. In 1985, all cephalopods were identified, sexed, weighed, and frozen whole. Beaks were extracted and stored in 70% isopropyl alcohol. Sex and age determination and body length mea- surements were made on a subsample of up to 200 walleye pollock from each trawl. Fork lengths were measured to the nearest centimeter. Saccular otoliths were collected for age determination (Smith and Bakkala, 1982) and stored in 70% isopropyl alcohol. Walleye pollock CPUE was calculated by age and body length. For purposes of this study, age- length frequencies for male and female walleye pol- lock were combined for each of the three years. Midwater trawls were made in 1985 with a Dia- mond midwater net (n=8) ( 10-16 fm mouth opening; 3.2-cm codend liner mesh; 354-mesh circumference; and 160-mesh depth with 2-m bridles) and a Marinovich herring trawl (rc = 15) (6.1-m width, 6.1- m height mouth opening; 1-cm codend liner mesh; 150-mesh circumference; and 350-mesh depth with 10-m bridles). Specific trawling positions were cho- sen within the vicinity of northern fur seal collec- tion areas based on the presence of fish or squid as Sinclair et al.: Prey selection by Callorhinus ursinus 147 indicated on 38 kHz echosounders and a chromoscope. Midwater towing depths measured by an attached transducer ranged from 22 to 340 m (x=143 m). All species of fish and cephalopods collected in midwater trawls were identified and counted. The CPUE and frequency of occurrence of each species, LRL and sex of gonatid squid, and walleye pollock frequency of occurrence by age and length were cal- culated separately for each trawl type. Comparison of seal diet and trawl collections The Odds Ratio (Fleiss, 1981) was used to compare prey availability (as determined by midwater and bottom trawls) with selection of prey by fur seals for each sample year: = pV where pi = % of diet comprised by a given prey taxon, ql = % of diet comprised by all other prey taxon, p2 = % of food complex in environment com- prised by a taxa, and q2 = %> of food complex in environment com- prised by all other taxa. Values were calculated for number of each prey species and percent frequency of occurrence among seals, and CPUE values (no./ha) for each trawl type. Values for p2 and ql were also calculated for the trawl types combined in order to provide a compre- hensive description of the water column. The natu- ral log of the calculated Odds Ratio represents ei- ther positive or negative selection- The Odds Ratio was chosen because, unlike other electivity indices, the significance of the distance of calculated values from zero (null hypothesis that prey were consumed non-selectively) can be tested with the Z-statistic (Gabriel, 1978). In order to quantify the degree of overlap in the composition of bottom trawls, midwater trawls and fur seal GI contents, percent similarity (PS) values (Langton, 1982) were calculated: PS =100-0.5^a -b, where a = %> number of a given prey for seals, and b = % number of the same prey for trawls. Results Fur seal diet Eighty-three fur seals were collected. Ten of the 17 GI tracts collected in 1981 were empty and were excluded from the analysis. Of the 73 animals in- cluded in the analysis, 13 were juvenile males, 3 were juvenile females and 57 were adult females. Most fur seals were collected over depths less than 200 m within the outer shelf domain (Fig. 1). Fish represented 89% and cephalopods 11% of prey numbers for all three sample years combined. One-hundred percent of the GI tracts had fish re- mains and 82% of all samples contained walleye pollock. A total of 2,658 walleye pollock otoliths were measured. In all years combined, juvenile walleye pollock (3-20 cm FL) were the most numerous and frequently occurring prey species. Sixty-five percent of prey walleye pollock were from the 0-age group (3-13 cm FL) and 31% were from age group 1 (13- 20 cm FL). Only 4% of prey pollock were from age group 2 (20 + cm FL) and older. Gonatid squids occurred in 36% of the samples, but in comparison with pollock, they were not con- sumed in large numbers (Fig. 2). Gonatus madokai- G. middendorffi and Gonatopsis borealis-Berry- teuthis magister were the second most frequently occurring prey in all years combined. Seventy-nine percent of the 389 beaks measured were from squid 5-12 cm DML. Northern smoothtongue (Leuroglossus schmidti), a bathylagid deepsea smelt, was the second most numerous fish prey overall (Fig. 2) even though it was found only in 1985 (Table 1). Northern smooth- tongue composed a higher percentage of the total number offish than walleye pollock >2 years old for all sample years combined. Atka mackerel (Pleurogrammus monopterygius) composed 23.9% of the 1981 prey sample and was present in five of seven stomachs collected in 1981 that had prey re- mains, but the species was identified from the prey remains of only one other individual among the six collected in the same area in 1982 (Table 1). Although walleye pollock were eaten by fur seals in all 3 years, marked differences in age and body size were found between years (Table 1; Fig. 3). In 1981, the few walleye pollock otoliths found were from fish 3-4 years of age. Fur seal GI tracts con- tained primarily age-0 pollock in 1982 and age-1 pollock in 1985. Exclusion of otoliths that were in fair condition caused a downward shift in modal FL frequencies of 1 to 2 cm, but did not change our es- timation of the age categories of pollock eaten by fur seals. The species of forage fishes and squids consumed by fur seals varied between samples taken on and off the continental shelf (200 m) (Fig. 4). The GI tracts of fur seals collected over oceanic and conti- nental slope regions contained primarily northern smoothtongue and squids, especially Gonatopsis 148 Fishery Bulletin 92f 1). 1994 10 20 Percent number/frequency 30 40 50 60 70 walleye pollock (Theragra chalcogramma) gonatid squid (Gonatidae) Atka mackerel (Pleurogrammus monopterygius) northern smoothtongue (Leuroglossus schmidti) Salmoniformes fOsmendae) Percent of total number of prey, all years Percent frequency of occurrence of prey, all years Figure 2 Percent of total number and frequency of occurrence of primary prey in northern fur seal (Callorhinus ursinus) gastrointestinal tracts for sample years 1981, 1982, and 1985 combined. Species shown include the top three prey from each sample year. borealis-Berryteuthis magister. Seals collected over the continental shelf contained the remains of wall- eye pollock of all ages and squids, especially Gonatus madokai-G. middendorffi. Adult walleye pollock, although rare in stomach contents, were found in greatest frequency in fur seals collected from the outer domain of the continental shelf. Juvenile wall- eye pollock were consumed primarily over the midshelf and outer domain. Atka mackerel was found only in samples collected over the outer shelf domain north of Unimak Island. Comparisons with trawl samples Of the five top-ranked species collected in bottom trawls, only walleye pollock was found in fur seal GI contents (Figs. 2 and 5). Walleye pollock from bottom trawls ranged from 1 to over 12 years of age and had mean body lengths of 38.9 cm (3-4 years old) in 1981, 39.7 cm (4-5 years old) in 1982, and 44 cm (5-6 years old) in 1985 (Fig. 6). All but four of the cephalopods caught in 1985 bottom trawls were Berryteuthis magister ranging from 17.5 to 31.2 cm DML ( x = 2 1.6). As in the seal samples, B. magister was collected in trawls conducted over the outer continental shelf domain along the 200-m contour, or over the continental slope between 200 and 1000 m. Otherwise, the bottom trawl catch for all three years was so dissimilar to the midwater trawl catch (Figs. 5 and 6) and fur seal GI contents (Fig. 2) that electivity computations were not mean- ingful (Odds Ratio=0). Calculation of the Odds Ratio and Z-statistic on 1985 data with midwater and bottom trawl catch combined showed statistically significant positive selection by fur seals for age-0 pollock (P=0.0002), age-1 pollock (P<0.0001), northern smoothtongue (P<0.0001), and gonatid squid (P=0.02). Negative selection for adult walleye pollock was suggested but was not statistically significant (P=0.13). A similarity index of 81% was calculated for spe- cies composition and prey size in the 1985 GI samples and midwater trawls. Fur seals fed on three of the four top-ranked species caught in midwater trawls (Figs. 2 and 5). Midwater trawls and seals caught predominantly juvenile walleye pollock. Gonatid squids (Gonatus madokai, G. middendorffi, and Gonatopsis borealis) had low CPUE values but were second in frequency of occurrence in both fur seal GI tracts and midwater trawls. The modal length of walleye pollock and gonatid squids was 5- 20 cm in both midwater trawl and GI samples in 1985. Few adult walleye pollock and no large squid were collected in midwater trawls or seal GI samples. Seals and midwater trawls caught the same prey species at the same general locations on and off the continental shelf (Fig. 4). As in GI contents, age-0 and age-1 walleye pollock were collected in midwater trawls made on the middle and outer shelf and near the continental slope. Gonatopsis borealis were found on the continental slope and near-slope. Gonatus madokai and G. middendorffi were found throughout the sampling area, but primarily on the outer continental shelf and near-slope sampling areas. Sinclair et al.: Prey selection by Callorhinus ursinus 149 Table 1 Gastrointestinal contents of 73 northern fur seals (Callorh nus ursinus) collected from the Bering Sea in 1981 (n=7), 1982 (n=23), and 1985 (n=43). Tentative identifications are designated as (t). Prey species % number in each year % frequency occurrence 1981 1982 1985 1981 1982 1985 Fish Clupea pallasi — 0.1 — — 4.4 — Osmeridae (t) 8.7 — — 42.9 — — Salmonidae 5.4 — — 42.9 — — Leuroglossus schmidti — — 12.7 — — 9.3 Gadus macrocephalus (t) — — 0.1 — — 7.0 Theragra chalcogramma 54.4 87.3 74.1 100 95.7 72.1 3-5cm fork length — (8.8) (5.7) 5-10cm fork length (4.3) (63.9) (2.3) 10-20cm fork length — — (55.6) >20cm fork length (38.0) (1.4) (1.7) T. chalcogramma (t) — 0.1 0.1 — 8.7 4.7 unidentified Gadidae — — 0.9 — — 20.9 Lycodes sp. 1.1 — 0.5 14.3 — — Pleurogrammus monopterygius 23.9 0.1 — 71.4 4.4 — P. monopterygius (t) — 0.1 — — 4.4 — unidentified percoid 1.1 — — 14.3 — — unidentified fish 5.4 0.4 0.5 14.3 13.0 25.6 Squid Gonatus berryi — — 0.1 — — 2.3 G. pyros — — 0.1 — — 2.3 G. tinro — — 0.1 — — 2.3 G. tinro (t) — — 0.1 — — 2.3 Gonatus madokai-middendorffi — 0.1 4.8 — 4.4 34.9 Gonatus sp. — — 0.1 — — 2.3 Berryteuthis magister — 0.6 — — 8.7 — Gonatopsis borealis-B. magister — 10.2 6.4 — 17.4 20.9 unidentified Gonatidae — — 0.1 — — 7.0 unidentified squid — 1 — — 34.8 — Total number prey 92 1638 2189 Total number fish 92 1445 1936 100 100 100 Total number squid 193 253 52.2 46.5 Discussion The modal size distribution of walleye pollock in GI contents of female and juvenile male fur seals re- flected year-class strength projections of walleye pollock (Fig. 7). Walleye pollock have highly variable recruitment rates (Smith, 1981), and year-class strength varied five-fold between 1977 and 1982 (Bakkala et al., 1987). Population estimates based on bottom trawl and midwater acoustic surveys in the eastern Bering Sea indicated that the 1980 year class (age 1 in 1981) was about half the average year-class size; the 1981 year class (age in 1981) was the weakest observed prior to 1983; and the 1978 year class (age 3 in 1981) was the strongest observed. The 1982 and 1984 year classes were strong and the 1985 year class was considered av- erage (Bakkala et al., 1987). Similarly, walleye pol- lock as prey in 1981 were primarily adults 3 and 4 years of age (from the 1977 and 1978 year class); in 1982, seals ate age-0 pollock exclusively; and in 1985, prey pollock were primarily from the 1984 year class. The concordance of pollock recruitment and fur seal GI content analysis indicates that the variable recruitment of walleye pollock affects prey consumption by northern fur seals. The three basic dive patterns described for adult females in the Bering Sea are shallow, pelagic night- time diving (most commonly to 50—60 m); deep day- and-night diving over the continental shelf (most commonly to 175 m); and some combination of both, including shallow diving over the continental shelf 50 Fishery Bulletin 92(1). 1994 1981 n.39 ' □ 1982 n-1191 1985 n-1428 Figure 3 Age-length frequencies of walleye pollock (Ther chalcogramma) based on otoliths in northern fur [Callorhinus ursinus) gastrointestinal tracts by year. agra seal and both shallow and deep diving along the conti- nental slope. Dive pattern information is based on time-depth recordings (Gentry et al., 1986; Loughlin et al., 1987; Goebel et al., 1991), radio telemetry (Loughlin et al., 1987), stomach volume estimates (Mead, 1953; Taylor et al., 1955'; Spalding, 1964; Wada, 1971; Kajimura, 1984), and stomach clear- ance studies (Miller, 1978 5 ; Bigg, 1981 6 ; Bigg and Fawcett, 1985; Murie and Lavigne, 1985). Based on fur seal and trawl collections in this study and on distributional information of prey (Smith, 1981; Dunn, 1983; Kubodera and Jefferts, 1984; Lynde, 1984), shallow diving fur seals over the continental shelf concentrated on juvenile walleye pollock and juvenile gonatid squid (Gonatus madokai-G. middendorffi), while shallow divers off-shelf targeted juvenile gonatid squid (Berryteuthis magister-Gonalopsis borealis) and bathy- lagid smelt. Daytime deep diving over the continental shelf would be advantageous to seals concentrating on prey (i.e., adult wall- eye pollock) that tend to school at depth during daytime hours and disperse as they rise in the water column at night. Adult gonatid squid probably occur in schools at the bottom on the continental shelf and re- main deep along the shelf edge during both day and night. The location and degree of concentration of prey may be closely associ- ated with the hydrography of the foraging region. The hydrography of the foraging re- gion may have the most direct influence on the diving patterns of fur seals. Hydrographic characteristics of the Bering Sea continental shelf, include a two-layered midshelf and a three-layered outer shelf domain that may stratify and concentrate prey by species and age in a vertical plane. Nishiyama et al. (1986) proposed that ver- tical stratification within the eastern Bering Sea shelf serves as a "nursery layer" to con- fine young-of-the-year pollock in the upper 40 m of the water column within the bound- ary region between the upper and lower lay- ers. Copepod nauplii are also concentrated in this area, providing a ready source of food for larval walleye pollock (Bailey et al., 1986). Wada ( 1971) determined that primary foods of fur seals off the Sanriku Coast in Japan consisted of migrating species closely related to boundary regions, especially transition zone regions. The horizontal temperature and salinity structures that occur on either side of frontal regions within our study area (Kinder and Schumacher, 1981 ) may 4 Taylor. F II (' , M. Fuginaga, and F. Wilke. 1955. Distribu- tion and food habits of the fur seals of the North Pacific Ocean Rept. of Coop. Invest by the Govts, of Can., Japan, and the U.S.A. Feb. -July, 86 p Available Alaska Fish. Sci. Cent.. NOAA, NMFS, 7600 Sand Point Waj \K , BinC 15700, Seattle, WA 98115-0070. ' Miller, L. K. 1978. Energetics of the northern fur seal in rela- tion to climate and food resources of the Bering Sea. Final Rep. to U S. Mar. Mamm. Comm. MMC-75/08, 27p. K Bigg. M. A. 1981. Digestion rates of herring (Clupea harengus pallasi i and squid iLali^o npalfsri-nsi in northern fur seals. Submitted to the 24th Annual Meeting of the Standing Sci. Comm., N. Pac. Fur Seal Comm., 6-10 April, Tokyo, Japan. Available: Alaska Fish. Sci. Cent.. 7600 Sand Point Way NE., BmC15700, Seattle, WA 98115-0070. Sinclair et al.: Prey selection by Callorhmus ursinus 151 Percent number 100 90 80 70 60 50 40 30 20 10 10 20 30 40 50 60 70 80 90 100 northern smoothtongue (Leurogkxsus Schmidt) gonatid squid (Gonatopsis borealis/ Berryteu&tts magister) gonatid squid (Gonatus madokairmiddendotlrl) walleye pollock (all ages) (Theragra chalcogramma) Oceanic domain/Continental slope seals midwater trawls Continental shell (<200m) seals midwater trawls Figure 4 Primary species identified in fur seal (Callorhinus ursinus) gastrointestinal tracts and midwater trawls col- lected on and off the eastern Bering Sea continental shelf in 1985. walleye pollock (Themgra chalcogramma) lanternfish (Myaophxlae) northern smoothtongue (Leuroglossus schmidti) gonatid squid (Gonatidae) 40 60 80 100 Q Marinovich midwater trawl Diamond midwater trawl 50 I 100 150 200 walleye pollock (Theragra chalcogramma) yellowfin sole I jronectes asper) iMmnmu (Pleuronectes asper) rock sole (Lepidopsena bilineala) I Pacific cod (Gadus macrocephalus) Bottom trawls 1981 ^ 1982 □ 1985 Mean CPUE values (no. /ha) Figure 5 Catch per unit of effort (CPUE) number/hectare (no. /ha) values for species caught in 1985 midwater trawls and bottom trawls in 1981, 1982, and 1985. 152 Fishery Bulletin 92(1), 1994 400 300- 1 1 200 $ E 100- 1985 Trawls | Midwater □ Bottom N-4140 Jll^l 10 20 25 30 Fork length (cm) J I 45 _L_L 50 2 3 Age (years) 6 7 Figure 6 Age-length frequencies of walleye pollock (Therag chalcogramma) collected in bottom and midwater trawls 19H5. also form boundaries that concentrate prey. Diving depths of 175 m coincide with the depth break of the outer continental shelf. Diving depths of 50-60 m coincide with the depth break of the frontal systems between the midshelf and inner shelf. Previous analyses of fur seal diet in the eastern Bering Sea were based primarily on a sample of 3,530 stomachs collected pelagically in 1960, 1962- 64, 1968, 1973, and 1974 (North Pacific Fur Seal Commission Reports 1962, ' 1975, 2 and 1980 3 ; Fiscus et al. 1 D64; Fiscus et al. 1965; Fiscus and Kajimura 1965;. i:tviews of the pelagic data cite walleye pol- lock (Kajimura, 1985; Perez and Bigg, 1986), Pacific herring (Clupea pallasi), capelin (Mallotus villosus), Atka mackerel, gonatid squids (Gonatus spp., Berryteuthis magister and Gonatopsis borealis), and intermittently, northern smoothtongue (Kajimura, 1984) as principal fur seal prey in the eastern Bering Sea. Published reports and reviews of fur seal feeding habits prior to the pelagic collections (1892-1950's) also described walleye pollock, cape- lin, gonatid squid, and bathylagid smelt as primary prey in seal spewings or stomachs (Scheffer, 1950a; Wilke and Kenyon, 1952; Wilke and Kenyon, 1957). In terms of prey species composition, the summer diet of female and juvenile male northern fur seals does not appear to have changed dramatically since the turn of the century. Pollock and gonatid squid are still the predominant prey of northern fur seals in the eastern Bering Sea. More subtle changes, such as a decrease in pollock size may have occurred (Smith, 1981; Swartz- man and Haar, 1983) and could play a criti- cal role in foraging success of northern fur seals. Unfortunately, records of prey size in historical fur seal diet studies are incom- plete. It should be noted that Pacific herring and capelin were absent from fur seal di- ets in this study, despite collections in ar- eas where they occurred as important prey in the past. Fluctuation in the population status of Pacific herring and capelin in the Bering Sea has been attributed to the spo- radic and localized nature of their abun- dance (Turner, 1886; Meek, 1916; Favorite et al. 1977 7 ; Lowe 1991 8 ), overharvesting and displacement by walleye pollock (Wespestad and Barton, 1981; Swartzman and Haar, 1983; Wespestad and Fried, 1983; Bakkala et al., 1987), and/or environ- mental change such as the pronounced warming in the Gulf of Alaska and Bering Sea over the past decade (Royer, 1989). The absence of these previously important prey may be critical to seals during successive years of weak walleye pollock year-class abundance. Fur seals select juvenile walleye pollock as prey despite a wide availability of other prey types within their dive range. Fur seals may select their prey by size and schooling behavior, whether the prey are myctophids in oceanic waters off Japan (Wada, 1971); Pacific herring, capelin, market squid iLoligo opalescens) and Pacific whiting (=Pacific hake, Merluccius productus) in the eastern North Pacific (Kajimura, 1984; Perez and Bigg, 1986); or walleye pollock in the eastern Bering Sea (Kajimura, 1984). The most consistent prey characteristic between feeding studies across the northern fur seal range 7 Favorite, F., T. Laevastu, and R. R. Straty. 1977. Oceanogra- phy of the northeastern Pacific Ocean and eastern Bering Sea, and relations to various living marine resources. NWAFC Proc. Rep. 280p. Alaska Fish. Sci. Cent., NMFS, NOAA, 7600 Sand Point Way NE., Bin C 15700, Seattle, WA 98115-0070, 280p. R Lowe, S. A. 1991. Atka mackerel. In Stock assessment and fish- ery evaluation report for the groundfish resources of the Bering Sea/ Aleutian Islands region as projected for 1992, p. 11-2 to 11-40. North Pacific Fishery Management Council, P.O. Box 103136, Anchorage, AK 99510. Sinclair et al.: Prey selection by Callorhmus ursinus 53 B * C l/i IB C £: O ra E a> = >• c SI o « °- g. o CL 90 - 80 - 70- 60 - 50 - 40- 30 - 20 - 10- - 100 1978 1979 1980 1981 1982 1983 1984 1985 Pollock year class 80 70 - 2 50 1981 □ 1985 1978 1979 1980 1981 1982 1983 1984 1985 Pollock year class Figure 7 Estimates of walleye pollock iTheragra chalcogramma) year-class strength 1978- 85 (Bakkala et al., 1987), and the relative abundance of specific year classes in northern fur seal gastrointestinal tracts. is size and the tendency to form dense schools. In this sense, a "juvenation" of walleye pollock in the Bering Sea (Swartzman and Haar, 1983) may have provided fur seals with a newly abundant but un- stable resource, due to large fluctuations in the annual year-class strength of walleye pollock and due to potential displacement of other prey species (Pacific herring and capelin). During years of low pollock recruitment, fur seals may switch to other prey such as capelin and Pacific herring, and expe- rience food limitation only if these alternate prey resources have been displaced or depleted. Histori- cal records of northern fur seal diet are inadequate to either support or refute an "alternate prey" ar- gument. However, we suggest that when juvenile walleye pollock are unavailable, such as in our 1981 sampling season, female and juvenile fur seals se- lect other specific prey of the same size and eat adult walleye pollock only if these other preferred prey are not available. During their summer breeding season, northern fur seals consume the most abundant and available fish and squid in the eastern Bering Sea. Walleye pollock make up an estimated 50% of the ground- fish biomass in the eastern Bering Sea and Aleutian Islands area (Walters et al., 1988) and dense aggre- gations of 0-age pollock occur off the Pribilof Islands June through mid-August (Smith, 1981). Kubodera and Jefferts (1984) suggested gonatids are the ma- jor pelagic cephalopod group in the Bering Sea, where large increases in abundances of larval and postlarval gonatid squid occur in early June. Among Bering Sea gonatids, Gonatopsis borealis and Berryteuthis magister are considered to be among the most numerically dominant (Jefferts, 1983; Kubodera and Jefferts, 1984). Selection by northern fur seals of a wide variety of numerically dominant prey species throughout their migratory range has led to the general conclu- sion that they are non-specific, opportunistic feed- ers (Kajimura, 1985). Northern fur seals are flexible in their feeding habits, as indicated by the variation in GI contents of seals collected between California and Alaska. Nonetheless, fur seals concentrate on an average of three primary species within each oceanographic subregion (Perez and Bigg, 1986). In addition, fur seal consumption of walleye pollock, gonatid squid, and bathylagid smelt in the eastern Bering Sea is consistent throughout historical records, despite the wide variety of prey available to fur seals within their diving range. Based on this study, we conclude that female and young male fur seals select juvenile and small-sized fish and squid, despite the availability of larger prey types within their diving range. This study demonstrates that female and young male fur seals are size-selective midwater shelf and mesopelagic feeders, at least during the breeding and haul-out season in the east- ern Bering Sea. Acknowledgments Otolith identifications for the 1981 samples were made by the late J. Fitch. Otolith identifications for 1982 and 1985 were based on the otolith reference collections at the National Marine Mammal Labo- 54 Fishery Bulletin 92(1). 1994 ratory (NMML) and Los Angeles County Museum (LACM). Cephalopod identifications were based on the reference collections of the NMML and Oregon State University (OSU). Voucher specimens of prey material (statoliths, beaks, otoliths, teeth, and bones) are archived at the NMML. Identifications of squid and squid beaks were confirmed by C. Fiscus (NMML, retired), K. Jefferts (OSU), and W. Walker (LACM). Identification of fish otoliths and bones were confirmed by G. Antonelis Jr. (NMML) and J. Dunn (University of Washington) respectively. Voucher samples of juvenile pollock otoliths were confirmed by A. Brown (Alaska Fisheries Science Center [AFSC]), K. Frost (Alaska Department of Fish and Game [ADF&G], and L. Lowry [ADF&G]). Gary Walters (AFSC) helped interpret bottom trawl values, and W Carlson and C. Leap of the AFSC Graphics Unit helped produce the figures. The following individuals contributed to the qual- ity and content of this manuscript: G. Antonelis Jr., J. Baker, L. Fritz, R. Gentry, P. Livingston, and two anonymous reviewers. These data were first pre- sented in part as a Northwest and Alaska Fisher- ies Center Processed Report (Hacker and Antonelis, 1986 9 ) and in full as a Masters Thesis from Oregon State University (Sinclair, 1988). Literature cited Bailey, K., R. Francis, and J. Schumacher. 1986. Recent information on the causes of variabil- ity in recruitment of Alaska pollock in the eastern Bering Sea: physical conditions and biological interactions. Int. North Pac. Comm. Bull. 47:155-165. Bakkala, R. G., V. G. Wespestad, and J. J. Traynor. 1987. Walleye pollock. In R. G. Bakkala and J. W. 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Zool. (97), 159 p. Abstract. — The stomach con- tents of 1,215 anadromous ale- wives collected during winter and summer groundfish research sur- veys (1990-91) off Nova Scotia were examined to 1) describe the diet by season, area, bottom depth (<101 m, 101-200 m, >200 m), time of day and fish size (<151 mm, 151-200 mm, 201-250 mm, >250 mm FL), 2) evaluate diel feeding periodicity, and 3) estimate daily ration. Euphausiids, particu- larly Meganyctiphanes norvegica, were the most important prey and represented more than 82% by vol- ume of total stomach contents sea- sonally and geographically. Contri- butions by other prey groups (hyperiid amphipods, calanoid copepods, crustacean larvae, poly- chaetes, chaetognaths, mysids, pteropods, and fish larvae) were small and varied temporally and spatially. The proportion of eu- phausiids in the diet of alewives from the Scotian Shelf (winter) and Bay of Fundy (summer) tended to increase with increasing depth. Day and night differences in diet composition indicate that alewives may particulate-feed on macrozooplankton when prey vis- ibility is high and filter-feed on microzooplankton when prey vis- ibility is low. Diet composition was relatively homogenous among ale- wife size groups with euphausiids composing most of the total food volume. Alewives of different size groups ate similarly sized M. norvegica, generally the largest M. norvegica available. Diel feeding activity (stomach fullness) peaked at mid-day (summer collections) and mid-afternoon (winter collec- tions); feeding activity was re- duced at night. In all areas, feed- ing activity and the proportion of feeding fish was highest in regions where bottom depths exceeded 200 m. Mean stomach fullness was highest during summer in the Bay of Fundy and during winter on the Scotian Shelf; these regions are seasonally important foraging ar- eas for alewives off Nova Scotia. Daily ration was 1.2% of body weight during winter and 1.9% during summer. Manuscript accepted 17 August 1993 Fishery Bulletin 92:157-170 (1994) Feeding habits of anadromous alewives, Alosa pseudoharengus, off the Atlantic Coast of Nova Scotia Heath H. Stone Department of Fisheries and Oceans, Biological Sciences Branch PO. Box 550, Halifax. Nova Scotia B3J 2S7 CANADA Present address: Biological Station, Department of Fisheries and Oceans St. Andrews, New Brunswick, EOG 2XO CANADA Brian M. Jessop Department of Fisheries and Oceans, Biological Sciences Branch, RO. Box 550, Halifax, Nova Scotia, B3J 2S7 CANADA The anadromous alewife, Alosa pseu- doharengus, is a clupeiform fish whose range extends from New- foundland to North Carolina (Bigelow and Schroeder, 1953). Off Nova Scotia, alewives occur through- out the year in regions characterized by strong tidal mixing and up- welling in the Bay of Fundy-east- ern Gulf of Maine and are abun- dant during spring in the warmer, deeper waters of the central Scotian Shelf and areas of warm slope water intrusion along the Scotian Slope and the edges of Georges Bank (Stone and Jessop, 1992). In the Maritime provinces of Canada and Atlantic coastal United States, alewives and blueback herring, A. aestivalis, are fished commercially during their spring spawning migrations and are often marketed together as gaspereau or river herring. Little is known about the importance of ale- wives as predators in the marine environment or about their feeding habits and food consumption rates. Alewives are generally classified as size-selective, particulate and filter-feeding microphagists and can actively feed on individual zooplankton or passively feed by filtering the water with their gill rakers (Janssen, 1976; Durbin, 1979; James, 1988). Feeding mode depends on prey density, size, and visibility, and on predator size (Janssen, 1976, 1978a, 1978b; Durbin, 1979). The ability to switch feeding modes enables alewives to consume a wide size range of prey in a variety of environmental con- ditions. Size-selective predation by juvenile and nonanadromous fresh- water alewives can shift the species and size composition of zooplank- ton communities towards smaller forms (Brooks and Dodson, 1965; Brooks, 1968; Wells, 1970; Wars- haw, 1972; Vigerstad and Cobb, 1978). No information is available on size-selective predation in the ocean; however, in Minas Basin, a turbid macrotidal estuary, alewives were generally particulate feeders of larger, benthic prey rather than smaller pelagic prey (Stone and Daborn, 1987). Information on the feeding hab- its of anadromous alewives in the ocean is limited to qualitative as- sessments but is better known for freshwater juveniles (Vigerstad and Cobb, 1978; Gregory et al., 1983; Jessop, 1990) and estuarine resident subadults during summer (Stone and Daborn, 1987). Eu- phausiids, calanoid copepods and, to a lesser extent, hyperiid amphi- pods, chaetognaths, mysids, ptero- pods, decapod larvae, and salps 157 58 Fishery Bulletin 92|1|. 1994 have been identified as prey for alewives in conti- nental shelf waters from North Carolina to Nova Scotia (Holland and Yelverton, 1973; Edwards and Bowman, 1979; Neves, 1981; Vinogradov, 1984; Bow- man, 1986). However, none of these studies were comprehensive. We examined the stomach contents of anadromous alewives obtained from winter and summer ground- fish research surveys on the Scotian Shelf, Georges Bank, and in the Bay of Fundy to determine the importance of these regions as foraging areas for these fish. Seasonal, spatial, diel and size-related vari- ability in feeding are examined. Daily ration is esti- mated from information on diel feeding periodicity. Materials and methods Data collection Alewives were collected from seven groundfish re- search surveys conducted by the Canadian Depart- ment of Fisheries and Oceans in three regions (Georges Bank, central Scotian Shelf, and outer Bay of Fundy) during winter (February-March) and summer (July) over a two-year period (1990-91) (Table 1). All surveys used a Western IIA bottom trawl with a 10-mm stretched-mesh liner in the cod end. Thirty-minute tows at each sampling station were conducted throughout the 24-hour day. Up to 40 fish of representative size range from each set were frozen for later analysis. Bottom water tem- perature CO, time of tow deployment, latitude, lon- gitude, and bottom depth (m) were recorded for each set. Stomach content data were grouped by season and sample location: Winter-Fundy, Winter-Shelf, Winter-Georges, and Summer-Fundy (Fig. 1). Stone and Jessop (1992) provide additional details of the survey area and procedures, and seasonal distribu- tion of fish. Fork length (mm), weight (g), sex and species (de- termined by peritoneal colour (Leim and Scott, 1966)) were recorded for each fish. Whole digestive tracts, individually identified, were preserved in 4% buffered formalin. Diet analysis Stomachs were weighed (±0.01 g) and the contents ranked subjectively using a fullness code (0=empty, 1 = 12% full, 2=25% full, 3=50% full, 4=75% full, 5=100% full) and a digestion code ( l=finely digested, nothing recognizable; 2=medium digestion, some recognizable parts; 3=some digested, some undi- gested material; 4=undigested whole animals). The stomach content weight was obtained by subtract- ing the weight of the empty stomach from the total stomach weight. Stomach content weight, as a per- centage offish body weight C#BW), was used as an index of fullness to evaluate feeding activity and estimate daily ration. Stomach contents were iden- tified (to species where possible), enumerated, and the volume of each food type estimated by means of a points system (Swynnerton and Worthington, 1940; Stone and Daborn, 1987). For diet analysis, prey taxa (Table 2) were grouped into nine categories based on taxonomy and ecology: 1) euphausiids (Meganyctiphanes norvegica and some Thysanoessa spp.); 2) hyperiid amphipods (Parathemisto gaudichaudiy, 3) calanoid copepods (Calanus spp., Centrophoges spp. and Metridia spp.); 4) polychaetes (Nereis spp. and unidentifiable spe- cies); 5) fish larvae (Ammodytes dubius and uniden- tifiable species); 6) mysids {Neomysis americana); 1) Table 1 Stomach and fork length statistics, by season and geographic area, tained from groundfish research surveys conducted off Nova Scotia for al (1990 ewives, -1991). Al osa pseudoharengus. ob- Season and area Collection date Number Fork length (mm) 1990 1991 Sets Stomach Stomachs with prey Mean ± SD Range Winter-Fundy 2-10 Feb :i 112 58 201.9 ± 5.38 100-303 Winter-Georges 28 Feb-Mar 7 Feb 16-26 29 438 147 193.6 ± 1.83 118-305 Winter-Shelf 13-19 Mar Mar 15-18 29 489 322 223.8 ± 2.82 95-302 Summer-Fundy 6-10 Jul Jul 05-09 15 176 141 242.6 + 2.48 142-302 Total 82 1,215 668 213.6 ± 1.86 95-305 Stone and Jessop: Feeding habits of Alosa pseudoharengus 159 40°N 70°W Figure 1 Set locations for alewives, Alosa pseudoharengus, obtained from groundfish re- search surveys off the Atlantic coast of Nova Scotia (1990-91) grouped by season and geographic area. Offshore banks are delineated by the 100-m depth contour; the outer edge of the continental shelf is delineated by the 200-m depth contour. chaetognaths; 8) crustacean larvae (furciliae of Thysanoessa spp. and some decapod larvae); and 9) pteropods. The percent frequency of occurrence (%FO), percent of total stomach content number (%N), and percent of total stomach content volume (%V) of prey categories were estimated for stomachs containing recognizable food (digestion code >2). The Index of Relative Importance (IRI=(%N+%V) x <7rFO) was calculated for each prey category (Pinkas et al., 1971) and used for various diet comparisons. Diets were analyzed by season and geographic area (Win- ter-Fundy, Winter-Georges, Winter-Shelf, Suramer- Fundy), as well as by depth range within season and area, to compare food items from shallow regions and offshore banks <<100 m), mid-depths (101-200 m) and the shelf edge or deep basins (>200 m). Diel differences in diet composition (day and night, based on time of gear deployment) were examined for the entire data set. Ontogenetic differences in diet within season/area were examined by grouping fish lengths into four size classes (<151, 151-200, 201- 250 and >250 mm FL), which were assumed suffi- cient for detecting shifts in prey composition. Data from 1990 and 1991 were combined for all compari- sons because the ranks of IRI values for all prey categories between years were highly correlated (Spearman rank correlation coefficient (r s )=0.67; P<0.05; n=9). Predator-prey size analysis Total lengths (±1 mm, tip of rostrum to end of tel- son) of undigested, whole M. norvegica in the stom- achs of 55 alewives (>200 mm FL, since most intact prey occurred only in larger fish) from Winter- Georges, Winter-Shelf, and Summer-Fundy cruises were compared with predator size. Thysanoessa spp. were not measured because of poor condition. Lengths of M. norvegica from Emerald Basin col- lected in June 1991, by Sameoto et al. (1993) using the Bedford Institute of Oceanography Net and Environmental Sensing System (BIGNESS) were compared with euphausiid length frequencies from stomach contents to estimate the proportion of the available size range of M. norvegica consumed by alewives. The BIONESS is not considered to be size- selective for euphausiids (Sameoto et al., 1980). 160 Fishery Bulletin 92(1). 1994 Diel feeding periodicity and daily ration estimate Diel feeding periodicity and daily ration were exam- ined separately for winter (Bay of Fundy, Scotian Shelf, and Georges Bank combined) and summer (Bay of Fundy) collections because of seasonal dif- ferences in photoperiod. Stomach fullness data from tows within each successive 3-hour (winter cruises) and 4-hour (summer cruises) interval were grouped and assigned to the midpoint of the time period. Small sample sizes precluded grouping of summer collections into 3-hour intervals. Daily ration (DR) of alewives during winter and summer and by size class during winter (<151 mm, 151-200 mm, 201-250 mm, >250 mm) was esti- mated in terms of % body weight from the model of Elliott and Persson (1978): c _ (*-*■-) a . 1-e ■Rt where the consumption of food (C t ) during the time interval t to t t is calculated from the average amount of food in the stomach at time t (S„), the amount in the stomach at time t t (S,) and the instan- taneous evacuation rate R. The estimates of C t cal- culated for each time interval are then summed to give the total daily ration (DR). Feeding is assumed constant within each time interval. R is assumed exponential and temperature dependent (Elliott, 1972), as R = ae hT . The slope (6) may be fairly constant for different prey types and fish species (mean=0.115), but the intercept (a) changes with prey type and can be estimated from gastric evacuation experiments (Durbin et al., 1983). Gastric evacuation rate data are unavailable for anadromous alewives; therefore, an intercept (a=0.0406) was obtained from Durbin et al. based on values for a variety of small invertebrates fed to several freshwater and marine fishes. High fat levels in the prey may retard evacu- ation (Durbin et al., 1983) but the principal food item in this study (M. norvegica) has a low lipid content (Ackman et al., 1970). Average bottom tem- peratures for winter (mean=7.16°C) and summer (mean=7.43°C) collections were used to estimate R. Statistical analysis Differences in the rankings of IRI values for prey categories (n =8 1 between three or more groups were tested for significance with the Kendall coefficient of concordance (w) (Siegel, 1956); for paired groups, the Spearman rank correlation coefficient (rj was used (Fritz, 1974). Euphausiids, which consistently ranked highest in importance in all comparisons, were excluded from correlation analysis to reduce bias and emphasize correlations among remaining prey groups. One-way ANOVA was used to examine feeding activity, represented by the index of fullness (arc- sine Vp transformed) by season and geographic area, by depth range within season and geographic area and by diel sampling period (winter and summer collections) and to compare total lengths of eu- phausiid prey. Paired means, adjusted for unequal sample sizes, were compared with the Tukey- Kramer test (Sokal and Rohlf, 1981). The relation between predator fork length and mean prey length was examined by linear regression for alewives with three or more M . norvegica present in their stomachs. Results Alewives examined for stomach contents measured 95 to 305 mm FL (mean=213.6 mm, n=l,215); fish from summer cruises in the Bay of Fundy were larger on average than those from other collections (Table 1). Capture depths ranged from 36 to 269 m, although most (75%) specimens were obtained from regions 101 to 200 m deep. Recognizable prey from over 20 different taxa oc- curred in 55% (668 of 1,215) of stomachs examined (Table 2). Over 95% of the total prey number, vol- ume, and frequency of occurrence were crustaceans (Table 2). Euphausiids were the most prevalent (91% by volume); Meganyctiphan.es norvegica were domi- nant by volume (61%) and furcilia larvae of Thysanoessa spp. were dominant numerically (32%). Other prey, including hyperiid amphipods, calanoid copepods, crustacean larvae, mysids, polychaetes, chaetognaths, pteropods, and fish larvae contributed little and varied temporally and spatially in relative importance. Diet composition by season and area Euphausiids were the most important food of ale- wives during winter and summer for all areas (Fig. 2). During winter, alewives from the outer Bay of Fundy and Georges Bank fed almost exclusively on euphausiids (99% and 95% of total volume, respec- tively). On Georges Bank, small (%V<3) proportions of calanoid copepods, hyperiid amphipods, and ptero- pods were also consumed. Prey diversity was great- est for alewives from the Scotian Shelf; euphausi- ids dominated by volume (82%) but were numeri- Stone and Jessop: Feeding habits of Alosa pseudoharengus 161 Table 2 Prey items found in the stomachs of alewives, Alosa pseudoharengus , collected from groundfi sh research surveys off Nova Scotia, 1990 -91. %FO = percent frequency of occurrence, %N = percent by number, %V = percent by volume. Prey item %FO %N %V Prey item %FO %N %v Crustacea 97.6 95.0 97.3 Decapoda 0.5 0.1 <0.1 Euphausiacea 91.3 72.4 91.0 Zoea 0.2 <0.1 <0.1 Meganyctiphanes norvegica 37.7 29.4 60.9 Megalopa 0.3 0.1 <0.1 Thysanoessa spp 6.9 4.5 6.0 Cirripedia Cypris larvae 0.2 <0.1 <0.1 Thysanoessa spp furcillia Unidentified Euphausiacea 3.7 40.1 32.1 6.3 1.2 23.0 Insecta Hymenoptera 0.5 <0.1 <0.1 Amphipoda 15.9 4.7 4.8 Polychaeta Nereis spp 1.8 1.1 0.1 0.1 0.5 0.4 Hyperiidea Parathemisto gaudichaudi Unidentified Hyperiidea 15.6 9.9 5.7 4.2 3.1 1.0 4.4 2.9 1.5 Unidentified Polychaeta Chaetognatha 0.8 1.1 <0.1 3.6 0.1 <0.5 Gammaridea Caprellidea 0.3 0.2 0.5 <0.1 0.4 <0.1 Hydrozoa 0.3 — <0.1 Gastropoda Pteropoda (Limacina) 5.1 0.8 0.3 Copepoda Calanoidea 8.2 17.4 1.2 Teleost larvae 3.9 0.5 1.4 Centrophages spp Calanus spp 3.1 0.5 1.3 0.7 0.2 <0.1 Ammodytes dubius Unidentified fish larvae 2.7 1.2 0.5 <0.1 1.0 0.4 Metridia spp Unidentified calanoids 2.0 6.9 1.1 14.3 <0.1 0.9 Algae Organic material 1.2 0.6 — 0.2 <0.1 Mysidacea Unidentified remains 6.6 — 0.8 Neomysis americana 0.2 0.4 0.2 Total stomachs with food Total prey number 668 14,752 Cumacea 0.3 <0.1 <0.1 Total prey volume (points) 25,232 cally less than in other areas. Hyperiid amphipods, (Parathemisto gaudichaudi), ranked second in im- portance (%V=10), followed by crustacean larvae (furciliae), calanoid copepods, and fish larvae, (Ammodytes dubius). During summer in the Bay of Fundy, alewives fed heavily on euphausiids (%V=95) but also consumed chaetognaths, mysids, and poly- chaetes (second, third, and fourth in importance). Rankings of IRI values (excluding euphausiids) for Winter-Georges, Winter-Shelf, and Summer-Fundy samples were not significantly correlated (u^O.22, P=0.701), indicating seasonal and geographic differ- ences in the dietary importance of these lesser prey categories. Winter-Fundy samples contained too few prey categories to be analyzed. Diet composition by depth range For Winter-Shelf and Summer-Fundy collections, the proportion of euphausiids in the diet increased with increasing depth (Fig. 3). At bottom depths less than 101 m on the Scotian Shelf, euphausiids com- posed 64% of total volume and 22% of total number; at 101 to 200 m, %V = 83 and %N = 23 and at depths greater than 200 m, %V = 96 and %N = 95. During summer in the Bay of Fundy, euphausiid consump- tion increased with depth such that at less than 101 m, %V = 82 and %N = 35; at 101 to 200 m, %V = 97 and %N = 97; while at depths greater than 200 m, both %V and %N = 100. Other prey categories gen- erally decreased in number with increasing depth as did their relative proportion. For both Winter- Shelf and Summer-Fundy collections, prey diversity and abundance were greatest where bottom depths were less than 101 m. Multiple correlations of IRI values for prey cat- egories (excluding euphausiids) between the three bottom-depth interval groups were not significant (u>=0.54, P=0.12) for Scotian Shelf collections and reflect the decreasing number of prey categories with increasing depth. For Summer-Fundy samples, the Spearman rank correlation of IRI values for the two shallower depth-intervals was not significant (r s =-0.35, P>0.05) and euphausiids were the only prey at depths greater than 200 m. Depth-related differences did not occur in the euphausiid-dominated diet of alewives from the Winter-Fundy and Winter-Georges collections at 162 Fishery Bulletin 92(1), 1994 Winter-Fundy (n = 58) Winter-Georges (n = 147) %N %v 1UU- 60- 20- j^L 20- 60- 100^ I I I I I I 1 1 Winter-Shelf %N %v n = 322) 100- 60- 20- y 20- 60- 100 i I I I I I I I %N %v Summer-Fundy (n - 141) OO-i Legend Eup 60- Cop Pter Amp 20- I CruLar 20- =1 Chaet M Mys 1 Poly 60- 00- 1 1 1 1 1 H- l=J !0% FO t— I = 20% FO Figure 2 Relative importance of prey categories in the diet of alewives, Alosa pseudoharengus, collected from groundfish research surveys off Nova Scotia (1990-91), ranked from highest Index of Relative Im- portance (left to right) by season and area, n = number of stom- achs with prey; '#FO = % frequency of occurrence; %N = % of total prey number; %V = % of total prey volume; Eup = euphausiids; Cop = calanoid copepods; Pter = pteropods; Amp = hyperiid amphipods; CruLar = crustacean larvae; FishLar = fish larvae; Chaet = cha- etognaths; Mys = mysids; Poly = polychaetes. bottom depths exceeding 101 m (no fish were ob- tained at bottom depths less than 101 ml. IRI rankings of" prey categories between depth groups r Georg nk collections were highly correlated • .=0 !9, P fi.01 ' / prey i igorie n present for analysis of Winter-Fundy collections. In both winter and summer, most euphausiids con- med at depths less than 101 m were Thysanoessa pp. v than M. m vegica a) f, r shallower regions < (Table 3). el vari< t t h ou gh i numbers and volumes were ingested during the day (7rN=74, %V=92) than at night ( r /rN=16, %V=85) (Fig. 4). IRI values for day and night collections were not significantly correlated (r s =0.26, P>0.05) reflecting the greater consumption of hyperiid am- phipods during the day and copepods, crustacean larvae and fish larvae at night. Diet composition by size class Diet composition was relatively homo- geneous among alewife size groups (<151 mm, 151-200 mm, 201-250 mm, >250 mm) with euphausiids com- posing most of the total food volume (Fig. 5). Multiple correlations of IRI values for prey categories (excluding euphausiids) by fish length group were significant for both the Scotian Shelf (u;=0. 58, P=0.024) and Georges Bank (w=0.65, P=0.011). For Sum- mer-Fundy collections, diets of the two largest size groups were nearly identical; IRI values were not signifi- cantly correlated (r s =0.38, P>0.05) due to slight differences in the rankings of minor prey categories (i.e., amphipods, mysids, polychaetes, chaetognaths). Prey size composition Alewives ingested similar sizes of M. norvegica during winter (Georges Bank, Scotian Shelf) and summer (Bay of Fundy) (Fig. 6). Modal peaks in euphausiid size appeared at 25-27 mm and 30 mm on the Scotian Shelf and at 30-35 mm for Georges Bank and the Bay of Fundy. In com- parison, M. norvegica from Emerald Basin BIONESS collections in June 1991 were bimodally ibut( it 25-27 mm and 34 mm. Euphausiids larger than 29 mm were proportionately less fre- quent than in stomach contents. Mean lengths of M. norvegica consumed by ale- vives varied by season/area group (F._, 7II| =65.5, P<0.001), although differences between means were small (Winter-Georges: mean=32.1±3.13; Winter- Shelf: mean=28.7±3.72; Summer-Fundy: mean ll.2±3.64). The average size of euphausiids con- d did not differ (F, 50 =3.31, P =0.075) with ale- wife stum;. I tion -ize i ran: 225-300 mm FL). Stone and Jessop: Feeding habits of Alosa pseudoharengus 163 Feeding activity Feeding activity, as indicated by mean stomach fullness index values, varied by season/geo- graphic area (F 3 1910 =46.20, P< 0.001). Mean stomach fullness was highest for Summer-Fundy and Winter-Shelf collections and lowest for Winter-Fundy and Winter-Georges collections (Ta- ble 4). The proportion of feeding fish was highest during summer in the Bay of Fundy (80.6%) and lowest during winter on Georges Bank (33.6%). Stomach fullness was significantly higher at bot- tom depths greater than 200 m for all but the Winter-Shelf col- lections, where mean fullness did not differ among depth groups (Table 4). Similarly, the proportion of feeding fish was highest in areas exceeding 200 m deep for all collections. Alewife feeding activity varied throughout the diel period dur- ing winter (F 1 1(m =24.97, P< 0.001) and summer (F 5 196 = 7.98, P<0.001) with maximum full- %N %V Winter-Shelf 100-| < 101 m (n = 49) 60 20- 20- 60 100 Summer-Fundy 100-1 < 101 m (n = 33) Chaot %N %v 60 20- 20" 60 100 Jzj_P" %N %v 100- 101-200 m (n = 262) 60- fl j L 20" 60- 1 1 1 I I 1 i i %N %V 100- 101-200 m 200 m (n = 11) 60- 20- M 20" 60- - 100- I I I I i i i %N %V > 200 m (n = 36) 60- 20- 20-1 60- 1 'III 1 1 1 i = 20% FO i = 20% FO Figure 3 Relative importance of prey categories in the diet of alewives, Alosa pseudoharengus, obtained from groundfish research surveys off Nova Scotia (1990-91), ranked from highest Index of Relative Importance (left to right), by depth range, for Scotian Shelf (winter) and Bay of Fundy (summer) collections. (Symbols as in Fis;. 2). at 7.43°C during summer (Table 5). The winter daily ration of alewives generally decreased from 1.95% BW for fish less than 151 mm FL to 1.13% BW at 151-200 mm FL, 1.19% BW at 201-200 mm FL and 1.00% BW at larger than 250 mm FL. Discussion Our study clearly indicates that alewives off Nova Scotia feed primarily on euphausiids, particularly Meganyctiphanes norvegica; much smaller contribu- tions are made by other prey. Alewives from the 164 Fishery Bulletin 92(1), 1994 Table 3 Mean number of Meganyctiphanes norvegi :a and Thysanoessa spp. in the stomachs of alewives , Alosa pseudoh irengus , by depth interval within season and geogra phic area from groundfi sh researc h surveys off Nova Scotia (1990-91). n = nx. mber of s tomach s with prey. Depth M in i vegica Thysanoessa spp . Season and area (mi Mean t SD n Mean + SD n Winter-Fundy 101-200 11.3 + 7.36 3 5.4 + 0.81 5 >200 10.2 + 2.20 9 2.5 i 1.50 2 Winter-Georges 101-200 5.9 ± 0.81 26 11.8 * 6.55 6 >200 20.3 + 1.68 28 — — — Winter-Shelf <101 — — — 32.0 + 14.63 10 101-200 14.7 + 1.41 89 9.6 ± 3.79 20 >200 5.8 1 3.47 1 — — — Summer-Fundv <101 18.9 + 3.47 14 12.0 ± 2.00 2 101-200 21.9 - 2.67 48 — — — >200 27.5 ± 2.39 31 23.0 — 1 100-1 %N %v 100 Night (n = 290) %N %V Legend Eup 100 100 "i — i — i — r i = 20% FO Figure 4 Relative importance of prey categories in the diet of alewives, Alosa pseudoharengus, obtained from groundfish research surveys off Nova Scotia ( 1990-91), ranked from highest Index of Relative Importance (left to right) for day and night collections (Symbols as in Fig. 2). Atlantic seaboard of the United States consumed relatively fewer euphausiids 137-56% by weight) (Edwards and Bowman, 1979; Vinogradov, 1984) than off Nova Scotia (82-99% by volume). Euphausiids represent a large component of the marine zooplankton community and are abundant in the Bay of Fundy (Kulka et al., 1982; Locke and Corey, 1988), Gulf of Maine (Bigelow, 1926), the deep basins of the Scotian Shelf (Herman et al., 1991) and the outer shelf and shelf slope (Sameoto, 1982). Given their two-year life cycle (Hollingshead and Corey, 1974; Berkes, 1976), the availability and rela- tive abundance of euphausiids is more seasonally stable than for other prey spe- cies (i.e., chaetognaths, hyperiid amphipods, calanoid copepods, mysids), most of which undergo fluctuations in abundance progressing from a spring low to a summer high before declining in fall and win- ter (Evans, 1968; Sherman and Schaner, 1968; Corey, 1988; McLaren et al., 1989). Small seasonal differences in diet composition reflect the op- portunistic foraging behaviour of alewives and the availability of food types from offshore re- gions during winter as com- pared with the Bay of Fundy in summer. During winter, the diet diversity of alewives was greatest on the Scotian Shelf probably because the late win- ter (mid-March) sampling period co- incides with the hatching and occur- rence of the larval forms of Thy- sanoessa spp. (Berkes, 1976) and Ammodytes dubius (Scott, 1972), both of which occurred only in the diet of alewives from the Scotian Shelf. In the Bay of Fundy, alewife consumption of chaetognaths and mysids in the summer reflects their increased abundance and availabil- ity (Sherman and Schaner, 1968; Corey, 1988). The increased proportion of eu- phausiids in the diet of alewives from the Scotian Shelf (winter) and the Bay of Fundy (summer) coin- cides with an increased relative abundance of euphausiids with in- creasing depth. In the Scotian Shelf Basins, M. norvegica occur between 170 m and the bottom with highest concentrations generally below 200 m (Sameoto et al., 1993). In the Bay of Fundy, M. norvegica is most abundant where bottom depths are between 165 and 200 m, while Thysanoessa inermis occur between 95 and 155 m (Kulka et al., 1982). The greater proportion and number of other prey categories at depths less than 101 m on the Scotian Shelf and in the Bay of Fundy likely result from decreased euphausiid abundance (thereby in- creasing the relative contribution of other prey) rather than an absolute increase in the abundance of other zooplankters. Depth-related variation in Stone and Jessop: Feeding habits of Alosa pseudoharengus 165 euphausiid species composition in the diet of alewives from all regions matches differences in the bottom depth preferences of M. norvegicci (>150 m) and Thysanoessa spp. (100- 150 m) (Berkes, 1976; Kulka et al., 1982; Sameoto et al., 1993). Diel differences in the diet of ale- wives may reflect the influence of vary- ing light intensity on prey availability and on their relative success in locat- ing and capturing prey. Consumption of microzooplankters (crustacean larvae, calanoid copepods) was greater at night perhaps because of increased filter- feeding activity (Janssen, 1978b). Con- versely, ingestion of macrozooplankters (euphausiids, hyperiid amphipods) may be highest during the day because visual cues favour a particulate-feeding mode. Large size, darkly pigmented eyes, and a habit of forming large concentra- tions (Mauchline and Fisher, 1969) may make M. norvegica easily detect- able by alewives during daylight whereas at night, photophores along the abdomen of M. norvegica may as- sist detection. Most euphausiid species migrate vertically over the diel period, rising from deep water (150-200 m) towards the surface at dusk, remaining near surface throughout the night, and then migrating to the depths at dawn (Mauch- line, 1984). Alewives also have a diel pattern of vertical migra- tion in the marine environment (Neves, 1981; Stone and Jessop, 1992) and may encoun- ter sufficient light higher in the water column at night to par- ticulate feed on euphausiids. Ontogenetic differences in diet composition were not ap- parent; euphausiids dominated the diet of alewives ranging in length from 95 to 305 mm. Ale- wives switch from feeding pri- marily on microzooplankton to macrozooplankton at some point during their marine de- velopment and like other simi- larly sized clupeids, concentrate their feeding at intermediate trophic levels (James, 1988). Winter- Shelf 100 E O > CD 1) u n. 80 GO 40 20 35 12 141 134 .;;:;' Winter- Georges 13 65 34 3S Summer- Fundy 66 S3 Legend Eup Cop Ptar Amp CruLar FIshLar Chaet Mys Poly B C C D Predator length group c D Figure 5 Percentage of total volume of prey categories in the diet of ale- wives, Alosa pseudoharengus, for different size classes (mm FL) obtained from groundfish research surveys off Nova Scotia (1990- 91). Euphausiids were the only prey category in Winter-Fundy cruises. A: <151 mm; B: 151-200 mm; C: 201-250 mm; D: >250 mm; Eup = euphausiids; Cop = calanoid copepods; Pter = ptero- pods; Amp = hyperiid amphipods; CruLar = crustacean larvae; FishLar = fish larvae; Chaet = chaetognaths; Mys = mysids; Poly = polychaetes; n = number of stomachs with food. Winter-Georges Winter-Shelf Summer-Fundy BIONESS (n = 257) (n - 269) (n = 178) (n = 785; 25 30 35 Total length (mm) 45 Figure 6 Size frequency distributions of M. norvegica consumed by alewives, Alosa pseudoharengus, obtained from winter (Georges Bank. Scotian Shelf) and summer (Bay of Fundy) groundfish surveys off Nova Scotia (1990-91) and from BIONESS samples in Emerald Basin (Spring, 1991 ). n = sample size. 166 Fishery Bulletin 92(1), 1994 Table 4 Mean stomach fullness index (arcsine Vp transformed) by season and geographic area and by depth interva for al ewives, Alosa pseudoharengus, obtained from groundfish research surveys off Nova Scotia (1990-91) Mean fullness inde x values lacking a letter in com- mon are significantly differe nt (Tukey HSD, P<0.05). n = number of stomachs examined (including empty stomach s). Full ness index (%BW) % with Season and area Depth (m) n food Mean i SD Maximum Winter-Fundy all 112 51.8 2.3z i 0.25 9.9 Winter-Georges all 438 33.6 2.1z ± 0.13 9.9 Winter-Shelf all 489 65.0 3.8y ± 0.10 10.0 Summer-Fundy all 175 80.6 3.9y + 0.21 10.0 Winter-Fundy 101-200 60 28.3 1.5z t 0.29 9.4 >200 52 78.8 3.7y + 0.32 9.9 Winter-Georges <101 7 28.6 2.1z i 0.87 5.9 101-200 376 28.7 1.7z t 0.12 9.9 >200 55 67.3 4.6y * 0.46 9.9 Winter-Shelf <101 92 55.3 3.4z i 0.16 7.4 101-200 385 68.1 3.9z + 0.12 10.0 >200 12 91.7 3.4z + 0.46 8.0 Summer-Fundy <101 48 liS s 3.5z + 0.33 8.8 101-200 87 82.8 3.6z + 0.30 10.0 >200 40 90.1 5.0y t 0.51 9.8 Gilmurray (1980) found mainly microplanktonic prey (e.g., calanoid copepods, cypris larvae, insects) in the diet of alewives less than 80 mm FL obtained from tidal creeks in the upper Bay of Fundy. The shift towards consumption of macrozooplankton likely occurs at fish sizes smaller than those examined in the present study (i.e., <95 mm FL). Diel feeding activity during winter and summer, as indicated by the mean fullness index, reached a maximum near mid-day and is typical of size-selec- tive predators which rely on visual cues (Eggers, 1977). Summer resident subadult alewives in Minas Basin display a similar feeding pattern, although peak feeding occurred later in the afternoon (1500 hours), coincident with the time of high tide when turbidity was lowest and prey visibility highest (Stone, 1985). Summer feeding activity by juvenile anadromous alewives in freshwater also peaks dur- ing the day but ceases or declines overnight ( Jessop, 1990). Nocturnal feeding by alewives was more ap- parent during winter than summer; the significance of this seasonal difference in feeding activity is un- clear. Alewives can and do feed efficiently at night using both particulate (Janssen and Brandt, 1978) and filter-feeding (Janssen, 1978b) modes. Alewives greater than 200 mm FL generally consumed the largest Meganyctiphanes avail- able. Length-frequency distri- butions of M. norvegica, which has a life span of about two years, are typically bimodal (Hollingshead and Corey, 1974; Berkes, 1976). Alewives selec- tively favor larger prey (Brooks and Dodson, 1965; Brooks, 1968; Wells, 1970) and likely use a particulate feeding strat- egy in doing so. Slight seasonal and geographic differences in the average size of M. norvegica ingested likely reflect size differences in euphausiid populations rather than selec- tion by the predator. Daily ration calculations were based on the model of El- liott and Persson (1978) which was originally intended for field samples collected within a given area from the same popu- lation over time. Our stomach fullness data for alewives from the Bay of Fundy, Georges Bank, and the Scotian Shelf covered a wide area geographically and may involve more than one popu- lation. The broad temporal and spatial coverage reduces the effect of day-to-day and regional varia- tions in diet which would arise from more restricted sampling. Calculated daily ration levels for alewives off Nova Scotia were similar to those reported for other teleosts (Fange and Grove, 1979). Lower esti- mates were obtained during winter (1.22% BW at 7.16°C) than for summer ( 1.88% BW at 7.43°C) since temperature is related to metabolic requirements and to the evacuation rate of stomach contents (Durbin et al., 1983). Both estimates are well above maintenance ration levels for temperatures in the 7-8°C range and are sufficient for positive growth (Brett and Groves, 1979). Alewife daily ration de- clined with increasing fish size; small fish, includ- ing marine species such as North Sea cod, Gadus morhua (Daan, 1973), winter flounder, Pseudo- pleuronectes arnericanus (Huebner and Langton, 1982) and silver hake, Merluccius bilinnearis (Durbin et al., 1983), generally consume proportion- ally more food per unit weight than large fish (Windell, 1978). Overall, our estimates of daily Stone and Jessop: Feeding habits of Alosa pseudoharengus 167 Table 5 Mean amount of food (%BW) in the stomachs of alewives, Alosa pseudoharengus , obtained from groundfish surveys off Nova Scotia (1990-91), with estimates of food consumption (C.) and daily ration {DR = XCJ, by season and size c lass, n = number of stomachs examined (including empty stomachs. For winter collections, R = 0.0925, temperature = 7.16°C; 'or summer collections, R = 0.0954, tern perature = 7.43°C. Stomac h contents (%BWl Season (size class) Time period (hr) c, (%BW) DR n Mean + SD (%BW) Winter (all) 2400-0300 84 0.75 + 0.100 1.216 0300-0600 184 0.65 * 0.053 0.098 0600-0900 97 0.23 ± 0.038 -0.308 0900-1200 122 0.52 + 0.079 0.401 1200-1500 96 1.09 i 0.104 0.792 1500-1800 150 0.20 ( 0.032 -0.709 1800-2100 177 0.52 ± 0.043 0.421 2100-2400 189 0.42 i- 0.053 0.033 0.488 Summer (all) 2400-0400 11 0.22 i 0.041 1.880 0400-0800 5 0.58 + 0.198 0.515 0800-1200 61 2.32 ± 0.161 2.316 1200-1600 74 1.32 • 0.190 -0.320 1600-2000 19 0.48 i 0.249 -0.508 2000-2400 5 0.03 + 0.031 -0.357 0.234 Winter 2400-0400 29 0.95 .i 0.233 1.949 (<151 mm FL) 0400-0800 31 1.13 ± 0.146 0.563 0800-1200 4 0.90 + 0.382 0.143 1200-1600 3 1.32 + 0.439 0.842 1600-2000 13 0.42 ± 0.110 -0.593 2000-2400 87 0.55 i 0.103 0.311 Winter 2400-0300 22 1.10 + 0.199 1.126 (151-200 mm FL) 0300-0600 IS 0.61 + 0.095 -0.253 0600-0900 26 0.12 + 0.048 -0.396 0900-1200 29 0.22 i 0.063 0.151 1200-1500 9 0.84 ± 0.532 0.765 1500-1800 119 0.14 t 0.039 -0.565 1800-2100 26 0.74 t 0.153 0.719 2100-2400 25 0.16 * 0.053 -0.457 Winter 2400-0300 30 0.52 ± 0.094 1.189 (201-250 mm FL) 0300-0600 60 0.62 i 0.093 0.256 0600-0900 23 0.27 + 0.062 -0.222 0900-1200 :.] 0.53 * 0.123 0.372 1200-1500 27 0.93 i 0.192 0.600 1500-1800 56 0.31 + 0.065 -0.453 1800-2100 42 0.61 + 0.075 0.434 2100-2400 36 0.50 + 0.093 0.046 0.156 Winter 2400-0300 14 0.24 t 0.052 1.000 O250 mm FL) 0300-0600 34 0.32 + 0.042 0.161 0600-0900 17 0.27 t 0.067 0.029 0900-1200 39 0.68 + 0.173 0.545 1200-1500 57 1.19 ± 0.124 0.772 1500-1800 22 0.11 I 0.032 -0.902 1800-2100 39 0.31 t 0.076 0.257 2100-2400 11 0.50 * 0.141 0.302 168 Fishery Bulletin 92(1], 1994 x •a c 200 m) and temperature, were suitable for M. norvegica (Kulka et al., 1982; Sameoto et al., 1993). Alewives prefer bottom temperatures of 7— 11°C off- shore at mid-depths in spring ( 101-183 m), in shal- lower nearshore waters in summer (46-82 m) and in deeper offshore waters in fall ( 119-192 m) (Stone and Jessop, 1992). During winter, Meganyctiphanes seeks deeper, warmer water rather than the cold upper layers (Bigelow, 1926; Hollingshead and Corey, 1974). While the seasonal pattern of move- ment by alewives (inshore and northward during spring and offshore and southward during fall) is partially linked with spawning migrations, it is apparent that their marine distribution is also in- fluenced by the distribution, availability, and abun- dance of their main prey, M. norvegica. Acknowledgments We thank D. Sameoto and R. Cutting for critically reviewing earlier drafts of the manuscript. We also wish to thank M. Strong, P. Fanning, and J. Martell for collecting the alewives used in our analyses, S. Wilson and J. Tremblay for taxonomic assistance, and D. Ingraham for helping with laboratory work. Literature cited Ackman, R. G., C. A. Eaton, J. C. Sipos, S. N. Hooper, and J. D. Castell. 1970. Lipids and fatty acids of two species of North Atlantic krill (Meganyctiphanes norvegica and Thysanoessa inermis) and their role in the aquatic food web. J. Fish. Res. Board Can. 27:513-533. Berkes, F. 1976. Ecology of euphausiids in the Gulf of St. Lawrence. J. Fish. Res. Board Can. 33:1894- 1905. Bigelow, H. G. 1926. Plankton of the offshore waters of the Gulf of Maine. Bull. U.S. Bur. Fish. 40:1-509. Bigelow, H. B., and W. C. Schroeder. 1953. Fishes of the Gulf of Maine. Bull. U.S. Fish Wildl. Serv. 74, 577 p. Bowman, R. E. 1986. Effect of regurgitation on stomach content data of marine fishes. Env. Biol. Fish. 16:171-181. Stone and Jessop: Feeding habits of Alosa pseudoharengus 169 Brett, J. R., and T. D. D. Groves. 1979. Physiological energetics. In W. S. Hoar, D. J. Randall, and J. R. Brett (eds.), Fish physiology, Vol. 8, p. 280-344. 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Convergence of accoustic, optical, and net- catch estimates of euphausiid abundance: use of artificial light to reduce net-avoidance. Can. J. Fish. Aquat. Sci. 50:334-346. Scott, J. S. 1972. Eggs and larvae of northern sand lance tAmmodytes dubius) from the Scotian Shelf. J. Fish. Res. Board Can. 29:1667-1671. Sherman, K., and E. G. Schaner. 1968. Observations on the distribution and breed- ing of Sagitta elegans (Chaetognatha) in coastal waters of the Gulf of Maine. Limnol. Oceanogr. 13:618-625. Siegel, S. 1956. Nonparametric statistics for the behavioral sciences. McGraw-Hill Book Co., Toronto, 312 p. Sokal, R. R., and F. J. Rohlf. 1981. Biometry, 2nd ed. H. Freeman, San Fran- cisco, 859 p. Steedman, H. F. 1976. General and applied data on formaldehyde fixation and preservation of marine zoo- plankton. In H. F Steedman (ed.), Zooplankton fixation and preservation, p. 103-154. Unesco Press, Paris, 350 p. Stone, H. H. 1985. Composition, morphometric characteristics and feeding ecology of alewives (Alosa pseudoharengus) and blueback herring (A. aestivalis) (Pisces: Clupeidae) in Minas Basin. Master's thesis, Acadia Univ., Wolfville Nova Scotia, 191 p. Stone, H. H., and G. R. Daborn. 1987. Diet of alewives, Alosa pseudoharengus and blueback herring, A. aestivalis (Pisces: Clupeidae) in Minas Basin, Nova Scotia, a turbid macrotidal estuary. Env. Biol. Fish. 19:55-67. Stone, H. H., and B. M. Jessop. 1992. Seasonal distribution of river herring Alosa pseudoharengus and A. aestivalis off the Atlantic coast of Nova Scotia. Fish. Bull. 90:376-389. Swynnerton, G. H., and E. B. Worthington. 1940. Notes on the food of fish in Haweswater (Westmoorland). J. Anim. Ecol. 9:183-187. Vigerstad, T. J., and J. S. Cobb. 1978. Effects of predation by sea-run juvenile ale- wives ( Alosa pseudoharengus) on the zooplankton community at Hamilton Reservoir, Rhode Island. Estuaries 1:36-45. Vinogradov, V. I. 1984. Food of silver hake, red hake and other fishes on Georges Bank and adjacent waters, 1968- 74. NAFO Sci. Counc. Studies 7:87-94. Warshaw, S. J. 1972. Effects of alewives (Alosa pseudoharengus) on the zooplankton of Lake Warskopmic, Con- necticut. Limnol. Oceanogr. 17:816-825. Wells, L. 1970. The effects of alewife predation on zooplankton in Lake Michigan. Limnol. Oceanogr. 15:556-565. Windell, J. T. 1978. Digestion and the daily ration of fishes. In T Bagenal (ed.), Fish production in fresh waters, p. 227-254. Blackwell Scientific Pubis., London, 365 p. Abstract. A survey of queen conch [Strombus gigas) popula- tions near Lee Stocking Island, Exuma Cays, Bahamas, showed that 74% of all adults were on the narrow island shelf adjacent to the Exuma Sound, in 10-18 m of wa- ter. None were found deeper than 25 m, and relatively few adults were found shallower than 10 m. Numbers of juveniles were great- est on the Great Bahama Bank and decreased with increasing depth on the island shelf. No juve- niles were found in shelf regions greater than 15 m in depth. Pat- terns of shell morphology, which were related to growth rates in juveniles, suggest that adults that mature on the Great Bahama Bank rarely move to deep water, and that the most important sources for deep-water stocks are small, nearshore nurseries on the island shelf. The mostly unfished deep-water populations are prob- ably now the primary source of larvae for queen conch in the Exuma Cays. Because virtually all of the conch are within the limits of SCUBA diving, it will be impor- tant to identify and to protect criti- cal nursery habitats for reproduc- tive stocks. Queen conch, Strombus gigas, reproductive stocks in the central Bahamas: distribution and probable sources Allan W. Stoner Kirsten C. Schwarte Caribbean Marine Research Center. 805 E 46th Place Vera Beach, Florida 32963 Manuscript accept 8 September 1993 Fishery Bulletin 92:171-179 (1994) Queen conch (Strombus gigas), once abundant throughout the Car- ibbean region, have been fished to near extinction or to a level at which there is no longer a viable fishery in many localities (Appel- doorn et al., 1987; Berg and Olsen, 1989). This is particularly true in nations where the fishery has been open to SCUBA divers. Stock depletion resulted in at least tem- porary closures of the conch fishery in Bermuda, Florida, Cuba, Bon- aire, and the U.S. Virgin Islands. Regulations including size limits, catch quotas, gear restrictions, and closed areas have been instituted in other countries. This study was conducted in an attempt to understand reasons for the rapid depletion of queen conch populations in the Caribbean re- gion, and to evaluate the signifi- cance of deep-water conch stocks. Several authors have suggested that these deep-water conch, living beyond the normal range of free divers, are the primary source of larvae for shall-water populations and the fishery (Berg and Olsen, 1989; Wicklund et al., 1991; Stoner et al., 1992; Stoner and Sandt, 1992). Therefore, we surveyed the density and age structure of queen conch in the vicinity of Lee Stock- ing Island in the central Bahamas. Differences in shell morphology and growth rate between conch found on Great Bahama Bank and on the windward island shelf adja- cent to Exuma Sound were used as indicators of geographic source for reproductive stocks. The impor- tance of deep-water populations is discussed in terms of fisheries management. Methods and materials Study site An assessment of the adult conch population was conducted between 1989 and 1991 in a 12-km long sec- tion of the Exuma Cays, central Bahamas, adjacent to Lee Stocking Island (Fig. 1). To the west and south of the Cays lies the Great Bahama Bank, a shallow, sand- and seagrass-covered platform that extends to the Tongue of the Ocean. To the east and north is a narrow (1-2 km) island shelf, a steep shelf- break beginning at an approxi- mately 30-m depth, and the deep Exuma Sound. Great Bahama Bank in the re- gion of the study site is character- ized by strong tidal currents that carry oceanic water from Exuma Sound onto the bank through chan- nels between the islands. Approxi- mately 909r of the bank area is less than 3.5 m deep; the remainder is tidal channels with depths to 8 m near the inlets and between the Brigantine Cays. For this study the I 71 172 Fishery Bulletin 92(1). 1994 Figure 1 Map of the survey area near Lee Stocking Island, Exuma Cays, Bahamas. Flood tidal cur- rents (arrows) and the locations of queen conch, Strombus gigas, nursery habitats (cross- hatched) are shown. Dashed lines separate the inner and outer bank regions and delineate the study site. Areas south and west of the Brigantine Cays were not surveyed. bank was divided into an inner section from the Brigantine Cays to a line mid-way between the Brig- antines and Lee Stocking Island, and an outer sec- tion from the mid-line to the cays at the eastern side of the bank (Fig. 1). Each section is approximately 5 km wide. The rationale for this division was that the outer section of the bank is flushed with oceanic water on every tide, while the inner bank is flushed only on extreme tides. Virtually all queen conch nurs- eries in the Exuma Cays are found within the outer 5.0 km of the bank (Stoner et al., in press.) (Fig. 1). The eastern shores of the Exuma Cays are char- acterized primarily by steep aeolianite cliffs and beach rock interspersed with a few high-energy sandy beaches in coves, particularly on Lee Stock- ing Island and Children's Bay Cay. The seagrass Thalassia testudinum is found on shallow, soft-sedi- ment platforms extending a short distance off the sandy beaches. Most of the shallow nearshore, how- ever, is hard-bottom covered with a short turf of the green alga Cladophoropsis sp. The hard-bottom habitat, interspersed with small patches of sand and hard corals, is characteristic to 10-m depth. From 10 to 20 m the bottom comprises mixed hard-bottom and bare sand. Off Lee Stocking Island, corals form a 2-km long steep ledge from 10 to about 18 m, but a gradual slope to 25 m is typical of most of the study area. Patchy sand, coral, and hard-bottom are found between 20 and 30 m. Detailed hydrographic charts are not available for the Exuma Cays; therefore, shelf bathymetry was mapped with 540 electronic depth-sounder points, corrected for tidal state, and positions acquired with Global Positioning System (GPS) from the RV Chal- lenger during summer 1991. GPS positions taken at close intervals along the eastern shores of the is- lands were used as zero-depth data points. Three- dimensional plotting features of Systat 5.0 software were used to provide a bathymetric chart for the shelf region with 0, 2.5, 5, 10, 15, 20, 25 and 30 m contours for depth at mean low tide. Total surface area for each of the seven depth intervals was cal- culated with a digitizing board and SigmaScan 3.9 software. The surface area of the inner and outer bank regions was determined in a similar way with the aid of topographic maps. Stoner and Schwarte: Distribution of Strombus gigas 173 Survey methods The shelf region was surveyed in each of seven depth zones between 2.5 and 30 m (described above) along nine transects (perpendicular from the Cays into Exuma Sound) placed at approximately 1.0-km intervals. At each of the 63 shelf stations, divers swam parallel to the isobaths for a distance mea- sured with a calibrated General Oceanics flow meter equipped with a large propeller for low velocity flows. Calibration was performed by towing the meter repeatedly (n>6) through calm water at the side of a small boat over a pre-measured distance of 100 m. Precision was ±2%. Current velocity on the shelf adjacent to Lee Stocking Island is generally low (<3 cm/sec) and to the northwest, parallel to the isobaths (Smith, 1992 1 ). Recognizing the potential effect of current on the calculated distance, each dive included two legs, one up-current and one down- current in parallel lines of equal length separated by approximately 20 m. Two dives were made at most stations for density determinations and shell measurements (described below). For density, all queen conch were counted in an 8-m wide path defined by a line held between two divers. The average swim distance was 380 m, resulting in coverage of just over 3000 m 2 . Conch density was calculated by using only those conch in the 8-m band. Shell measurements were made for animals outside the 8-m band in areas with low conch densities. Underwater visibility was usually high and the area of bottom searched was actually much larger than the swim path alone. Conse- quently, all conch within approximately 30 m could be collected for measurement. In areas where conch densities were high, one dive was made to collect density data and another to collect only measure- ment data. An attempt was made to measure at least 100 adults from each depth zone, but this was not possible in the 0-5, 5-10, and 25-30 m zones because of low densities in these zones. Statistical differences in density among the survey zones were evaluated with the non-parametric Kruskal-Wallis test (Sokal and Rohlf, 1969) with stations used as replicates (n-9). The shallowest depth zone (0-2.5 m) was limited primarily to sandy coves on the major islands of the survey area. Adult queen conch were few in these areas, and juveniles were distributed unevenly; therefore, the important seagrass areas of the shal- low coves were thoroughly searched. Density mea- sures were not made but all conch encountered were measured (as described below). 1 N. P. Smith, Harbor Branch Oceanography Inst., Fort Pierce. FL, pers. commun. 1992. Sparse distribution of adult conch and the large surface area of the Great Bahama Bank required the use of different survey methods from those applied on the shelf. Because the bank waters are shallow and conch were easily seen, large areas were sur- veyed by towing a diver at the surface in continu- ous lines. The bank region was divided into 95 — 1 x 1 km squares oriented along lines of latitude. Then, in a systematic grid of lines running diago- nally through the squares, every square was crossed at least once during the survey. Additional tows were made in areas already known to have concentrations of adults, i.e., near nurseries previously mapped (Fig. 1; Stoner et al., in press.). Divers were towed a total distance of 126 km. Although water clarity on the bank was not as high as that on the island shelf, the towed diver could usually see at least 2.5 m on either side of the transect line. Surveys were not conducted on a few days when visibility was restricted. While being towed at approximately 50 cm/sec, the diver signaled numbers of adult queen conch to the boat operator, who recorded position. Positions for the ends of all straight line transects were determined with GPS, tow distance was estimated by chart, and conch density was calculated on the basis of the 5-m wide path examined. During the bank survey, 472 adults were gathered and measured. Presence of juveniles on the bank was noted but not quantified in this study. For comparison with shelf sites, a random collection of 322 juvenile conch was made from a nursery west of Lee Stocking Island during August 1991. These conch were measured for shell length. The total number of adult queen conch was esti- mated crudely for each bank and shelf area by ex- trapolating the average density of conch for an in- dividual zone over the total surface area for the same zone. Because variances in the density data were large, confidence intervals for the extrapolated numbers of conch were not calculated. Shell measurements Queen conch reach sexual maturity between 3.5 and 4 years of age, a few months after the shell edge has formed a broadly flared lip (Appeldoorn, 1988). Af- ter the lip flares, queen conch stop growing in length but continue to deposit shell material on the inside of the lip (Egan, 1985; Appeldoorn, 1988). Therefore, with certain limitations, thickness of the shell lip is an indicator of approximate age (Stoner and Sandt, 1992). In this study, shell-lip thickness was mea- sured with calipers in the area of greatest thickness, about two-thirds of the distance posterior from the 174 Fishery Bulletin 92(1), 1994 siphonal groove and 35 mm in from the edge of the shell, according to the methods of Appeldoorn (1988) and Stoner and Sandt (1992). Shell length was measured from the tip of the spire to the end of the siphonal canal in both adults and juveniles. Re- peated measures made by different persons showed that both length and lip thickness measurements were made to ±1 mm. Differences in length-fre- quency and thickness-frequency distributions were tested with the non-parametric Kolmogorov- Smirnov test. Morphological differences between bank and shelf populations were tested with canonical discriminant function analysis from shell length and lip thickness data. This multivariate technique is well suited for differentiating two types where individual charac- teristics do not separate the types. The analysis computes a third variable Z, which is a linear func- tion of both variables (length and thickness, in this case) such that the equation for the new line maxi- mizes the distance between the two types ( Sokal and Rohlf, 1969). The significance of the discriminant function Z was determined with the Hotelling- Lawley trace test statistic (Morrison, 1976). Results of the canonical analysis were then examined to determine what percentage of the individuals were correctly classified according to collection site. Observations were also made on general shell thickness (particularly in juveniles), length of api- cal spines and resultant shell diameter, and num- ber of spines per whorl. None of these characteris- tics were quantified systematically. Shell growth experiment Early observations suggested that shell phenotypes were different between shelf and bank conch. Adults from the shelf appeared to be longer and to have thicker shell lips than those from the bank. Juve- niles from the shelf were more narrow, thin-lipped, and had shorter apical spines than those on the bank (Martin-Mora, 1992). To examine the potential relation between shell morphology and growth rates, juveniles were tagged in two different nursery sites: in the well-studied nursery west of Children's Bay Cay and in seagrass areas off Charlie's Beach in the northeast cove of Lee Stocking Island (Fig. 1). Ju- veniles were individually marked with spaghetti tags (Floy Tag & Manufacturing Co.) tied around the spire and measured to the nearest millimeter with calipers. Charlie's Beach conch between 108 and 150 mm (mean=137 mm, n=281) were measured and released in the last week of August 1990. Children's Bay Cay conch, somewhat smaller than the Charlie's Beach conch (106 to 133 mm, mean=118 mm. n=292), were tagged and released in early Septem- ber 1990. Conch from both populations were remeasured for shell length five months later, at the end of February 1991. Forty-eight conch were recov- ered at Charlie's Beach and 135 were recovered at the Children's Bay Cay site. Daily growth rate was calculated for individuals by dividing increase in length by the number of days between measure- ments. Differences in growth rate between the two sites were evaluated by using the Mann-Whitney U- test. Results Conch densities and abundance Densities of adult queen conch in the survey area were highest between 15 and 20 m depth on the island shelf (Table 1) with nearly 88 conch/ha (Fig. 2). Density was also high between the 10- and 15-m isobaths. In both of these depth zones densi- ties of adults were highly variable, but there was no apparent pattern across transect lines. There was a highly significant difference in the density of adult conch in the survey zones (Kruskal-Wallis test, H adj =36.195, P<0.001KFig. 2). No conch were found deeper than 25 m, despite an abundance of appar- ently suitable habitat of sand and algae-covered hard-bottom. Adults were most sparsely distributed 50 20 • 10 o c If) c; v Q U C o O 120 100 80 60 40 20 r+-. Eh Inner Outer Bank •- «- CN ^ Depth in (Meters) Shelf Figure 2 Density of queen conch, Strombus gigas, on the Great Bahama Bank and in six different depth zones of the island shelf hear Lee Stocking Island, Bahamas. Values are ± mean standard error of the Stoner and Schwarte: Distribution of Strombus gigas 175 Table 1 Estimated total number o f adu It queen conch, Strombus gigas, in a 12- km section o fth e Exuma Cays , Bahamas, between Adderly Rocks and Rat Cay. Density (no. /ha) Region Total area (h a) (mean ± SE of mean) Total no. of conch Bank Inner 4,979 0.19 ± 0.14 946 Outer 3,997 3.16 ± 1.69 12,631 Bank total 8,976 13,577 Shelf 0-2.5 m 161 Low — not qualified Negligible 2.5-5 m 198 2.24 ± 1.70 444 5-10 m 465 7.21 ± 4.11 3,353 10-15 m 429 60.1 ± 46.8 25,800 15-20 m 454 87.9 ± 31.5 39,902 20-25 m 320 18.3 ± 9.1 5,843 25-30 m 151 ± Shelf total 3,687 75,342 Grand Total 12,663 88,919 in the inner section of the Great Bahama Bank with only 0.19 conch/ha (SD=0.15, n=28). Density of adult conch in the outer (seaward) section of the bank was close to the value for the 2.5-5 m depth zone on the shelf. Although not quantified, numbers of adults in the nearshore (0-2.5 m) zone of the shelf were negligible. Few juvenile conch were observed on the shelf between 2.5-and 15-m depth (Fig. 2). A total of 372 juveniles were found in densely aggregated patches on seagrass beds off the eastern beaches of Lee Stocking Island. On the Great Bahama Bank, most juveniles were aggregated in specific nursery loca- tions documented previously (Stoner et al., in press.). None was found deeper than 15 m. The area between 10 and 20 m depth on the is- land shelf was a particularly important habitat for adult queen conch (Table 1). Approximately 74% of all conch in the 12-km long survey area reside in this narrow depth zone. It is also clear that large expanses of shallow bank habitat support a rela- tively small proportion (15.2%) of the adult popula- tion. Mating conch and demersal egg masses were very abundant during summer months at shelf sites deeper than 10 m, but none were observed on the bank. Shell morphology The shelf sites were characterized by large adult queen conch, primarily between 200 and 260 mm (mean=227, SD=23, n=572), whereas most adult conch on Great Bahama Bank were between 170 and 210 mm shell length (mean=187, SD=16, n=472). Pooling all adults measured, there was a highly significant difference in the length-frequency distribu- tion of conch on the shelf and on the bank (Kolmogorov- Smirnov test, P<0.001). The distributions (Fig. 3) show clearly the separation in size of adults between bank and shelf sites, particularly when com- paring nearshore (0-5 m) shelf zones with those from the bank. The distributions show a decrease in shell length be- tween the nearshore shelf and deeper zones, while those be- tween 5 and 25 m are obviously similar. Bank conch had thin shell lips (mean=10, SD=6); conch from nearshore (2.5-5 m) regions of the shelf were intermediate in lip thickness (mean=18, SD=5); and deep-shelf (5-25 m) conch had the thickest shell lips (mean=30, SD=7)(Fig. 4). All three of these groups were significantly different from one another in terms of lip thickness distribution (Kolmogorov- Smirnov tests, P<0.01). There was obvious similar- ity in the thickness distributions of shells in depth zones between 5 and 25 m; therefore, these four depth categories were pooled. Distinctness of the morphs collected on the bank and shelf is further suggested by a plot of shell length and lip thickness for 250 randomly chosen individuals from each of the two regions (Fig. 5). Also, when length and lip thickness data for all 1,029 conch measured in the survey were used in canonical discriminant function analysis, a highly significant separation was found between conch col- lected in the two different regions (Hotelling-Lawley Trace, F= 1,854, P<0.001). Less than 5% of the conch in the survey were not collected in the region pre- dicted by the multivariate equation. Bank conch were small and had thin shell lips, whereas conch from the island shelf were large and had thick shell lips. Results of the analysis, however, do not rule out the possibility that the smallest adult conch from the shelf region, particularly apparent in the 5-10 m depth zone, could be older animals from the bank. Length-frequency distributions of juvenile queen conch were different on the Great Bahama Bank and island shelf (Fig. 6). Both the bank and nearshore 176 Fishery Bulletin 92(1). 1994 40 N - 472 Boik 30 20 10 40 n . m Shelf 0-5 m 30 C 20 j_ O 10 Jl _l 1 - -■■ o « N .57 Shelf 5-10 m D x Q_ 20 o l0 CL v, «o N . 22» Shelf 1 0- 1 5 m O 30 -1 — ' 20 J rcen o o o -- ^. :, , ,50 Shelf 15-20 m CL) 0- 20 I 10 ^m 40 . N _ loo Shelf 20-25 m JO 20 10 100 120 140 160 180 200 220 240 260 280 300 She!! Length (mm) Figure 3 Length-frequency distributions for adult queen conch, Strombus gigas, on the Great Bahamas Bank and in five different depth zones of the island shelf near Lee Stocking Island, Bahamas. 40 N - 472 Bonk 30 20 A 10 JL to N . j6 Shelf 0-5 m 30 c 10 J_ o 10 Jl 1 , 40 . JHH o K ,i, Shelf 5-10 m 13 so CL 20 O CL 10 .^4. ^_ 40 N . 2 29 Shelf 10-15 m o 30 20 c CD (J 10 4 _jL n. iso Shelf 15-20 m CD 30 CL 20 1 10 ^jjL 40 n . ioo Shelf 20-25 m 30 20 J 10 _jB_ 100 120 140 160 180 200 220 240 260 280 300 Shell Length (mm) Figure 4 Distribution of shell lip-thickness for adult queen conch, Strombus gigas, on the Great Bahama Bank and in five different depth zones of the island shelf near Lee Stocking Island Bahamas. (0-2.5 m) shelf had juveniles less than 100 mm in shell length; however, these were rare in the shelf environment, and few juveniles less than 160 mm were found on the shelf between 2.5- and 15-m depth. None of the juveniles on the bank were near the 227-mm average length of adults on the shelf, but many juveniles collected in deeper water were close to adult size. Other differences were observed in the shells of queen conch from bank and shelf regions. Juvenile conch from the bank differed from shelf juveniles because of thicker shells and longer lateral spines Stoner and Schwarte: Distribution of Strombus gigas 177 w c -XL Q. 50 40 30 20 10 ° Bonk • Shelf - ^i t • • v Jn ♦. 400. 10 15 20 25 Shell Length (cm) 30 Figure 5 Scatterplot of shell lip-thickness vs. shell length for adult queen conch, Strombus gigas, collected from the Great Bahama Bank and from the Lee Stocking Island shelf. Two-hundred and fifty randomly chosen points were plotted for each site. (5—6 spines/whorl vs. 7-9 spines/whorl in shelf ju- veniles). Bank conch had a maximum shell diameter between 80 and 90% of shell length at 100 mm length, whereas juveniles from the shelf had diam- eters between 50 and 60% of shell length. These characteristics persisted to adult stages with bank conch having longer spines. The outer whorls of shelf adults, even young individuals, were often nearly smooth. Growth rates Juvenile queen conch on the island shelf at Charlie's Beach grew in length at a rate approximately 2.4 times the rate observed at the Children's Bay Cay site. Conch recovered at Charlie's Beach grew 0.139 mm/day (SD=0.025, n=135). At Children's Bay Cay, mean growth rate was 0.058 mm/day (SD=0.021, n=48). The differences in growth rate between bank and shelf juveniles were highly significant (Mann- Whitney [/-test, P<0.001). Discussion The rapid increase in adult queen conch density at depths greater than 10 m is probably a direct func- tion of fishing, which is limited to free-diving on the bank and shallow nearshore shelf areas around Lee Stocking Island. This conclusion is substantiated by observations of conch depth distribution in other localities. In unfished areas of Islas Los Roques, Venezuela, Weil and Laughlin (1984) found that 60 B0 100 120 140 160 180 200 220 240 260 280 Shell Length (mm) Figure 6 Length-frequency distribution for juve- nile queen conch, Strombus gigas, from the Great Bahama Bank and from two depth zones on the island shelf near Lee Stocking Island, Bahamas. density of queen conch was highest in 4.0 m of wa- ter and density decreased with depth to 18 m. This may represent the natural distribution of queen conch. In comparable 4-m deep habitats not pro- tected from fishing, densities were 5 times less than those in the protected area. Similarly, in the Exuma Land and Sea Park, a 500-km 2 fishery reserve 90 km north of Lee Stocking Island, there are large numbers (unquantified) of adult conch at 2-4 m depth, and many of these shallow-water conch have been observed laying eggs (Stoner, pers. observ.); whereas adults are uncommon in shallow water near Lee Stocking Island and spawning has never been observed at less than 5 m depth. Similar to the pattern reported in this study for Lee Stocking Is- land, Torres-Rosado ( 1987) found maximum density of adult queen conch between 10 and 20 m in Puerto Rico, where fishing is heavy in shallower waters. It is recognized that queen conch move to greater depths with age and size (Randall, 1964; Weil and Laughlin, 1984); this has been confirmed in the Lee Stocking Island area by the recovery of individuals that were tagged as juveniles at Charlie's Beach and subsequently found in deeper offshore waters Fishery Bulletin 92(1). 1994 (Stoner, unpubl. data). However, our morphological analyses of conch suggest that very few conch us- ing the bank for a nursery actually reach the off- shore spawning sites. Furthermore, similarities in length frequency and shell morphology between ju- veniles found immediately off the east (windward) side of the Cays on isolated seagrass beds and adults in deep water suggest that the small aggregations of juveniles found on the shelf serve as the primary source for the offshore reproductive stocks. Given that mating and egg-laying are rare on the Great Bahama Bank, it is likely that recruitment to bank nurseries is sustained by deep-water reproductive populations (Wicklund et al., 1991; Stoner et al., 1992; Stoner and Sandt, 1992). Differences in shell morphology between bank and shelf conch are not well understood but appear to be related to growth rate. Alcolado (1976) reported that large, thin shells and short spines in queen conch in Cuba were associated with rapid growth. A similar phenomenon may explain the shell differ- ences observed in this study. Juveniles in the nearshore shelf environment of Charlies' Beach grew rapidly and had the large, thin-shelled, short- spined morphotype typical of the shelf adults. The small, thick-shelled, long-spined conch on the bank had growth rates less than half of those on the shelf. A recent transplant experiment at Lee Stocking Is- land demonstrated that shell form and spination in juvenile conch is an environmentally mediated char- acteristic associated with habitat type and indi- vidual growth rate (Martin-Mora, 1992). The large size of the deep-water reproductive stock may explain high productivity of queen conch in the Exuma Cays. It is likely, however, that abun- dance of conch in the region is now dependent upon the small, isolated pockets of fast-growing juveniles that inhabit the nearshore shelf habitat during the first two or more years of life then recruit to deep- water reproductive populations. Stoner and Sandt (1992) found that the adult population at an 18-m deep site off Lee Stocking Island was relatively stable between 1988 and 1991, but most individu- als were old and thick-lipped. The predominance of old conch in deep water may or may not be a func- tion of low recruitment rates from shallow water in recent years, and the significance of shallow-water spawning to conch abundance is unknown. In an comparison of data from Glazer and Berg (in press.), densities of queen conch in the Exuma Cays are 10 to 100 times higher than those reported for many other localities in the Caribbean region. This may be related to geographic differences in habitat quality, recruitment processes, and fishing methods. The Exuma Cays probably represent a particularly efficient system for retaining conch lar- vae because of unique geographic and oceanographic conditions such as an alongshore current and nu- merous tidal inlets leading to nursery grounds (Stoner et al., in press), but fishing methods can play a large role in the population structure of queen conch. Fishing in the Bahamas is restricted to free- diving and limited diving with surface-supply air for adults with flared shell lips; therefore, conch deeper than 10 m are rarely exploited. Depth distribution of queen conch near Lee Stocking Island suggests that virtually every conch in the Exuma Cays is within the range of SCUBA divers and that popu- lations of S. gigas could be decimated quickly if the fishery were opened to this latter gear. On the other hand, if the source of deep-water conch is shallow- water nurseries, protection of deep-water reproduc- tive stocks only delays the effects of overfishing, and certain nurseries should be protected as well. Analy- sis of larval transport and recruitment processes will be crucial to the sound management of this already threatened commercial species. Acknowledgments This research was supported by a grant from the Undersea Research Program of NOAA, U.S. De- partment of Commerce. We thank P. Bergman, G. Donnly, R. Gomez, J. Lally, D. Mansfield, M. Ray, V. Sandt, D. Wicklund, and E. Wishinski for assis- tance in the field work. R. I. Wicklund prompted us to examine the important juvenile stocks off the windward beaches. The manuscript was improved with helpful criticism by R. Appeldoorn, R. Hardy, E. Martin, M. Ray, and an anonymous reviewer. Literature cited Alcolado, P. M. 1976. Crecimiento, variaciones morfologicas de la concha y algunos datos biologicos del cobo Strombus gigas L. (Mollusca, Mesogas- tropoda). Acad. Ciencias de Cuba, Inst, de Oceanol. No. 34, 36 p. Appeldoorn, R. S. 1988. Age determination, growth, mortality, and age of first reproduction in adult queen conch, Strombus gigas L., off Puerto Rico. Fish. Res. 6:363-378. Appeldoorn, R. S., G. D. Dennis, and O. Monterrosa-Lopez. 1987. Review of shared demersal resources of Puerto Rico and the lesser Antilles region. In R. Mahon (ed.), Report and proceedings of the expert consul- Stoner and Schwarte: Distribution of Strombus gigas 179 tation on shared fishery resources of the lesser Antilles region. FAO Fish. Rep. 383:36-106. Berg Jr., C. J., and D. A. Olsen. 1989. Conservation and management of queen conch (Strombus gigas) fisheries in the Caribbean. In J. F. Caddy (ed.), Marine inverte- brate fisheries: their assessment and management. Wiley & Sons, NY, p. 421-442. Egan, B. D. 1985. Aspects of the reproductive biology of Strombus gigas. M.S. thesis, Univ. British Co- lumbia, Vancouver, Canada, 147 p. Glazer, R. A., and C. J. Berg Jr. In press. Current and future queen conch, Strombus gigas, research in Florida. In R. S. Appeldoorn and B. Rodriguez (eds.), The biology, fisheries, mariculture, and management of the queen conch. Fundacion Cientifica Los Roques, Caracas, Venezuela. Martin-Mora, E. 1992. Developmental plasticity in the shell of the queen conch, Strombus gigas. M.S. thesis, Florida State Univ., Tallahassee, 52 p. Morrison, D. F. 1976. Multivariate statistical methods. McGraw- Hill, New York. Randall, J. E. 1964. Contributions to the biology of the queen conch, Strombus gigas. Bull. Mar. Sci. 14:246-295. Sokal, R. R., and F. J. Rohlf. 1969. Biometry. W. H. Freeman, San Francisco, 776 p. Stoner, A. W., and V. J. Sandt. 1992. Population structure, seasonal movements. and feeding of queen conch, Strombus gigas, in deep-water habitats of the Bahamas. Bull. Mar. Sci. 51:287-300. Stoner, A. W., V. J. Sandt, and I. F. Boidron- Metairon. 1992. Seasonality in reproductive activity and lar- val abundance of the queen conch, Strombus gigas. Fish. Bull. 90:161-170. Stoner, A. W., M. D. Hanisak, N. P. Smith, and R. A. Armstrong. In press. Large-scale distribution of queen conch biology, fisheries, and mariculture: implications for stock enhancement. In R. S. Appeldoorn and B. Rodriguez (eds.), The biology, fisheries, maricul- ture, and management of the queen conch. Fundacion Cientifica Los Roques, Cara- cas, Venezuela. Torres-Rosado, Z. A. 1987. Distribution of two mesogastropods, the queen conch, Strombus gigas Linnaeus, and the milk conch, Strombus costatus Gmelin, in La Parguera, Lajas, Puerto Rico. M.S. thesis, Univ. Puerto Rico, Mayaguez, 37 p. Weil, E., and R. Laughlin. 1984. Biology, population dynamics, and reproduc- tion of the queen conch, Strombus gigas Linne, in the Archipielago de Los Roques National Park. J. Shellfish Res. 4:45-62. Wicklund, R. I., L. J. Hepp, and G. A. Wenz. 1991. Preliminary studies on the early life history of the queen conch, Strombus gigas, in the Exuma Cays, Bahamas. Proc. Gulf Caribb. Fish. Inst. 40:283-298. Abstract. — Regression and time series analyses were used to investigate the relation between Apalachicola River flows and blue crab, Callinectes sapidus, harvests in and around Apalachicola Bay, Florida. Apalachicola River flows in one year were positively corre- lated with Franklin County blue crab landings during the next year (r 2 =0.32, P<0.001, 1952-90), and the strength of the correlation in- creased when only more recent years were examined (r 2 =0.49, P=0.001, 1973-90). In this area, blue crabs mature to a harvestable size by one year of age. Apala- chicola River flows were also cor- related with neighboring Wakulla County blue crab landings with a one-year time lag (r 2 =0.52, P=0.001, n=l7), but were not asso- ciated with blue crab landings for the remaining west coast of Florida. The mean monthly flow from September to May, termed the growout period, was the pa- rameter most highly correlated with the following year's blue crab landings. Of five north Florida riv- ers examined, the Apalachicola River was most highly correlated with Franklin and Wakulla County blue crab landings. Results of this study further document the influence of Apal- achicola River flows on estuarine productivity. The positive relation between flows and blue crab har- vests a year later suggests that low flow conditions in the estuary during the growout period nega- tively affect juveniles. Although the underlying causes of the corre- lations are not known, the effect of inflows on estuarine salinity is one of several possible mechanisms that warrants further investigation. The influence of Apalachicola River flows on blue crab, Callinectes sapidus, in north Florida Dara H. Wilber 1 640 Oak Ridge Road. Vicksburg. MS 39 1 80 Manuscript accepted 20 July 1993 Fishery Bulletin 92:180-188 1 1994) River flow affects many character- istics of estuaries, including salin- ity, turbidity, and nutrient and de- trital concentrations. Changes in flow, therefore, may significantly affect estuarine biota, the extent to which may be inferred by examin- ing historical relations between flow and productivity. Apalachicola Bay, Florida, like many estuaries, is subject to changes in freshwater inflow related to factors such as rainfall and upstream demands for agricultural, municipal, and indus- trial uses. Plans to reallocate fresh- water resources (U.S. Army Corps of Engineers, 1989 1 ) have renewed interest in the question of how freshwater inflows are related to productivity in the Apalachicola River and Bay system. This study examined the historical relation- ship between Apalachicola River flows and estuarine productivity. One method of characterizing the importance of freshwater inflow to estuarine productivity is to corre- late historical flow data with the commercial catch (landings) of es- tuarine-dependent species (Fun- icelli, 1984). Commercial landings are used to estimate estuarine pro- ductivity because they are often the only available long-term records from which species abun- dance can be inferred. Long-term records are available for several commercially important species in Apalachicola Bay, including oysters and blue crabs, which have differ- ent trophic requirements and es- tuarine residency patterns. By ex- amining associations between these species and Apalachicola River flows, effects of freshwater delivery upon estuarine productiv- ity can be evaluated. Associations between freshwater inflows and Apalachicola oyster harvests have been previously addressed (Wilber, 1992). The present study examines the influence of Apalachicola River flows on local and regional commer- cial blue crab, Callinectes sapidus, landings. Other north Florida riv- ers were also examined to estimate the relative importance of the Apalachicola River to blue crab landings with respect to these drainages. Blue crabs in the Gulf of Mexico reach a harvestable size within a year of age (Perry, 1984) and com- prise a significant portion of the commercial landings by 18-months of age (Steele, 1992 2 ). Blue crabs enter the Apalachicola estuary as megalopae and young juveniles, reaching peak juvenile abundances in the winter (Livingston, 1983). Young crabs concentrate in the less saline portions of the bay, whereas egg-bearing females remain in the higher-salinity gulf waters where they spawn. It has been proposed that adult female blue crabs along the Florida gulf coast migrate to 1 U.S. Army Corps of Engineers, Mobile District. 1989. Draft Post Authorization Change Notification Report for the Real- location of Storage from Hydropower to Water Supply at Lake Lanier, Georgia, 320 p. 2 P. Steele, Florida Marine Research Inst., 108th Ave. SE, St. Petersburg, FL 33701, pers. commun. 1992. 180 Wilber: Influence of Apalachicola River flows on Callinectes sapidus 181 gulf waters near Apalachicola Bay to spawn and that the larvae are distributed to the south by loop currents (Oesterling and Evink, 1977). Evidence supporting this hypothesis was examined in this study. Methods Fisheries data Several aspects of the blue crab fishery may lead to inaccurate fishery representation of adult stock abundance. For example, unreported landings from the recreational fishery and crab bycatch from the shrimp fishery are potential sources of bias in blue crab landings statistics (Perry, 1984). Although these sources of error cannot be controlled, if they are independent of river flow and account for a rela- tively constant proportion of the total landings over time, a valid, although perhaps conservative, rep- resentation of environmental effects on the species can be obtained. The Florida Department of Natural Resources (FDNR) provided monthly landing data for blue crabs from Franklin and Wakulla Counties for 1979- 90, monthly effort data (number of trips) for 1987- 90, annual landing data from Wakulla County from 1973 to 1990 (excluding 1977), and annual landing data from the Florida west coast from 1960-1990. Franklin County annual landing data from 1952 to 1979 were also obtained (Herbert et al., 1988 3 ). Sta- tistical analyses (Wilkinson, 1990) were conducted by using the full 39-year Franklin County dataset, as well as a shorter (1973-90) dataset, which al- lowed comparisons between Franklin and Wakulla Counties that were not confounded by differences in time periods. The limited amount of effort data pre- cluded analyses of catch per unit of effort. Flow and rainfall data The Apalachicola River begins at the Florida state line by the confluence of the Chattahoochee and Flint Rivers. Apalachicola flow data were collected at the United States Geological Service gauge at Blountstown, Florida, which is the closest station to the estuary (105 km upstream) with an adequate period of record. This station is not immediately adjacent to the estuary, therefore fresh water from local inputs and storm events are not included. The drainage area downstream from the Blountstown gauge is less than 9% of the total area drained by the Apalachicola-Chattahoochee-Flint River system (Leitman et al., 1983 4 ). Parameters examined included the highest and lowest average flows for 7 and 120-consecutive days each year (referred to as the 7- and 120-day maxi- mum and minimum flows). Monthly minimum, mean, and maximum values, and the mean monthly flow during the growout period (September-May) were also examined. By using these flow durations, associations between landings and seasonal high and low flows could be examined, which was not possible when analyses included only mean annual flow. The growout-flow time period was adapted from a similar study correlating blue crab landings in Georgia with river discharges (Rogers et al., 1990 5 ). Sufficient historical flow data were also available for the Suwannee, Econfina, St. Marks, and Ochlockonee Rivers (Fig. 1), thus permitting a re- gional analysis of associations between flows and blue crab landings. For each river, the annual one- day minimum, one-day maximum, and annual mean flows were used. One-day high and low flow magni- tudes were used because of their availability and because preliminary analyses which substituted other flow durations (annual minimums and maxi- mums) on the Apalachicola River did not change results considerably. Statistical analyses Blue crab landings and flow data were tested for monthly, seasonal, and inter-annual dependencies through autocorrelations. Data were adjusted to remove dependencies when autocorrelations were significant. If autocorrelations between successive months were present, data were replaced by the difference between each month and the preceding month. If seasonal autocorrelations were present, the effects were removed by dividing each value by a seasonal factor. For instance, if landings exhibited a significant autocorrelation with a 12-month time lag, which reflected a similarity in catches for the same month among years, each monthly value was divided by the month's mean value and replaced by the quotient. Similar analyses were conducted with seasonal (three-month averages) landings and flow data following adjustments to remove significant autocorrelations. Flow data were log 10 transformed. 3 Herbert, T. A., and Associates. 1988. The Franklin County Fisheries Options Report, 164 p. 4 Leitman, H. M., J. E. Sohm, and M. A. Franklin. 1983. Wet- land hydrology and tree distribution of the Apalachicola River flood plain, Florida. U.S. Geological Survey Water-Supply Pa- per 2196, 52 p. 5 Rogers, S. G., J. D. Arrendondo, and S. N. Latham. 1990. As- sessment of the effects of the environment on the Georgia blue crab stock. Final Rep. Georgia Dep. Natl. Resources, 69 p. 182 Fishery Bulletin 92(1), 1994 . . A .Jl A .£ A _M_A_ A- Apalachicola R. O Ocklockonee R. SM- St. Marks R. E- Econfina R. S- Suwannee R. " EST ^..-•'^•° °° Figure 1 Percentage of total Florida west coast blue crab (Callinectes sapidus) landings caught by area (Steele, 1982). The five rivers used in the multivariate regres- sion analyses (Apalachicola, Ochlockonee, St. Marks, Econfina, and Suwannee) are depicted. Autoregressive order 1 (ARIMA) models were con- ducted on the Franklin and Wakulla County blue crab annual data and the residuals from these analyses were correlated with flow. This approach provided statistically rigorous estimates of P-values for the flow/landings relationships that were inde- pendent of any effects resulting from the one-year autocorrelations in landings. Analyses that used the ARIMA residuals and those that used unadjusted blue crab landings data were reported because both methods impart useful information. Correlations that used unadjusted annual blue crab data, i.e., significant autocorrelations were not removed, were biologically relevant because feedback mechanisms inherent to these autocorrelations (such as reproduc- tion and recruitment) may also be associated with flow. Results of analyses that used unadjusted data are also more readily compared to results of other studies. Use of ARIMA models statistically validated the significant relations between blue crab landings and flow data, but may have removed some biologi- cally relevant information. This paper primarily refers to unadjusted regression results. Regression analyses incorporating a one-year time lag between flows and landings were conducted to examine the effects of flow on early blue crab life history stages. Contemporaneous analyses were con- ducted to assess the effect of flow on adults. Univariate and stepwise multivariate regression analyses were conducted to estimate the amount of variability in blue crab landings accounted for by five major rivers on Florida's northern gulf coast. The criterion for admitting a flow variable into the stepwise regression models was an F-statistic greater than 4.0 for its partial correlation with land- ings. Data on blue crab landings for the west coast of Florida were used as a dependent variable in some analyses. To more specifically examine Wilber: Influence of Apalachicola River flows on Callinectes sapidus 183 whether there was evidence that the Apalachicola River affects blue crab landings on a regional basis, Franklin and Wakulla landings were removed from the west coast dataset. Regression analyses were conducted to test whether Apalachicola flows and the remaining west coast landings were significantly related. Results Annual landings Blue crab landings varied nearly 10-fold over the period of record examined in each county (Fig. 2). Significant autocorrelations between consecutive years were present in both Franklin (r 2 =0.19, P=0.006) and Wakulla (r 2 =0.37, P=0.016) County landings. Annual flow parameters did not exhibit any significant autocorrelations. Annual Franklin County blue crab landings were most highly correlated with Apalachicola River flows of the previous year and these correlations were positive (Table I). The growout flow with a one-year time lag accounted for the greatest amount of varia- tion in blue crab landings (r 2 =0.32, P<0.001; Fig. 3A). The regression analysis of ARIMA residuals (autocorrelation in blue crab landings removed) and growout flows of the previous year was also signifi- cant (r 2 =0.21; P=0.004). Wakulla County landings ANNUAL BLUE CRAB LANDINGS 1.5 O.S 0.0 WAKULLA FRANKLIN —i — i — i — [ — i — i — i — i — | — i — i — i — i — [ — r 1950 1960 1970 1980 YEAR 1990 Figure 2 Annual blue crab (Callinectes sapidus) landings for Franklin (closed squares) and Wakulla (open squares) Counties in millions of kilograms. Table 1 R 2 values from regression analyses for Franklin (n=39) and Wakulla (n = 17) Coun ty blue crab (Callinectes sap idus) landings and Apalachicola River flows. All correlations were positive. Flow parameter Franklin Wakulla no lag period 7-day low 0.16* 0.12 120-day low 0.14* 0.18 7-day high 0.04 0.07 120-day high <0.01 <0.01 growout 0.08 0.10 one-year lag 7— day low 0.25** 0.29* 120-day low 0.21** 0.21 7-day high 0.18* 0.17 120-day high 0.21** 0.31* growout 0.32*** 0.52*** * = P < 0.05. ** = P < 0.01. *** = P < 0.001. were significantly correlated only with Apalachicola flows of the previous year, with the growout flow also accounting for the greatest amount of variation in annual blue crab landings (r 2 =0.52, P=0.001; Fig. 3B). The regression analysis of ARIMA residuals and growout flows one year previous was significant (r 2 =0.35, P=0.02). The shorter (1973-90) data record for Franklin County landings was more strongly correlated with growout flows with a one-year time lag (r 2 =0.49, P=0.001; Fig 3C) than was the full 39- year dataset. Monthly and seasonal landings As expected, the monthly Franklin and Wakulla County blue crab landings (1979-90) exhibited sig- nificant autocorrelations for 1- and 12-month time lags. All monthly river flow parameters (minimum, mean, and maximum) also exhibited significant cor- relations between successive months and with 12- month lags. Correlations between monthly landings and flow parameters (without any adjustments for significant autocorrelations) were positive for time lags of 3, 4, and 5 months. Significant negative cor- relations were present for flows that lagged 2-4 months behind landings. Correlations that used landings and flow data with the 1- and 12-month autocorrelation effects removed were not significant for either county. Peak harvests generally occurred between May and September in both counties. There were also no significant correlations between the seasonal (three- 184 Fishery Bulletin 92(1), 1994 ONE-YEAR TIME LAG FRANKLIN COUNTY (1952-1990) ONE-YEAR TIME LAQ WAKULLA COUNTY (1873-1990) 2.0 -i O 1.6 1.0 2 ° 0.6 0.0 r - 0.62 B 300 500 700 800 1100 300 500 700 000 1100 QROWOUT FLOW (lnT/SEC) QROWOUT FLOW (M /SEC) ONE-YEAR TIME LAQ FRANKLIN COUNTY (1973-1990) 1.0 0.8 o j 0.6 I Q < cr o 0.4 - 0.2 - 0.0 T~ 300 500 700 900 QROWOUT FLOW (M^SEC) 1100 Figure 3 Apalachicola River growout (flows m 3 /sec, mean flow from September through May) plotted against the following year's (A) Franklin County blue crab landings (1952-90), (B) Wakulla County blue crab landings (1973-90 ex- cluding 1977), and (C) Franklin County blue crab landings (1973-90). Flow data were log transformed in the statistical analyses. Wilber: Influence of Apalachicola River flows on Callinectes sapidus 185 Apalachicola 1.00 Ochlockonee 0.59** St. Marks 0.32 Econfina 0.50* Suwannee 0.60** * = P < 0.01. ** = P < 0.001. month average) flow and land- ings data with autocorrelations removed. The timing of peak monthly harvests was not re- lated to the magnitude of the annual harvests. Regional analysis Given the close geographical proximity of the five rivers ( Fig. 1) used in the multiple regres- sion analyses, significant corre- lations between annual flow pa- rameters may be expected among the rivers. Apalachicola River an- nual mean flows, although significantly correlated with other river flows (except the St. Marks), had the lowest correla- tions with the other drainages (Table 2). Significant correlations with blue crab landings were more common for Apalachicola River flows than for any other north Florida river tested (Table 3). Franklin County landings were correlated only with Apalachicola flows, whereas Wakulla County and west coast landings were also correlated with Suwannee and Ochlockonee flows, respectively (Table 3). These significant univariate correlations incorporated a one-year time lag. The Franklin County multivariate re- gression model included Apalachicola and Ochlockonee minimum flows of the previ- ous year (r 2 =0.45, P<0.001; Table 4). The Wakulla multivariate model accounted for the most variation in blue crab landings (r 2 =0.64; Table 4) and included Apalachicola mean and Ochlockonee minimum flows of the previous year. The west coast multivariate model with a one-year time lag in- cluded Apalachicola maximum and Ochlockonee minimum and mean flows (r 2 =0.53; Table 4). The only significant multivariate model that in- cluded parameters with and without time lags was for west coast landings, which used both no-lag Suwannee minimum flows and Apalachicola maximum flows of the previous year (r 2 =0.49). Analyses that examined associations between Apalachicola River flow and west coast landings with Franklin and Wakulla County landings removed were not significant. Discussion Several consistent results appeared in the correla- tions of annual blue crab landings with Apalachicola Table 2 Pearson correlation matrix of annual mean river flows for all possible combinations of five north Florida rivers. Apalachicola Ochlockonee St. Marks Econfina Suwannee 1.00 0.77* 0.76* 0.93* 1.00 0.64** 0.77** 1.00 0.79* 1.00 Table 3 Univariate correlations between Wakulla, Franklin, an d west coast blue crab (Callinectes sapidus ) landings and th a river flows from five north Florida drainages (Suwannee, Econfina, St. Marks, Ochlockonee, and Apalachicola) wi th a one-year time lag. Signs of the correlations are given in parentheses. Region Correlation r 2 P Franklin Apalachicola minimum ( + ) 0.31 0.001 Apalachicola mean ( + ) 0.25 0.004 Apalachicola maximum ( + ) 0.14 0.039 Wakulla Apalachicola minimum (+) 0.29 0.031 Apalachicola mean ( + ) 0.38 0.010 Suwannee minimum ( + ) 0.30 0.028 West Coast Apalachicola mean ( + ) 0.15 0.035 Apalachicola maximum ( + ) 0.26 0.004 Ochlockonee minimum (-) 0.22 0.009 Table 4 Multiple regression results for Franklin, Wakulla, and west coast landings of blue crabs (Callinectes sapidus) with a one-year time lag incorporated into the analyses. The independent variables are the five river drainages listed in Table 3. Listed below are the signs of the correlations in paren- theses, Student's ^-statistics, and associated P- values. Region Variable Franklin Wakulla West Coast Apal. min ( + ) Och. min. (-) Apal. mean ( + ) Och. min. (-) Apal. max. ( + ) Och. mean ( + ) Och. min. (-) 4.38 -2.68 4.57 -3.05 2.99 3.12 -2.61 <0.001 0.012 0.001 0.009 0.006 0.004 0.015 0.45 (I 64 0.53 186 Fishery Bulletin 92(1), 1994 River flows. Statistically significant correlations were positive and primarily restricted to a time lag of one year, indicating higher flows were associated with higher blue crab landings the following year and lower flows with poorer landings the next year. The mean flow during the growout period (Septem- ber through May) of the previous year was the most highly correlated flow parameter with blue crab landings in both counties. A number of explanations are consistent with the observation that more fresh water (within a certain range) was associated with higher blue crab land- ings the following year. Greater freshwater inflows reduce estuarine salinities, thereby increasing the area of suitable habitat in the middle, and perhaps lower, estuary where juvenile blue crabs can forage and develop (Livingston et al., 1976; Perry, 1984). Increases in low salinity habitat may reduce preda- tion by marine species on juvenile blue crabs. Greater freshwater flows may also broaden an estuary's signal to offshore female migrants and/or megalopae, thus increasing the potential recruit- ment population base (Perry and Stuck, 1982; Mense and Wenner, 1989). In addition, higher in- flows carry more detrital and nutrient matter (Mattraw and Elder, 1982), which may either di- rectly or indirectly enhance food availability. In both Franklin and Wakulla counties, flows be- low approximately 600 m 3 /sec appear more closely related to the following year's landings than higher growout flows, i.e., the regression equation fits the data better at the low end of the flow spectrum (Fig. 3). Several factors may explain this phenomenon. Food availability may limit blue crab production at flows below a certain level but may not be limiting at flows above this level and, therefore, crab produc- tivity is not influenced by further increases in flow. Prey limitation at low flows may also lead to canni- balism, further limiting blue crab population size (Lipcius and Van Engel, 1990). The finding that more recent years produce a stronger correlation between blue crab landings and river flows was also observed in Georgia (Rogers et al., 1990 5 ). Total discharges from September to May (growout period) of five Georgia rivers were posi- tively correlated with landings (r 2 >0.8). Shorter time periods (the most recent 14 and 19 years of land- ings statistics) produced better correlations with flow than the full period of record (37 years). The authors concluded increased fishing pressure in more recent years resulted in only one year class being fished, and, thus, environmental effects were more obvious on a single year class in the shorter dataset. Similarly, that more recent landings for Franklin County were more highly correlated with Apalachicola River flows than landings for the longer 39-year period may reflect a trend toward harvesting a single year class. The significant 1- and 12-month time lags in Franklin and Wakulla County reflect similarities in catches between successive months and a seasonal component, respectively. The 12-month auto- correlation indicates that trends in landings occur at the same time of year (e.g., summer peaks) and should not be confused with an annual auto- correlation, which is indicative of a similarity in harvests between entire years. The positive corre- lations between unadjusted monthly flow and land- ings data correspond to the summer peak in blue crab landings following 3-5 months after the spring peak in flows, and low winter landings following low late-summer and fall flows. The negative correla- tions with 2-4 month time lags reflect fall low flows following peak summer harvests and high spring flows occurring after low winter harvests. The ab- sence of significant correlations between monthly landings and flows, once these data were adjusted to remove seasonal autocorrelations, indicates that residual (non-seasonal) variation in monthly flows is unrelated to the non-seasonal variation in mon- thly landings. Livingston (1991) found a positive contemporane- ous correlation between monthly Apalachicola River flows and blue crab abundances in trawl surveys conducted from 1972 to 1985. This finding corre- sponds to high juvenile abundances during high-flow months. The positive correlation in the present study between monthly flows and blue crab landings 3-5 months later may reflect the maturation of ju- veniles into adults in the summer, and thus the observed time lag in the correlation. The majority of the Apalachicola-Chattahoochee- Flint basin is in Georgia and is subject to different climatic conditions than are the other north Florida rivers examined, which may explain the relatively small correlations between Apalachicola River flows and flows on these other rivers. Georgia rainfall is more strongly correlated with Apalachicola River flows than Florida rainfall (Meeter et al., 1979). A consistent and important finding of the multivari- ate regression analyses was that Apalachicola flows were more highly correlated with Franklin, Wakulla, and Florida west coast landings of the next year than any other river drainage tested. Regressions comparing Apalachicola flows to west coast landings, after Franklin and Wakulla County landings were removed, were not significant, suggesting the influ- ence of the Apalachicola drainage is restricted pri- marily to Franklin and neighboring Wakulla County. Thus, there was no evidence supporting the hypoth- Wilber: Influence of Apalachicola River flows on Callmectes sapidus 187 esis of mass blue crab spawning near Apalachicola Bay and larval transport down the gulf coast of Flor- ida via the loop current (Oesterling and Evink, 1977). Several studies have addressed factors that influ- ence interannual variation in blue crab abundance, primarily concentrating on larval and post-larval recruitment (reviewed in Lipcius and Van Engel, 1990). Lipcius and Van Engel (1990) found high interannual, seasonal, and spatial variation in blue crab abundances in a 17-year fishery-independent dataset collected in the Chesapeake Bay. They ob- served that years with high blue crab abundances appeared to be dominated by the previous year class because peak catches occurred in the summer. Years with low abundances had peak abundances in the fall, suggesting the dominance of the new year class. This observation supports the contention that varia- tion in recruitment plays a major role in determin- ing interannual fluctuations. No interaction between annual abundance and seasonal peak catch was apparent for the Franklin or Wakulla County blue crab landings, which may indicate either the true absence of such a relation, the inadequacies of us- ing fishery statistics, or a difference in growth rates between the two regions that invalidates the use of the same analysis. Interestingly, the fishery-inde- pendent trawl data from the Chesapeake were sig- nificantly (r 2 =0.33) correlated with the commercial landings data. The influence of physical factors on blue crab abundances has been documented in other areas, such as a positive relationship between blue crab landings and freshwater inflows in Georgia (Rogers et al., 1990 5 ), an inverse relation between salinity and juvenile blue crab abundances on the Texas coast (More, 1969), and a positive relation between blue crab productivity and vegetated area in the Gulf of Mexico (Orth and van Montfrans, 1990). The positive correlation between blue crab landings and Apalachicola River flows of the previous year pro- vides additional evidence of the importance of fresh- water inflows to juvenile blue crabs. Apalachicola River flows have a significant impact on estuarine productivity, as indicated by commer- cial harvests of oysters (Wilber, 1992) and blue crabs. Although statistical correlations do not indi- cate the causal mechanisms underlying these asso- ciations, the river's influence on estuarine salinities as a mediating factor is deserving of further exami- nation. Undoubtedly, the Apalachicola River affects estuarine biota via mechanisms other than salinity (Livingston, 1991). Factors such as the transport of nutrients and organic matter, however, are unlikely to result in a significant correlation between low flows and oyster harvests two years later, unless food limitation is only measurably important for newly settled oyster spat. In addition, the majority of nutrient and detrital transport from the river occurs during high flow periods in the spring (Mattraw and Elder, 1982). There was no evidence that above-average flows were associated with either oyster or blue crab productivity. In both fisheries, flows on the low end of the spectrum were most sig- nificantly associated with landings. These signifi- cant correlations were positive and incorporated time lags, suggesting estuarine conditions during low minimum flow periods were not favorable for juveniles of either species. Acknowledgments The careful reviews of R. Hardy, G. Lewis, D. Meeter, R. Lipcius, P. Steele, and R Wilber are grate- fully acknowledged, as well as the technical support of J. Bennett, J. McKenna, G. Miller, and D. Tonsmeire. This work was supported by the North- west Florida Water Management District and the State of Florida's Surface Water Improvement and Management (SWIM) Program. Literature cited Funicelli, N. A. 1984. Assessing and managing effects of reduced freshwater inflow to two Texas estuaries. In V. S. Kennedy (ed. ), The estuary as a filter, p. 435-446. Lipcius, R. N., and W. A. Van Engel. 1990. Blue crab population dynamics in Chesa- peake Bay: variation in abundance (York River, 1972-1988) and stock-recruit functions. Bull. Mar. Sci. 46:180-194. Livingston, R. J. 1983. Resource atlas of the Apalachicola estuary. Florida Sea Grant College Publication No. 55, 64 p. 1991. Historical relationships between research and resource management in the Apalachicola River-estuary. Ecological Applications 1(4):361- 382. Livingston, R. J., G. J. Kobylinski, F. G. Lewis HI, and P. F. Sheridan. 1976. Long-term fluctuations of epibenthic fish and invertebrate populations in Apalachicola Bay, Florida. Fish. Bull. 74(2):311-321. Mattraw, H. C, and J. F. Elder. 1982. Nutrient and detritus transport in the Apalachicola River, Florida. U.S. Geol. Surv. Water-Supply Pap. 2196-C. Meeter, D. A, R. J. Livingston, and G. C. Woodsum. 1979. Long-term climatological cycles and popula- 188 Fishery Bulletin 92(1). 1994 tion changes in a river-dominated estuarine system. In R. J. Livingston (ed.), Ecological pro- cesses in coastal and marine systems. Marine Science 10:315-338. Mense, D. J., and E. L. Wen nor. 1989. Distribution and abundance of early life his- tory stages of the blue crab, Callinectes sapidus, in tidal marsh creeks near Charleston, South Carolina. Estuaries 12:157-168. More, W. R. 1969. A contribution to the biology of the blue crab (Callinectes sapidus Rathbun) in Texas, with a description of the fishery. Texas Parks Wildl. Dep. Tech. Ser. 1:1-31. Oesterling, M. L., and G. L. Evink. 1977. Relationship between Florida's blue crab population and Apalachicola Bay. In R. J. Livingston and E. A. Joyce (eds.), Proceedings of the conference on the Apalachicola drainage sys- tem; 23-24 April 1976, Gainesville, Florida. FL Mar. Res. Pub. 26:101-121. Orth, R. J., and J. van Montfrans. 1990. Utilization of marsh and seagrass habitats by early stages of Callinectes sapidus: a latitudinal perspective. Bull. Mar. Sci. 46:126-144. Perry, H. M. 1984. A profile of the blue crab fishery of the Gulf of Mexico. Gulf State Mar. Fish. Comm. No. 9, 80 p. Perry, H. M., and K. C. Stuck. 1982. The life history of the blue crab in Mississippi with notes on larval distribution: proc. blue crab colloquium; 18-19 October 1979, Biloxi, Mississippi. Gulf States Mar. Fish. Comm. 7:17-22. Steele, P. 1982. A synopsis of the biology of the blue crab Callinectes sapidus Rathbun in Florida: proc. blue crab colloquium; 18-19 October 1979, Biloxi, Mississippi. Gulf States Mar. Fish. Comm. 7:29-35. Wilber, D. H. 1992. Associations between freshwater inflows and oyster productivity in Apalachicola Bay, Florida. Estuarine, Coastal and Shelf Sciences 35:179-190. Wilkinson, L. 1990. SYSTAT: the system for statistics. SYSTAT, Inc. Evanston, IL, 676 p. Oocyte maturation in Hecate Strait English sole [Pleuronectes vetulus) Jeff Fargo Department of Fisheries and Oceans, Pacific Biological Station Biological Sciences Branch. Nanaimo, British Columbia V9R 5V6 Albert V. Tyler School of Fisheries and Oceans University of Alaska, Fairbanks, Alaska 99775 English sole, Pleuronectes vetulus, is an important component of the bottom trawl fishery in Hecate Strait, British Columbia, Canada. It is a small-mouthed flounder that feeds on sedentary inverte- brates associated with sandy sub- strate and is most common at depths of 80-150 m (Hart, 1973). The species is characterized by moderate growth (&=0.22), mortal- ity (M=0.20) and longevity (20 years) (Fargo, 1993). It recruits to the fishery at an age of four years, which is roughly equivalent to the age of sexual maturity (Ketchen, 1956; Tyler et al., 1987 1 ). Most of the exploited population is under 12 years of age (30-45 cm in length) (Fargo, 1993). Results from tagging studies (Ketchen, 1956; Fargo et al., 1984) and analysis of landing statistics and age composition data (Fargo, 1993) indicate that a single stock exists in Hecate Strait. Since 1955, abundance for this stock has fluctuated, primarily be- cause of changes in recruitment (Fargo, 1993). Factors influencing recruitment for this stock are poorly understood. Ocean tem- perature and circulation have 1 Tyler, A. V., J. Fargo, R. P. Foucher, and J. B. Lucas. 1987. Studies on the repro- ductive biology of Pacific cod and En- glish sole in Hecate Strait from the cruise of the FR/V W.E. Ricker, Novem- ber 25-29, 1986. Can MS. Rep. Fish. Aquat. Sci. 1937, 43 p. been found to influence spawning time and oocyte maturation for the stock off the Oregon coast (Kruse and Tyler, 1989). These authors postulated that 1) the rate of gonadal development for English sole was inversely related to summer bottom temperatures in the same manner as is somatic growth, and 2) spawning was de- layed by rapid increases in bottom temperature caused by upwelling. In Hecate Strait, where Ekman transport is weak, these tempera- ture changes may be brought about by the fall transition when strong winds from the south cause mixing of the warm surface wa- ters to depths of 150 metres (Dodimead, 1980 2 ). Relatively little information exists on spawn- ing time and egg development for the Hecate Strait stock. We inves- tigated oocyte growth and devel- opment to examine the length of the oocyte maturation period and the time and duration of spawn- ing for the English sole stock in Hecate Strait. Materials and methods Samples of English sole ovaries were obtained from research cruises and at ports-of-landing from commercial vessels between November 1987 and November 1990. The fish were caught with bottom trawls at five locations throughout Hecate Strait (PMFC Areas 5C-D, Table 1, Fig. 1). Length-stratified samples were collected to ensure that ovaries were obtained throughout the size range of fish collected. For each collection we attempted to sample fifteen sexually mature fish from each 5-cm length interval over a range of 30-50 cm, though this was not always possible. The minimum size fish (30 cm) from which an ovary was dissected cor- responds to the length at first maturity for this stock (Ketchen, 1956; Tyler et al., 1987 1 ). Total length and the condition of matu- rity for each fish sampled was re- corded. The right ovary was then removed and preserved in a buff- ered formalin-saline solution (Foucher et al., 1987 3 ). Sampling methods have been described in previous reports (Foucher et al., 1987 3 ; Tyler et al., 1987 1 ). A list of ovary samples examined is given by sample type and month in Table 1. Preserved ovaries were pre- pared for histological examination by soaking in Davidson's fixative for approximately 24 hours. Sub- sequently, tissue sections were dissected from the anterior por- tion of the ovary (which contained the greatest amount of eggs), em- bedded in paraffin wax, sectioned at 5 u, stained with haematoxylin and counterstained with eosin (Yasutake and Wales, 1983). Oocyte diameter was measured with a light microscope calibrated 2 Dodimead, A. J. 1980. A general review of the oceanography of the Queen Char- lotte Sound-Hecate Strait-Dixon En- trance region. Can. MS. Rep. Fish. Aquat. Sci. 1574, 248 p. 3 Foucher, R. P., J. Fargo, and J.B. Lucas. 1987. Cruise of the FV Nucleus. Janu- ary 5-17, 1987 to Hecate Strait to study reproductive biology of Pacific cod and English sole. Can. MS Rep. Fish. Aquat. Sci. 1941, 25 p. Manuscript accepted 8 October 1993. Fishery Bulletin 92:189-197 (1994) 189 190 Fishery Bulletin 92(1), 1994 Figure 1 Location of trawling grounds in the study area, Hecate Strait, British Columbia, Canada. to the nearest 5 p, or with a projection microscope calibrated to the nearest 4 u. Three hundred oocytes were measured from at least one fish for every cm length interval for each sample (Table 1). Measure- ment of 300 oocytes per fish was necessary to pro- vide complete information on the size composition of developing oocytes. Only oocytes that had been sectioned through the nucleus, close to the center of the oocyte, were measured. Mean diameter was es- timated as the mean of the minimum and maximum diameters for each oocyte (Foucher and Beamish, 1980). For smaller oocytes (10-20 p), precision of the measurement was lower because of distortion of the oocyte by surrounding maturing oocytes (Dunn, 1970). A description of the histological stage of oo- cyte development (Fargo and Sex- ton, 1991 4 ) was also recorded. We were unable to obtain oocyte measurements from ovaries col- lected from ripe fish in October and November 1990. These samples were taken from com- mercial vessels at ports of land- ing. Ovaries from these samples had combinations of hydrated and non-hydrated oocytes with many burst cells. These fish had been held in chilled seawater for sev- eral days prior to sampling, prob- ably exacerbating the state of hy- drated oocytes and causing them to burst. Since oocyte diameter data for these samples would have been biased (because most measurable oocytes would not have reached the hydrated state) the slides from these samples were used only to assess the his- tological stage of the oocytes. This problem did not occur with the November 1987 sample collected at sea on a research vessel. Prior to statistical testing of the data, we tested oocyte size distri- butions for normality using the Shapiro-Wilk test. We applied two sample £-tests to test for differ- ences in the mean diameter of previtellogenic and vitellogenic oocytes between months within years and among years. We used linear regression to investigate the relation 1) between fish length and mean oocyte diameter within months and 2) between fish length and mean oocyte diameter at the time of spawning. Results Oocyte development Ovaries were examined from 174 fish (Table 1) caught at five locations in Hecate Strait (Fig. 1). The sampling period encompassed seven different months over three years. Descriptions and micro- 4 Fargo, J., and T. Sexton. 1991. A quide to the ovarian histol- ogy of English sole iParophrys vetulus). Can. MS. Rep. Fish. Aquat. Sci. 2133, 19 p. NOTE Fargo and Tyler: Oocyte maturation in Pleuronectes vetulus 191 graphs of the stages of matura- tion for English sole oocytes have been summarized by Fargo and Sexton (1991). 4 Ex- amples of oocyte size distribu- tions for fish of different lengths sampled during the same period, August 1988, are presented in Figure 2. For all sizes of English sole collected, we observed the simultaneous presence of only two modes in the oocyte size distributions. The smaller mode (10-150 u> consisted of previtellogenic oo- cytes and the larger mode (150-500 u) of vitellogenic oo- cytes. No previtellogenic oo- ctyes >150 u were observed. The size modes for previtel- logenic oocytes were similar among fish ranging in size from 33 to 46 cm. The mode for vitellogenic oocytes shifted to the right (increased) with in- creasing fish length. Vitellogenic oocytes in- creased in size from early sum- mer until they became hy- drated prior to spawning in the fall (Fig. 3). We observed no trend in the size composition of previtellogenic oocytes over the same period. As the month of spawning was approached a complete separation between the two modes became appar- ent. The irregular shape of the modal distribution for vitel- logenic oocytes in Figure 3 is caused by combining data for fish of different lengths and de- veloping at different rates. The more normal distribution for this mode during the month of spawning is due to two factors. First, the size range of fish for this sample was smaller than for other samples and, second, egg diameter at the time of spawning was similar for fish of different length. Fargo and Sexton (1991) 4 described the events of oocyte maturation for English sole in detail. Briefly, Table 1 A summary of ovary samples examined in the study of oocyte matura- tion in Hecate Strait English sole (Pleu ronectes vetulus) Length class (cm) (No. ovaries Date Sample type Location examined) 7-13 January 1987 Research cruise Two Peaks 30-34 (2) White Rocks 35-39 (6) 40-44 (5) 45-49 (5) 50-54 (4) 55-59 ( 1 ) Total (23) 19 January 1988 Port sample White Rocks 30-34 ( 1 ) 35-39 (1) 40-44 ( 1 ) Total (3) 17 March 1987 Research cruise Horseshoe 30-34 (2) 35-39 (2) 40-44 (3) 45-49 (2) 50-54 (1) Total (10) 16 March 1988 Port sample Horseshoe 30-34 ( 1 ) 35-39 (4) 40-44 (2) 45-49 (1) Total (8) May 5 1988 Port Sample Horseshoe- 30-34 ( 1 ) White Rocks 35-39 (1) 40-44 (2) 45-49 (6) Total (10) 6 June 1987 Research cruise Horseshoe- 30-34 ( 1 i Bonilla 35-39 ( 3 1 40-44 (3) 45-49 (3) 50-54 (3) Total (12) 2 June 1988 Port sample Horseshoe 30-34 ( 1 ) 35-39 (3) 40-44 (3) 45-49 (3) 50-54 (2) Total (12) 27 August 1987 Research cruise Horseshoe 30-34 (1) 34-39 ( 7 ) 40-44 (5) 50-54 1 1 ) Total (14) 192 Fishery Bulletin 92(1). 1994 Table 1 (continued) Length class (cm) (No. ovaries Date Sample type Location examined) 22 August 1988 Port sample Horseshoe Total 30-34 (2) 34-39 (5) 40-44 (8) 45-49 (6) 50-54 (1) (22) 28 August 1990 Port sample Two Peaks Total 35-39 (4) 40-44 (4) 45-49 (4) 50-54 (1) (13) 27 January 1988 Port sample Two Peaks- Butterworth Total 30-34 (4) 35-39 (2) 40-44 (3) 45-49 (1) 50-54 (1) (11) 19 January 1990 Port sample Horseshoe Tota 30-34 (1) 35-39 (6) 40-44 (8) 45-49 (1) (16) 5-6 November 1987 Research cruise Horseshoe- 30-34 (5) Butterworth- 35-39 (7) White Rocks 40-44 (4) 45-49 (2) 50-54 (1) 3 November 1990 Port sample Butterworth Total 30-34 (D 35-39 (ll 40-44 (2) 45-49 (1) 50-54 (1) (6) vitellogenesis occurred when oocytes reached a di- ameter of about 150 p. Vacuolization occurred in oocytes ranging from 180 u to 250 p Deposition of yolk in the outer cortex occurred in oocytes ranging in size from 200 p to 430 p, and hydra ted oocytes ranged in size from 375 (i to 550 p. We began our investigation of the timing and duration of oocyte maturation by examining the size composition and histological stage of oocytes col- lected from fish sampled between January and No- vember. Ovaries examined from 68 of 72 fish col- lected during winter and spring (January 1987-88 and March 1987-88) contained mainly pre- vitellogenic oocytes. The fish examined from the January samples contained previtellogenic oocytes only. Four of 22 fish examined from samples collected during the month of March contained vitellogenic oocytes. Three of these (36-40 cm in length) con- tained vitellogenic oocytes that were hydrated and translucent (405-429 p mean diameter). The fourth fish (46 cm in length) contained oocytes that had recently undergone vitello- genesis (mean diameter=230 p). Vitellogenesis for most fish occurred in the early summer. In May 1988, we observed vitellogenic oocytes in six of nine fish examined, ranging from 40 to 49 cm in length. All of these oocytes were in the early stages of development, prior to vacuolization, with mean diameters ranging from 174 to 263 p. Smaller fish (length range 33-42 cm) con- tained previtellogenic oocytes only . In June (1987, 1988) vi- tellogenic oocytes, ranging in mean diameter from 178 p to 269 p, were present in 23 of 24 fish examined (length range 36-52 cm). Vitellogenic oocytes in one fish of 52 cm were at an advanced stage of development (mean diameter=252 p), with yolk granules formed in the outer cortex. The relation be- tween mean diameter of vitel- logenic oocytes and fish length was not significant for the months of May ( 1988) and June (1987, 1988) (linear regression, P>0.1 for all three, n=6, 11, 12) By August the oocytes in some of the larger fish (45-50 cm) were nearing hydration. Mean diameters for vitellogenic oocytes from fish sampled in August (1987, 1988, 1990) ranged from 226 p to 429 p. There were significant, positive linear relationships between fish length and mean oocyte diameter for all of these samples (Table 2, Fig. 4). The size distributions for previtellogenic and vitellogenic oocytes did not differ significantly (Shapiro- Wilk test, P<0.05) from that of the normal distribution for any of the following cases. There was no significant difference in mean diameter of previtellogenic oocytes for the same months across NOTE Fargo and Tyler: Oocyte maturation in Pleuronectes vetulus 193 33 cm prevHellogenlc 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) 42 cm prevHellogenlc 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) 38 cm 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) 46 cm | vltellogenlc prevHellogenlc 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) Figure 2 Oocyte size compositions determined from ovary samples collected from Hecate Strait English sole {Pleuronectes vetulus) in August 1988. the two years (Table 3). However, there were signifi- cant differences in mean diameter for previtellogenic oocytes among months within both years (Table 4). No obvious trend in mean diameter over time was apparent for previtellogenic oocytes. There were sig- nificant differences in the rate of oocyte development between 1987 and 1988 (Table 3). The mean diam- eter of vitellogenic oocytes in June and August of 1987 was significantly larger than for the same months in 1988, suggesting that vitellogenesis oc- curred earlier in 1987 than in 1988. There were also significant differences in the mean diameter of vitellogenic oocytes among months within years (Table 5). The mean diameter of vitellogenic oocytes increased significantly, coinciding with advancing oocyte development, between June-November in 1987 and June-October in 1988. Spawning Ovaries obtained from spawning fish (October 1988, 1990 and November 1987, 1990) were examined to investigate 1) size-dependent spawning and 2) the relation between fish length and egg diameter at the time of spawning. For the October 1988 sample, we observed the presence of vitellogenic oocytes only in fish smaller than 40 cm. The mean diameter of vitellogenic oocytes in these fish ranged from 287 to 408 p. Fish ranging in length from 43 to 52 cm con- tained spent ovaries with previtellogenic oocytes only. Thus, we concluded that the larger fish had spawned prior to the time of the sample collection. In the October 1990 sample, taken two weeks ear- lier than the 1988 sample, some of the fish larger than 40 cm contained hydrated oocytes while oth- ers had spent ovaries with resorbing oocytes, sug- gesting that they were spawning in early October. Oocytes examined from samples collected in Novem- ber (1987, 1990) also indicated that larger fish had spawned previous to this time. Fish larger than 42 cm contained only pre-vitellogenic oocytes and there was no sign of resorbing oocytes. Most smaller fish were in spawning condition during this month. Vitellogenic oocytes were present in fish ranging from 30 to 42 cm. Mean diameter ranged from 373 to 483 p and these oocytes were hydrated and trans- 194 Fishery Bulletin 92(1). 1994 January n=1001 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (micron*) May previtellogenic vltellogenk; 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) June prevttellooenlc 1/ n=676 ^ vttellogenlc Ik 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) 600 — 500 J.400 ?300 V I 200 "- 100 August prevttellooenlc 1/ n= )564 vltellogenlc / 10 60 110 160210260310360410460 Oocyte diameter (microns) October (spawning) prevttellogenlc n=377 vttellogenlc A 10 60 110 160 210 260 310 360 410 460 Oocyte diameter (microns) Figure 3 Oocyte size composition determined from ovary samples collected from Hecate Strait English sole iPleuronectes vetulus) during January-October in 1988 (samples combined). lucent. We then combined all the data on mean egg diameter for spawning fish and there was no relationship between mean egg di- ameter at the time of spawning (hydrated and translucent) and fish length (linear regression, P>0.1, rc = 19). Discussion Oocyte development Dunn and Tyler ( 1969) and Dunn (1970) determined the length of time required for oocyte matura- tion in winter flounder iPleuronectes americanus). They observed two size modes of previtellogenic oocytes at any particular time. They documented the rate of increase in size for these modes for three consecutive years and concluded that the oocyte maturation period for this species was three years. We observed only a single mode for both previtellogenic and vitellogenic oocytes in fish sampled during all the months examined in our study. Johnson et al. ( 1991) reported similar results in their study of Puget Sound English sole. If oocytes Table 2 Linear regression statistics for the relationship bet ween vitell ogenic oocyt e mean diameter and fis h length "or Engl sh sole (Pleuronectes vetul us) for the month of August 1987 1988, and 1990. Degrees o f Year freedom F-statistic P Regression equation' r 1987 13 10.72 0.007 Y = 122 + 5.93X 0.687 1988 20 20.93 - n iiiiiii Y = -93 + 9.44X 0.910 1990 12 44.01 . I) 1)001 Y = - 237 + 12. 6X 0.724 ' Y = oocyte mean diameter (a.). X = total length of fish (cm ) produced in year i were spawned in year i+1, we would expect to see two size classes of immature oocytes in year i+1, corresponding to those oocytes that were produced in year i (large immatures) to be spawned in year i+1 and those that were pro- duced in year i+1 (small immatures) to be spawned in year i+2. The fact that there were no significant differences in the mean diameter of previtellogenic oocytes for the same months in consecutive years (1987-88) suggests that the oocyte maturation pe- riod for Hecate Strait English sole is probably one year. NOTE Fargo and Tyler: Oocyte maturation in Pleuronectes vetulus 195 500 450 400 350 300 250 200 150 100 50 500 450 T 40 ° I 350 \ 4 300 | 250 -I 200 § 100 50 30 500 450 1" 400 § 350 4 300 I 250 -1 200 -£. 150 <§ 100 50 30 Aub-37 ° g ° 6 o o ° o; o provttolloflonlc • vltsllogenic — regression 30 35 40 45 50 Total length (cm) 55 Aug-88 oggooe °86 oo 35 40 45 Total length (cm) 50 55 Aug-90 • • OO OOq nOO 35 40 45 Total length (cm) 50 55 Figure 4 Mean oocyte diameter vs fish length determined from ovary samples collected from Hecate Strait English sole {Pleuronectes vetulus) during the month of August, 1987. 1988, and 1990. We also found no trend in the mean size of previtellogenic oocytes among months within years, contrary to results reported by Dunn and Tyler (1969). One explanation for this is that the recruit- ment of small immature (previtellogenic) oocytes from the germinal epithelium is a continual process for Hecate Strait English sole. Alternatively, there may be a short time period, following spawning for example, during which previtellogenic oocytes recruit and quickly grow to a size of around 80 p. Additional work is needed to resolve these possibilities. Table 3 Results of two sample r-tests of mean diameters of previtellogenic and vitellogenic oocytes for English sole (Pleuronectes vetulus) determined from samples collected during the same month in 1987 and 1988. Month and year n mean diameter (microns) P previtellogenic January 1987 January 1988 6,264 1,001 69.0 69.4 >0.1 March 1987 March 1988 1,603 1,737 59.2 59.8 >0.1 June 1987 June 1988 1,389 1,132 72.4 72.9 >0.1 August 1987 August 1988 1,812 2,071 66.7 65.8 >0.1 vitellogenic June 1988 June 1988 953 1,029 219.1 203.3 <0.0001 August 1987 August 1988 1,774 3,584 362.1 318.1 <0.0001 Spawning In general larger fish produced yolk earlier and spawned earlier than smaller fish. Most of the spawning fish were obtained from samples collected in October and November but there was also evi- dence of spring (March) spawning for smaller fish. Egg size at the time of spawning did not appear to be dependent on fish length. However, there is some evidence from this study to suggest a possible mini- mum size limit for eggs at the time of spawning. That is, the difference in the mean diameter of vitellogenic oocytes between smaller and larger fish decreased over time until there was no apparent difference at the time of spawning. Observations made during this study indicate that atresia was not as prevalent for Hecate Strait English sole as that reported for English sole in Puget Sound by Johnson et al. (1991). Marine fish species show wide variability in the reproductive process, which enables them to miti- gate the uncertain conditions in the marine environ- ment (Murphy, 1968; Roff, 1981). English sole dem- onstrate considerable phenotypic plasticity with regard to spawning. In Hecate Strait the spawning season extends from early fall through the follow- 196 Fishery Bulletin 92|1), 1994 Table 4 Results of two sample t-tests of the mean diameter ( |i ) of previtellogenic oocytes in English sole (Pleuronectes vetulus) among months for samples collected in 1987 and 1988. Year and Month January March June August November 1987 January P<0.0001 P<0.0001 P=0.0004 P<0.0001 (n=6264, 7=69. Out March P<0.0001 P<0.0001 P<0.0001 June P<0.0001 P<0.0001 (/i = 1132, 7=72. 9u) August — P<0.0001 (/i = 2071, 7=65. 8u) October — — (o = 1500, 7=56.9u) Table 5 Results of two sample r-tests of the mean diameter (u) of vitellogenic oocytes in E riglish sole (Pleuronectes vetulus) among months for samples collected in 1987 and 1988. Year and month June August November Year and month May June August October 1987 1988 June — <0.0001 <0.0001 May >0.1 <0.0001 <0.0001 (n= 953, 7=219.1u) (n=191, .7=201. 4u I June — <0.0001 <0.0001 August <0.0001 (/i = 1029. 7=203. 3u) (re=1774, 7=362. lu) August (7i=3584, 7=318. In) <0.0001 <0.0001 November — — — October — — — (n= 488, i=413.7m (71=710, 7=342. lu) ing spring. Johnson et al. (1991) reported a similar spawning period for Puget Sound English sole as did Kruse and Tyler ( 1989) in their study of English sole off the Oregon coast. This reproductive strategy may increase the probability of encountering favorable conditions for larval survival by spreading the re- productive effort over the longest possible time span. Based on our results it is unlikely that cohort-spe- cific spawning occurs as in Pacific herring, Clupea pallasi (Ware and Tanasichuk, 1989), and Norwe- gian Atlantic herring, Clupea harengus (Lambert, 1990). However, in view of the relation between oocyte maturation and fish length and the duration of the spawning period, it is possible that first time NOTE Fargo and Tyler: Oocyte maturation in Pleuronectes vetulus 197 spawners spawn at a different time than the rest of the stock. We can suggest no mechanism to account for this and more data are required to corroborate these results. There is also evidence of interannual variability in oocyte maturation and this process appears to be size-related. Smaller fish matured later and spawned later than larger fish. Our results indicate that the time of peak spawning and the duration of the spawning season are variable from year to year. The results from this study provide baseline infor- mation for an investigation of the recruitment biol- ogy of this stock. Acknowledgments We wish to acknowledge John Bagshaw, Serge Villeneuve, Tammy Laberge, Christina Horvath, Tracy Sexton, and Corinne Kikegawa for their aid in preparing the slides for histological examinations and photography of specimens. Ron Tanasichuk and Doug Hay reviewed the manuscript and provided advice regarding the spawning characteristics for the species. The scientific editor and three anony- mous reviewers provided a number of suggestions which improved the paper. Literature cited Dunn, R. S. 1970. Further evidence for a three year oocyte maturation time in the winter flounder (Pseu- dopleuronectes americanus). J. Fish. Res. Board Canada. 27:957-960 Dunn, R. S., and A. V. Tyler. 1969. Aspects of the anatomy of the winter floun- der (Pseudopleuronectes americanus) with hypoth- eses on oocyte maturation time. J. Fish. Res. Board Canada 26:1943-1947. Fargo, J. 1993. Flatfish. In B. M. Leaman and M. Stocker (ed.), Groundfish stock assessments for the west coast of Canada in 1992 and recommended yield options for 1993. Can. Tech. Rep. Fish. Aquat. Sci. 1919:95-131. Fargo, J, R. P. Foucher, S. C. Schields, and D. Ross. 1984. English sole tagging in Hecate Strait, R/V G.B. REED, June 6-24, 1983. Can. Data Rep. Fish. Aquat. Sci. 427, 49 p. Foucher, R. P., and R. J. Beamish. 1980. Production of nonviable oocytes by Pacific hake (Merluccius productus). Can. J. Fish. Aquat. Sci. 37:41-47. Hart, J. L. 1973. Pacific fishes of Canada. Fish. Res. Board Can. Bull. 180, 740 p. Johnson, L. L, E. Casillas, M. S. Myers, L. D. Rhodes and O. P. Olson. 1991. Patterns of oocyte development and related changes in plasma 17-B estradiol, vitellogenin and plasma chemistry in English sole Parophrys vetulus Girard. J. Exp. Mar. Biol. Ecol. 152: 161-185. Ketchen, K. S. 1956. Factors influencing the survival of the lemon sole (Parophrys vetulus) in Hecate Strait, British Columbia. J. Fish. Res. Board Canada, 13(5): 647-694. Kruse, G. H., and A. V. Tyler. 1989. Exploratory simulation of English Sole (Parophrys vetulus) recruitment mechanisms. Trans. Am. Fish. Soc. 118:101-118. Lambert, T. C. 1990. The effect of population structure on recruit- ment in herring. J. Cons. int. Explor. Mer 47:249-255. Murphy, G. I. 1968. Pattern in life history and the environment. Am. Nat. 102:391-403. Roff, D. A. 1981. Reproductive uncertainty and the evolution of iteroparity: why don't flatfish put all their eggs in one basket? Can. J. Fish. Aquat. Sci. 38:968- 977. Ware, D. M., and R. Tanasichuk. 1989. Biological basis of maturation and spawning waves in Pacific herring (Clupea harengus pallasi). Can. J. Fish. Aquat. Sci. 46(101:1776- 1784. Yasutake, W. T., and J. H. Wales. 1983. Microscopic anatomy of salmonids: an atlas. Fish. Wild. Ser. U.S. Dep. Int. Res. Pub. 150, 189 p. Estimation of weight-length relationships from group measurements William H. Lenarz Tiburon Fisheries Laboratory National Marine Fisheries Service, NOAA 3 1 50 Paradise Drive. Tiburon, CA 94920 Catch sampling provides data that are basic to fisheries re- search and is often an important component of research budgets. Samplers typically select fish ran- domly, measure length, remove ageing structures, and determine sex for each individual. In many schemes for sampling commercial (e.g., Sen, 1986; Tomlinson, 1971) and survey catches (e.g., Gun- derson and Sample, 1980), sample weight is needed to expand the sample results to the total catch. Individual weights are usually not needed to satisfy the main objectives. Often only the aggre- gate weight of the sample is taken to save time, and if at sea, to avoid difficult logistics. While sampling costs are easily justified by program objectives, scientists frequently use the data for addi- tional research. Investigators often use weight- length relations to study possible correlations between condition of fish and environmental factors or population density (e.g., Pat- terson, 1992). A literature search revealed only two previous devel- opments of methods of estimating weight-length relations from samples of individual lengths and aggregate weights (WLRAW). Cammen (1980) used a general nonlinear regression program from the BMDP package (Dixon, 1983) as a WLRAW method. He tested the method with simulated data and compared the results of regression using unweighted ob- servations to using observations weighted by the inverse of sample weights, and with various esti- mates made when individual weights were known. Since the data were simulated, assuming a multiplicative error term, it would have been more appropriate to use the inverse of sample weight squared for weighting. The non- linear method produced good fits to the simulated data, and weighted parameter estimates were closer to the true values than unweighted estimates. Damm ( 1987) developed two non- linear WLRAW methods. One method is a biased approxima- tion, and his report indicated that the other method did not always produce estimates of the param- eters. In this note I describe a new WLRAW method, compare it with Cammen's method, explore error term characteristics, and describe bootstrap estimates of confidence limits of estimates. The methods of Damm (1987) were not studied because his biased approximation method requires as much calcula- tion as my new method and his other method does not always work. Methods The relation between expected weight and length of an indi- vidual fish is usually assumed to be the power equation, E(W l ) = alf l Where V^ = weight of fish i, a - parameter. (1) L, = length of fish i, p = parameter. For the new WLRAW method I modeled the weight-length rela- tionship as W, flK) + £, (2) J i=i where W = L. = e. = T = average weight of fish in sample j, number of fish in sample j, length of fish i in sample j, error term for sample j, 1, • • , T, number of samples. I assumed that error was additive because under field conditions much of the error was due to lim- its to the accuracy in measure- ment of sample weights. Because the dependent variable in Equa- tion 2 was a sample average, its variance should contain a compo- nent which is proportional to the inverse of n . Thus in the new es- timation procedure, I weight each observation by n to stabilize the variance. I made the assumption that, after weighting by sample size, error was random and inde- pendent of,/. The new method treated esti- mation of parameters of (Eq. 2) as a separable least-squares problem (Seber and Wild, 1989). For a trial value of (3 (P'l, y was calculated for each sample, /,=<2X>/», (3) With the new notation, Equation 2 becomes W- = a y ; + e j~ (4) Manuscript accepted 16 August 1993 Fishery Bulletin 92:198-202 (1994) 198 NOTE Lenarz: Estimation of weight-length relations from group measurements 199 I then obtained an estimate of a (a') corresponding to p" by using the standard least squares linear re- gression with zero intercept method. I used a non- linear least squares procedure to obtain the estimate of (3 ( B )■ This procedure was analogous to finding the transformation, Lf, that minimized the sum of squares about the linear regression (Eq. 4). Using this procedure, I estimated brackets for ensuring that the searching range included P with the proce- dure MNBRAK (Press et al., 1989). Then I used the iterative procedure BRENT (Press et al., 1989) to obtain the final estimate. BRENT uses parabolic interpolation to minimize the sum of squares as a function of (3'. Convergence is assumed when the procedure does not change the value of P' more than a tolerance specified by the user. As previously stated, observations were weighted by n to stabilize the variance. I implemented the WLRAW method in double precision using Sun FORTRAN for a Sun SPARC2 work station. Bootstrap approximations of confidence intervals about the line were calculated for the new method. The literature contains a variety of bootstrap meth- ods proposed to approximate confidence intervals (e.g., DiCiccio et al., 1992). I used the nonparamet- eric BC method of Efron (1987) because it often a produces good results and is relatively easy to use. BC a stands for accelerated bias corrected boot- strap confidence intervals. Efron (1987) showed that, in the parametric case, the method is approximately correct if a transformation to a normally distributed variable exists. The transformation does not need to be known and the variance does not need to be con- stant. While the correctness of the BC has not been a mathematically proven for nonparametric cases, such as the WLRAW, Efron (1987) stated, "...empirical re- sults look promising." The BC a confidence limits of an estimate of parameter 8, 0, are IBS(N(z[a]))<6J ;=i (7) where U , 3(A) ef ] -e A estimate of 6 whenyth sample has a very small amount of extra weighting (A). If a and z are zero, then Equation 7 becomes the percentile method that is the most frequently used bootstrap method in the fisheries literature (e.g., Sigler and Fujioka, 1988). I chose to approximate 90% confidence bands rather than 95% or 99% bands because 90% non- parametric bootstrap intervals tend to perform bet- ter than intervals that cover a wider portion of the distribution (Efron, 1988). Following the advice of Efron, I used 1,000 bootstrap replicates. Cammen (1980) used the general nonlinear re- gression program of BMDP to estimate the param- eters of Equation 2, except that he assumed that the error term is multiplicative and used total sample weight instead of average weight as the dependent variable. The BMDP program uses the Gauss-New- ton algorithm. I used the same algorithm in the nonlinear regression program of the SAS package (SAS Institute Inc., 1989) on a Sun SPARC2 to com- pare parameter estimates and execution times with the new method. Since the correct error model is not known, I also estimated the parameters using no _weighting__and weight set to 1/W ,1/W, 2 , n l IW r and n ] I W", and compared asymptotic stan- dard errors of the parameter estimates. The new es- timation procedure is simpler than the Gauss-New- ton approach because it searches for the least squares by iteratively changing the value of one parameter instead of two. I used data collected on chilipepper rockfish {Sebastes goodei) by a cooperative landing sampling program of the California Department of Fish and 200 Fishery Bulletin 92|1). 1994 Game and National Marine Fisheries Service to examine utility of the WLRAW method. Samplers collected two groups of fish from each sampled land- ing. For each group a container that holds 22.7 kg of fish was filled with fish regardless of species. Then the sampler obtained total group weights to the nearest lb (0.45 kg) for each species and the total length of each fish was measured to the nearest mm. I converted weights to kg. I changed lengths to deci- meters to minimize potential scaling problems in the computations. Before using the WLRAW method, I combined groups within a landing because they may not be independent. I first used data for all months during 1991 from all ports between Morro Bay and Crescent City, California, to develop, test, and time the software. Results of the test runs are described briefly in the Results and Discussion section. More detailed re- sults are presented for a more typical application of the method. Investigators are more likely interested in results from a smaller number of samples taken from more restrictive scales of time and area than from data sets like the one used in the preceding example. I used data for chilipepper rockfish taken during July and August 1991 from Morro Bay to illustrate use of the method. Results and discussion The data from all ports consisted of measurements from 7,687 fish taken in 186 samples. The procedure required 1.6 seconds, compared with 18.8 seconds for the Gauss-Newton method. The Gauss-Newton and new methods produced parameter estimates that were identical to six decimal places. Predicted weights were very close to the results of Phillips (1964), who used data from individually measured fish. Sums of squares plotted against P' indicated that there were no local minima. Residuals were not related to weight, indicating that the additive error assumption is correct. Sometimes transformation of (3' to ln(P') when estimating parameters of power equations avoids problems due to curvature (Rat- kowsky, 1983). Transformation was tried and pa- rameter estimates were identical to the results when P' was not transformed. When P' was transformed, the procedure required more time to complete, so the transformation was not used. Data were available for 583 fish taken from 13 samples taken in Morro Bay, during July and Au- gust 1991. There were no strong trends between the residual and expected weight (Fig. 1). There was a tendency for absolute values of residuals to be nega- tively correlated with the number offish in a sample (Fig. 2A). The tendency was reduced when residu- 004 D 0.02 Residual (kg) 8 o B o D D D D D n -0.04 -0.06 D 04 0.5 06 07 0.8 0.9 1 Expected Weight (kg) Figure 1 Residual of average weight (kg) as a function of expected weight (kg) for chilipepper rockfish (Sebastes goodei) collected in samples taken from Morro Bay during July and August 1991. als were multiplied by sn , as expected under the assumption that variance is proportional to the in- verse of sample size (Fig. 2B). Also, n produced the lowest asymptotic standard errors of the parameter estimates of the six weighting factors explored (Table 1). The results shown in Table 1 and Figures 1 and 2 indicated that the additive error model with weighting by n was appropriate for these data. Bootstrap estimates of standard error using the new method were higher than asymptotic estimates us- ing the Gauss-Newton method. The bootstrap and asymptotic normal confidence intervals were narrow and similar within the range of most observed av- erage weights but diverged when expected weight was greater than 0.75 kg even though individual fish of larger size occurred in many of the samples (Table 2). The bootstrap confidence intervals were skewed at the larger sizes. However, the bootstrap estimates of absolute bias were less than 0.01 kg except they were -0.01 kg for 450-mm fish and -0.02 kg for 500-mm fish. All estimates of the absolute value of a were about 0.015, which indicated that a could have been ignored for this set of data. The new WLRAW method performed well. Good fits to the data were obtained and the residuals agreed with the assumptions. Approximate confi- dence limits indicated that precise estimates of ex- pected weight are obtained with a small number of samples under field conditions for sizes of fish within the range of most observed average weights. The method is fast when used on a work station or on a modern personal computer. The new method is 10 times faster than using the Gauss-Newton ap- NOTE Lenarz: Estimation of weight-length relations from group measurements 201 0.04 A D 0.02 •0.02 a D a D n o O D a ■0 04 ■0.08 -0.08 D 1 1 i 40 60 Sample Size 0.2 B a fo, CO * o o n L> D to D ■o i-o.t ir o D o D O -0.2 -0.3 D _l 1 - 1 40 60 Sample Size Figure 2 (A) Residual of average weight (kg) as a function of sample size for chilipepper rockfish (Sebastes goodei ) collected in samples taken from Morro Bay during July and August 1991. (B) Residual multi- plied by yrij as a function of sample size. proach with a standard statistical package. Some of the difference is probably due to the overhead in- volved with using the statistical package. When computationally intensive methods such as bootstrapping are used, time saved by using the new method is significant. The widening confidence limits for expected weights beyond the range of most observed average weights indicated use of expected weights beyond the observed range is extrapolation and should not be done. This also applies to comparison of param- eter estimates from different sets of data. If the range of observed average weights differ much among the data sets, comparison of parameter esti- mates is not meaningful. Estimates of the two pa- Table 1 Estimates of standard errors of parameter esti- mates of weight-length model for chilipepper rock- fish iSebasted goodei) collected from Morro Bay during July and August 1991. The Gauss-New- ton method was used with observations weighted by six factors to estimate the parameters, and the new method with rij as the weighting factor. As- ymptotic standard errors are shown for the Gauss- Newton method and bootstrap standard errors for the new method. Coefficients of variation of the pa- rameter estimates are shown in parentheses. Standard error Weighting factor Gauss-Newton method none 0.0028 (0.30) n, _ 0.0019 (0.21) raj/Wj 0.0020 (0.20) n/W, 2 0.0022 (0.20) 1/W, 0.00.30 (0.29) 1/W, : New method 0.0032 (0.28) 0.0046 (0.50) 0.2159 (0.07) 0.1489 (0.05) 0.1528 (0.05) 0.1547 (0.05) 0.2129 (0.07) 0.2069 (0.07) 0.2211 (0.07) Table 2 Expecte d weigh ts for chilipepper roc kfish iSebastes goodei ) collected from Morro Bay dur- ing July and August 1991, and 909t confidence about th 3 line. C onfidence limits were approxi- mated us \>iv, 1 he bootstrap BC (bootstrap ) and the asymptotic normal method 3 ( normal ) . Ex- pected weights were ca culatec from the esti- mated weight- ength relation (0.0091819 Length 3 L 758673 , Confidence limits Normal Bootstrap Total Expected length weight Lower Uppei Lower Jpper (dm) (kg) (kg) (kg) (kg) (kg) 3.00 0.30 0.28 0.32 0.28 0.33 3.50 0.49 0.47 0.51 0.47 0.50 3.75 0.61 0.60 0.62 0.60 0.62 4.00 0.75 0.74 0.76 0.73 0.76 4.50 1.09 1.04 1.14 0.99 1.12 5.00 1.52 1.42 1.63 131 1.61 rameters of the weight-length relation are highly correlated even when individuals are weighed and standard linear regression is used (Lenarz, 1974). Thus, regardless of the type of data or statistical 202 Fishery Bulletin 92(1). 1994 procedure, I recommend comparison of weight- length relations among data sets by comparison of expected weights of fish at sizes within the range of observed average weights common to all data sets of interest. The results of this study suggest that an additive error term is more appropriate than a multiplica- tive error term for modeling weight-length relations. Most previous studies have assumed multiplicative error, which is implied when the log-log transforma- tion is used to estimate parameters of the model from individually measured fish by linear regres- sion. The multiplicative error assumption has not been demonstrated correct even when data are available from fish weighed individually. While good fits to data are usually obtained under the multi- plicative assumption, if the assumption is not valid, statistical inferences may be erroneous. Pienaar and Thomson (1969) assumed that the error term was additive for their data and discussed statistical as- pects of the assumption. Further examination of the error term form would be interesting. Copies of the FORTRAN code used in this study are available from the author. Acknowledgments I thank James Bence for considerable statistical advice, particularly on the bootstrap procedure. James Bence, Alec MacCall, and Steve Ralston con- structively reviewed drafts of the note. I also thank David Woodbury for his assistance during an early stage of this study and Dale Roberts for his help with the use of SAS. Literature cited Cammen, L. M. 1980. Estimation of biological power functions from group measurements. Can. J. Fish. Aquat. Sci. 37:716-719. Damm, U. 1987. The estimation of weight at length from the total weight and the length distribution of a sample. ICES CM 1987/D:16, 9 p. DiCiccio, T. J., M. A. Martin, and G. A. Young. 1992. Fast and accurate approximate double boot- strap confidence intervals. Biometrika 79(2): 285-95. Dixon, W. J. 1983. BMDP statistical software. Univ. California Press, Berkeley, 733 p. Efron, B. 1987. Better bootstrap confidence intervals. J. Am. Statist. Assoc. 82(3971:171-185. 1988. Bootstrap confidence intervals: good or bad? Psychol. Bull. 104(2):293-6. Gunderson, D. R., and T. M. Sample. 1980. Distribution and abundance of rockfish off Washington, Oregon, and California during 1977. Mar. Fish. Rev. 42 (3-41:2-16. Lenarz, W. H. 1974. Length-weight relations for five eastern tropical Atlantic scombrids. Fish. Bull. 72:848- 851. Patterson, K. R. 1992. An improved method for studying the condi- tion offish, with an example using Pacific sardine Sardinops sagax (Jenyns). J. Fish Biol. 40: 821-831. Phillips, J. B. 1964. Life history studies on ten species of rockfish (genus Sebastodes). Calif. Dep. Fish Game Fish Bull. 126, 70 p. Pienarr, L. V., and J. A. Thomson. 1969. Allometric weight-length regression model. J. Fish. Res. Board Canada 26:123-131. Press, W. H., B. P. Flannery, S. A. Teukolsky and W. T. Vetterling. 1989. Numerical recipes the art of scientific com- puting (FORTRAN version). Cambridge Univ. Press, Cambridge, 702 p. Ratkowsky, D. A. 1983. Nonlinear regression modeling. Marcel Dekker, NY, 276 p. SAS Institute Inc. 1989. SAS/STAT® User's guide, version 6, 4th edi- tion, Vol. 2. SAS Institute Inc., Cary, NC, 846 p. Seber, G. A., and C. J. Wild. 1989. Nonlinear regression. J. Wiley & Sons, NY, 768 p. Sen, A. R. 1986. Methodological problems in sampling com- mercial rockfish landings. Fish. Bull. 84:409- 421. Sigler, M. F., and J. T. Fujioka. 1988. Evaluation of variability in sablefish, Anoplopoma fimbria, abundance indices in the Gulf of Alaska using the bootstrap method. Fish. Bull. 86:445-452. Tomlinson, P. K. 1971. Some sampling problems in fishery work. Biometrics 27:631-41. Spiny lobster recruitment and sea level: results of a 1 990 forecast Jeffrey J. Polovina Honolulu Laboratory, Southwest Fisheries Science Center National Marine Fisheries Service, NOAA 2570 Dole Street, Honolulu, Hawaii 96822-2396 Joint Institute for Marine and Atmospheric Research (JIMAR) University of Hawaii, Honolulu. Hawaii 96822 Department of Oceanography, School of Ocean and Earth Science and Technology University of Hawaii, Honolulu, Hawaii 96822 Gary T. Mitchum Joint Institute for Marine and Atmospheric Research LIIMAR) University of Hawaii, Honolulu, Hawaii 96822 Department of Oceanography, School of Ocean and Earth Science and Technology University of Hawaii, Honolulu. Hawaii 96822 A relation between recruitment to the fishery and sea level for the spiny lobster Panulirus mar- ginatus, in the Northwestern Ha- waiian Islands, was supported by data from 1985 through 1990 (Polovina and Mitchum, 1992). A forecast of future recruitment was made based on projected sea lev- els (Polovina and Mitchum, 1992). This note updates that forecast with two more years of data. Fishery data from 1985 to 1990 indicated considerable inter- annual variation in recruitment strength of spiny lobster, Pan- ulirus marginatus, between the two principal fishing grounds (Necker Island and Maro Reef), although separated by about 700 km (Fig. 1; Polovina and Mitchum, 1992). Recruitment strength variation between the two fishing areas was measured as the ratio of the commercial landings from Maro Reef divided by the combined commercial land- ings from Necker Island and Maro Reef. A strong correlation was ob- served between this measure of recruitment strength at Maro Reef and the sea level gradient along the Northwestern Hawaiian Islands, advanced by four years (Polovina and Mitchum, 1992). The sea level gradient was mea- sured as the difference in sea level between tide gauges at French Frigate Shoals, southeast of Maro Reef, and Midway Island, northwest of Maro Reef. A high proportion of the commercial landings came from Maro Reef following a steep gradient, while relatively few spiny lobsters were caught at Maro Reef following a flat gradient. The four-year lag is based on the minimum legal har- vest size which, for the spiny lob- ster is about three years old, af- ter benthic settlement. Prior to benthic settlement, the larvae are planktonic for about one year. Since sea level gradient appears to lead recruitment to the fishery by four years, the relation can provide up to a four-year forecast. Based on data through 1990, it was forecast that in 1991 recruit- ment to the fishery at Maro Reef would be weak but would recover in 1992 relative to recruitment at Necker Island (Fig. 2). The 1991 and 1992 fishery data show this forecast correct (Fig. 2), although the fishery for the entire North- western Hawaiian Islands was relatively weak in 1992. Thus, while sea level gradient index does forecast the relative strength of recruitment at Maro Reef, it is not, by itself, an index of absolute recruitment strength. It has been argued that sea level gradient measures the strength of the Subtropical Counter Current, which appears to intersect the Hawaiian ridge as three narrow eastward flowing bands at 20, 24, and 26 degrees north latitude (Polovina and Mitchum, 1992; White and Walker, 1985). Recent studies of P. marginatus larval distribution find a relatively high abundance of late stage larvae consistently present near lat. 26°N, and tracks from Argos drifter buoys drogued at 30 m indicate buoy entrap- ment along lat. 26'N. 1 These re- sults provide some additional sup- port to our original hypothesis that a positive relationship exists between the strength of the Sub- tropical Counter Current and lo- cal larval survival, retention, and recruitment to the fishery at Maro Reef (Polovina and Mitchum, 1992). Literature cited Polovina, J. J., and G. T. Mitchum. 1992. Variability in spiny lob- ster Panulirus marginatus re- 1 Polovina, J.J.. and R.B. Moffitt. In re- view. The spatial and temporal distribu- tion of the larvae of the spiny lobster {Panulirus marginatus) in the North- western Hawaiian Islands. Manuscript accepted 11 August 1993 Fishery Bulletin 92:203-205 (1994) 203 204 Fishery Bulletin 92(1), 1994 Figure 1 The Hawaiian Archipelago. o 3 a 3 .s purchase from the Superintendent of Documents. U.S. Government Printing Office, Washington, DC 20402. 206 Fishery Bulletin Guide for Contributors Preparation Title page should include authors' full names and mailing addresses and the senior author's telephone and FAX number. Abstract should not exceed one double-spaced typed page. It should state the main scope of the research but emphasize its conclusions and relevant findings. 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U.S. Department of Commerce Seattle, Washington Volume 92 Number 2 April 1994 Fishery Bulletin Contents iii Errata 207 223 236 254 262 275 292 APR 2 8 1994 Woods Hole, MA 02543 Blood, Deborah M., Ann C. Matarese, and Mary M. Yoklavich Embryonic development of walleye pollock, Theragra chalcogramma, from Shelikof Strait, Gulf of Alaksa Brodeur, Richard D., and William C. Rugen Diel vertical distribution of ichthyoplankton in the northern Gulf of Alaska Clark, Malcolm R., and Dianne M. Tracey Changes in a population of orange roughy, Hoplostethus atlanticus, with commercial exploitation on the Challenger Plateau, New Zealand Daniel, Louis B., Ill, and John E. Graves Morphometry and genetic identification of eggs of spring-spawning sciaenids in lower Chesapeake Bay Ditty, James G., Richard F. Shaw, and Joseph S. Cope A re-description of Atlantic spadefish larvae, Chaetodipterus faber (family: Ephippidae), and their distribution, abundance, and seasonal occurrence in the northern Gulf of Mexico Ditty, James G., Richard F. Shaw, Churchill B. Grimes, and Joseph S. Cope Larval development, distribution, and abundance of common dolphin, Coryphaena hippurus, and pompano dolphin, C. equiselis (family: Coryphaenidae), in the northern Gulf of Mexico Kastelle, Craig R., Daniel K. Kimura, Ahmad E. Nevissi, and Donald R. Gunderson Using Pb-2 1 0/Ra-226 diseguilibria for sablefish, Anoplopoma fimbria, age validation Fishery Bulletin 92(2). 1994 302 Moltschaniwskyj, Natalie A., and Peter J. Doherty Distribution and abundance of two juvenile tropical Photololigo species (Cephalopoda: Loliginidae) in the central Great Barrier Reef Lagoon 313 Murie, Debra J., Daryl C. Parkyn, Bruce G. Clapp, and Geoffrey G. Krause Observations on the distribution and activities of rockfish, Sebastes spp., in Saanich Inlet, British Columbia, from the Pisces IV submersible 324 Perrin, William F, Gary D. Schnell, Daniel J. Hough, James W. Gilpatrick Jr., and Jerry V. Kashiwada Reexamination of geographic variation in cranial morphology of the pantropical spotted dolphin, Stenella attenuata, in the eastern Pacific 347 Powell, Eric N., John M. Klinck, Eileen E. Hofmann, and Sammy M. Ray Modeling oyster populations. IV: Rates of mortality, population crashes, and management 374 Prager, Michael H. A suite of extensions to a nonequilibrium surplus-production model 390 Stoner, Allan W., and Megan Davis Experimental outplanting of juvenile queen conch, Strombus gigas. comparison of wild and hatchery-reared stocks 412 Taylor, David M., Paul G. O'Keefe, and Charles Fitzpatrick A snow crab, Chionoecetes opilio (Decapoda, Majidae), fishery collapse in Newfoundland 420 Warlen, Stanley M. Spawning time and recruitment dynamics of larval Atlantic menhaden, Brevoortia tyrannus, into a North Carolina estuary 434 Reilly, Stephen B., and Paul C. Fiedler Interannual variability of dolphin habitats in the eastern uupical Pacific. I: Research vessel surveys, 1986-1990 451 Fiedler, Paul C, and Stephen B. Reilly Interannual variability of dolphin habitats in the eastern tropical Pacific. II: Effects on abundances estimated from tuna vessel sightings, 1 975-1 990 Notes 464 Aurioles-Gamboa, David, Maria Isabel Castro-Gonzalez, and Ricardo Perez-Flores Annual mass strandings of pelagic red crabs, Pleuroncodes planipes (Crustacea: Anomura Galatheidae), in Bahia Magdalena, Baja California Sur, Mexico 471 Ennevor, Bridget C. Mass marking coho salmon, Oncorhynchus kisutch, fry with lanthanum and cerium 474 Hazin, Fabio H. V, Clara E. Boeckman, Elizabeth C. Leal, Rosangela R T. Lessa, Kohei Kihara, and Kazuyuki Otsuka Distribution and relative abundance of the blue shark, Prionace glauca. in the southwestern equatorial Atlantic Ocean Errata (i) Bigelow, Keith A. Age and growth of the oceanic squid Onychoteuthis borealijaponica in the North Pacific Fish. Bull. 92(l):13-25 Figure 5 should read as shown below. — 300 250 200 150 100 50 400 350 300 250 200 150 100 50 Males • • - 1 1 1 1 i i 1 1 1 50 100 150 200 250 300 350 400 450 Males 400 - 300 - °^ D# 200 - o o Jr 100 - - A n. ~+t-^ — , — , — , — , 50 100 150 200 250 300 350 400 450 Females • V? 1 1 I 1 1 1 1 1 1 X £ S: 1000 -, Females o H .-, » _ <*u 50 100 150 200 250 300 350 400 450 AGE (days) 50 100 150 200 250 300 350 400 450 AGE (days) Figure 5 Relation between age (determined by number of increments within statoliths) and mantle length (mm) and weight (g) for male and female Onyclwteuthis borealijaponica. Western North Pacific 1990 (open circles), central North Pacific 1990 (closed circles), central North Pacific 1991 (closed triangles = juveniles-subadults, open triangles = unknown sex), and eastern North Pacific 1990 (open squares). (2) Perryman, Wayne L., and Morgan S. Lynn Examination of stock and school structure of striped dolphin {Stenella coeruleoalba) in the eastern Pacific from aerial photogrammetry Fish. Bull. 92(1):122-131 Figures 3, 4, and 7, and Tables 1, 2, and 3 show an incorrectly typeset species name for striped dol- phin. The correct name should read striped dol- phin, Stenella coeruleoalba. The National Marine Fisheries Service (NMFS) does not approve, recommend, or endorse any proprietary product or proprietary material mentioned in this publication. No references shall be made to NMFS, or to this publication fur- nished by NMFS, in any advertising or sales promotion which would indicate or imply that NMFS approves, recommends or endorses any proprietary pro- duct or proprietary material mentioned herein, or which has as its purpose an intent to cause directly or indirectly the advertised product to be used or pur- chased because of this NMFS publication. Abstract. Eggs of walleye pollock, Theragra chalcogramma, from Shelikof Strait, Alaska, were reared at three temperatures: 3.8°, 5.7°, and 7.7°C. Development was divided into 21 stages. A piece- wise regression model with mid- points of each stage describes the relation between time to each stage of development and tempera- ture. Preserved eggs of each stage are described, illustrated, and pho- tographed. Midpoint of hatch was 393 hours at 3.8°C, 303 hours at 5.7°C, and 234 hours at 7.7°C. Mean length of larvae at hatch in- creased linearly with temperature. We compared rate of develop- ment, time to 50% hatch, and mor- phological development with other studies of walleye pollock eggs. Rate of development and time to 50% hatch were similar among populations of eastern North Pa- cific walleye pollock. Western North Pacific walleye pollock re- quired longer incubation times than eastern North Pacific walleye pollock. Morphological develop- ment of Shelikof Strait eggs differs from development of western North Pacific walleye pollock eggs: optic vesicles, myomeres, eye lenses, heart, and otic capsules appear earlier than in Shelikof Strait eggs, and eye pigment ap- pears later. The differences in de- velopment may be exacerbated by the condition of the eggs in which they were examined (e.g. pre- served vs. live). Developmental differences between stocks are dis- cussed with the conclusion that model components for egg mortal- ity and spawning biomass must be based on specimens collected in the area of interest. Embryonic development of walleye pollock, Theragra chalcogramma, from Shelikof Strait, Gulf of Alaska* Deborah M. Blood Ann C. Matarese Mary M. Yoklavich** Alaska Fisheries Science Center, National Marine Fisheries Service. NOAA 7600 Sand Point Way N.E., Seattle, WA 981 15-0070 Walleye pollock, Theragra chalco- gramma, is the most abundant member of the family Gadidae in the subarctic Pacific Ocean and Bering Sea, supporting the largest single-species commercial fishery in the world (Megrey, 1991). In the Gulf of Alaska, Shelikof Strait is the principal spawning area (Kendall and Picquelle, 1990) and has been the site of intensive re- search to understand processes leading to recruitment variability of walleye pollock (Schumacher and Kendall, 1991). Age determination of fertilized eggs is a basis for investigating biotic and abiotic impacts on the earliest life-history stage and thus for understanding interannual variability in walleye pollock re- cruitment. Age of walleye pollock eggs has been crucial to several studies. Egg mortality and spawn- ing biomass are estimated by mod- eling age-specific egg abundance over time (Picquelle and Megrey, 1993; Bates 1 ). Patterns in horizon- tal or vertical distribution and abundance of walleye pollock eggs in the western Gulf of Alaska have been described by grouping devel- opmental stages into broad age groups (Kendall and Kim, 1989; Kendall and Picquelle, 1990). Egg age is an independent vari- able in the models used to estimate egg production and mortality. Therefore, increasing the accuracy in measuring egg ages should im- prove estimates of these values. In past studies, walleye pollock eggs have been incubated in the labora- tory to develop temperature-spe- cific equations that estimate dura- tion of development or age of the eggs, to describe morphological de- velopment, to observe effects of light on egg buoyancy and hatching rate, and to obtain larvae for ex- periments (Table 1). Although these incubation studies provide pertinent data on ontogeny of wall- eye pollock, none can be used with accuracy to determine age of eggs Bates, R. D. 1987. Estimation of egg pro- duction, spawner biomass, and egg mor- tality for walleye pollock, Theragra chalcogramma, in Shelikof Strait from ichthyoplankton surveys during 1981. U.S. Dep. Commer., NOAA, Nat. Mar. Fish. Serv., Northwest Alaska Fish. Cent., 7600 Sand Point Way N.E., Bin C15700, Bldg. 4, Seattle, WA 98115-0070. Proc. Rep. 87-20, 192 p. Manuscript accepted 3 November 1993 Fishery Bulletin 92: 207-222 (1994) * Contribution 0148 of the Fisheries Oceanography Coordinated Investigations, NOAA, Seattle. ** Present address: Southwest Fisheries Science Center, Pacific Fisheries Environmen- tal group. National Marine Fisheries Service, NOAA, P.O. Box 831, Monterey, CA 93942 207 208 Fishery Bulletin 92(2). 1994 Table 1 Summary of Theragra chalcogramma egg incubation stuc ies. Reference Temperature (°C) Source and region Stages' Regression equation Morphological description Illustrations Photographs Gorbunova, 1954 3.4, 8.2 (means) western Pacific Ocean N ii n i • No Yes Yes No Yusa, 1954 6.0-7.0 Ishikan Bay, Japan 27 2 No Yes Yes Yes Hamai et al., 1971 2.4-2.5, 6.5-6.7, 9.9-10.1, ( means) Funka Bay, Japan 4 Stage-specific equation to predict age (d) at any stage No No No Hamai et al., 1974 5.0 (mean) Funka Bay, Japan 6 No No No No Matarese 1983 unpubl.'' 5.0 N. Gulf of Alaska 21 Stage-specific equation to predict age (h) at any stage'' Nn No No Haynes and Ignell, 1983 2.0, 5.0, 6.0, 8.0, 11.0 Stephens Passage, SE Alaska 7 General equation to predict age (h) at any stage No No No Nakatani and Maeda. 1984 -1.0, 0.0, 2.0, 4.0, 7.0, 10.0, 13.0 Funka Bay, Japan 5 To 507r hatch No No No Paul, 1984, unpubl. 1 5.0 N. Gulf of Alaska 21 No No No No Bailey and Stehr, 1986 5.6, 8.5 Puget Sound, Washington None No No No No Olla and Davis, 1993 6.0 Shelikof Strait, Alaska Nunc Nil No No No ; Prior to hatch. 2 Reported as intervals of time. 3 A. C. Matarese, Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Poin 4 In Bates 1987 (Footnote 1). 1 A J. Paul, University of Alaska Fairbanks, Institute of Marine Science, Seward Marine Center Lab, t Way P.O. B NE., Seattle ox 730, Sewa WA 98115. rd, AK 99664. from Shelikof Strait. Eggs need to be obtained from the study area and incubated at a range of tempera- tures occurring in the area. Categorizing the con- tinuous process of egg development into a large number of stages should increase the precision of the egg-age estimate The first objective of our study was to incubate Shelikof Strait walleye pollock eggs at the mean water temperature for Shelikof Strait, bracketing that temperature to include upper and lower ex- tremes. Egg development times were used to pro- duce a stage duration table and a regression model to estimate egg age based on water temperature and developmental stage. Morphological development is described for 21 developmental stages. These de- scriptions are accompanied by illustrations and photographs to facilitate identification of body struc- tures and stage hallmarks. The second objective was to compare our rates of egg development to other walleye pollock incubation studies. Morphological development is included in this comparison. Blood et al.. Embryonic development of Theragra chalcogramma 209 Methods Incubation Adult walleye pollock were collected with a rope trawl off Cape Kekurnoi (57°42.5'N, 155°16.2'W) in Shelikof Strait, Alaska, on 7 April 1989 from the NOAA research vessel Miller Freeman. Eggs from one female and milt from three or four males were hand stripped into glass petri dishes, gently mixed, and left undisturbed for one minute. Eggs were then rinsed, transferred to 3°C (surface water tempera- ture) seawater in glass jars (3.8 L), and held two hours. Floating eggs with a perivitelline space were assumed to be fertilized (Blaxter, 1969; Alderdice, 1988). Viable eggs were poured into eighteen 0.5-L jars filled with 3°C seawater. Eggs were not counted but apportioned similarly among the jars at a con- centration of about one egg/mL. Six capped jars were held in each of three water bath incubators onboard the Miller Freeman. Initial incubation temperatures were set to include the range of temperatures in the area. Mean water temperature at depths of 150-200 m in Shelikof Strait, where most eggs are found (March-May) (Kendall and Kim, 1989), is 5°C; ex- tremes of 3.6° and 5.9°C have been reported (Reed and Schumacher, 1989). Incubators were sealed to minimize light and movement and placed in sepa- rate refrigerators adjusted to 3°, 5°, and 7 C. One- half of the water in the jars was replaced every day with seawater of the same temperature. Eggs were preserved in phosphate buffered formalin (5%) 2 or Stockard's solution 3 (Velsen, 1980). Stockard's solu- tion cleared the chorion and darkened embryonic tissue, easing examination of embryonic develop- ment. Phosphate buffered formalin did not darken embryonic tissue as much as Stockard's solution, yielding better definition of some structures like somites and otic capsules. Live, newly hatched lar- vae were measured (standard length in millimeters) and preserved (5% buffered formalin). Detailed exami- nation and morphological description of embryos were completed after eggs were returned to the laboratory. During the first 24 hours after fertilization, eggs were sampled at 2-3 hour intervals. After 24 hours, intervals were increased to about 6 hours. When an interval was less or greater than 6 hours, the sub- sequent sampling time was adjusted to return to the original 6-hour schedule. Data were not recorded for three sampling times late in development because intervals were inadvertently extended to 12 hours (236, 258, and 282 hours). At each interval, 10 to 50 eggs were sampled from one jar per incubator; only one jar was sampled to ensure there would be enough eggs and larvae left to sample near the end of the incubation period. Jars were sampled in rotation throughout the duration of the experiment until no eggs remained. When eggs began to hatch, all jars were checked and newly hatched larvae were removed in addition to eggs scheduled to be sampled. Dead eggs were removed from the designated sample jar at each interval. Water bath temperatures were recorded for every sampling interval. Frequent opening of refrigerators during the initial short sampling intervals increased temperatures in the refrigerators despite thermostat adjustments. Water bath temperatures stabilized after 48 hours to 3.8°, 5.7°, and 7.7°C. Morphological descriptions Eggs were examined with the aid of a dissecting microscope (6-50x magnification) and described ac- cording to a 21-stage scheme adapted from Naplin and Obenchain (1980) (Table 2). Morphological terms follow Trinkaus ( 1951) with one exception: the term "blastodisc," in this paper, includes the germi- nal area from the time of cytoplasm polarization until embryonic shield formation (Markle and Table 2 Stages of embryonic development of Theragra chalcogramma (adapted from Naplin and Obenchain, 1980). Stage Developmental stage 2 50 mL 37% formaldehyde, 4.0 g sodium phosphate monobasic, 6.5 g sodium phosphate dibasic, made up to 1 L with distilled water. 3 50 mL 371 formaldehyde, 40 mL glacial acetic acid, 60 mL glycerin, and 850 mL distilled water. 1 Precell 2 2 cell 3 4 cell t 8 cell 5 16 cell 6 32+ cell 7 Blastodermal cap 8 Early germ ring 9 Germ ring 1/4 down yolk 10 Germ ring 1/2 down yolk 11 Germ ring 3/4 down yolk 12 Late germ ring L3 Early middle (blastopore closure) 14 Middle middle (appearance of pigment 15 Late middle (tail bud thickens) \h Early late (tail bud lifts from yolk) 17 Tail 5/8 around yolk 18 Tail 3/4 around yolk 19 Tail 7/8 around yolk 20 Full circle around yolk 21 Tail 1-1/8 around yolk 210 Fishery Bulletin 92(2). 1994 Waiwood, 1985, in part). Eggs preserved in Stockard's solution were photographed with a Nikon F2 camera fitted with a PB6 200-mm bellows exten- sion and a 24-mm 1:2.8 reverse-mounted lens. This configuration produced a 47x magnification. Re- flected light was supplied by two synchronized flash units. Other photographs (stages 5 and 6) were taken with a single-lens reflex adapter (0.32x) on a Wild M-8 dissecting microscope with transmitted light. At 50x, the phototube and adapter increased magnification to 66x. Analysis Endpoint, midpoint, and duration of stage (in hours) were estimated for eggs incubated at each tempera- ture. For stages 1-20, stage endpoint was deter- mined by the presence of two stages during a sam- pling time; if stages n and n + \ were present, the time at which the eggs were sampled was consid- ered a transition and therefore the endpoint for stage n. If there was no transition, the endpoint for stage n was the midpoint between the last sampling time during which stage-rc eggs were present and the first time stage-« + l eggs were observed. Dura- tion and midpoint of stage n were determined as Duration Stage n = Endpoint Stage (n) - Endpoint Stage in - 1); Midpoint Stage n = Endpoint Stage (n Duration Stage n li Endpoint of stage 21 was the sampling interval when the last embryo had hatched. With the mid- points and time of 50% hatch, a piece-wise least- squares linear regression model (SAS, 1985) was derived to estimate age (hours) of eggs at a specific stage incubated at any temperature within the lim- its of this experiment. Differences in mean lengths of larvae hatched from the three temperature groups were analyzed by a Student-Newman-Keuls test. Lengths of larvae hatching at stages 20 and 21 were analyzed by a two-way analysis of variance (ANOVA) by using stage and temperature. We chose five representative developmental stages and compared time to midpoint of each stage among incubation studies. Comparison with Hamai et al. (1971, 1974) was possible for only three stages. We grouped data on time to 509f hatch into western and eastern North Pacific studies and performed a log- transformed analysis of covariance to test for differ- ences in time to 50% hatch between these two ar- eas with incubation temperature as the covariate. Results Incubation rates Temperatures of the three water baths increased at the beginning of sampling (Fig. 1). Temperature spikes that occurred after 288 hours in the 5.7°C jars and after 396 hours in the 3.8°C jars, were associ- ated with the appearance of large numbers of lar- vae; water baths may have warmed when refrigera- tors were opened frequently to measure larvae. Eggs developed at similar rates among incubation temperatures for the first 36 hours through stage 6 (Fig. 2). After stage 6, at about 36 hours, when tem- peratures had stabilized, development rates began to diverge. Duration of stages 7-21 was variable (Table 3). Usually the duration of a stage was longer at cooler temperatures. However, this was not al- ways the case, and stages 12 and 20 required simi- lar amounts of time regardless of temperature. At all temperatures, hatching began during stage 20; the percentage of eggs hatched by the beginning of stage 21 was 35% at 3.8°, 40% at 5.7°, and 8.1% at 1.1'C Four larvae from the 7.7°C group hatched after 192 hours; another 18 hours elapsed before other larvae hatched at this temperature. These early larvae were not included in this analysis be- cause we assumed that the hiatus in hatching times indicated that early hatching was anomalous, i.e. hatching may have been mechanically induced. Af- ter hatching began, time required for 50% hatch decreased as temperature increased: 48, 36, and 24 hours at 3.8°, 5.7 , and 7.7°C. The elapsed time be- tween hatching of the first and last larvae was 72 hours at 3.8°C and 60 hours at both 5.7 and 7.7°C. Eggs developed normally at 5.7° and 7.7°C; how- ever, curvature of the spine was observed in some late-stage embryos incubated at 3.8C These abnor- mal eggs hatched, but most larvae were not mea- sured because of curvature. Mean length at hatch of all larvae increased with incubation temperature: 4.15 (SD 0.380, n = 100), 4.29 (SD 0.272, rc=192), and 4.55 mm (SD 0.303, rc=84> at 3.8°, 5.7°, and 7.7'C (Fig. 3). Mean lengths of larvae from the three tem- perature groups were significantly different (P<0.05). In addition, larval lengths increased as the hatching period progressed at all temperatures. Length of larvae hatching at stages 20 and 21 was significantly different at all temperatures (P<0.01); larvae that hatched at stage 21 were 9-13% longer than larvae that hatched at stage 20. Blood et al.: Embryonic development of Theragra chalcogramma 21 1 O / J& * **.^*^w**** **•**.*** * ********* *.. ; 3.8 C -+- 5.7 C * 7.7 C I 1 1 1 1 I : 1 1 1 1 1 1 1 1 1 1 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 Incubation time (h) Figure 1 Temperatures recorded in water baths during incubation of Theragra chalcogramma eggs. o> CO *-■ CO c CD E Q. o CD > CD Q 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 Incubation time (h) Figure 2 Development of Theragra chalcogramma eggs incubated at 3.8°, 5.7°, and 7.7°C. Points represent occurrence of stages at scheduled sampling times. The piece-wise regression model (SAS, 1985) has two separate components and is discontinuous be- tween stages 6 and 7 (Fig. 4). This type of model was necessary because of the rapid divergence of devel- opmental rates at all temperatures after stage 6; it was not possible to fit one equation to the entire incubation time. The two components are described by the following equations: component 1: stages 1-6 Age = 3.27 - 0.13 (stage) (temperature) + 0.47 (stage 2 ); component 2: stages 7-21 Age = 17.82 + 7.05(stage) - 0.656 (stage) (temperature) + 0.043(stage 3 ) - 0.0032 (temperature) (stage 3 ), 212 Fishery Bulletin 92(2). 1994 Table 3 Endpoint, midpoint, and duration in hours (h) of stage of deve opment of Theragra chalcogramma eggs incu- bated at 3.8°, 5.7°, and 7.7°C 3.8'C 5.7"C 7.7°C Stage Endpoint (h) Midpoint (h) Duration (h) Endpoint (h) Midpoint (h) Duration (h) Endpoint (h) Midpoint (h) Duration (h) 1 4.00 2.00 4.00 4.00 2.00 4.00 3.50 1.75 3.50 2 6.00 5.00 2.00 6.00 5.00 2.00 4.00 3.75 0.50 3 8.00 7.00 2.00 7.00 6.50 1.00 5.00 4.50 1.00 4 10.25 9.12 2.25 9.00 8.00 2.00 7.00 6.00 2.00 5 12.50 11.37 2.25 10.25 9.62 1.25 10.25 8.62 3.25 6 22.50 17.50 10.00 22.50 16.37 12.25 19.50 14.87 9.25 7 64.00 43.25 41.50 51.00 36.75 28.50 40.00 29.75 20.50 8 78.00 71.00 14.00 68.00 59.50 17.00 48.00 44.00 8.00 9 90.00 84.00 12.00 75.00 71.50 7.00 54.00 51.00 6.00 10 105.00 97.50 15.00 87.00 81.00 12.00 57.00 55.50 3.00 11 120.00 112.50 15.00 93.00 90.00 6.00 68.00 62.50 11.00 12 138.00 129.00 18.00 108.00 100.50 15.00 84.00 76.00 16.00 13 153.00 145.50 15.00 114.00 111.00 6.00 87.00 85.50 3.00 14 180.00 166.50 27.00 135.00 124.50 21.00 102.00 94.50 15.00 15 195.00 187.50 15.00 164.00 149.50 29.00 111.00 106.50 9.00 16 219.00 207.00 24.00 174.00 169.00 10.00 117.00 114.00 6.00 17 252.00 235.50 33.00 189.00 181.50 15.00 132.00 124.50 15.00 18 312.00 282.00 60.00 219.00 204.00 30.00 144.00 138.00 12.00 19 336.00 324.00 24.00 258.00 238.50 39.00 180.00 162.00 36.00 20 378.00 357.00 42.00 300.00 279.00 42.00 219.00 199.50 39.00 21 414.00 393.00' 36.00 330.00 303.00' 30.00 270.00 234.00' 51.00 ' 50% hatch. where age of the egg is expressed in hours. The value of R 2 is 0.96 for component 1 and 0.99 for component 2. We compared our rates of egg development to other walleye pollock incubation studies in the 5- 7°C range (Table 4). There was a significant differ- ence between regression equations of incubation time to 50% hatch and temperature for western versus eastern North Pacific studies (P<0.01), but the slopes were not different (P=0.18). Based on the 95% confidence interval about the parameter esti- mates, time to 50% hatch of western North Pacific walleye pollock tended to be 1.2 to 1.3 times longer on average than that of the eastern North Pacific fish at a specific temperature. Morphological descriptions Walleye pollock eggs are pelagic and have a smooth, clear chorion and homogeneous yolk. No oil globules are present. Preserved eggs range from 1.2 to 1.8 mm in diameter, although most are 1.35-1.45 mm (Matarese et al., 1989). Appearance of the egg var- ies with type of preservative. There was little or no shrinkage of yolk material in Stockard's solution, whereas yolk of formalin-preserved eggs decreased in volume and the yolk membrane frequently col- lapsed. This effect of formalin preservation was helpful in determining how much of the tail had lifted away from the yolk in late-stage embryos. Development of walleye pollock eggs and embryos, from fertilization to just before hatching, was di- vided into the following 21 stages (Table 2): Precell (stage 1) Cytoplasm at the animal pole forms a blastodisc; bands of cytoplasm extend from below the equator to the blastodisc (Fig. 5A), which is without distinct margins (Fig. 6A). When intact, the yolk membrane almost touches the inner wall of the chorion. The perivitelline space is most vis- ible over the blastodisc. 2 cells (stage 2) The first cell division of the blastodisc is in the horizontal plane. Cell material may not be equally divided (Figs. 5B and 6B). 4 cells (stage 3) The second cleavage is perpen- dicular to, and in the same plane as, the first. Cells are roughly equal in size and form a square (Figs. 5C and 6C). 8 cells (stage 4) The third cleavage is perpen- dicular to the second cleavage (parallel to first cleav- age). Each cell divides in half in the horizontal Blood et al.: Embryonic development of Theragra chalcogramma 213 E E O) c 9 C CO 9 s 5.5 5.0 4.5 4.0 3.5 "T O 7./C D 6.7°C A 3.8° C 3.0 204 248 292 336 380 424 Total incubation time (h) Figure 3 Mean hatch lengths at each sampling interval during the hatching period of Theragra chalcogramma larvae incubated at 3.8°, 5.7°, and 7.7°C. Stage of de- velopment at hatch is also shown. Shaded circle indicates overall mean length for each temperature. Vertical bars are standard deviations; numbers indicate sample size. plane. Cells form a rectangle with the four cells in the center smaller than those at the corners of the rectangle (Figs. 5D and 6D). 16 cells (stage 5) The fourth cleavage is perpen- dicular to the third; this is the last stage in which cell division is restricted to the horizontal plane. Most eggs have a square or rectangular block of cells with four cells on each side; all cells are in contact with yolk through this stage (Figs. 5E and 6E). 32 cells (stage 6) Initially, the single layer of cells has a flat, irregular square or rectangular shape. Cell division continues in horizontal and vertical planes, transforming the blastodisc into a hollow cap of cells on the yolk resembling a rasp- berry (Figs. 5F and 6F). Cells increase in number but the size of the blastodisc remains constant. The perivitelline space widens between yolk and chorion. Blastodermal cap (stage 7) The blastodisc progresses through two steps: at first, cell size de- creases from continued cleavage; cell material ap- pears granular and the blastodisc resembles a flat- tened dome on the yolk surface. Then, the base of the cell mass sinks below the yolk surface; the periblast extends beyond the equator of the blasto- disc, giving the appearance of a "flying saucer" in lateral view (Figs. 5G and 6G). Early germ ring (stage 8) The center of the blasto- disc flattens and the periph- ery (germ ring) thickens in preparation to overgrow the yolk (epiboly). The blasto- coel, visible on one side of the blastodisc, appears grainy and pale (Fig. 5H). The margin between blasto- coel and blastodisc is indis- tinct (Fig. 6H). Germ ring 1/4 around yolk (stage 9) The blasto- disc, now the embryonic shield, expands as the germ ring begins to overgrow the yolk. The margin of the fu- ture anterior end of the em- bryo is slightly curved and sharply defined. Cell mate- rial covering the blastocoel appears less grainy than in the previous stage. After preservation, this thin cellu- lar layer appears concave in lateral view. The germ ring margin is thin and flattened, extending 1/4 around yolk (Figs. 51 and 61). Germ ring 1/2 around yolk (stage 10) The germ ring envelopes half the yolk and the anterior margin of the embryonic shield is sharply curved and thick (Figs. 5J and 6J). The beginning of neu- ral development is visible; a neural keel extends from the anterior margin of the embryonic shield to 2/3 its length (Fig. 5K). Germ ring 3/4 around yolk (stage 11) Head and upper body region begin to differentiate but no distinct brain lobes are apparent. Optic vesicles develop. Prospective head and body mesoderm out- lines the hour-glass shape of the developing embryo (Fig. 7A). The notochord is visible ventrally. The germ ring has progressed 3/4 down the yolk (Fig. 6K). Late germ ring (stage 12) Myomere differen- tiation begins; separate myomeres are not visible. The midbody expands dorsoventrally; prospective head and body mesoderm forms a narrow outline of the embryo. The blastopore is open and the germ ring envelopes more than 7/8 of the yolk (Figs. 7B and 8A). Early middle stage (stage 13) The blastopore is closed. The notochord and 7-12 incomplete myomeres are visible. Tail margin is indistinct and 214 Fishery Bulletin 92(2). 1994 a> c o a. ■o E o 0) E i- flat; the medial portion of the tail bud is thicker (Figs. 7C and 8B). The body of the embryo appears flattened. Although not distinguish- able in preserved specimens, Kupffer's vesicle is visible in the live egg. Middle middle stage (stage 14) Embryos have 14-16 myomeres. Differen- tiation begins in eyes and mid- and hindbrain. Fore- brain very small and under- developed. The tail bud mar- gin is defined but still flat- tened (Fig. 8C). The entire length of the body is thicker. Small melanophores are scattered along the dorsum between the hindbrain and 4/5 of body length (Fig. 7D). Late middle stage (stage 15) About 20-25 myomeres are visible. Eye lenses are formed. The liver appears as a slight bulge in the body wall, and the gut area is de- lineated. The tail bud is thick and appears lifted from the yolk surface with the margin attached (Fig. 7E). Pigment is darker than in the previous stage and dendritic, extending from midbrain to tip of tail bud and confined mostly to the dorsum. Nares, mid- and hindbrain, and pectoral bud anlagen are visible dorsally (Fig. 8D). 450 400 350 300 250 200 150 100 50 n — i — i — i — i — i — i — i — i — i — i r n — i — i — i — i — i — i — r -B-- 3.a°c -e- 9.7 C ▲ 7.7 °C I I I I I I I I L J I L 1 2 3 S 7 8 10 11 12 13 14 18 Ifl 17 18 10 20 BOM Developmental stage Figure 4 Time (h) to midpoint of stage of Theragra chalcogramma eggs incubated at 3.8°, 5.7°, and 7.7°C. Fitted lines are results of regression model; symbols are observed values. Early late stage (stage 16) Heart tissue begins to expand when the embryo has about 24-36 myo- meres. Forebrain differentiates from midbrain. The tail bud lifts from the yolk surface (Figs. 7F and 8E) and pigment forms two parallel rows dorsoposteriorly. Tail 5/8 around yolk (stage 17) The embryo has 27-36 myomeres. More of the tail lifts from the Table 4 Comparison of time (h) to estimated midpoint of five developmental milestones of Theragra chalcogramma embryos incubated at 5-7°C. Eastern North Pacific incubation stuc ies Western North Pacific incubation studies Matarese, Paul, 1984, Haynes and This Hamai et al., Yusa, Nakatani and Hamai et al., 1983, unpubl. unpubl. - Ignell, 1983' study 1971* 1954 Maeda, 1984 5 1974 Stage 15.0-C) 15.0 C) 16.0 C) (5.7'C) (6.5-6.7'C) (6.0-7.0 Cl (7.0"C) (5.0 Cl Blastodermal cap 37.5 39.5 35 36.8 28.5 31 Blastopore closure 114 UK 105 108 100 102 Its 139 Tail 3/4 211.7 217 li,l 204 234 192 Tail full circle 274.5 264 250 279 250 270 216 330 Vi<, hatch 349 320 285 303 345 288+ 298 411 A C Matarese. Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way N.E., Seattle, WA 98115. 2 A .1 Paul, University of Alaska Fairbanks. Institue of Marine Science, Seward Marine Center Lab, P.O. Box 730, Seward, AK 99664. Values except for hatch estimated from Table 3 in Haynes and Ignell (1983). 50% hatch from Table 7. 1 Values except fur hatch estimated from Fig. 3 in Hamai et al. (1971). ' Values except for hatch estimated from Fig 5 in Nakatani and Maeda (1984). Blood et al.: Embryonic development of Theragra chalcogramma 215 cytoplasm - blastodtsc perivitelline space E periblast germ ring blastocoel neural keel Figure 5 Illustrations of preserved Theragra chalcogramma eggs. (Al Stage 1 (precell); (B) Stage 2 (2 cell); (C) Stage 3 (4 cell); (D) Stage 4 (8 cell); (E) Stage 5 (16 cell); (F) Stage 6 (32 cell); (G) Stage 7 (blastodermal cap); (H) Stage 8 (early germ ring); (I) Stage 9 (germ ring 1/4, lateral view); (J) Stage 10 (germ ring 1/2, lateral view); (K) Stage 10 (dorsal view). 216 Fishery Bulletin 92(2), 1994 E II Figure 6 Photographs of preserved Theragra chalcogramma eggs. (A) Stage 1 (precell); (Bi Stage 2 (2 cell); (C) Stage 3 (4 cell); (D) Stage 4 (8 cell); (E) Stage 5 (16 cell); (F) Stage 6 (32 cell); (G) Stage 7 (blastodermal cap); (H) Stage 8 (early germ ring); (I) Stage 9 (germ ring 1/4); (J) Stage 10 (germ ring 1/2); (K) Stage 11 (germ ring 3/4). Blood et al.: Embryonic development of Theragra chalcogramma 217 optrc vesicle prospective head and body mesoderm optic vesicle myomeres blastopore lens E otic capsule H hatching gland: hatching glands J K L Figure 7 Illustrations of preserved Theragra chalcogramma eggs. (A) Stage 11 (germ ring 3/4); (B) Stage 12 (blastopore almost closed); (C) Stage 13 (early middle); (D) Stage 14 (middle middle); (E) Stage 15 (late middle); (F) Stage 16 (early late); (G) Stage 17 (tail 5/8 circle); (H) Stage 18 (tail 3/4 circle); (I) Stage 19 (tail 7/8 circle); (J) Stage 20 (tail full circle, lateral view); (K) Stage 20 (dorsal view); (L) Stage 21 (tail 1-1/8 circle). 218 Fishery Bulletin 92(2). 1994 Figure 8 Photographs of preserved Thcragra chaleogramma eggs. (A) Stage 12 (blastopore almost closed); (B) Stage 13 (early middle); (C) Stage 14 (middle middle); (D) Stage 15 (late middle); (E) Stage 16 (early late); (F) Stage 17 (tail 5/8 circle); (G) Stage 18 (tail 3/4 circle); (H) Stage 19 (tail 7/8 circle); (I) Stage 20 (tail full circle); (J) Stage 21 (tail 1-1/8 circle). Blood et al.: Embryonic development of Theragra chalcogramma 219 3.5 mm SL Figure 9 Illustration of preserved Theragra chalcogramma yolk-sac larva (Matarese et al., 1989). yolk surface (Fig. 8F). The dorsal finfold is formed on the posterior 1/3 of the body and pigment on the head extends at least to the posterior margin of the eye (Fig. 7G). The liver is prominent and the heart is beating in the live egg. Tail 3/4 around yolk (stage 18) The embryo has 36-41 myomeres. The tip of the tail is tapered and curves away from the longitudinal axis of the embryo (Fig. 7H). The dorsal finfold extends to midbody and pectoral fin buds are prominent. Otic capsules are formed. Large stellate melanophores are scattered over the dorsum, extending just to the midlateral surface; posterior to the anus, two rows of melanophores are seen dorsally and a few are found along the ventral midline (Fig. 8G). The tip of the tail is unpigmented. Tail 7/8 around yolk (stage 19) When the embryo has 44-48 myomeres, the dorsal finfold ex- tends anteriorly 2/3 body length, inserting just pos- terior to the pectoral fin buds and centered over the liver (Figs. 71 and 8H). Pigment on the head extends to the middle of the eye. At midbody, pigment is scat- tered on either side of the dorsal midline, extend- ing to just above the lateral midline. Postanal pigment migrates toward the dorsal and ventral midlines. Tail full circle around yolk (stage 20) The embryo has 48—49 myomeres and the pancreas is visible adjacent to the liver (Fig. 7J). The embryo now encircles the yolk and the tail tip may reach from near the snout to as far back as the posterior margin of the eye (Fig. 81). Hatching glands, simi- lar to those of other teleosts (Yamagami, 1988), are discernible on the surface of the snout and may extend over the dorsal surface of the eye (Figs. 7 J and 7K). The posterior portion of the eye is pig- mented. Postanal pigment migrates and begins to form the postanal bars found in yolk-sac larvae (Matarese et al., 1989) (Fig. 9). Tail 1 1/8 times around yolk (stage 21) The embryo has 49-50 myomeres and the tail tip elon- gates, extending beyond the posterior margin of the eye (Fig. 8J). The urinary bladder is visible posterior to the anus (1/3 body length; not shown on figure) and the dorsal finfold extends to mid- brain. Head pigment extends to the anterior margin of the eye (Fig. 7L). The dorsal half of the eye is pigmented. Most body pigment coalesces to three ar- eas: dorsally, on gut; a bar at 1/2 body length; and a bar at 3/4 body length. In the postanal bars, most pigment is along dorsal and ventral midlines; some pig- ment extends onto the lateral body. Pig- ment is scattered on the preanal body. Discussion Time from first hatch to 50% hatch was inversely related to temperature. Hatch times reflected the effects of temperature described by Yamagami (1988), who demonstrated that the hatching enzyme secreted by the embryo solubilizes the chorion more rapidly at higher temperatures. The first larvae to hatch were stage 20. Early hatching may have been an artifact of rearing conditions. However, hatching glands were present at this stage, which, with the appearance of eye pigment, may correspond to a level of development that would enable these larvae to survive. Early hatching may occur naturally with some frequency. Within batches of walleye pollock larvae from Puget Sound that had been incubated in the laboratory, larvae hatching early grew to an equivalent size as larvae hatching later (larvae hatched on day 1 were the same length at day 3 as larvae hatched on day 3). Those early hatched lar- vae also began to feed at the same time as larvae hatched later. 4 Rate of development and time to 50% hatch were similar among studies of walleye pollock from the eastern North Pacific, specifically the Gulf of Alaska (Matarese, unpubl. data; Haynes and Ignell, 1983; and this study; Paul 5 ). From data on time (days) to 50% hatch for all temperatures reported in all in- cubation studies (Fig. 10), incubation times of west- ern North Pacific walleye pollock are longer than eastern North Pacific walleye pollock. This finding appears to conflict with Haynes and Ignell's (1983) comparison with Yusa's (1954) study in which they report similar rates of development 4 Olla, B. Mark O. Hatfield Marine Science Center, Oregon State University, 2030 Marine Science Drive, Newport, OR 97365- 5297. Pers. commun. 18 August 1992. s Paul, A. J. University of Alaska Fairbanks, Institute of Marine Science, Seward Marine Center Lab, P.O. Box 730, Seward, AK 99664. Unpubl. data. 220 Fishery Bulletin 92(2). 1994 for eastern and western stocks. However, their com- parison was made with midpoints of stages calcu- lated from a regression model instead of observed midpoints. Also, Yusa (1954) reported a temperature range of 6— 7"C instead of a mean; our interpreta- tion of Haynes and Ignell's (1983) classification and calculation of Yusa's (1954) data suggests incubation temperatures were always above 6.5°C (see their Table 6 and our Table 4). Finally, Haynes and Ignell (1983) monitored midpoints of stages more closely than midpoint of hatch and did not specifically re- fer to 50% hatch. 6 We assumed the values reported as observed midpoints of hatch (their Table 7) were close to 50% hatch. Yusa's (1954) study could not be compared with ours with regard to time to 50% hatch. Time of hatch is often a result of how eggs are treated during incubation and may vary with differ- ent batches. 7 However, walleye pollock eggs from Japanese waters are larger than those from the Gulf of Alaska (mean=1.4— 1.6 mm and 1.3—1.4 mm, re- spectively; Bailey and Stehr, 1986). At similar tem- peratures, larger eggs take longer to develop (Pepin, 1991). The difference in incubation time emphasizes the need to collect data from fish specific to the area of interest. This will reduce the sources of variation in develop- ment time for laboratory- reared eggs; failure to identify and improve these sources would compromise the useful- ness of models predicting egg age based on water tem- perature. Development is a continuous process. The sampling intervals and arbitrary designation of stage endpoints break develop- ment into subjective units. Us- ing the 21-stage scheme, we did not see a clear decrease in each stage duration with an increase in temperature. However, this will not affect the usefulness of our results. When stages are grouped to encompass a greater degree of morphological devel- opment, as in Haynes and Ignell (1983) and Picquelle and Megrey (1993), de- velopment time is inversely related to temperature. A greater number of stages within a group increases the accuracy of prediction of egg age. A large num- ber of stages also allows others greater flexibility in grouping those stages. Our regression model predicts temperature-spe- cific development time for purposes of computing rates of egg production and egg mortality. There is no biological basis upon which the regression is predicated because stages that are assigned to the eggs are arbitrary; stages are ordinal data that are based on morphological criteria without consider- ation for development time. An alternative method to estimate development time from temperature is to fit a separate regression for each stage. The dis- advantage of this alternative method is that many parameters are fitted with few data points. Two studies describing morphological develop- ment, Gorbunova (1954) and Yusa (1954), have been published. Gorbunova ( 1954) was not comparable to our study. We compared our descriptions of morpho- logical development with Yusa (1954). We assigned stages to descriptions of hourly morphological devel- 6 Haynes, E., National Marine Fisher- ies Service, Auke Bay Laboratory, 11305 Glacier Highway, Juneau, AK 99801-8626. Pers. commun. April 1991. 7 Paul, A. J., University of Alaska Fairbanks, Institute of Marine Sci- ence, Box 730, Seward, AK 99664. Pers. commun. 17 March 1992. O -*- CO o CO Q 30 25 20 15 10 -&- Matarese 1 983 (see caption) —8- Paul 1 984 (see caption) — This study -B- Haynes and Ignell 1983 — Hamaietal. 1974 -A- Hamai et al. 1971 O Nakatani and Maeda 1 984 Western North Pacific ncubation Studies Eastern North Pacific Incubation Studies 4 6 8 Incubation temperature (°C) 10 12 Figure 10 Days to 50% hatch for Theragra chalcogramma eggs at various temperatures of incubation. (A. C. Matarese, unpubl. data, Alaska Fisheries Science Cen- ter, National Marine Fisheries Service, 7600 Sand Point Way N.E., Seattle, WA 98115. A. J. Paul, unpubl. data, University of Alaska Fairbanks, Insti- tute of Marine Science, Seward Marine Center Lab, P.O. Box 730, Seward, AK 99664.) Blood et al.: Embryonic development of Theragra chalcogramma 221 opment of walleye pollock embryos incubated at 6.0— 7.0°C (Yusa, 1954) for comparison with morphologi- cal characteristics of eggs reared at 5.7°C in this study We used hallmarks of each stage (e.g. num- ber of cells, germ ring advancement, number of myomeres, tail growth around yolk) to distribute Yusa's (1954) descriptions into 21 stages. Yusa's (1954) descriptions were similar to ours up to stage 11. Beginning with stage 11, Yusa (1954) described the development of some structures occurring one or more stages earlier than this study: myomeres and nares were sighted one stage earlier; brain dif- ferentiation and eye lenses, two stages earlier; the heart, three stages earlier; and the otic capsules, five stages earlier (Table 5). Otoliths sighted by Yusa ( 1954) were not visible in our specimens. Conversely, eye pigment was observed in our study one stage earlier than that observed by Yusa (1954). Other structures appeared at the same stage in each study: optic vesicles, Kupffer's vesicle, liver, gut, and pec- toral-fin anlagen. Also, after stage 13, similar num- bers of myomeres were visible at like stages in both studies as was the beating of the heart. Differences between the two studies may be the result of egg condition when examined: Yusa (1954) described live eggs, whereas most of our descriptions were of preserved eggs. Formalin preservation may obscure myomeres or destroy structures such as embryonic otoliths (McMahon and Tash, 1979). Stockard's solution darkens embryonic tissue and obscures fine details. Also, morphological develop- ment may differ between western and eastern North Pacific walleye pollock, further emphasizing the need to restrict data collection to specific areas of interest to increase accuracy of interpretation. Acknowledgments We thank the following people whose combined ef- forts helped us accomplish our research and produce this paper: William Rugen assisted with shipboard experiments; Kevin Bailey, Gail Theilacker, and Steve Porter helped us interpret the morphology of late-stage eggs and yolk-sac larvae; Trish Brown provided statistical analyses; Morgan Busby photo- graphed the eggs; and Beverly Vinter illustrated eggs and helped interpret many morphological struc- tures. We thank Art Kendall, A. J. Paul, Kevin Bailey, Susan Picquelle, and Bori 011a for prelimi- nary reviews of the manuscript. Gail Theilacker and Richard Brodeur helped refine later versions. We also thank the members of FOCI who assisted with field collections. Literature cited Alderdice, D. F. 1988. Osmotic and ionic regulation in teleost eggs and larvae. In W. S. Hoar and D. J. Randall (eds.), Fish physiology, Vol. XI, Part A, p. 163-251. Academic Press, Inc., San Diego. Table 5 Descri (1954) ptions of morphological development of Theragra c and this study. halcogramma embryos at comparable stages by Yusa Stage Yusa (6.0-7.CTC) This study (5.7"C) 11 medullary plate and optic vesicles visible optic vesicles visible 12 5-7 myomeres; 3 sections of brain visible myomeres begin to differentiate 13 9-13 myomeres; heart, otic capsules, otoliths, eye lenses, and Kupffer's vesicle visible 7-12 myomeres; Kupffer's vesicle visible 14 16-17 myomeres; nares and pigment along dorsum visible 14-16 myomeres; pigment along dorsum visible; mid- and hindbrain differentiation 15 18-30 myomeres; liver, gut, and pectoral anlagen visible; 3 sections of brain formed 20-25 myomeres; eye lens, nares, pectoral anlagen, liver, and gut visible 16 35 myomeres 24-36 myomeres; heart visible; 3 sections of brain formed 17 37 myomeres; heart beating 27-36 myomeres; heart beating 18 40 myomeres 36-41 myomeres; otic capsules visible 19 44-48 myomeres 20 48-49 myomeres; eye pigment appears 21 eye pigment appears 222 Fishery Bulletin 92(2). 1994 Bailey, K. M., and C. L. Stehr. 1986. Laboratory studies on the early life history of the walleye pollock, Theragra chalcogramma (Pallas). J. Exp. Mar. Biol. Ecol. 99:233-246. Blaxter, J. H. S. 1969. Development: eggs and larvae. In W. S. Hoar and D. J. Randall (eds.), Fish physiology, Vol. Ill, p. 177-252. Academic Press, New York. Gorbunova, N. N. 1954. Reproduction and development of the wall- eye pollock, Theragra chalcogramma (Pallas). Tr. Inst. Okeanol. Akad. Nauk. SSSR 11:132-195. (In Russian, transl. by S. Pearson, 1972, Natl. Mar. Mammal Lab., NMFS, 7600 Sand Point Way N.E., Seattle, WA 98115-0070.) Hamai, I., K. Kyushin, and T. Kinoshita. 1971. Effect of temperature on the body form and mortality in the development and early larval stages of the Alaska pollock, Theragra chalco- gramma (Pallas). Bull. Fac. Fish. Hokkaido Univ. 22:11-29. Hamai, I., K. Kyushin, and T. Kinoshita. 1974. On the early larval growth, survival, and variation of body form in the walleye pollock, Theragra chalcogramma (Pallas), in rearing ex- periment feeding the different diets. Bull. Fac. Fish. Hokkaido Univ. 25:20-35. Haynes, E. B., and S. E. Ignell. 1983. Effect of temperature on rate of embryonic development of walleye pollock, Theragra chalcogramma. Fish. Bull. 81:890-894. Kendall, A. W., Jr., and S. Kim. 1989. Buoyancy of walleye pollock (Theragra chalcogramma) eggs in relation to water proper- ties and movement in Shelikof Strait, Gulf of Alaska. In R. J. Beamish and G. A. McFarlane (eds.), Effects of ocean variability on recruitment and an evaluation of parameters used in stock assessment models, p. 169-180. Can. Spec. Publ. Fish. Aquat. Sci. 108. Kendall, A. W., Jr., and S. J. Picquelle. 1990. Egg and larval distributions of walleye pol- lock Theragra chalcogramma in Shelikof Strait, Gulf of Alaska. Fish. Bull. 88:133-154. Markle, D. F., and K. G. Waiwood. 1985. Fertilization failure in gadids: aspects of its measurement. J. Northw. Atl. Fish. Sci. 6:87-93. Matarese, A. C., A. W. Kendall Jr., D. M. Blood, and B. M. Vinter. 1989. Laboratory guide to early life history stages of Northeast Pacific fishes. U.S. Dep. Commer., NOAA Tech. Rep. NMFS 80, 652 p. McMahon, T. E., and J. C. Tash. 1979. Effects of formalin (buffered and unbuffered) and hydrochloric acid on fish otoliths. Copeia 1979:155-156. Megrey, B. A. 1991. Population dynamics and management of walleye pollock (Theragra chalcogramma) in the Gulf of Alaska, 1976-1986. Fish. Res. 11:321- 354. Nakatani, T., and T. Maeda. 1984. Thermal effect on the development of wall- eye pollock eggs and their upward speed to the surface. Bull. Jpn. Soc. Sci. Fish. 50:937-942. Naplin, N. A., and C. L. Obenchain. 1980. A description of eggs and larvae of the snake eel, Pisodonophis cruentifer (Ophichthidae). Bull. Mar. Sci. 30:413-423. Olla, B. L., and M. W. Davis. 1993. The influence of light on egg buoyancy and hatching rate of the walleye pollock, Theragra chalcogramma (Pallas). J. Fish. Biol. 42:693-698. Pepin, P. 1991. Effect of temperature and size on develop- ment, mortality, and survival rates of the pelagic early life history stages of marine fish. Can. J. Fish. Aquat. Sci. 48:503-518. Picquelle, S. J., and B. A. Megrey. 1993. A preliminary spawning biomass estimate of walleye pollock, Theragra chalcogramma, in the Shelikof Strait, Alaska, based on the annual egg production method. Bull. Mar. Sci. 53:728-749. Reed, R. K., and J. D. Schumacher. 1989. Transport and physical properties in central Shelikof Strait, Alaska. Cont. Shelf Res. 9:261-268. SAS Institute, Inc. 1985. SAS® user's guide: basics, version-5 edition. SAS Institute, Inc., Cary, NC, 1290 p. Schumacher, J. D., and A. W. Kendall Jr. 1991. Some interactions between young walleye pollock and their environment in the western Gulf of Alaska. Calif. Coop. Oceanic Fish. Invest. Rep. 32:22-40. Trinkaus, J. P. 1951. A study of the mechanism of epiboly in the egg of Fundulus heteroclitus. J. Exp. Zool. 118:269-319. Velsen, F. P. J. 1980. Embryonic development in eggs of sockeye salmon, Oncorhynchus nerka. Can. Spec. Pub. Fish. Aquat. Sci. 49, 19 p. Yamagami, K. 1988. Mechanisms of hatching in fish. In W. S. Hoar and D. J. Randall (eds.), Fish physiology, Vol. XI, Part A, p. 447^99. Academic Press, Inc., San Diego. Yusa, T. 1954. On the normal development of the fish, Theragra chalcogramma (Pallas), Alaska pollock. Bull. Hokkaido Reg. Fish. Res. Lab. 10:1-15. Abstract. — The diel vertical distribution patterns of several abundant ichthyoplankton taxa were examined from depth-strati- fied tows off Kodiak Island in the western Gulf of Alaska during 1986 and 1987. Most larvae were found in the upper 45 m of the water column throughout the diel period but were concentrated in higher densities near the surface (0-15 m) in daylight hours and at greater depths at night. Four of the five dominant taxa examined in detail showed significantly greater weighted mean depths during the night than during the day. This pattern was the opposite to that previously reported for the numerically dominant taxa (Ther- agra chalcogramma) in this area. Since there was no clear relation between the diel vertical distribu- tion of these taxa and the vertical distribution of water temperature and density or copepod nauplii prey, we hypothesize that this re- verse migration is either a strat- egy to minimize spatial overlap with predators that follow a nor- mal diel migration pattern or one to optimize light levels for feeding. Diel vertical distribution of ichthyoplankton in the northern Gulf of Alaska* Richard D. Brodeur William C. Rugen Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA 7600 Sand Point Way NE, Seattle, WA 98 1 1 5 Planktonic eggs and larvae of ma- rine fishes exist in three dimen- sions in the open ocean. Unfortu- nately, traditional ichthyoplankton surveys, which use non-closing sampling gear, provide information only on two dimensions, integrat- ing the vertical indirectly into the horizontal dimensions. It is well known that vertical current shear can be substantial over short dis- tances and that light, temperature, hydrostatic pressure and food show much stronger gradients in the ver- tical relative to the horizontal di- mensions in the water column (Laprise and Dodson, 1993). Thus, a larva can often change not only its geographic position, but also its immediate environment by altering its vertical position in the water column. Diel vertical migration is well documented for larval, juvenile, and adult life history stages of ma- rine fishes (see review by Neilson and Perry, 1990). The adaptive sig- nificance of these migrations is presently in dispute, but it has been attributed to position mainte- nance, bioenergetic optimization, thermoregulation, and predator avoidance (Kerfoot, 1985; Lampert, 1989). In addition, the degree of migration and amplitude of depths over which a species vertically mi- grates often changes during ontoge- netic development (Brewer and Kleppel, 1986; de Lafontaine and Gascon, 1989). Knowledge of vertical distribu- tion patterns of marine fish larvae is crucial not only in understand- ing ecological processes but also has practical implications in the assessment of abundance. Sam- pling just the upper depths of a species range can lead to substan- tial underestimates of abundance, whereas sampling the entire water column for surface-dwelling taxa may waste limited ship time. De- spite the importance of the larval phase in recruitment of marine fishes, relatively little is known about larval vertical distribution patterns off the continental shelf in the North Pacific Ocean. With the exception of walleye pollock, Ther- agra chalcogramma, which has been fairly well studied through much of its geographic range (Kamba, 1977; Kendall et al., 1987; Pritchett and Haldorson, 1989; Kendall et al. 1 ), the only compre- hensive studies on vertical distri- bution of coastal ichthyoplankton in the northeast Pacific Ocean are from the California Current region (Ahlstrom, 1959; Boehlert et al., 1985; Brewer and Kleppel, 1986; Lenarz et al., 1991). This paper presents information on the verti- cal distribution of five abundant ichthyoplankton taxa (other than walleye pollock) collected in the Manuscript accepted 18 October 1993 Fishery Bulletin 92:223-235 (1994) "Contribution No. 0181 of the Fisheries Oceanography Coordinated Investigations. 223 224 Fishery Bulletin 92|2), 1994 coastal waters of Alaska during spring and examines diel differences in these patterns in relation to en- vironmental and biotic factors. Materials and methods Samples examined were collected from two cruises of the NOAA ship Miller Freeman in the area south- west of Kodiak Island in the north- ern Gulf of Alaska (Fig. 1). During May 1986 and 1987, 22 depth-strati- fied tows were made with a 1-m 2 Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) (Wiebe et al., 1976) equipped with 153-um mesh. The net was towed obliquely and nets were opened sequentially at the de- sired depth strata. The primary pur- pose of the sampling was to collect information on the vertical distribu- tion of walleye pollock larvae, which are generally found in the upper 50 m (Kendall et al. 1 ), and their prey. Therefore, the emphasis during the sampling was on the upper part of the water column. The nets sampled the following nominal depths: 0-15, 15-30, 30-45, 45-60, 60-80, 80-100, and >100 m. Maximum sampling depth varied (range 150-252 m) depending on the depth of the water column at a particular station. There were eight depth strata sampled at most sta- tions but the cutoff depth between the seventh and eighth net was variable. Therefore, we pooled the catches from these two nets into a single depth stra- tum (>100 m) for analysis. The actual sampling depths are given in Table 1. More complete station and catch information is given in Siefert et al. 2,3 -i 1 1 1 1 " 1- cX S j / { Kodiak i y Island A) / + 4 / FJ -57 00 <^y <-, ;• c \ / -£> +5 > C^> +9 ' Sutwik 1. I , /?Ck. r ~ ' (/ Trinity Is. +7 ' / i f Semidi Is. ^ / 0-6 > N + - " Chirikof 1. -56 00 ;- 4 / 1 V / ' , s * 00 TL , . 1 . 157 00W 156 00 1 ' (■ 1 55 00 1 54 Figure 1 Location of MOCNESS sampling series in Shelikof Strait used to de- termine vertical distribution of larvae in 1986 and 1987. 1 Kendall, A. W., Jr., L. S. Incze, P. B. Ortner, S. R. Cummings, and P. K. Brown. In review. The vertical distribution of eggs and larvae of walleye pollock {Theragra chalcogramma i in Shelikof Strait, Gulf of Alaska. Submitted to Fish. Bull. 2 Siefert, D. L. W., L. S. Incze, and P. B. Ortner. 1988. Vertical distribution of zooplankton, including ichthyoplankton, in Shelikof Strait, Alaska: data from Fisheries Oceanography Coordinated Investigations (FOCI) cruise in May 1986. NWAFC Processed Rep. 88-28, 232 p. 1 Siefert, D. L. W., L. S. Incze, and P. B. Ortner. 1990. Vertical distribution of zooplankton, including ichthyoplankton, in Shelikof Strait, Alaska: data from Fisheries Oceanography Coordinated Investigations (FOCI) cruise in May 1987. NWAFC Processed Rep. 90-05, 129 p. The 22 tows were grouped into five collection se- ries (Table 1) based upon date and location of sam- pling (see Kendall et al. 1 ) and included two complete diel series. The first diel series (Series 4) attempted to sample the same body of water over a four day period during 1986 by following a radar-tracked drifter drogued at 35 m (Incze et al., 1990). The second diel series (Series 9) sampled the same loca- tion on three successive days during 1987. Other collections (Series 5, 6, and 7) were taken at vari- ous times of the day but in the same general area as these two series (Fig. 1, Table 1). Retrieved nets were thoroughly washed and con- tents were preserved in 5% buffered formalin. Samples were sorted to the lowest possible taxon and life history stage at the Polish Sorting Center in Szczecin, Poland. The volume filtered was esti- mated from a mechanical flowmeter mounted on the MOCNESS frame and abundances were converted to number per 1000 m 3 . Up to 50 preserved larvae of each taxon from each net were measured to the nearest 0. 1 mm standard length. Net depth, temperature, and Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 225 Table 1 Station and tow data for collection subset used in the diel series ordered by time of day. Net depths (m) Bottom Local Time Series Tow Year Date depth (m) time period 1 2 3 4 5 6 7 4 5 1986 10 May 293 0745 Dawn 2- -15 15- -30 29- 45 46- -58 59 -78 79- -99 101- -229 4 1 1986 8 May 220 0746 Dawn 1- 15 15- -31 35- 45 45- -61 61- -80 80- -100 101- -200 9 4 1987 23 May 201 0747 Dawn 0- 15 15- -30 30- -45 45- -60 60- -80 80- -100 100- -150 9 8 1987 24 May 190 0800 Dawn 0- 15 15 30 30 -45 45 ■60 60- -80 80- -100 100- -150 6 2 1986 L5 May 223 0940 Day 2- -14 15- 29 30 44 45 -60 61 -80 80- -100 100- -175 5 1 1986 13 May 210 1010 Day 2- 15 16 -30 30- -45 45- -61 61 -80 80 -100 101- -152 4 6 1986 111 May 296 1341 Day 2- 14 30- 45 77- -99 99- -214 5 2 1986 13 May 210 1351 Day 2- -14 15 -29 30- 4 4 45- -59 59- -78 80- -99 100- -176 4 2 1986 8 May 227 1356 Day 2- -14 15- -30 30- -47 79- -99 99- -200 9 5 1987 23 May 179 1422 Day 0- 15 15- -30 30- -45 45 -60 60- 80 80- -100 100- -150 9 9 1987 24 May 191) 1543 Day 0- 15 15 -30 30- -45 45 -60 60 -80 80- -100 100- -150 7 2 1986 18 May 123 1911 Dusk 2 15 15 30 30 45 45 -59 60 -79 so -100 4 3 1986 9 May 235 2006 Dusk 2- -13 13- 28 29- -45 46 59 60- -79 100- -200 4 7 1986 11 May 293 2011 Dusk 3- -14 14 -30 31- 44 43- -59 60- -78 78 -97 98- -252 9 6 1987 24 May 179 2107 Dusk 0- -15 15 -30 30- -45 45 -60 60 -80 80- -100 100- -150 9 Hi 1987 25 May 196 2122 Dusk 0- -15 15- -30 30- -45 45 -60 60 -80 80- -100 100- -150 5 3 1986 14 May 210 2200 Night 2- -14 14- -30 30 45 46 -60 60 -80 81- -101 100- -163 7 1 1986 17 May 126 2416 Night 1 15 16 30 31- 45 45- -59 60 -80 80 -99 4 4 1986 9 May 242 0135 Night 2- -15 15- 30 30- -58 59- -80 80- -100 100- -202 9 11 1987 25 May 198 0218 Night 0- -15 15 30 30- 45 45 -60 60 -80 80- -100 100- -150 9 7 1987 24 May 195 0219 Night 0- 15 15 30 30- 45 45- 60 60- -80 80- -100 100- -150 6 1 1986 15 May 229 0353 Night 2- -15 15- 29 30 -44 45- -60 60 80 80 -100 100- -172 salinity were measured continuously in real time dur- ing the tow and stored for later analysis. To examine diel variations in density and size of larvae with depth, collections from the 22 tows were grouped into one of four time periods (hours): dawn (0530-0830), day (0830-1830), dusk (1830-2130), and night (2130-0530). Diel-depth variation in den- sity of eggs and larvae at each depth was examined by using a two-way ANOVA on log-transformed data. The log (X+l) transformation was used to achieve homogeneous variances (Bartlett's Test, all P>0.05). In addition, a weighted mean depth of occurrence of eggs or larvae of the dominant species for each time interval was calculated as follows: IIXa,, A- m where n ( = number of tows in time interval t , N = number of larvae in net j in tow i in '.it time interval t, D = midpoint depth of net j in tow i in time interval t with a variance equal to Var(D t ) = ( "' ] w=i ; 2>2(Z) a -A> 2 , K-i) & where N (/ = number of larvae in tow i in time interval t. Differences in the weighted mean depths over the four time periods were tested with ANOVA, and Tukey multiple-comparison tests were conducted when significant differences were observed. Untransformed larval lengths for the three most abundant species were entered as dependent vari- ables in two-way ANOVAs, with time of day and depth as factors. Results Species composition Eggs and larvae of species other than walleye pol- lock were found in 134 of the 145 samples collected 226 Fishery Bulletin 92(2). 1994 during the 1986 and 1987 cruises. Flathead sole (Hippoglossoides elassodon) eggs were the only pe- lagic eggs other than walleye pollock collected and were found in 28.4% of the samples. This species had a mean density of 62.99 eggs/1000 m 3 (SD=179.66) and comprised 74.9% of the total egg abundance in the 22 tows. A total of 33 larval taxa were identified but only a few taxa occurred in more than 10% of the samples (Table 2). Larvae other than walleye pollock oc- curred in 92.4% of the collections but made up only 26.3% of the overall total abundance of larvae (to- tal mean density=143.61 larvae/1000 m 3 ; SD=257.03). Larvae of three taxa (H. elassodon , Am- modytes hexapterus, and Bathymaster spp. 4 ) were found at sufficient densities to enable examination of their vertical abundance and length distribution patterns in detail for the four time periods. Two other species (Gadus macrocephalus and Pleuro- nectes bilineatus) were found at relatively high den- sities during day and night but at low densities during the twilight periods; hence, these taxa were examined only for day-night differences. Vertical distribution The distribution of//, elassodon eggs showed little variation in weighted mean depth by time of day (F=3.10, P>0.05); the highest abundances were 4 Larvae of three Bathymaster species known to occur in the study area are presently not identifiable to species. Based on the abundance and distribution patterns of the adults, most of the larvae present in our collections are probably B. signatus. (A. Matarese, Alaska Fisheries Science Center, Seattle, WA 98115. Pers. commun. 1992). Table 2 Summary of all larvae including walleye pollock collected in the 1986-87 vertical distribution study. Percent Mean Length occurrence density range Scientific name Common name (n=145) (no./1000m :i ) (mm) Osmerus mordax rainbow smelt 0.69 0.25 21 Leuroglossus schmidti northern smoothtongue 0.69 0.02 9-15 Stenobraehius leueopsarus northern lampfish 4.14 3.05 4-7 Protomyctophum thompsoni bigeye lanternfish 0.69 0.04 10 Myctophidae unidentified myctophid 0.69 0.06 3 Gadus macrocephalus Pacific cod 15.86 27.51 3-11 Theragra chalcogramma walleye pollock 93.79 402.71 3-8 Gadidae unidentified gadid 2.76 0.36 4 Sebastes spp. unidentified rockfish 1.38 0.17 4-5 Hexagrammos decagrammus kelp greenling 1.38 0.05 8-11 Dasycottus setiger spinyhead sculpin 0.69 0.07 8 Gymnocanthus spp. unidentified sculpin 0.69 0.07 7-8 Hemilepidotus hemilepidotus red Irish lord 1.38 0.13 11-13 Icelinus spp. unidentified sculpin 7.59 0.65 4-5 Malacocottus zonurus darkfin sculpin 0.69 0.05 6-7 Radulinus asprellus slim sculpin 1.38 0.10 4-5 Ruscarius meanyi Puget Sound sculpin 0.69 0.04 4 Agonidae unidentified poacher 10.34 0.84 5-10 Nectoliparis pelagicus tadpole sculpin 0.69 0.07 4-8 Cyclopteridae unidentified snailfish 2.07 0.19 4-5 Bathymaster spp. unidentified ronquil 13.10 30.67 4-7 Anoplarchus spp. unidentified prickleback 2.07 0.26 8-10 Lumpenella longirostris longsnout prickleback 1.38 0.05 10-11 Lumpenus maculatus daubed shanny 6.21 0.64 12-23 Poroclinus rothrocki whitebarred prickleback 4.83 0.97 10-15 Cryptacanthodes aleutensis dwarf wrymouth 1.38 0.19 14 Pholis spp. unidentified gunnel 0.69 0.04 13-17 Zaprora silenus prowfish 3.45 0.40 12-14 Ammodytes hexapterus Pacific sand lance 40.69 12.76 6-19 Hippoglossoides elassodon flathead sole 17.93 59.81 4-19 Pleuronectes bilineatus rock sole 13.10 3.71 3-10 Pleuronectes vetulus English sole 0.69 0.13 8 Psettichthys melanostictus sand sole 0.69 0.14 4-5 Pleuronectidae unidentified flounder 0.69 0.10 4 Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 227 found in the surface layer (0-15 m) during all four time periods (Fig. 2). Although there were signifi- cant (P=0.005) differences in density by depth strata, neither the diel density differences alone (P=0.838) nor the interaction between time and depth (P=0.996) was significant. The majority of larvae, excluding pollock larvae, from all collections combined were collected from the upper three depth strata (Fig. 3). The maximum density overall occurred at the second depth stra- tum ( 15-30 m), below which larval density declined with depth. However, this overall vertical distribu- tion pattern was apparently confounded by higher larval densities found during the night when the larvae were mainly caught in the 15-30 m stratum; Dawn Day ro CD c Q. 100 - 0-16 15-30 30-45 45-60 60-80 80-100 > 100- } 100 200 300 No/1000 m 3 100 200 300 400 No/1000 m 3 Dusk Night -15 ■30 •45 15 30 45 -60 - I 60 -80 - I 80- 100 100 - 15-30 30-45 } 45-60 60-80 ^ 80-100 ) > 100 100 200 300 400 NO./1000 m' 100 200 300 400 No/1000 m 3 Figure 2 Diel vertical distribution of Hippoglossoides elassodon eggs. Bars are mean abundances per 1000 m 3 at each depth interval and error bars are ± one standard deviation about the mean abundance. during the other three time periods the highest den- sities were in surface waters (Fig. 4). The weighted mean depth of larvae overall was significantly (P<0.05) greater at night than during the other three time periods (Table 3) and the interaction between time and depth was marginally significant (P=0.05; Table 4), suggesting that there were diel differences in overall larval depth distribution. Four of the five most abundant larval taxa showed the greatest weighted mean depths (Table 3) and the lowest surface densities (Fig. 4) at night. This gen- eral pattern was also evident in the two time peri- ods examined for the fifth species, G. macro- cephalus, but the diel differences were not signifi- cant (Table 3). Only A. hexapterus and G. macro- cephalus showed significant diel dif- ferences in larval density, with high- est densities occurring at night (Table 4). None of the dominant taxa, however, showed a significant interaction between time and depth strata. Length distributions The distribution of larval lengths by time of day and depth showed no consistent pattern among the three most abundant species (Fig. 5). Al- though time and time-depth interac- tions were significant (all P<0.03) factors in explaining the variation in mean length of H. elassodon and Bathymaster spp., none of the fac- tors was significant for A. hexap- terus. Examining only the strata where more than two lengths were available, we found that the small- est larvae of both Bathymaster spp. and A. hexapterus were caught in the surface stratum at night but in deeper strata during daylight hours (Fig. 5). However, H. elassodon showed an increase in mean length with depth during daylight hours and the reverse pattern at night (Fig. 5). Hippoglossoides elassodon was the only taxon to show a signifi- cant difference in length distribu- tions between night and day collec- tions (Kolmogorov-Smirnov Test; Z=3.881; P=0.001). Although the lack of larger larvae in daytime col- lections might suggest some daytime gear avoidance by this species 228 Fishery Bulletin 92(2). 1994 (Fig. 6), there were few small larvae caught at night, which cannot be explained by gear avoidance. Since the majority (>95%) of these lengths were from lar- 0-15 I I r I 15-30 I i E — 30-45 N I n | 45-60 3 .c cj 60-80 - a 80-100 > 100 100 200 300 400 500 600 NO./1000 m J Figure 3 Vertical distribution of all larvae ex- cluding walleye pollock (Theragra chalcogramma) combined over all time periods. Bars are mean abundances per 1000 m 3 at each depth interval and error bars are ± one standard deviation about the mean abundance. vae collected from the same location (Series 9), sam- pling variability cannot be invoked as an explana- tion for this pattern. Discussion Our results indicate that the vast majority (>99%) of pelagic eggs and larvae (excluding walleye pol- lock) are distributed in the upper 100 m of the wa- ter column during the spring months. Therefore, sampling to this depth should be sufficient to char- acterize the horizontal distribution patterns of these species. Of the common taxa we examined, all but H. elassodon have demersal eggs (Matarese et al., 1989). The transit time to surface waters following hatching from demersal eggs is apparently of such short duration that even newly hatched larvae were rarely collected below 100 m. However, this does not appear to be the case for walleye pollock, which spawn at depths greater than 200 m in Shelikof Strait, with mean depths of eggs and yolk-sac lar- vae generally greater than 100 m (Kendall and Kim, 1989; Kendall et al. 1 ). The diel vertical distribution pattern that we ob- served for several taxa is not the pattern typically observed for most ichthyoplankton and for zooplank- ton in general. The more common pattern, termed a 'Type F migration (Neilson and Perry, 1990), in- volves a nocturnal ascent into surface waters and is undertaken by larvae of a diversity offish species. Table 3 Weighted mean depths (m) and standard deviations of the mean depths (in parentheses) for each taxon and for all larvae excluding walleye pollock by time of day and overall depth for all times combined. Also given are the results of the ANOVAs testing for diel differences in weighted mean depth and the significant (P< 0.05) Tukey multiple-comparison tests between time periods. Dawn Day Dusk Night Overall F-value Tukey test All Larvae (excluding walleye pollock) 16.59(2.72) 17.46(1.52) 15.45(2.25) 25.74 (1.52) 21.75 (1.87) 33. 17**' Night>Day=Dawn=Dusk Hippoglossoides elassodon 14.82(0.63) 16.94(2.73) 10.80 (0.42) 20.10 (0.05) 18.06 (1.33) 3] 89 Nigh t > Day = Da wn> Dusk Ammodytes hexapterus 31.21(11.45) 27.67 (4.59) 22.67 (2.38) 37.51 (4.45) 32.85 (3.39) 6 05" ' Night>Dusk = Day Bathymaster spp. 8.21 (0.11) 11.25 (0.08) 11.28 (1.08) 37.95 i2.84) 18.12 (6.05) 441.48*** Nigh t>Dusk= Day > Dawn Pleuronectes bilineatus 19.75 (1.83) 30.73 (1.63) 25.47 (4.64) 128.36*** Night>Day Gadus macrocephalus 20.36(10.35) 24.92 (0.12) 22.12 (6.56) 1.14 n.s. P<0.001, " P<0.01; n.s. P>0.05. Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 229 However, the reverse pattern ('Type II' migration), although less frequently documented, has been ob- served for larvae of several fish species, including many of the taxa we examined. For example, Boehlert et al. (1985) observed larval G. macrocephalus at lower depths at night than dur- ing the day off the Oregon coast. Walline 5 found that Bathymaster spp. in the Bering Sea generally mi- grated downward at night. Larvae of A. hexapterus collected in bays around Kodiak Island were concen- trated from 10 to 30 m during the day but were found at lower depths at night (Rogers et al. 6 ), and larvae of a congener (A. personatus) collected off Ja- pan also exhibited reverse migration (Yamashita et al., 1985). Rogers et al. 6 and Pritchett and Haldor- Table 4 Results of two-way ANOVAs testing for differences in density of larvae by depth and time of day. Sum of Mean Source of variation df squares square F-ratio P-value All larvae (excluding walley B pollock) Time 3 14.60 4.86 9.85 0.00 Depth 6 51.63 8.61 17.40 0.00 Time x depth 18 14.11 0.78 1.58 0.05 Error 4868 2406.73 0.49 Hippoglossoides elassodon Time 3 4.97 1.66 0.48 0.69 Depth 6 93.59 15.60 4.54 0.00 Time x depth 18 15.49 0.86 0.25 0.99 Error 116 398.59 3.44 Ammodytes hexapterus Time 3 55.08 18.36 11.70 0.00 Depth 6 94.13 15.69 9.99 0.00 Time x depth 18 34.27 1.90 1.21 0.26 Error 116 182.03 1.57 Bathymaster spp. Time 3 8.84 2.95 1.24 0.30 Depth 6 44.33 7.39 3.10 0.01 Time x depth 18 21.35 1.19 0.49 0.96 Error 116 276.73 2.39 Pleuronectes bilineatus Time 1 1.68 1.68 1.35 0.25 Depth 6 19.01 3.17 2.55 0.03 Time x depth 6 3.47 0.58 0.47 0.83 Error 69 85.78 1.24 Gadus macrocephalus Time 1 9.12 9.12 4.09 0.05 Depth 6 22.46 3.74 1.68 0.14 Time x depth 6 7.06 1.18 0.53 0.79 Error 69 153.81 2.23 son (1989) found that rock sole (P. bilineatus), as well as larvae of several other taxa, showed reverse diel migrations during the spring. We believe that sampling bias could not have re- sulted in the observed reverse distributions. Eggs of H. elassodon, as expected, showed no differences by time of day in our study and walleye pollock larvae in these same collections exhibited a normal diel mi- gration pattern (Type I), occurring mainly in the 30- 45 m range during daytime and above 30 m at night (Kendall et al. 1 ; see also Kendall et al., 1987). Net avoidance, although suggested by the higher night catches overall as well as the larger mean size of larvae collected at night, is not a plausible expla- nation for the observed diel pattern. Light-aided daytime avoidance would be ex- pected to influence the catch of lar- vae in the surface strata more than those in deeper strata, thus leading to underestimates of near-surface daytime abundances and the mag- nitude of reverse migration. The prevalence of the reverse diel migration pattern in our study suggests an adaptive role for this behavior. Temperature gradients are relatively minor (<1°C) over the upper 50-60 m where most of the migration oc- curs (Fig. 7), and the majority of the larvae appear to be above the seasonal thermocline at all times of the day. Thus, we see no possi- bility of temperature-mediated energetic advantage related to migration at any time of the day. Similarly, observed density gradi- ents are not pronounced (<0.5 o t units) within this surface layer (Fig. 7; Kendall et al. 1 ) and there appears to be no physical mecha- nism that would aggregate either 5 Walline, P. D. 1981. Hatching dates of walleye pollock (Theragra ehalco- gramma) and vertical distribution of ichthyoplankton from the eastern Bering Sea, June-July 1979. NWAFC Processed Rep. 81-05, 22 p. 6 Rogers, D. E., D. J. Rabin, B. J. Rogers, K. J. Garrison, and M. E. Wangerin. 1979. Seasonal composition and food web relationships of marine organisms in the nearshore zone of Kodiak Island including ichthyoplankton, mero- plankton (shellfish), zooplankton and fish. Univ. Washington, Fish. Res. Inst. Rep. FRI-UW-7925, 291 p. 230 Fishery Bulletin 92(2). 1994 DAWN DAY DUSK NIGHT All non-Pollock larvae 0-15 15-30 30-45 1 45-60 60-80 J 80-100 > 100 3- i 40 80 120 50 100 150 200 40 80 120 Hippoglossoides elassodon 0-15 15-30 30-45 45-60 60-80 80-100 > 100 ' • 50 100 150 200 100 200 300 40 80 120 160 Bathymaster spp. 0-15 I I *—* 15-30 t 30-45 >m^ 45-60 CO — 60-80 J~ 80-100 *- > 100 } I 4. 200 400 600 800 3- i 10 20 30 40 50 100 200 300 40 80 120 160 20 40 60 80 Ammodytes hexapterus C 0-15 I I 15-30 +- 30-45 © 45 " 6 ° p| 60-80 80-100 > 100 | I 4. Gadus macrocephalus Pleuronectes bilineatus * T 3- 10 20 30 10 20 30 40 50 20 40 60 80 100 10 20 30 40 3- 12 8 10 8 12 5 10 15 20 10 20 30 40 50 Density (no. /1 000m 3 ) Figure 4 Diel changes in the vertical distribution of all larvae (excluding walleye pollock), and Hippoglossoides elassodon, Bathymaster spp., Ammodytes hexapterus, Gadus macrocephalus, and Pleuronectes bilineatus larvae. Bars are mean abundances per 1000 m 3 at each depth interval and error bars are ± one standard deviation about the mean abundance. Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 231 DAWN DAY DUSK NIGHT 0-15 15-30 30-45 Hippoglossoides ^ 45-60 elassodon £ 60-80 80-100 CO » 100 ~-, -^ ^^ • • co < 5 6 7 8 9 10 <■ 56789 10 456789 10 456789 10 > 0-15 1- 15-30 ® 30-45 Bathymaster tT 45-60 S PP- •- 60-80 80-100 -*- > 100 . — . — , ^^ Q. 15 6/ i 5 6 r t 5 6 7 4 5 6 7 0) 0-15 Q 15-30 Ammodytes 30-45 hexapterus 45 . 60 60-80 80-100 ' 100 • * ^_ — — • , . , • 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 4 8 12 16 20 Length (mm) Figure 5 Diel vertical distribution of larval lengths of Hippoglossoides elassodon, Bathymaster spp., and Ammodytes hexapterus. Circles are mean length at each depth interval and error bars are ± one standard deviation about the mean length. The plus signs indicate actual lengths measured when less than three lengths were available from a particular depth stratum. 0.30 0.25 0.20 c o £ 0.15 a. o £ 0.10 0.05 0.00 ! UDay □ Night 2 6 7 8 Length (mm) 10 Figure 6 Day versus night proportional length distribu- tions of Hippoglossoides elassodon larvae. larvae or their prey at certain depths or inhibit them from migrating to different depths. The fact that walleye pollock larvae, which are the dominant fish larvae in this area representing 70- 80% of the larvae present in Shelikof Strait in the spring (Rugen 7 ; this study), show a normal migra- tion pattern (Kendall et al., 1987) suggests one po- tential explanation for reverse migration patterns of other larvae. If other larvae feed on the same microzooplankton prey as larval walleye pollock and these prey resources were limiting, then the pres- ence of these other larvae in surface waters at dif- ferent times of the day than those of walleye pol- lock would reduce competition with the numerically dominant taxon. Copepod nauplii, an important 7 Rugen, W. C. 1990. Spatial and temporal distribution of lar- val fish in the Western Gulf of Alaska, with emphasis on the period of peak abundance of walleye pollock tTheragra chalcogramma) larvae. NWAFC Processed Rep. 90-01, 162 p. 232 Fishery Bulletin 92(2), 1994 Water temperature and density DAWN NIGHT DAY DUSK Sigma-t 2S 2 253 254 26.5 266 25.2 25 3 254 25 5 25 6 25 1 25 2 253 25 4 25 5 25.2 25.3 254 25 5 25 6 E 20 -— - -C •*—• a. a> 60 Q BO sigma-t temp 3 4 6 60 30 4 5 6 30 4 5 6 3.0 4 5 6 Temperature (°C) Figure 7 Diel vertical profiles of temperature and density (o t ). Data are means of at least four casts within each time interval and were collected at 1 m depth intervals. component of the diet of many larval fishes includ- ing walleye pollock (Kendall et al., 1987), were the most abundant microzooplankton category found in Shelikof Strait, mostly in the upper 30 m during May 1986 and 1987 (Incze and Ainaire 8 ). During diel Series 4, copepod nauplii had overall mean depths between 20 and 34 m but showed no obvious diel pattern in depth distribution (Kendall et al. 1 ). Al- though feeding at a different time of day from wall- eye pollock might reduce interference competition (i.e. behavioral interactions) with the dominant spe- cies, it is highly unlikely, based on typical larval fish and copepod naupliar densities, that prey resources could ever be depleted by larval fish (Cushing, 1983; MacKenzie et al., 1990). Moreover, if food were lim- iting, then it would be advantageous for all larvae to stay in the layer of maximum food concentration throughout the diel period to maximize total intake. Thus, we do not see a trophic benefit accruing from a reverse migration pattern for these larvae. If feeding by these larvae is periodic and depen- dent on some minimum light level, then the verti- cal distribution pattern can be partially explained by larval feeding response. Assuming light levels were limiting feeding at depths below 30 m, then it would be necessary for larvae to ascend to a shal- lower depth during the daytime when light is at a maximum. Following the cessation of feeding at dusk, larvae would be expected to become inactive and passively sink to deeper levels at night. Such a mechanism has been postulated for Japanese sand lance (A. personatus) by Yamashita et al. ( 1985) who demonstrated a nocturnal cessation of feeding in this species. Although we lack data on the diel feed- ing chronology of any of the taxa examined here, it is possible that feeding occurs mainly in the crep- uscular periods, with a temporary cessation of in- gestion occuring during midday as observed in the field for larval walleye pollock (Canino and Bailey 9 ). The shallowest mean depth occurs at either dawn or dusk for the three common species that were ex- amined over the four time periods with slightly greater depths occurring during midday. If larvae were not feeding during the middle of the day, it would be advantageous to cease swimming alto- gether and sink through the water column to avoid being sensed by mechanoreceptive or visual preda- tors. Following a particular isolume would produce a similar daytime pattern but could not account for the deeper distribution at night that we observed. Larval walleye pollock in the laboratory have been shown to avoid high light levels (Olla and Davis, 1990) but they also require relatively low light lev- els to initiate feeding (Paul, 1983). Unfortunately, we have no data available on the light levels neces- sary for feeding in the taxa we examined with which we can evaluate this hypothesis. 8 Incze, L. S., and T. Ainaire. In review. Zooplankton of Shelikof Strait, Alaska. I. Micro-zooplankton prey of larval pollock, Theragra chalcogramma. Submitted to Fish. Bull. 9 Canino, M. F., and K. M. Bailey. In review. Gastric evacuation of walleye pollock, Theragra chalcogramma (Pallas), larvae in response to feeding. Submitted to Journal of Fish Biology. Brodeur and Rugen: Vertical distribution of ichthyoplankton in the northern Gulf of Alaska 233 A potential disadvantage to a diurnal ascent is increased susceptibility to visually feeding planktivorous fishes. However, acoustic and trawl survey data suggest that epipelagic fish predators are rare during the spring in this area and the majority of the nekton biomass is found in midwater or near the bottom (Brodeur et al., 1991), well be- low the depth of most larvae. On the other hand, euphausiids, which are possibly the major inverte- brate predator on walleye pollock yolk-sac larvae, undergo a nocturnal ascent to surface water and descend to greater depths during the day in Shelikof Strait (Bailey et al., 1993). If euphausiids were also predators on non-pollock larvae and feed only in the surface layer above the nightime depths of these lar- vae, then a distinct advantage would be conferred upon individuals adopting a reverse diel migration pattern, as has been postulated for copepods (Ohman et al., 1983; Ohman, 1990). Based on field and experi- mental results, it has become increasingly apparent that predators can alter the diel vertical distribution patterns of invertebrate prey (Ohman et al., 1983; Gliwicz, 1986; Bollens and Frost, 1989; Levy, 1990; Neill, 1990; Frost and Bollens, 1992), but evidence for this effect on larval fish as prey is presently lacking. Although a variation in depth by time of day was apparent for all species and consistent among spe- cies, it was not substantial enough to be statistically significant in all cases (e.g. G . macrocephalus). This may be due in part to the lack of resolution of our sampling intervals. The smallest average migration that we could detect is -15 m; thus, diel vertical migrations less than that were not likely to be de- tected. Although a daily ambit of 30 m is not excep- tional for larger larvae, it may be excessive for newly hatched individuals. For a study specifically exam- ining the diel vertical distribution of the species considered here, we recommend sampling with a multiple net system every 5 m over the upper 40 m of the water column. Some bias may have also re- sulted from combining tows from different years, weeks, or geographic areas into our four time peri- ods, which was necessitated by the relatively low oc- currence rate and densities of these taxa. However, the remarkably strong and consistent diel differ- ences among the different taxa, despite this intro- duced sampling variability, lend credence to our findings. If there was differential migration by size classes of larvae, this condition might also obscure some of our results. The vertical distribution of larval lengths of the dominant species did not show any consistent patterns by time of day. The mean length by depth varied significantly for H. elassodon; smaller larvae were found at greater depths during the daytime and at the surface at night. This can- not be explained by visual gear avoidance alone since the nighttime pattern would then be expected to be random rather than exhibit the increasing mean length with depth that we observed. A possible explanation for this pattern might be that larger larvae may migrate a greater distance than smaller larvae, a pattern frequently observed in other fish larvae (Neilson and Perry, 1990). It is also possible that the migration of different size classes is asyn- chronous (Pearre, 1979). However, the available size ranges of the dominant species in our data was not extensive enough to examine diel migration patterns of different size classes. Moreover, caution should be exercised in examining larval length data in mul- tiple net systems. Since larvae shrink upon death (Theilacker, 1980; Hay, 1981) and the likelihood of death may be related to time in net, we may assume that larvae caught in the first (deepest) net may have undergone more shrinkage than those in the last (surface) net. In conclusion, this study shows that all the com- mon larvae exhibit a reverse vertical migration pat- tern, opposite to that of the overall dominant spe- cies, walleye pollock. In Auke Bay, an inland embayment in Southeast Alaska (58°22' N) on the eastern side of the Gulf of Alaska, Haldorson et al. ( 1993) found a Type I migration for the numerically dominant osmerid larvae in their sampling and a Type II migration for the five next most abundant taxa {T. chalcogramma, H. elassodon, P. bilineatus, Leuroglossus schmidti, and Agonidae). These au- thors attribute this diel-depth distribution pattern to temperature preferences by each species, al- though their vertical temperature gradients were more pronounced than what we observed in our study. Since most abiotic variables (other than light intensity) and food resources varied little over the depths through which much of the migration oc- curred in Shelikof Strait, we hypothesize that the reverse migration pattern that we documented was either a predator-avoidance mechanism or else an optimization of light levels for feeding. The preva- lence of reverse migration in this and other studies suggests that it may be more common than previ- ously suspected, especially in higher latitude ecosys- tems, and the factors contributing to this phenom- enon merit further investigation. Acknowledgments The MOCNESS tow collections used in this study were made available by Lew Incze (Bigelow Labo- ratory) and Peter Ortner (Atlantic Oceanographic 234 Fishery Bulletin 92(2). 1994 and Meteorological Laboratories, NOAA). Field as- sistance was provided by Shailer Cummings (AOML) and the crew of the NOAA ship Miller Free- man. Susan Picquelle and Patricia Brown (Alaska Fisheries Science Center) assisted in statistical analysis. Art Kendall, Gary Stauffer, JeffNapp, Bori Olla, Susan Sogard, and Michael Davis (AFSC), R. Ian Perry (Pacific Biological Station), Steven Bollens (Woods Hole Oceanographic Institution), and two anonymous reviewers provided valuable comments on earlier versions of the manuscript. Literature cited Ahlstrom, E. H. 1959. Vertical distribution of pelagic fish eggs and larvae off California and Baja California. U.S. Fish. Wild. Serv., Fish. Bull. 60:107-146. Bailey, K. M., R. D. Brodeur, N. Merati, and M. M. Yoklavich. 1993. 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Vinter. 1989. Laboratory guide to early life history stages of Northeast Pacific fishes. U.S. Dep. Commer., NOAA Tech. Rep. NMFS 80, 652 p. Neill, W. E. 1990. Induced vertical migration in copepods as a defence against invertebrate predation. Nature 345:524-526. Neilson, J. D., and R. I. Perry. 1990. Diel vertical migrations of marine fishes: an obligate or facultative process? Adv. Mar. Biol. 26:115-168. Ohman, M. D. 1990. The demographic benefits of diel vertical migra- tion by zooplankton. Ecol. Monogr. 60:257-281. Ohman, M. D., B. W. Frost, and E. B. Cohen. 1983. Reverse diel vertical migration: an escape from invertebrate predators. Science 220:1404- 1407. OUa, B. L., and M. W. Davis. 1990. Effects of physical factors on the vertical dis- tribution of larval walleye pollock Theragra chalco- gramma under controlled laboratory conditions. Mar. Ecol. Prog. Ser. 63:105-112. Paul, A. J. 1983. 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Diel vertical migration and feeding rhythm of the larvae of the Japanese sand-eel Ammodytes personatus. Bull. Japan. Soc. Sci. Fish. 51:1-5. Abstract. — The commercial fishery for orange roughy on the Challenger Plateau developed in 1981, increased markedly through- out the mid-1980s, and then de- clined rapidly by 1990. Data from research trawl surveys and com- mercial fishing returns over the period are examined, and changes in the population are described. The distribution of orange roughy changed over the period examined; there was a contraction of the areas of high density and apparent fishing-out of aggrega- tions on relatively fiat bottom. Ag- gregations are now largely con- fined to pinnacles. Biomass of or- ange roughy, measured by bottom trawl survey indices and commer- cial catch per unit of effort, de- clined substantially and is cur- rently estimated to be about 20% of virgin levels. Most other inci- dental species in the trawl surveys have also declined in abundance, and there are no indications of 'species replacement.' Data on size, reproductive stage, size at maturity, and feeding have also been examined. Size structure of the population has not changed over time. Time of spawning (July) and the pattern of gonad develop- ment have been consistent over the years. Diet composition has also remained similar; dominant prey groups are natant decapod crustaceans and small fish. It is suggested that biological changes have not been apparent because orange roughy are a long- lived, slow-growing species, with low productivity. There could be a long response time to fishing pres- sure, yet orange roughy popula- tions can be quickly reduced to low levels by commercial fishing. Changes in a population of orange roughy, Hoplostethus atlanticus, with commercial exploitation on the Challenger Plateau, New Zealand Malcolm R. Clark Dianne M. Tracey MAF Fisheries Greta Point. PO. Box 297 295 Evans Bay Parade, Wellington, New Zealand Manuscript accepted 4 November L993 Fishery Bulletin 92:236-253 (1994) Orange roughy (Hoplostethus atlanticus Collett) has a worldwide distribution on the continental slope at depths of 700 to 1,500 m. However, it is fished commercially only off New Zealand, Australia, and in the northeastern Atlantic Ocean. The New Zealand fishery is the most established, having started in 1978; the others date from 1988 and 1991, respectively. Orange roughy is one of the most valuable commercial species in New Zealand waters, with annual landings of 40-50,000 metric tons (t) and export earnings of NZ $100- 150 million (Robertson, 1991). The New Zealand fishery for or- ange roughy occurs in a number of areas (Fig. 1), including the Chal- lenger Plateau, a broad submarine plateau off the west coast of New Zealand. The commercial fishery on the Plateau developed in late 1981 and rapidly expanded into one of the most important orange roughy fisheries in New Zealand waters, with annual catches up to approxi- mately 16,000 t (Table 1). The fish- ery operates primarily during win- ter (June-August), when the fish form large spawning aggregations at depths of 850-900 m (Clark, 1991a). The fishery has been managed by a Total Allowable Catch (TAC) sys- tem since 1982. Tracey et al. (1990) and Clark (1991a) discussed details of this management regime. Ini- tially, catches were limited to 7,000 t by the TAC for all areas of the New Zealand Exclusive Economic Zone (EEZ), outside the established fishing grounds on the east coast. A TAC of 4,950 t was set for 1983- 84 and 1984-85 (October-Septem- ber fishing year) on the Challenger Plateau and west coast of the South Island. This was raised to 6,190 t specifically for the Chal- lenger fishery in 1985-86 based on biomass estimates from a trawl survey in winter 1984. The quota was raised to 10,000 t in 1986-87, and further to 12,000 t in 1987-88, in order to assess the effects of heavier fishing on the population dynamics of orange roughy ("adap- tive management"). In the follow- ing fishing year only 8,200 t of quota were allocated, the rest with- held because of signs that the or- ange roughy population was declin- ing rapidly. During 1989-90 the TAC was reduced to 2,500 t after new stock assessments showed the population was overexploited and had declined to low levels (Clark and Francis, 1990). The TAC was further reduced for 1990-91 to pro- mote rebuilding of the population. These changes occurred against a background of increasing infor- 236 Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 237 /175°W * EEZ boundary Figure 1 Map of New Zealand and offshore waters of the Exclusive Economic Zone (EEZ), showing the location of the Challenger Plateau and other major fishing areas for orange roughy (Hoplostethus atlanticus). Table 1 Reported catch es (t) of orange roughy (Hop- lostethus atlanti cus) from the Challenger Plateau (ORH 7A and outside EEZ) (from Clark 1992; to- tal estimated catch includes allowance for re- search survey catches an d a correction for 15-30% under-estimation of true catch in reported catch figures because of bu rst trawls. fish dis- cards, and incorrect official conversion factor). Total Total Fishing year reported estimated (Oct-Sept) catch catch TAC 1980/81 33 43 1981/82 4,248 5,522 1982/83 11,839 15,409 1983/84 9,527 12,514 4,950 1984/85 5,117 6,707 4,950 1985/86 7,753 10,251 6,190 1986/87 11,492 15,750 10,000 1987/88 12,181 15,830 12,000 1988/89 10,241 12,627 12,000' 1989/90 4,309 5,171 2,500 1990/91 1,357 1,560 1,900 ' 8,219 t allocated. mation on orange roughy. It has only recently been realized that orange roughy is very slow-growing and long-lived. Mace et al. (1990) recorded a growth rate of about three cm per year for the first four years of life (validated ages), and an estimated age at maturity of 24 years and maximum age over 50 years. They estimated natural mor- tality to be low (less than 0-1 yr -1 ) and concluded that sustainable yields of orange roughy would be relatively low and show slow recov- ery from over-fishing. Recent esti- mates of the maximum age of or- ange roughy from Australian wa- ters approach 150 years (Fenton et al., 1991). Quotas for orange roughy har- vest from New Zealand have been reduced in recent years on the ba- sis of information which suggests much lower productivity than origi- nally assumed. However, the Chal- lenger Plateau population had al- ready declined markedly and pro- vides some insight into the effects of heavy fishing pressure on orange roughy population dynamics. There is an extensive literature on general re- sponses offish populations to exploitation, covering lake ecosystems (e.g. Regier and Loftus, 1972; Spangler et al., 1977), coral reef fisheries (e.g. Russ and Alcala, 1989), and relatively shallow-water marine environments (e.g. Hempel, 1978; Pauly, 1979; Grosslein et al. 1980). There have been few studies on deep-water or long-lived species such as orange roughy. The closest is probably Pacific ocean perch (Sebastes alutus) which is found at depths to 600 m in the North Pacific Ocean and has a maxi- mum age of 90 years (e.g. Gunderson, 1977; Lea- man, 1991). There are a number of general population re- sponses to exploitation, which include 1 Decline in abundance of fished species. 2 Contraction of distribution or areas of high density. 3 Change in age structure or size structure, or both, with fewer old, large fish and the population dominated by new recruits. 4 Increase in growth rate of individuals, with a decrease in age for a given length. 5 Lower age at maturity or size at maturity, or both. 238 Fishery Bulletin 92(2), 1994 6 Possible change in species composition over time ('species replacement'). Such responses are often observed in short-lived, fast-growing species (e.g. Pauly, 1979; Grosslein et al., 1980). Some have also been noted with Sebastes alutus (Gunderson, 1977; Leaman, 1991), but it is not clear whether these changes would occur in such a long-lived species as orange roughy, or over what time period such changes would become apparent. Orange roughy on the Challenger Plateau have been exploited for only 10 years, and hence it seems un- likely that marked changes in biological character- istics could occur over such a relatively short time period in relation to the longevity of the species. Therefore, we might expect to observe changes in biomass and distribution, as well as age and size struc- ture of the population, but not changes in growth rate or reproductive potential. In this paper, we summarize some of the available data on distribution, abundance, and biology of or- ange roughy on the Challenger Plateau, primarily over the period 1984-90. This period covers the early years of the developing fishery, to maximum levels of exploitation, and subsequent decline of the population. We describe the reduction in size and distribution of the stock and investigate associated changes in size structure, aspects of reproduction, and feeding. Methods Research trawl surveys Trawl surveys have been carried out in the winter (June-July) of each year from 1984 to 1990. The vessel used, area covered, intensity of trawling, and survey design differed between years, and all are not directly comparable (Table 2). Surveys from 1987 to 1989 were treated as fully comparable, but only se- lected data have been used from other surveys: distribution from 1984 and 1990, and biology (size, reproductive, and feeding data) from 1984, 1985, 1986, and 1990. The general survey design was two-phase stratified random (af- ter Francis, 1984). The survey area was divided into a number of strata based on depth and certain bottom features (e.g. pinnacles). General stratification is shown in Figure 2. The depth range covered was 800 to 1,200 m. New, random station positions within strata were selected each year, except in strata 10 and 11 on pinnacles where a random tow direction was ad- justed to avoid untrawlable ground, and these trawls were repeated each year. A similar net de- sign and gear set up was used for each survey. Tow length was standardized where possible at 1.5 nau- tical miles (nmi). Trawling speed was 3.0-3.5 knots. Biomass indices were calculated by the area swept method as described by Francis ( 1981). Biomass and its standard error were calculated from the follow- ing formulae: and B=^(X,a,)/cb S B = J^sfaf/c 2 b 2 where B is biomass (t), X is the mean catch rate (kg-km -1 ) in stratum i, a i is the area of stratum i (km 2 ), b is the width swept by the gear (defined as doorspread (m) by MAF Fisheries), c is the catch- ability coefficient (an estimate of the proportion of fish available to be caught by the net), S B is the standard error of the biomass, s ( is the standard error of X t The catchability coefficient was assigned a value of 0.27, which represents the wingend spread di- vided by the doorspread, because orange roughy form schools which are not believed to be herded substantially by doors or sweeps. 1 Approximate 95% confidence limits (CD were calculated as CL = B±2S B . 1 Orange Roughy Working Group, MAF Fisheries, Greta Point, P.O. Box 297, Wellington, New Zealand, pers. commun. 1991. Table 2 Trawl surveys carried out on the Cha lenger Plateau for orange roughy (Hoplostethus a 'In nticus). Area Number Vessel Year Date (km 2 ) of trawls Survey design Arrow 1984 3/7-18/7 11,956 118 2 phase SRTS' Arrow 1985 4/7-20/7 209 16 1 phase SRTS Arrow 1986 4/7-17/7 94 10 transect grid Amaltal Explorer 1987 18/6-13/7 8,270 129 2 phase SRTS Amaltal Explorer 1988 4/7-24/7 8,270 85 2 phase SRTS Amaltal Explorer 1989 8/7-30/7 8,270 160 2 phase SRTS Will Watch 1990 7/7-29/7 8,270 141 2 phase SRTS ' Stratified random tra wl survey Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 239 EEZ boundary Figure 2 The Challenger Plateau survey area, showing bathymetry (depth contour in m) and details of survey stratification. ment of the fishery through to maximum exploitation. It is felt that the fishery was not constrained much by the TAC over this time. CPUE in winter months from 1983 to 1991. This in- cludes data from 1990 and 1991, following a substantial reduction in TAC and effort. CPUE in non-winter months from 1983 to 1991. Trawl survey indices from 1987 to 1989. These sur- veys covered the same area, had the same design, and used the same vessel. Trawl survey indices from 1984 and 1987 to 1990. This series incorporated data from a smaller area surveyed in 1984 and from the 1990 survey, both of which used a different ves- sel from 1987 to 89. The coefficient of variation (CV) is a measure of the precision of the biomass estimate, and was cal- culated by CV = 5 B /Bxl00. Stock reduction analysis A stock reduction technique was used to estimate virgin biomass based on the method of Francis (1990, 1992). This incorporated a complete catch history for the stock, a time series of abundance indices, and life history parameters used in a deter- ministic age-structured population model (see Clark, 1992). The latter were the von Bertalanffy growth parameters (L m =39.5 cm, &=0.059yr _1 , t = -0.3 yr), natural mortality=0.04yr _1 , weight-length parameters (a=0.0963, 6=2.68), age at maturity (24 yr), age at entry to the fishery (24 yr), and Beverton- Holt recruitment steepness of 0.75. Five sets of abundance indices were used from trawl surveys between 1984 and 1990, and commer- cial catch per unit of effort (CPUE) data (unstand- ardized mean catch per tow by monthly groupings): 1 CPUE in winter months (June-September) from 1983 to 1989. This covered the period of develop- The maximum likelihood method was used to estimate virgin biomass. Ninety-five percent confidence intervals were estimated by using bootstrapping techniques with the coefficient of variation fixed at 20%. The best estimate of virgin biomass was then used in an age-structured model (detailed in Francis, 1992) to es- timate current biomass. Biological data Standard procedure during trawl surveys was to take a random sample of about 200 fish from each tow. These were measured (standard length rounded down to the nearest whole cm [standard MAF Fish- eries procedure]) and sexed. Twenty of these fish were randomly selected, their otoliths extracted, and more detailed data collected: standard length (rounded down to the nearest whole mm), weight (rounded down to the nearest gm), sex, stage of gonad maturity (see below), gonad weight (rounded down to the nearest gm), fullness of stomach, state of digestion of contents, and stomach contents (to species level where possible). Size Length-frequency distributions have been con- structed to represent the total population where possible. In the years 1984 and 1987-90, data have 240 Fishery Bulletin 92(2). 1994 been scaled by percentage sampled to represent each catch and further scaled by stratum biomass to ap- proximate the population. Samples in 1985 and 1986 were scaled to represent solely the catch, as survey design was inadequate for biomass estimation. Length-frequency data are difficult to compare statistically and, for the purposes of this study, have not been attempted. However, to enable a general comparison, a single distribution was constructed combining length-frequency data from all years weighted by the number of tows each year. This distribution is plotted together with those from each year separately. Mean size by sex was calculated separately for three main regions of spawning within the survey area (strata 1, 4; 10; 9, 11) as it was unlikely these areas had been fished equally (see later 'Commer- cial Fishery' section). The sample sizes used in cal- culating the standard error were number of tows, not number of fish. Orange roughy can associate in size groups; between-tow variance was greater than within-'tow variance. Variance is represented by ±2.0 standard errors for all years except 1986, when ±2.2 standard errors was arbitrarily used because there were only 10 trawls. Reproduction Macroscopic staging of reproductive condition followed Pankhurst et al. (1987): Stage Female Male 1 2 3 Immature/resting Early maturation Maturation Immature/resting Early maturation Maturation 5 6 Ripe Running ripe Spent Ripe/running ripe Spent Relative frequency of gonad stages was examined. Analyses were based on the samples taken. They were not scaled in any way, as there were no appar- ent differences between the length frequencies of the samples and the distribution of the total population. Only data from females are presented, as their macroscopic gonad stages can be determined more accurately than those from males. Size at maturity was determined from samples taken over the total survey area using a 'probit analysis' approach (after Pearson and Hartley, 1976). It was assumed that length at maturity is normally distributed in the population. The regres- sion part of the analysis was repeated 10 times to ensure convergence of the estimate. 2 A standard lin- 2 Francis, C, MAF Fisheries, pers. commun. 1991 ear regression analysis was carried out on results to investigate trends over time by using the SAS sta- tistical package (SAS, 1988). Feeding Data on frequency of occurrence were available from all surveys. Frequency of occurrence was defined as the number of stomachs in which a food item occurs, expressed as a proportion of the total number of stomachs containing food. Only stomachs with part-full or full classifications, and with fresh or partly digested contents, were included in analyses. Commercial fishing data Data on the catch and position of each tow and the start and finish times have been collected since 1980. However, catch and effort information is dif- ficult to standardize and interpret for orange roughy. Fish can be highly aggregated at various times of the year, and 'windows' or escape panels in the net are frequently used to reduce catch size and mini- mise damage to nets. Fishing performance varies with experience of skipper and crew, and technol- ogy has advanced considerably in recent years (in particular, development of Global Positioning Sys- tem navigation, which enabled improved accuracy when fishing pinnacles). Fishing logbooks often do not have accurate information on length of tow on the bottom. Fishing for orange roughy on the Chal- lenger Plateau occurs on a variety of bottom terrain: on flat bottom, in troughs and steep slope, and on the tops and sides of pinnacles. In each case, the effective fishing time and fishing technique differ greatly, and they are almost separate types of fish- eries. In order to gain an indication of trends in catch rates, data were examined on the basis of mean catch per tow for two size classes of vessel (20— 60 m, generally domestic fresh fish boats; and 60- 90 m, domestic factory trawlers). Catch per unit of effort (CPUE) values were similar for both classes, and so data here are combined. Monthly data were amalgamated into two time periods: first, 'winter' (June, July, August) which covers the spawning period; second, 'out of season' (all other months). This division represents two distinct phases of or- ange roughy distribution, as well as differences in the mode of fishing (Clark, 1992). The former period is characterized by the formation of relatively stable, dense aggregations of fish, whereas in the latter period the orange roughy are more dispersed and widely distributed (Clark, 1991a). Fishing in win- ter generally involves shorter tows, often with smaller nets, than does out-of-season fishing. Clark and Tracey: Population changes of Hoplostethus atlanticus on the Challenger Plateau 24 1 In the following text, three colloquial area names have been used. These are given below with specific strata numbers (see Fig. 2): Central Flat Pinnacles Westpac Bank strata 1, 4 stratum 10 strata 9, 11 Results Distribution Trawl surveys The distribution of orange roughy in the survey area changed substantially between years (Fig. 3). In 1984 high catch rates were ob- served across much of the Central Flat area. (No trawls were made on the Pinnacles although heavy marks were observed on the echosounder; the sur- vey did not cover the Westpac Bank area.) In 1987 fish were still widely distributed in the Central Flat; -36' 1984 >48' •40°S Catch rate kg.knrr 1 100 j-s^ii -12' ^P 10 000 12' 24' 36' 48' 168° E 12' 24' -36' 1987 >36' 1988 -48

■48'----.. [®j -40°S -40°S r-'iS -12' 12' 12' 24' 36' 48' 168° E 12' 24' 1 2' 24' 36' 48' 168° E 12' 24' 36' 1989 - 3IV 1990 ■48' ; <3 ; .---. - € ''--. S; ■40°S r-<5/ ■40°S ■0 ■12' - 12' 1 2' 24' 36' 48' 168° E 12' 24' 12' 24' 36' 48' 168° E 12' 24' Figure 3 Contours of trawl survey catch rates (kg-km l ) of orange roughy (Hoplostethus atlanticus) in 1984 and 1987-90. there were two main schools and further concentra- tions around the Pinnacles and the Westpac Bank. In 1988 there was a marked contraction in the area of high catch rates; a single small aggregation was observed on the Central Flat, and by 1989 there were no aggregations in the Central Flat region. High catch rates still occurred on the Pinnacles and Westpac Bank in 1989, and these actually increased in 1990, after the TAC and fishing effort were greatly reduced. Commercial fishery The commercial fishery has been centered mainly inside the EEZ, targeting ag- gregations of orange roughy on the Central Flat and Pinnacles. Distribution of effort (number of tows) and catch between these two areas has changed over time (Table 3). In the period 1982-87, over 80% of the catch from the two areas was taken from the Central Flat with over 75% of the number of tows. In 1988 there was a marked increase in the propor- tion of catch and effort on the Pinnacles, and a corre- sponding reduction on the Cen- tral Flat. This shift continued in 1989 and 1990, during which the Pinnacles accounted for 65-70% of the catch. These changes reflect the change in distribution observed in the re- search trawl surveys. Relative abundance Trawl surveys Biomass indi- ces (estimates of relative bio- mass) from trawl surveys in 1987, 1988, and 1989 are given in Table 4. The indices indicate a marked decline in biomass over the period. The distribution of biomass among strata changed over the years 1987-90 (Table 5). In 1987 and 1988 over 60% of the bio- mass was in the Central Flat area, but only 30% in 1989 and 1990. Over this period, there was an increase in the proportion on the Pinnacles, especially between 1989 and 1990. Biomass levels in the surrounding areas have fluc- tuated but were particularly high in 1989. The proportion of biomass on the Westpac Bank has remained compara- tively constant. 242 Fishery Bulletin 92(2), 1994 Commercial fishery Mean catch per tow for all New Zealand vessels in the fishery from 1983 to 1991 is given in Table 6. Catch rates in winter, when the fish are aggregated for spawning, are generally higher than in other months. Although aggregations occur at other times, presumably for feeding, they are not as large or as stable as in winter. Catch rates in both periods declined steadily from 1983 to 1989 to between about 15% and 20% of original levels. The trend is slightly different in the two periods; winter catch rates declined more sharply to 1988, whereas in the other months the largest decrease was between 1983 and 1984. Catch rates increased in 1990, following a reduction of the TAC, when there were less vessels and fewer trawls on the grounds. Individual trawl catch rates for orange roughy can be highly variable, consisting of 'hits' and 'misses.' Therefore it is not useful to describe the variance around these mean catch rates, beyond commenting that there is wide variation. It should be stressed that the changes in catch rates presented here may give an indication of changes in stock size but should be treated with caution. Difficulties in interpreta- tion of such data for orange roughy are described in the 'Methods' section, and the form of relationship between mean catch per tow and stock abundance is uncertain. Stock reduction results Abundance indices used in, and estimates of virgin biomass from, the stock reduction analyses are given in Table 7. Point estimates of B Q range from 95,000 t to 278,000 t. The best fits of data to the model (those with the lowest CV) are from the winter CPUE se- ries. Results from trawl survey data have higher CVs but confirm that an estimate of the order of 100,000 t is reasonable. The 1987-89 trawl survey series gave the lowest virgin biomass estimate. It was not considered reli- able because there were only three indices, and high Table 3 Distribution of commercial catch (% of total catch taken tn the two areas) and effort (% of number of tows) for orange roughy (Hoplostethus atlan- ticus) for the Central Flat and Pinnacles ( winter period June to August). Year Central Flat Pinnae les % catch % tows % catch % tows 1982 97.2 95.6 2.8 4.4 1983 97.0 94.7 3.0 5.3 1984 95.2 93.6 4.8 6.4 1985 87.7 78.0 12.3 22.0 1986 87.3 83.2 12.7 16.8 1987 84.4 77.3 15.6 22.7 1988 52.9 56.0 47.1 44.0 1989 34.0 43.3 66.0 56.7 1990 30.2 45.0 69.8 55.0 Biomass indices tethus atlanticus) from 1987 to 1989 Table 4 (t) of orange roughy (Hoplos- from trawl surveys, conducted . (CV = coefficient of variation.) Year Biomass (t) CV 1987 1988 1989 78,661 30,946 11,746 26 27 11 fishing mortality rates were required to support the catch history. A maximum F of 1.0 is regarded as realistic for orange roughy (Francis et al., 1992). This constrains the virgin biomass to a minimum value of 94,000 t. The estimate from non-winter CPUE is compara- tively high. It has a large CV and is based on rela- tively low numbers of trawls (because most fishing effort is in winter). Such a biomass level would also Comparison of biomass estimates (t) o] 'orange Table 5 roughy (Hopl ostethus atlanticus) by region from 1987 to 1990. 1987 1988 1989 1990 Region Biomass (t) '; Biomass (t) ', Biomass (tl ', Biomass 0.85 mm) based on the morphological criteria of Joseph et al. ( 1964). Rear- ing chambers were returned to the laboratory and held for 3 to 14 days. In these, larvae were periodically sac- 256 Fishery Bulletin 92(2). 1994 rificed and preserved in 5-8% buffered formalin. Iden- tifications of preserved sciaenid larvae from pigment characters were based on Ditty (1989). Sciaenid eggs collected in the same area during spring 1991, 1992, and 1993 were sorted from fresh plankton samples. To avoid contamination by the morphologically similar eggs of the cynoglossid Symphurus plagiusa and the soleid Trinectes macula- tus that contain several oil globules and are abundant in lower Chesapeake Bay during the spring, all eggs with >3 oil globules were omitted from the samples. Although eggs of most spring-spawning sciaenids gen- erally possess three or fewer oil globules (usually two) those of Menticirrhus saxatilis may contain from 1 to 16 oil globules (Johnson, 1978). After sorting, eggs were measured, placed in scintillation vials with 26 ppt seawater, and frozen at -70°C for genetic analy- sis. Individual eggs were thawed and remeasured prior to homogenization to assess shrinkage. Sciaenid eggs were genetically typed by compar- ing mtDNA restriction fragment patterns of indi- vidual eggs with those of known adults. To obtain patterns of known adults, mature female sciaenids (B.chrysoura, C. nebulosus, C. regalis, M. saxatilis, and P. cromis) were collected by pound net, trawls, and hook and line in April and May 1990 and 1991. Ovarian tissue was excised and frozen at -70°C. MtDNA was purified from ovarian tissue by cesium chloride equilibrium density gradient ultracentrifu- gation following the protocols of Lansman et al. (1981). To determine a restriction enzyme that un- ambiguously identified the different sciaenid spe- cies, aliquots of mtDNA were individually digested with the following restriction enzymes: Apal, Aval, Banl, Banll, Hindlll used according to manu- facturer's instructions. The resulting fragments were separated electrophoretically on 1.0% agarose mini-gels run at 5 V/cm for four hours and visual- ized with ethidium bromide. MtDNA-enriched genomic DNA was isolated from individual eggs following the protocols of Graves et al. (1990). Entire DNA samples were digested with a single discriminating restriction endonuclease, separated electrophoretically, and transferred to a nylon filter (Southern transfer) following standard protocols (Sambrook et al., 1989). Filters were hy- bridized with highly purified black drum mtDNA, nick-translated with biotin-7-dATP, washed, blocked and visualized following the methods of Graves et al. (1990). Results A total of 10,803 sciaenid eggs was sorted from samples collected in 1990 and 1991. Outside egg diameter of all specimens ranged from 0.650 to 1.12 mm. Successive blind readings of samples of 75 to 100 eggs were used to assess measurement error. No differences were found in the size-frequency distri- butions indicating good agreement within the 0.025- mm size classes (two-sample <-test, P<0.05, n=79). Qualitative analysis of culture experiments using the two egg types of Joseph et al. (1964) revealed the presence of three species. Cultures containing eggs designated Type I (<0.80 mm) resulted in lar- vae of B. chrysoura, whereas cultures of eggs desig- nated Type II (>0.85 mm) resulted in larvae of C. regalis and P. cromis. Analysis of preserved ichthyoplankton samples from 1990 and 1991 revealed the presence of larvae of B. chrysoura, C. regalis, and P. cromis. No early life history stages of other sciaenids were identified; however, yolk-sac larvae could not be identified to species. Because rearing studies and analysis of field-caught plankton samples revealed the presence of more than two species, we could not rely on the criteria of Joseph et al. (1964) for specific identifi- cation. We therefore examined weekly frequency of occurrence of all sciaenid eggs during 1990 (Fig. 1) and 1991 (Fig. 2). Based on temporal occurrence and size frequency we identified three modes. The larg- est eggs (>0.975 mm), Type C, were most abundant during the period 23 April through 9 May. Type-C eggs declined in abundance throughout May in both years. Mid-sized eggs (0.850-0.950 mm), designated Type B, generally appeared later than Types A and C. Type-B eggs did not exceed 5% of the total fre- quency of sciaenid eggs until 15 May 1990 and 9 May 1991. Type-B eggs increased in abundance from mid-May until the end of sampling. The smallest eggs (<0.850 mm), designated Type A, co-occurred with Type-C eggs; however, they did not exceed 5% of the total sciaenid eggs until 8 May 1990 and 9 May 1991. In 1990, Type-A eggs peaked in abun- dance on 15 May and gradually declined through- out the sampling period. In 1991, Type-A eggs were most abundant during the last sample on 28 May. To test the hypothesis that eggs designated Types A, B, and C were separate species assemblages, the mtDNA restriction fragment patterns of known adult sciaenids were compared with those of fresh egg samples separated into Types A, B, and C. Re- striction fragment length polymorphism analysis of mtDNA, purified from adult B. chrysoura, C. nebulosus, C. regalis, Menticirrhus saxatilis, and P. cromis, revealed species-specific restriction fragment patterns for each of the five enzymes. Of the five en- zymes, Hindlll showed the greatest differences be- tween species, facilitating visualization with the Southern blotting procedure (Table 2). Daniel and Graves: Morphometry and genetic identification of sciaenid eggs 257 > o z LU o LU cc LU o a: LU a. 23 April 1990 60 15 May 1990 20 I I Type A Type B M Type C 25 20 15 10 0.65 0.75 85 95 1 05 115 -W III 0.65 0.75 085 0.95 1.05 1.15 1 May 1990 24 May 1990 30 20 -, f , p , Jl fi , , , i \ \ * -B-l -B — 10 0.65 75 85 0.95 8 May 1990 I HI , II p II p . ,l , i, J . l l Jp , 0.65 75 85 95 1 05 1 15 ?0 10 n „n ■lit] r ll 65 75 85 95 1.05 1.15 31 May 1990 .-JL JL Jl f" J_ WU 0.65 0.75 0.85 95 105 1 15 OUTSIDE EGG DIAMETER (mm) Figure 1 Frequency distributions of outside egg diameters of sciaenid eggs collected over six weeks during spring 1990 in lower Chesapeake Bay. mm and larger (n=32), all pos- sessed the restriction fragment pattern diagnostic for P. cromis. Discussion A total of 62 eggs, representing all sciaenid egg size classes collected in lower Chesapeake Bay, was identified with diagnostic Hi n dill restriction frag- ment patterns. Bairdiella chrysoura, C. regalis, and P. cromis were the only species of sciaenids identi- fied; no other restriction fragment patterns were observed. Genetic identification of eggs designated Type A (<0.850 mm, n=12) resulted in 11 individu- als of B. chrysoura and one specimen (0.825-mm OED size class) of C. regalis (Fig. 3). Cynoscion regalis composed the majority of type-B eggs (0.850- 0.975 mm, rc = 18) analyzed, but seven of the 10 larg- est type-B eggs (0.975-mm OED size class) were identified as black drum. Type-C eggs, those 1.00 Identifications of eggs of sci- aenids are often based on pub- lished diameter distributions or hatching experiments, or both. Results of hatching experiments and genetic analysis in this study indicate that samples of eggs of a single size class may represent the products of two or more species. For example, eggs designated Type I (<0.80 mm) and identified as silver perch by Joseph et al. (1964) were shown with genetic analysis to contain eggs of both weakfish and silver perch. Similarly, eggs designated Type II (>0.85 mm) and identi- fied as black drum by Joseph et al. (1964) were shown with rear- ing and genetic analysis to con- tain eggs of both weakfish and black drum. During the present study, nei- ther hatching experiments nor genetic analysis identified eggs as black drum that were smaller than 0.975 mm OED. While tem- porally limited, the results of this study suggest that the range in size for eggs of black drum (0.975-1.125 mm) in lower Chesapeake Bay may be more restricted than those previously reported. The ranges of egg diameter overlapped for silver perch and weakfish. Eggs genetically identified as silver perch ranged in size from 0.650 to 0.825 mm, in agreement with previously reported size ranges for silver perch in the northwestern Gulf of Mexico (0.59-0.82 mm, Holt et al., 1988) and Chesapeake Bay (0.625-0.775 mm, Joseph et al., 1964). Although Holt et al. (1988) identified eggs of silver perch as small as 0.590 mm, no sciaenid eggs smaller than 0.650 mm OED were collected in the present study. Sizes of eggs genetically identified as weakfish were found to range from 0.825 to 0.975 mm in diameter. These values are comparable with those reported by Wisner ( 1965, 0.84-0.96 mm) but are narrower than 258 Fishery Bulletin 92(2), 1994 30 20 10 I I Type A TypeB W TypeC o z LU o cc LU o a: LU Q. 20 15 the range (0.68-1.18 mm) given by Merriman and Sclar (1952) for Block Island Sound, New York. While the range in sizes for silver perch and weakfish re- ported in this study agree with past research, overlaps in these ranges preclude the sole use of egg size for identification. Neither Joseph et al. (1964), Olney (1983), nor the present study identified eggs of C. nebu- losus or M. saxatilis in samples collected in lower Chesapeake Bay. Fable et al. (1978) described laboratory-spawned eggs of C. nebulosus from a single female and reported a mean diameter of 0.77 mm (range 0.70-0.85 mm). Although based upon a limited sample size, Fable et al.'s data indicate that eggs of C. nebu- losus could be confused with eggs of B. chrysoura; however, no eggs in our limited sample of this size range (n=12) were ge- netically identified as C. nebu- losus. A possible explanation for the lack of eggs of C. nebulosus in the present study may be the tendency for adults to spawn in or around vegetated areas (Brown, 1981). The absence of eggs of Menticirrhus spp. in this genetic analysis may be ex- plained by our exclusion of eggs with greater than three oil glob- ules. Additionally, Menticirrhus saxatilis reportedly spawns off front beaches and possibly off- shore (deSylva et al., 1962); con- sequently, circulation in the bay may prevent eggs of this species from entering the survey area or they may be transported to areas that were not sampled in our study. The identification of species-specific restriction fragment patterns for spring-spawning sciaenids is based on the assumption that there is limited in- traspecific variation of the diagnostic restriction fragment patterns. Recent studies of the population genetics of spotted seatrout, black drum, and weak- fish (Graves et al., 1992; Gold et al., 1993) indicate that these species exhibit low intraspecific mtDNA variability. Furthermore, no variation of the Hi n dill fragment pattern was found in a survey of mtDNA 23 APR 1991 19 MAY 1991 20 10 I I n# II n I, IWr- 65 0.75 0.85 0.95 1.05 1.15 29 APR 1991 0.65 75 085 95 1.05 1.15 22 MAY 1991 30 25 10 JjlLl. I }l, .1^1.. 65 75 85 95 1 05 1 15 9 May 1991 065 0.75 085 0.95 1 05 1.15 28 MAY 1991 n l || , PP Ulfl.U, ll I -I 20 15 I I p I :, ,<}■,', =m^h a- 65 75 85 95 1.05 1.15 065 0.75 0.85 0.95 1.05 1.15 OUTSIDE EGG DIAMETER (mm) Figure 2 Frequency distributions of outside egg diameters of sciaenid eggs collected over six weeks during spring 1991 in lower Chesapeake Bay. isolated from 25 adult B. chrysoura (L. Daniel, unpubl. data). Consequently, the common restriction fragment patterns used to distinguish species in this study were deemed suitable for use in identifications. Variability in egg-size distributions with changing salinity and over the spawning season were not exam- ined in this study. Consequently, exact size groupings may only be applicable to the particular salinity re- gime (19-25 ppt) that we sampled. However, samples were taken throughout peak spawning for black drum and silver perch and may encompass the ranges that occur for these species in lower Chesapeake Bay. Results of our genetic analysis suggest that iden- tifications of eggs of spring-spawning Sciaenidae in Daniel and Graves: Morphometry and genetic identification of sciaenid eggs 259 Table 2 Common fragment sizes produced by restriction endonuclease {Hindlll) digestion of mtDNA purified from ovarian tissue of spring spawning sciaenids. Species Fragment sizes (Kbi Bairdiella chrysoura 5.0 Cynoscion nebulosus 8.5 Cynoscion regalis 5.6 Menticirrhus saxatilis 5.4 Pogonias cromis 3.3 3.9 4.5 4.3 3 2 2.9 2.8 1.9 3.8' 4.1 2.9 2.4 2.0 2.7 2.5 1.7 1.9 2.1 1.7 1.3 1.8 1.3 1.0 ' J. Gold, Texas A&M, College Station, TX, pers. commun. 1993. 07 0.7250750775 08 08250.850875 09 09250950975 1 1025105107511 1125 Outside Egg Diameter (mm) B. chrysoura | | C. regalis P. cromis Figure 3 Size distributions of all eggs morphologically typed as sciaenids and identi- fied using genetic techniques. wise, measures of spawning stock biomass will be similarly over-es- timated, results that could signifi- cantly impact management deci- sions. Comparable biases in esti- mates of egg production and spawning stock biomass of weak- fish could result from egg mis- identifications. However, the more protracted spawning season and greater area of spawning for weakfish in Chesapeake Bay (Olney, 1983) would make these impacts much less severe. Biochemical techniques are an important tool for the fur- ther study of eggs of sciae- nids. Genetic analysis has the potential to produce reliable results and permit the stor- age of samples for later analy- sis. Additional studies are needed to survey genetic identifications over the entire spawning season and area to determine if egg sizes change over time or are influenced by seasonal changes in hydrog- raphy or by age structure of the spawning stock. Finally, the use of genetic techniques, coupled with an extensive ex- amination of morphology could lead to the delineation of other characters that may be useful in separating the eggs of these species. Acknowledgments lower Chesapeake Bay based on OED are subject to error. These findings are particularly timely in light of the increased use of fishery-independent assess- ments of stock size that require precise estimates of egg abundance (egg production method). Because eggs of black drum and weakfish are spatio-tempo- rally coincident and OEDs overlap, estimates of egg production by black drum in lower Chesapeake Bay may be over-estimated by 50% or greater if identi- fication criteria are based solely on egg size. Like- J. McGovern, M. Cavaluzzi, J. Field, C. Baldwin, and K. Kavanaugh kindly assisted with sample collection. P. Crewe patiently processed plankton samples at sea and in the laboratory, and J. McDowell provided valuable technical assistance with mtDNA analyses. J. Olney helped in the de- sign and implementation of the sampling program and provided comments on the manuscript. Addi- tional reviews were provided by J. Musick, J. Cowan, and E. Heist. This study was funded in part by the Virginia Marine Resources Commission un- der U.S. Fish and Wildlife Contract F-95-R. 260 Fishery Bulletin 92(2). 1994 Literature cited Brown, N. J. 1981. Reproductive biology and recreational fishery for spotted seatrout, Cynoscion nebulosus, in the Chesapeake Bay area. M.A. thesis, College of William and Mary, Williamsburg, 119 p. Comyns, B. H., J. Lyczkowski-Shultz, D. L. Nieland, and C. A. Wilson. 1991. Reproduction of red drum, Sciaenops ocellatus, in the northcentral Gulf of Mexico: sea- sonality and spawner biomass. In R. D. Hoyt (ed.), Larval fish recruitment and research in the Americas: proceedings of the thirteenth annual larval fish conference, p. 17-26. Dep. Commer., NOAA Tech. Rep. NMFS 95. Cowan, J. H., R. S. Birdsong, E. D. Houde, J. S Priest, W. C. Sharp, G. B. Mateja. 1992. Enclosure experiments on survival and growth of black drum eggs and larvae in lower Chesapeake Bay. Estuaries 15(3):392-402. deSylva, D. P., F. A. Kalber Jr., and C. N. Shuster. 1962. Fishes and ecological conditions in the shore zone of the Delaware River estuary, with notes on other species collected in deeper water. Univ. Del. Mar. Lab. Inf. Ser. Publ. 51, 164 p. Ditty, J. G. 1989. Separating early larvae of sciaenids from the western North Atlantic: a review and comparison of larvae off Louisiana and Atlantic coast of the U.S. Bull. Mar. Sci. 44(3):1083-1105. Fable, W. A., Jr., T. D. Williams, and C. R. Arnold. 1978. Description of reared eggs and young larvae of the spotted seatrout Cynoscion nebulosus. Fish. Bull. 76 (l):65-72. Gold, J. R., L. R. Richardson, C. Furman, and T. L. King. 1993. Mitochondrial DNA differentiation and popu- lation structure in red drum (Sciaenops ocellatus) from the Gulf of Mexico and Atlantic Ocean. Mar. Biol. 116(2):175-185. Graves, J. E., M. A. Simovich, and K. M. Schaefer. 1988. Electrophoretic identification of early juve- nile yellowfin tuna, Thunnus albacares. Fish. Bull. 86(4):835-838. Graves, J. E., M. J. Curtis, P. A. Oeth, and R. S. Waples. 1990. Biochemical genetics of southern California basses of the genus Paralabrax: specific identifi- cation of fresh and ethanol-preserved eggs and early larvae. Fish. Bull. 88:59- 66. Graves, J. E., J. R. McDowell, and M. L. Jones. 1992. A genetic analysis of weakfish, Cynoscion regalis, stock structure along the mid-Atlantic coast. Fish. Bull. 90:469-475. Holt, G. J., S. A. Holt, and C. R. Arnold. 1985. Diel periodicity of spawning in sciaenids Mar. Ecol. Prog. Ser. 27:1-7. Holt, S. A., G. J Holt, and L. Young-Abel. 1988. A procedure for identifying sciaenid eggs. Contr. Mar. Sci. 30:99-108. Johnson, G. D. 1978. Development of fishes of the mid-Atlantic bight. Vol. rV: Carangidae through Ephippidae. Fish Wildl. Serv., U.S. Dep. Interior, 314 p. Joseph, E. B., W. H. Massman, and J. J. Norcross. 1964. The pelagic eggs and early larval stages of the black drum from Chesapeake Bay. Copeia 1964:425-434. Lansman, R. A., J. C. Avise, C. F. Aquadro, J. F. Shapira, and S. W. Daniel. 1981. The use of restriction endonucleases to mea- sure mtDNA sequence relatedness in natural populations. Ill: techniques and potential appli- cations. J. Mol. Evol. 17:214-226. Lippson, A. J., and R. L. Moran. 1974. Manual for identification of early develop- mental stages of fishes of the Potomac River estuary. Maryland Dep. Natl. Resources, Power Plant Sitting Prog. Rep. PPSP-MP-13, 282 p. Merriman, D. and R. C. Sclar. 1952. The pelagic fish eggs and larvae of Block Is- land Sound. Bull. Bingham. Oceanogr. Collect. Yale Univ. 13(3): 156-2 19. Morgan, R. P. 1975. Distinguishing larval white perch and striped bass by electrophoresis. Chesapeake Sci. 16:68-70. Olney, J. E. 1983. Eggs and early larvae of the bay anchovy, Anchoa mitchilli, and the weakfish, Cynoscion regalis, in lower Chesapeake Bay with notes on associated ichthyoplankton. Estuaries 6(l):20-35. Olney, J. E., and E. D. Houde. 1993. Evaluation and use of in situ silhouette pho- tography in studies of estuarine zooplankton. Bull. Mar. Sci. 52(2):845-872. Olson, R. R., J. A. Runstadler, and T. D. Kocher. 1991. Whose larvae? Nature 351:357-358. Pearson, J. G. 1929. Natural history and conservation of the red drum and other commercial sciaenids on the Texas coast. U.S. Bur. Fish. Bull. 44:129-214. Sambrook, J., E. F. Fritsch, and T. Maniatis. 1989. Molecular cloning: a laboratory manual, 2nd ed. Cold Spring Harbor Laboratory Press, NY. Saucier, M. H., and D. M. Baltz. 1992. Spawning site selection by spotted seatrout, Cynoscion nebulosus, and black drum, Pogonias cromis, in Louisiana. Environ. Biol. Fishes 36:257-272. Saucier, M. H., D. M. Baltz, and W. A. Roumillat. 1992. Hydrophone identification of spawning sites of spotted seatrout, Cynoscion nebulosus (Osteichthys: Sciaenidae) near Charleston, South Carolina. N.E. Gulf Sci. 12(2):141-145. Silberman, J. D., and P. J. Walsh. 1992. Species identification of spiny lobster phyllosome larvae via ribosomal DNA analysis. Mol. Mar. Biol. Biotech. l(3):195-205. Daniel and Graves: Morphometry and genetic identification of sciaenid eggs 261 Smith, P. J., and P. G. Benson. 1980. Electrophoretic identification of larval and 0- group flounders (Rhombosolea spp.) from Wellington Harbor, N.Z. J. Mar. Freshwater Res. 14:401^104. Welsh, W. W, and C. M. Breeder Jr. 1923. Contribution to life histories of Sciaenidae of the eastern United States coast. Bull. 39:141-201. U.S. Bur. Fish. Wisner, B. W. 1965. McClane's standard fishing encyclopedia. Holt, Rhinehart and Winston, Inc., New York, NY, 1057 p. Abstract. — The Atlantic spa- defish (Chaetodipterus faber) is the only member of the family Ephippidae in the western Atlan- tic Ocean and its life history is poorly understood. We redescribe Atlantic spadefish larvae, discuss their relationship to known larvae of other ephippid genera, and dis- cuss the distribution, abundance, and seasonal occurrence of Atlan- tic spadefish in the northern Gulf of Mexico. Larval Atlantic spade- fish are characterized by a small, peak-like, median supraoccipital crest with a single, dorsally di- rected spine; large preopercle spines, numerous serrate ridges, and other spines on the head; a deep, robust body which becomes laterally compressed; heavy body pigmentation; and early develop- ment of specialized spinous scales or "prescales" (at about 5.5-mm standard length [SL]). Transition to juvenile stage begins about 8.0- 8.5 mm SL. Developmental mor- phology and head spination of At- lantic spadefish is similar to that of Pacific spadefish, Chaetodipter- us zonatus. Sequence of fin com- pletion is pelvics — dorsal and anal soft rays — dorsal spines- pectorals. Overall, >85% of Atlan- tic spadefish larvae were found in waters >28.0°C and between 26.7 and 31.3 ppt. Larvae occur prima- rily in coastal waters, except near the Mississippi River delta, an area with a narrow shelf and rap- idly increasing water depths. Delta waters may offer additional habitat suitable to Atlantic spade- fish larvae because of lower salini- ties. Larvae are primarily collected between June and August and in the north-central Gulf of Mexico. Larval Atlantic spadefish are ap- parently rare in the eastern Gulf off Florida. Catch rates near the Mississippi River delta during August were higher than else- where in the north-central Gulf and suggest a possible association with riverine frontal areas which requires further study. A re-description of Atlantic spadefish larvae, Chaetodipterus faber (family: Ephippidae), and their distribution, abundance, and seasonal occurrence in the northern Gulf of Mexico James G. Ditty Richard F. Shaw Joseph S. Cope Coastal Fisheries Institute, Center for Coastal, Energy, and Environmental Resources Louisiana State University, Baton Rouge, LA 70803 The percoid family Ephippidae is usually considered to comprise five genera and 17 species (Nelson, 1984). The Atlantic spadefish (Chae- todipterus faber) is the only mem- ber of this family in the western Atlantic Ocean. Rare north of Chesapeake Bay, Atlantic spadefish inhabit coastal waters which ex- tend southward to Brazil (Johnson, 1978). Historically, Atlantic spade- fish represented a relatively minor portion of recreational fisheries. Nevertheless, fishing tournaments are currently being used to stimu- late interest in their fisheries (Schmied and Burgess, 1987). Ryder (1887) described eggs and yolk-sac larvae of Atlantic spade- fish, but Johnson (1978) questioned the identity of these specimens. Larvae >2.5 mm standard length (SL) are described and illustrated by Hildebrand and Cable (1938), but this study is insufficient to ex- amine important developmental details and is based on the static rather than dynamic approach to larval description (Berry and Richards, 1973). Finucane et al. 1 illustrated 5.1- and 6.4-mm SL At- lantic spadefish. Johnson (1984) commented on cranial morphology and provided insight on the value of larval characters in resolving the relations among ephippids and their relation to other families. Aspects of juvenile and adult life history are discussed for Atlantic spadefish from South Carolina wa- ters (Hayse, 1990), but the distri- bution, abundance, and seasonal occurrence of Atlantic spadefish larvae are poorly understood. Our objectives are to redescribe the de- velopment of Atlantic spadefish lar- vae, discuss their relation to known larvae of other ephippid genera, and to describe the distribution, abundance, and seasonal occurrence of Atlantic spadefish larvae in the northern Gulf of Mexico (Gulf). Materials and methods The distribution, abundance, and seasonal occurrence of larval Atian- Finucane. J. H., L. A. Collins, L. E. Barger, and J. D. McEachran. 1979. Ichthyoplankton/mackerel eggs and lar- vae. Environmental studies of the south Texas outer continental shelf, 1977. Final Rep. to Bur. Land Manage., Wash., DC, Southeast Fish. Cent., Natl. Mar. Fish. Serv. NOAA, Galveston, TX 77550, 504 p. Manuscript accepted 19 August 1993 Fishery Bulletin 92:262-274 (1994) Contribution No. LSU-CFI-92-7 of Louisiana State University Coastal Fisheries Institute. 262 Ditty et al.: A redescription of Chaetodipterus faber larvae 263 tic spadefish were determined from collections taken primarily during Southeast Area Monitoring and Assessment Program (SEAMAP) ichthyoplankton surveys of the Gulf between 1982 and 1986 (SEAMAP 2 ). These years represent the first time interval for which a complete set of data were cur- rently available. Latitude 24°30' N was the south- ern boundary of our study area in the eastern Gulf, a cutoff which approximates the continental shelf break off the southern tip of Florida. Latitude 26°00' N was the southern boundary of the central and western Gulf. These coordinates approximate the U.S. Exclusive Economic Zone (EEZ (/Fishery Con- servation Zone (FCZ). Standard ichthyoplankton survey techniques as outlined by Smith and Richardson (1977) were em- ployed in data collection. Stations sampled by Na- tional Marine Fisheries Service (NMFS) vessels were arranged in a systematic grid of about 55-km intervals. NMFS vessels primarily sampled waters >10 m deep. Each cooperating state had its own sampling grid and primarily sampled their coastal waters. Hauls were continuous and made with a 60- cm bongo net (0.333-mm mesh) towed obliquely from within 5 m of the bottom or from a maximum depth of 200 m. A flowmeter was mounted in the mouth of each net to estimate volume of water fil- tered. Ship speed was about 0.75 m/sec; net retrieval was 20 m/min. At stations <95 m deep, tow retrieval was modified to extend a minimum of 10 minutes in clear water or 5 minutes in turbid water. Tows were made during both day and night depending on when the ship occupied the station. Overall, 1,823 2 SEAMAP. 1983-1987. (plankton). ASCII characters. Data for 1982-1986. Fisheries-independent survey data/National Ma- rine Fisheries Service, Southeast Fisheries Center: Gulf States Marine Fisheries Commission, Ocean Springs, MS (producer). bongo net tows were made between 1982 and 1986. The SEAMAP effort between 1982 and 1984 also involved the collection and processing of 814 neus- ton samples taken with an unmetered 1x2 m net (0.947-mm mesh) towed at the surface for 10 min- utes at each station. SEAMAP sampling during April and May was conducted primarily off the con- tinental shelf; sampling during March, and from June through December, was conducted primarily over the shelf at stations <180 m deep. Additional information on the spatial and temporal coverage of SEAMAP plankton surveys is found in Stuntz et al. (1985), Thompson and Bane (1986, a and b), Thomp- son et al. (1988), and Sanders et al. (1990). Atlantic spadefish larvae were also obtained from surface- towed 1x2 m neuston net collections (0.947-mm mesh, 71 samples) made by the National Marine Fisheries Service (NMFS, Panama City, Florida) during August 1988. These NMFS collections were associated with riverine/oceanic frontal zones off the Mississippi River delta. Frontal zones near the delta were not sampled during either June or July. A detailed examination of Atlantic spadefish lar- vae was made to describe developmental morphol- ogy. Body measurements were made on 21 Atlantic spadefish larvae between 1.9 and 12.5 mm (Table 1) and follow Hubbs and Lagler (1958) and Richardson and Laroche (1979). Measurements were made to the nearest 0.1 mm with an ocular micrometer in a dissecting microscope. We follow Leis and Trnski's (1989) criteria for defining length of preopercular spines, body depth, head length, eye diameter, and the eye diameter/head length ratio. We consider notochord length in preflexion and flexion larvae synonymous with SL in postflexion larvae and re- port all lengths as SL unless otherwise noted. Speci- mens were field-fixed in 10% formalin and later transferred to 70% ethyl alcohol. Terminology for Table 1 Morphometries of larval Atlan surements are expressed as % tic spadefish (Chaetodipterus faber) from the northern Gulf standard length (SL) and rounded to the nearest whole num of M ber sxico. Mea- SL N Preanal length Head length Snout length Orbit diameter Body depth pectoral Prepelvic distance 1.8-2.9 3 42-55 21-31 3-8 13-15 34-44 — 3.0-4.9 3 54-65 35-43 5-7 15-17 50-60 30-36 5.0-6.9 1 60-61 30-42 5-6 15-18 56-63 30-35 7.0-8.9 4 61-64 35-39 7-9 11 55-64 27-37 9.0-10.9 4 59-61 34-35 6-8 13-14 60-65 27-34 11.0-11.9 2 54-56 34-35 7-9 11 60-65 27 12.5 1 60 36 K 1 l 68 36 264 Fishery Bulletin 92|2). 1994 location of head spines followed Gregory (1933). One larva was cleared with trypsin then stained with alizarin in each millimeter (mm) length interval to examine small serrate ridges around the orbit (i.e. circumorbital bones), and spines and ridges on the head. We examined spines on the occipital and fron- tal bones with a scanning electron microscope (SEM), and specialized spinous scales with a com- pound microscope. Fin rays were counted when first segmented and spines when present. Representative specimens were illustrated with the aid of a cam- era lucida. Estimates of larval density (number of larvae/ 100m 3 of water) and catch (number of larvae/10 tow) were calculated by month. Months were combined across years because not all months were sampled every year (Appendix Table). Densities for stations where larvae were collected (i.e. positive catch sta- tions) were calculated by dividing sum of larvae collected in bongo net tows by total positive catch station volume of water filtered (VWF) and multi- plying the result by 100. In addition, an overall (i.e. grand) density estimate was calculated by dividing sum of larvae by total VWF for all stations sampled that month and multiplying the result by 100. Over- all density more closely reflects the density of lar- vae throughout the area by including the total vol- ume of water filtered in calculations. Estimates of larval catch in neuston nets were calculated by di- viding sum of larvae by number of positive catch neuston stations or by total number of neuston sta- tions sampled and multiplying the result by 10. Estimates of larval density and catch included sta- tions at long. >88°00' W because only one Atlantic spadefish larva was collected east of Mobile Bay, Alabama. Similarly, estimates were calculated only for June through August because May and Septem- ber had but one positive catch station each. Temperature and salinity data were gathered from the sea surface. Positive catch station hydro- graphic data were multiplied by total number of larvae collected at each station to obtain a monthly median and mean. Hydrographic data were also combined across months to obtain an overall (i.e. grand) median and mean. This method gives weight to distribution of larvae rather than to distribution of stations. We used a percent cumulative frequency of >85%> for defining the relation between distribu- tion of Atlantic spadefish larvae and water tempera- ture, salinity, and station depth. Percent frequency indicates the range of hydrographic conditions most often associated with occurrences of larvae. Proc Univariate was used to calculate median, mean, and percent cumulative frequency statistics (SAS Insti- tute, 1985). Results Morphometries and pigmentation Early larvae were rotund and deep-bodied; body depth was >50% SL by 3.5 mm and >60% by 9 mm (Table 1). Atlantic spadefish became increasingly deep-bodied and laterally compressed after noto- chord flexion. There were 24 myomeres but these became obscured by pigment in postflexion larvae. The head was large and averaged about 35% SL in larvae >3.0 mm. Head profile became steep and in- creasingly deeper than long. The mouth was termi- nal and the upper jaw reached to about mid-eye. Eyes were round and large, ranging from 36 to 43% of head length in larvae >3.5 mm (i.e. about 14-15% SL). The gut was tightly coiled in a single loop and the anus was slightly beyond mid-body (usually 55-60% SL). Pigment was largely restricted to the anterior-half of the body in early preflexion larvae of Atlantic spadefish. On the head of a 1.8-mm larva, external pigment was scattered over the mid- and hindbrain, nape, opercle, branchiostegal membrane, and along the isthmus and quadrate. Internally, pigment was present along and above the anterior portion of the notochord, and a single median patch was observed on the roof of the mouth. On the abdomen, there was a patch of pigment on the visceral mass immediately anterior to and below the pectoral-fin base. In ad- dition, melanophores were scattered over the pecto- ral fin base and its finfold and were distributed lat- erally over the visceral mass and hindgut. A row of about 20-25 small, closely spaced melanophores were visible along the ventral midline of the tail in early larvae. Number of melanophores along the ventral midline of the tail decreased as larvae grew. Melanophores on the nape, opercle, pectoral-fin base, and visceral mass formed a "swath" of pigment over the anterior 55-60% of the body by 2.5-3.0 mm (Fig. 1). By 3.0—3.5 mm, internal melanophores were visible anteriorly on the forebrain and laterally on the midbrain above the eye. Melanophores were also scattered both internally and externally over the hindbrain both anterior to and posterior to the base of the supraoccipital crest. By early postflexion (i.e. 5.0 mm), the head and abdomen were densely pig- mented but the posterior portion of the body was sparsely pigmented. Pigmentation increased on the posterior-half of the body as larvae grew, and by 10.0 mm the entire body was pigmented (Fig. 1). Consoli- dation of pigment into bands began on the head of Atlantic spadefish larvae with one band visible above the eye by 10.0-11.0 mm. This band of pig- ment was enclosed by indefinite, pale crossbars. The Ditty et al.: A redescription of Chaetodipterus faber larvae 265 anterior pale crossbar was situated above the middle of the eye and the posterior crossbar was behind the eye, extending mid-way down the preopercle. Lar- vae <12.5 mm had only one band of pigment (Fig. 1). The pelvics were the first fins to have pigment. Pelvic fin buds were pigmented by 4.0 mm; the pelvics were densely pigmented thereafter. Pigment Figure 1 Larval development of Atlantic spadefish, Chaetodipterus faber, from the northern Gulf of Mexico. (All. 8 mm; (Bl 3.5 mm; (C) 5.0 mm; (D) 7.0 mm; (E) 11.6 mm. All measurements are standard length (SL). appeared on the pectoral fin along the proximal portion of the rays at about 4.0-4.5 mm. Melano- phores were lightly scattered over the pectoral fin in the largest specimen examined (Fig. 1). Melano- phores were scattered over the membrane covering the anterior-most dorsal spines by about 6.0 mm and the anal spines by about 8.0 mm. Melanophores were added along the dorsal and anal fins as larvae developed, cov- ering the proximal-third of each soft ray in the largest specimen examined. Pigment was present along the proximal portion of the central rays of the caudal fin by 11.0 mm (Fig. 1). Head and body spination Atlantic spadefish larvae develop two series of preopercular spines, one along the posterior margin of the outer shelf and the other along the inner shelf. Both the outer and inner shelf have dorsal and ventral limbs. Three pre- opercular spines were present along the outer shelf of a 1.8-mm larva, the largest of which was present at its preopercular angle (Fig. 1). A fourth and a fifth spine were added by 3.5 mm, one dor- sal and one ventral to the angle of the preopercle. A sixth preop- ercular spine, smaller than the others and often difficult to locate, was present by 5.0 mm. This sixth spine was the anterior-most spine along the ventral limb of the ex- terior shelf and was resorbed by 11.0-12.0 mm in some specimens. One larva we examined had seven preopercular spines along the outer shelf but most had two spines along the dorsal limb, one at the angle, and three along the ventral limb (Fig. 2). Spines along the outer shelf were simple. Two to three spines were also present along the inner shelf of the preopercle by 3.5 mm. Number of spines along the inner shelf in- creased as larvae grew, resulting in a serrate margin (Fig. 2). A small, poorly developed opercle 266 Fishery Bulletin 92|2). 1994 spine was forming by 5.0 mm and was difficult to locate on larvae not cleared and stained. A spine also was present along the poste- rior margin of the interopercle near its junction with the subopercle by 6.0 mm (Fig. 2). The interopercular spine often was hidden by the large spine at the preopercular angle but was more easily located as the preopercular angle spine regressed. Atlantic spadefish larvae have numerous spines and ridges scat- tered over the head. A thickened ridge was visible dorsally along the supraoccipital of 2.0-mm lar- vae. This thickened ridge became a small, peak-like, median supra- occipital crest with a single, dor- sally directed spine by 2.5 mm. The supraoccipital spine began to regress by 5.0 mm and was re- sorbed by 10.0-10.5 mm. A su- praorbital ridge was present by 3.5 mm. This ridge became ser- rate by 4.0 mm. Small serrate ridges were visible along the dor- sal margin of both the lacrimal and jugal bones (i.e. first and sec- ond suborbitals; Gregory, 1933) and third suborbital bone by 5.0 mm. Spines or spinous ridges were also visible along the fourth and fifth suborbitals, dermo- sphenotic (i.e. sixth suborbital), posttemporal, pterotic, tabular, and supracleithral bones by 6.0 mm. The ventral margin of the jugal bone near the posterior margin of the maxil- lary had a single, ventrally directed spine by 7.0 mm (Fig. 2). Individual spines were also scattered over the frontal and occipital bones of young Atlantic spadefish. The bases of these spines were covered by integument so that only a portion of each spine was visible (Fig. 3). All head spines and spinous ridges were present in the largest specimen exam- ined (12.5 mm) but were difficult to locate on lar- vae not cleared and stained because of heavy body pigment. Teeth in Atlantic spadefish were placed in an in- ner and outer band. Teeth first appeared in a single band on the premaxillary and anteriorly on the dentary at about 2.5 mm. Teeth were pointed and closely spaced. A second band of teeth formed along Figure 1 (Continued) the upper and lower jaws by 4.0 mm; the outer band was slightly larger than the inner band. Teeth were added along the upper and lower jaws as larvae de- veloped (Figs. 1 and 2). Specialized spinous scales or "pre-scales" began to develop at about 5.5 mm. Pre-scales were character- ized by a single, elevated, posteriorly directed spine that was positioned near the center of the scale. Pre- scales developed first on the head and later ap- peared anteriorly along the lateral midline. Pre- scales were added outward toward the dorsal and ventral midlines and proceeded in a posterior direc- tion, covering the body by 10.0 mm. The first bones to ossify were the preopercular spines, supraoccipital crest, premaxillary, dentary, and cleithrum. Three predorsal bones (i.e. supra- Ditty et al.: A redescription of Chaetodipterus faber larvae 267 SUPRAOCCIRTAL SUPRAORBITAL POSTTEMPORAL TABULAR SUPRACLEITHRAL OPERCLE CIRCUMORBITALS INTEROPERCLE INNER PREOPERCLE OUTER PREOPERCLE Figure 2 Location of head spines on a 7.0-mm SL larva of Atlantic spadefish, Chaetodipterus faber, from the northern Gulf of Mexico. neurals) were ossifying by 6.0 mm. The anteriormost precaudal vertebrae and dorsal- and anal-fin pterygiophores ossified first; ossification proceeded posteriorly. All caudal bones were ossifying by 8.0 mm. Six branchiostegal rays and 10+14 vertebrae were present in all cleared and stained specimens. Fin development A continuous median finfold extended around the body from the nape to the anus of early larvae. Fin ray anlagen began forming obliquely downward in the caudal finfold during flexion (usually 3.5-4.5 mm). Caudal-fin ray development proceeded out- ward from mid-base as the hypural complex shifted to a terminal position, with the adult complement of 9+8 principal rays attained at about 6.0 mm (Table 2). Development of the dorsal- and anal-fin bases coincided with notochord flexion. Both fin bases and their ray anlagen began to differentiate near mid-fin; development proceeded outward from mid-fin. All dorsal and anal soft rays were present by about 7.0 mm. Soft dorsal and anal fin ray complements were present before their spines (Table 2); dorsal and anal spines developed in an anterior Figure 3 Scanning electromicrograph of the frontal and occipital spines of a 7.0-mm SL Atlantic spade- fish, Chaetodipterus faber. Epithelium was partially digested with trypsin to enhance visibility of frontal and occipital spines. Magnification: 140x. 268 Fishery Bulletin 92(2). 1994 to posterior direction. Pelvic fins were precocious and heavily pig- mented. Pelvic buds were visible by 4.0 mm; pelvics had a full complement of elements (I, 5) by 6.0 mm. Pectoral rays began to develop by 5.0 mm and a full complement (17) was present by 8.0 mm. Sequence of fin comple- tion was pelvics - soft dorsal and anal rays - dorsal spines - pecto- rals. A full complement of elements in all fins by 8.0-8.5 mm marked the beginning of transition to the juvenile stage (Table 2). Table 2 Fin-ray counts of larval Atlantic spade fish (Chaetodipterus faber) from the northern Gulf of Mexico Length (mm SD' Dorsal Anal Pectoral Pelvic Caudal 4.3 III, Anlagen 8 Anlagen Anlagen 0-7+7-0 5.0 III, 14 11 7 4 0-6+6-0 6.1 VII, 24 II, 17 13 I, 5 3-9+8-3 7.0 VII, 23 II, 18 1(1 I, 5 4-9+8-5 8.3 IX, 21 III, 17 17 I, 5 4-9+8-4 9.3 IX, 21 III, 18 17 I, 5 5-9+8-4 10.0 VIII, 23 III, 18 17 I, 5 5-9+8-5 ; One larva of each length. Temporal and spatial distribution Alantic spadefish larvae were col- lected from May through Septem- ber primarily in the north-central Gulf. Larvae were usually col- lected between June and August, density being highest during June and catch highest during August (Table 3). Larval Atlantic spade- fish were especially abundant near the Mississippi River delta during August 1988, when 19 of 72 neuston tows (26%) associated with riverine frontal zones col- lected larvae. During August 1984, however, <5% of neuston tows (rc = 162) from other areas of the north-central and western Gulf not associated with the delta captured larvae. Only one Atlan- tic spadefish larva was collected east of Mobile Bay, Alabama (long. 88°00' W). This 4.0-mm specimen was found off Apalachicola Bay (Florida) during August 1984 at a station 13 m deep (Fig. 4). Salin- ity at this station (34.2 ppt) was the highest re- corded with a positive catch during the study. The largest specimen collected in surface-towed nets was 12.5 mm; this observation may indicate that larvae move out of surface waters by this size. Overall, >85% of Atlantic spadefish larvae were collected in surface waters >28.0°C (median: 28.TC, mean: 28.7°C, range: 25.0°-32.2 <, C), at salinities be- tween 26.7 and 31.3 ppt (median: 28.8 ppt, mean: 28.4 ppt, range: 11.8-34.2 ppt), and at station depths <238 m (median: 83 m, mean: 139 m, range: 9-470 m) Table 3 Density (number of larvae/100 m 3 ) and catch (number of larvae/10 neus- ton tows) of Atlantic spadefish larvae (Chaetodipterus faber) from the northern Gulf of Mexico. Months are combined across years (1982-1986, and August 1988). Not all months were sampled each year. Numbers in parentheses are positive catch stations over total stations sampled by month. Monthly density estimates were calculated by dividing sum of larvae by either sum of volume water filtered (VWF) overall, or sum of positive station VWF. Monthly catch estimates were calculated by di- viding sum of larvae by number of stations sampled overall or by num- ber of positive catch stations. Gear June July August Bongo Overall density Positive density Neuston Overall catch Positive catch 0.3' 6.2 (19/341) 4.0 42.6 (19/201) <0.l 2 - 3 1.3 (4/134) 0.4 13.3 (3/92) <0.\ 4S 1.5 (4/221) 17.0 131.6 (32/248) ' Total VWF - 43,730 m 3 , positive catch station VWF - 1,799 m 3 , number of larvae col- lected was 111. 2 0.02/100 m 3 . 3 Total VWF - 22,207 m 3 , positive catch station VWF - 381 m 3 , number of larvae collected was 5. 4 0.03/100 m 3 . 5 Total VWF - 35,174 m 3 , positive catch station VWF - 796 m 3 . number of larvae collected was 12. (Fig. 5). However, distribution of larvae versus sta- tion depth was strongly influenced by two very large neuston-net collections of 192 and 64 larvae during August 1985 which represented 40% of all larval Atlantic spadefish taken. These two stations were located in waters near the shelf edge, 50 and 75 km east of the Mississippi River delta (28.1°C, 30.1 ppt, 235 m deep; 27.9°C, 28.1 ppt, 238 m deep, respec- tively). Other stations had 27 or fewer larvae. Dis- tribution of larvae versus station depth without the two large collections shifted median station depth Ditty et al.: A redescription of Chaetodipterus faber larvae 269 shoreward from 83 to 26 m; larvae may, therefore, primarily inhabit coastal waters. This shoreward LONGITUDE Figure 4 Distribution of Atlantic spadefish larvae (Chaetodipterus faber) in the northern Gulf of Mexico by month. Months are combined across years ( 1982-1986, and August 1988). Not all months sampled each year. Plus ( + ) signs are total stations sampled and squares are positive catch stations. Distribution of stations are for both bongo and neuston net tows. shift in median station depth was reinforced by dis- tribution of larvae in bongo net tows and by distri- bution of larvae during June and July (Fig. 4, Table 4). About 86% of all Atlantic spade- fish larvae collected in bongo net tows (rc=128) were from waters <25 m deep. In addition, distribution of larvae during June and July was shoreward of that during August. Simi- larly, 51% of all stations where larvae were collected (i.e. 41 of 81) were inside 25 m; 64% were inside 50 m. Only 14% of positive catch stations were located beyond the 100 m isobath; most of these stations were near the Mississippi River delta, an area with a nar- row shelf and rapidly increasing water depths. Discussion Our observations on the morphological devel- opment of Atlantic spadefish larvae generally agree with Hildebrand and Cable (1938). These authors, however, do not discuss pig- ment on the roof of the mouth. The presence of a single, median patch of pigment on the roof of the mouth is helpful in identifying early Atlantic spadefish larvae before the supraoccipital crest is clearly visible. Hildebrand and Cable (1938) do not discuss small spines or ridges along the circumorbital bones (i.e. supraorbital, suborbitals, and dermosphenotic) or tabular bone (Fig. 2) but do illustrate serrate ridges above the eye and in the pterotic region (Hildebrand and Cable, 1938, their Figs. 26 and 27). Spination on the circumorbital bones has generally been found only in those larval percoids with cranial or- namentation (Johnson, 1984). Most of these larval percoids also have other specializa- tions, such as spinous scales and an elongate spine at the angle of the preopercle, among other characters (Johnson, 1984). Neither Hildebrand and Cable (1938) nor Johnson (1984) mention the supracleithral spines we found on Atlantic spadefish larvae (Fig. 2) and in larvae of Pacific spadefish, Chaetodipterus zonatus (Martinez-Pecero et al., 1990). The "short, hair-like spines on the upper surface of the head" noted by Hildebrand and Cable (1938) on 9.0-mm Atlantic spadefish larvae may be the same spines we found scattered over the frontal and occipital bones (Fig. 3). These frontal and occipital spines are difficult to see under a dissecting microscope because 270 Fishery Bulletin 92(2). 1994 • larvae (6) = 123 • larvae (N) - 478 GEAR TYPE Hi BONGO tXS NtuSTON 25 27 26 29 30 31 32 Temperature (°C) 12 IS 16 17 19 20 23 24 25 26 27 2B 29 X 31 32 33 34 Salinity (ppt) Depth (m) Figure 5 Summary of positive catch station hydrographic data for larval Atlantic spadefish (Chaetodipterus faber) from the northern Gulf of Mexico. Percent catch is sum of larvae by interval and gear divided by total number of At- lantic spadefish larvae collected overall. Discrepancies in number of larvae by month among parameters are the result of missing hydrographic data. Table 4 Summary of hydrographic data by month for Atlantic spadefish [Chaetodipterus faber) larvae from the northern Gulf of Mexico. Data are from the surface and for positive catch bongo and neuston net stations only. Station hydrographic data are multiplied by total number of larvae collected at each station to obtain monthly mean and median values.' W is the number of larvae used to obtain mean and median values. Discrepancies in W by month among parameters resulted from missing hydrographic data. Water temperature CO Sal nity (ppt) Station depth (m ) N Mean Median Range N Mean Median Range N Mean Median Range June July August 160 9 433 29.0 29.4 28.1 29.3 29.8 28.6 25.0-30.5 29.3-30.5 27.6-32.2 143 9 433 27.6 27.6 28.8 27.6 27.6 29.4 12.1-33.9 25.4-28.6 11.8-34.2 192 9 433 17.3 27.3 194 16 21 235 9-90 16-70 11-470 This method gives weight to distribution of larvae rather than distribution of stations. they are largely covered by integument. The supraoccipital crest was resorbed by about 10.0-10.5 mm in Gulf larvae but still present on a 11.5-mm specimen from the U. S. Atlantic coast (Hildebrand and Cable, 1938). The identity of Ryder's (1887) yolk-sac Atlantic spadefish larvae is uncertain (Johnson, 1978). Ryder's 3.5-mm and 4.0-mm larvae lack a supra- occipital crest and preopercular spines, both of which Hildebrand and Cable (1938) and we found by 2.5 mm in Atlantic spadefish larvae. Ryder's 4.0-mm larva also has an oil globule in the yolk sac and the gut does not have the single, tightly coiled loop we found in preflexion Atlantic spadefish. Nei- ther Hildebrand and Cable (1938) nor we found an oil globule in Atlantic spadefish larvae of 2.0 mm or Ditty et al.: A redescription of Chaetodipterus faber larvae 271 2.5 mm, respectively. Differences between Ryder's and our study do not support identification of Ryder's larvae <4.0 mm as Atlantic spadefish even if we allow for specimen shrinkage (also noted by Johnson, 1978) and for slower development times due to cooler waters of Chesapeake Bay during the summer when Atlantic spadefish spawn. Johnson (1984) characterized the sequence of fin completion in larval Atlantic spadefish as pattern A: dorsal and anal soft rays - spinous dorsal - pelvics - pectorals. We cleared and stained seven larvae and found the sequence of fin completion more closely resembles Johnson's (1984) pattern F with all ele- ments of the pelvic fin present before dorsal and anal soft rays. This difference in fin completion pat- tern, however, may be due to differences in how we and Johnson interpreted spine formation and fin completion. We counted rays when first segmented and spines when present; Johnson may have counted pterygiophores. Pattern F is found in Hapa- logenys, Monodactylidae, and Pempherididae (Johnson, 1984). Larvae of Atlantic spadefish are characterized by early development of specialized spinous scales or "prescales" (at about 5.5 mm, this study) that even- tually transform into adult ctenoid scales. Spinous larval scales are present to about 15.0 mm (Johnson, 1984). Ctenoid scales are well developed by 18.0 mm (Hildebrand and Cable, 1938). Developmental morphology and head spination of Atlantic spadefish is generally similar to that of Pacific spadefish ( Martinez-Pecero et al., 1990). Both species are deep-bodied (usually 55-60% SL) and preanal length is about 60% SL. Pigmentation and standard length at which fins develop also are simi- lar; a full complement of rays is present in all fins by 8.0-9.0 mm in both species (Hildebrand and Cable, 1938; Martinez-Pecero et al., 1990; this study). However, consolidation of pigment into lat- eral bands, resorption of the supraoccipital crest, and the beginning of transition to the juvenile stage occur earlier in Pacific spadefish than in Atlantic spadefish. Larvae of ephippids from the Indo-Pacific region differ from Chaetodipterus from the western Atlantic and Pacific Oceans in extent of head spination (Leis and Trnski, 1989; Martinez-Pecero et al., 1990; this study). Larvae of Platax from the Indo-Pacific have a median supraoccipital crest with a serrate leading edge (Leis and Trnski, 1989) but do not have the circumorbital series of spinous ridges, nor spines on the jugal, tabular, pterotic, or supracleithral bones found in Chaetodipterus (Martinez-Pecero et al., 1990; this study). Head spination in Ephippus larvae from the Indo-Pacific is similar to that of Chaetodipterus and these two genera are probably more closely related than either is to Platax. Other species-specific head spination found in Chaetodipterus larvae from the western Atlantic and Pacific Oceans, and in Ephippus orbis, Platax batavianus, and three Platax species from the Indo-Pacific region include a posttemporal spine which may be reduced to a ridge in some species, a supraorbital ridge that varies in size among species, and one or two subopercular spines (Leis and Trnski, 1989; Martinez-Pecero et al., 1990; this study). Early larvae of Atlantic spadefish could be con- fused with priacanthids, lobotids, some carangids and stromateoids, the wreckfish — Polyprion amer- icanus, and Menticirrhus spp. because of similari- ties in head spination or in body pigmentation. Priacanthids have an elongate, serrate, median supraoccipital crest that extends posteriorly over the mid- and hindbrain; serrations along the lower jaw and frontal bone; and the angle preopercular spine is elongate and serrate as is the pelvic spine. Trip- letail, Lobotes surinamensis, have a vaulted, serrate supraoccipital crest in early larvae, the pelvics are inserted behind the pectoral fins, and have fewer anal fin elements than Atlantic spadefish (Atlantic spadefish: A. Ill, 17-18, tripletail: A. Ill, 11-12). In carangids, the two anteriormost anal spines are separated from the third by a distinct gap and most species have a low, median supraoccipital crest that has serrations along the dorsal edge; other carangids lack a supraoccipital crest entirely. Some carangids also have a precocious dorsal fin with anterior spines or rays elongate, or with serrations along the angle preopercular spine. Some stromateoids (e.g. Ariommus spp., Nomeus gronovii) resemble Atlan- tic spadefish in early body pigmentation, body shape, and by having precocious pelvics, but stromateoids lack a median supraoccipital crest, a large preopercular angle spine, and all but Hyperoglyphe have >30 myomeres. Polyprion americanus larvae have a small, peak-like median supraoccipital crest, but with serrations along the leading edge, and lack a serrate pterotic ridge and spines on the tabular bone (Johnson, 1984). Wreckfish also have 27 myomeres, fewer dorsal (22- 24) and anal fin (11-13) elements, and the mouth is larger than in Atlantic spadefish. Larval Atlantic spadefish differ from early larvae of Menticirrhus spp. by lack of both preopercular spines and the median supraoccipital crest in the latter. We recently examined specimens reported by Dawson (1971) as larval black driftfish, Hypero- glyphe bythites. These specimens had a supra- occipital crest, pterotic ridge, spine on the inter- opercle, other head spination, and a pigmentation pattern identical to Atlantic spadefish. Vertebral, 272 Fishery Bulletin 92(2). 1994 dorsal, and anal fin counts overlap between black driftfish and Atlantic spadefish, but teeth are found in a single band on the dentary in black driftfish (Ginsburg, 1954) and in two bands in Atlantic spa- defish (Hildebrand and Cable, 1938; this study). Dawson's 5.7-7.9 mm specimens had teeth in two bands along the dentary. Because it is unlikely that black driftfish larvae have the same suite of char- acters as Atlantic spadefish, Dawson's specimens should be assigned to Atlantic spadefish. Atlantic spadefish spawn from May through Sep- tember based on seasonal abundance of Atlantic spadefish larvae in the northern Gulf; peak spawn- ing occurs between June and August (Ditty et al., 1988; this study). Density estimates were highest during June in this study (Table 3), during July in a previous study of coastal waters off central Loui- siana (Ditty, 1986), and during July and August off Mississippi Sound (Stuck and Perry, 1982). Neuston net collections were greatest during August (Table 3). Gonad maturity data off South Carolina support peak spawning of Atlantic spadefish during summer (Hayse, 1990). Spatial distribution data indicate that Atlantic spadefish larvae are apparently rare in the eastern Gulf. Only one larva was collected east of Mobile Bay (Alabama) during this study, and one larva by Houde et al. 3 in a survey of Gulf waters off Florida. In addition, distribution of both larvae and station depths where larvae were collected indicates that Atlantic spadefish occur primarily in coastal waters (Ditty and Truesdale, 1984; this study), except near the Mississippi River delta where waters may offer additional habitat suitable to larvae because of lower salinities. The relatively high number of posi- tive stations (26%) near the delta during August 1988 sampling of frontal zones suggests that fron- tal zones may concentrate larvae. Frontal zone wa- ters may also provide a richer environment for feed- ing and growth of larvae because of higher phy- toplankton and zooplankton biomass (Govoni et al., 1989; Grimes and Finucane, 1991). However, Powell et al. (1990) were unable to demonstrate consis- tently that larvae have a nutritional advantage when associated with the Mississippi River plume. A possible association of Atlantic spadefish larvae with riverine frontal areas requires further study. In conclusion, understanding the biology, life his- tory, and relations of Atlantic spadefish requires a knowledge of the morphology, distribution, and ecol- ogy of their larvae. Larval characters (e.g. degree of 3 Houde, E. D., J. C. Leak, C. E. Dowd, S. A. Berkeley, and W. J. Richards. 1979. Ichthyoplankton abundance and diversity in the eastern Gulf of Mexico. Univ. Miami Report BLM Con- tract No. AA550-CT7-28, Miami, FL 33149, 546 p. head spination) may also provide insight into the interrelationships among the Ephippidae and their relationship to other families. The potential use of larval characters in defining these relationships, however, cannot be clearly understood until larval development within the family is more fully docu- mented (Watson and Walker, 1992). Acknowledgments This study was supported by the Marine Fisheries Initiative (MARFIN) Program (contract numbers: NA90AA-H-MF111 and NA90AA-H-MF727). The authors would like to thank the Southeast Area Monitoring and Assessment Program (SEAMAP) and Gulf States Marine Fisheries Commission for providing specimens and environmental data; Churchill Grimes (NMFS, Panama City Lab, Florida) for access to neuston net collections off the Mississippi River delta during August 1988; and John Lamkin (NMFS, Pascagoula, MS) for provid- ing specimens of the reported black driftfish for examination. We also thank Laura Younger for pro- viding scanning electromicrographs of the head spines of Atlantic spadefish larvae. Finally, we thank the reviewers for their comments in substantially improving the manuscript. Jack Javech (NMFS, Miami, FL) illustrated the larvae. Literature cited Berry, F. H., and W. J. Richards. 1973. Characters useful to the study of larval fishes. Mid. Atl. Coast. Fish. Cent. Tech. Pap. 1:48-65. Dawson, C. E. 1971. Notes on juvenile black driftfish, Hypero- glyphe bythites, from the northern Gulf of Mexico. Copeia 1971(41:732-735. Ditty, J. G. 1986. Ichthyoplankton in neritic waters of the northern Gulf of Mexico off Louisiana: composi- tion, relative abundance, and seasonality. Fish. Bull. 84(4):935-946. Ditty, J. G., and F. M. Truesdale. 1984. Ichthyoplankton surveys of nearshore Gulf waters off Louisiana: January-February and July 1976. Assoc. Southeastern Biol. Bull. 31(2):55-56. Ditty, J. G., G. G. Zieske, and R. F. Shaw. 1988. Seasonality and depth distribution of larval fishes in the northern Gulf of Mexico above 26"00'N latitude. Fish. Bull. 86(4):811-823. Ginsburg, I. 1954. Four new fishes and one little known species from the east coast of the United States, includ- Ditty et al.: A redescription of Chaetodipterus faber larvae 273 ing the Gulf of Mexico. J. Wash. Acad. Sci. 44:256-264. Govoni, J. J., D. E. Hoss, and D. R. Colby. 1989. The spatial distribution of larval fishes about the Mississippi River plume. Limnol. Oceanogr. 34(1): 178-187. Gregory, W. K. 1933. Fish skulls: a study of the evolution of natu- ral mechanisms. Trans. Am. Philos. Soc. 23:75- 481. Grimes, C. B., and J. H. Finucane. 1991. Spatial distribution and abundance of larval and juvenile fish, chlorophyll and macrozoo- plankton around the Mississippi River discharge plume, and the role of the plume in fish recruit- ment. Mar. Ecol. Prog. Ser. 75:109-119. Hayse, J. W. 1990. Feeding habits, age, growth, and reproduc- tion of Atlantic spadefish Chaetodipterus faber (Pisces: Ephippidae) in South Carolina. Fish. Bull. 88(l):67-83. Hildebrand, S. F., and L. E. Cable. 1938. Further notes on the development and life history of some teleosts at Beaufort, N. C. Bull. U.S. Bur. Fish. 48(24):505-642. Hubbs, C. L., and K. F. Lagler. 1958. The fishes of the Great Lakes region. Univ. Mich. Press, Ann Arbor, 213 p. Johnson, G. D. 1978. Development of fishes of the mid-Atlantic Bight, an atlas of egg, larval, and juvenile stages. Vol. IV: Carangidae through Ephippidae. U.S. Fish. Wildl. Serv., Biol. Serv. Prog. FWS/OBS-78/ 12, 314 p. 1984. Percoidei: development and relationships. In H. G. Moser, W. J. Richards, D. M. Cohen, M. P. Fahay, A. W. Kendall Jr., and S. L. Richardson (eds.), Ontogeny and systematics of fishes, p. 464- 498. Am. Soc. Ichthy. Herp., Spec. Publ. No. 1. Leis, J. M., and T. Trnski. 1989. The larvae of Indo-Pacific shorefishes. Univ. Hawaii Press, Honolulu, 371 p. Martinez-Pecero, R., E. Matus-Nivon, R. Ramirez- Sevilla, D. E. Hernandez-Ceballos, and M. Contreras-Olguin. 1990. Huevo, larva y juvenil del peluquero Chaeto- dipterus zonatus (Girard) (Pisces: Ephip- pididae). Rev. Biol. Trop. 38(l):71-78. (In Spanish.) Nelson, J. S. 1984. Fishes of the world, 2nd ed. John Wiley & Sons, NY, 523 p. Powell, A. B., A. J. Chester, J. J. Govoni, and S. M. Warlen. 1990. Nutritional condition of spot larvae associ- ated with the Mississippi River plume. Trans. Am. Fish. Soc. 119:957-965. Richardson, S. L., and W. A. Laroche. 1979. Development and occurrence of larvae and juveniles of the rockfishes Sebastes crameri, Sebastes pinniger, and Sebastes helvomaculatus (family Scorpaenidae) off Oregon. Fish. Bull. 77(1):1^6. Ryder, J. A. 1887. On the development of osseous fishes, includ- ing marine and freshwater forms. Rep. U.S. Fish. Comm, Part 13, 1885 (1887):489-604. Sanders, N., Jr., T. Van Devender, and P. A. Thompson. 1990. SEAMAP environmental and biological atlas of the Gulf of Mexico, 1986. Gulf States Marine Fish. Comm., Ocean Springs, MS, No. 20, 328 p. SAS Institute, Inc. 1985. SAS User's Guide: statistics, 1985 ed. SAS Institute, Cary, NC, 584 p. Schmied, R. L., and E. E. Burgess. 1987. Marine recreational fisheries in the south- eastern United States: an overview. Mar. Fish. Rev. 49(2): 1-7. Smith, P. E., and S. L. Richardson. 1977. Standard techniques for pelagic fish egg and larva surveys. FAO Fish. Tech. Paper No. 175, 100 p. Stuck, K. C, and H. M. Perry. 1982. Ichthyoplankton community structure in Mississippi coastal waters. In Fishery monitor- ing and assessment completion report, 1 January 1977 to 31 December 1981, p. VI-I-1 thru VI-I- 53. Gulf Coast Res. Lab. (Ocean Springs, MS), Proj. No. 2-296-R. Stuntz, W. E., C. E. Bryan, K. Savastano, R. S. Waller, and P. A. Thompson. 1985. SEAMAP environmental and biological atlas of the Gulf of Mexico, 1982. Gulf States Marine Fish. Comm., Ocean Springs, MS, No. 12, 145 p. Thompson, P. A., and N. Bane. 1986a. SEAMAP environmental and biological atlas of the Gulf of Mexico, 1983. Gulf States Marine Fish. Comm., Ocean Springs, MS, No. 13, 179 p. 1986b. SEAMAP environmental and biological atlas of the Gulf of Mexico, 1984. Gulf States Marine Fish. Comm., Ocean Springs, MS, No. 15, 171 p. Thompson, P. A., T. Van Devender, and N. J. Sanders Jr. 1988. SEAMAP environmental and biological atlas of the Gulf of Mexico, 1985. Gulf States Marine Fish. Comm., Ocean Springs, MS, No. 17, 338 p. Watson, W, and H. J. Walker Jr. 1992. Larval development of sargo (Anisotremus davidsonii) and salema (Xenistius californiensis) (Pisces: Haemulidae) from the Southern Califor- nia Bight. Bull. Mar. Sci. 51(3):360-406. 274 Fishery Bulletin 92(2). 1994 Appendix Table Summary of total number of bongo net/neuston net stations examined for Atlantic spadefish larvae (Chaetodipterus faber) in the Gulf of Mexico. Acronyms are as follows: SEAMAP - Southeast Area Monitoring and Assessment Program; NMFS - National Marine Fisheries Service, Panama City, Florida. NS means no samples. Mar Apr May Jun Jul Aug Sep Oct Nov Dec SEAMAP 1982 77 VO 2 69/68 71/73 102/100 26/24 NS NS 3/8 29/3 NS 1983 15/13 27/27 84/84 55/45 44/42 NS NS 39/26 NS 24/23 1984 23/0 44/0 46/0 55/54 20/26 155/162 NS 24/0 6/0 36/36 1985 29/0 NS NS 85/0 39/0 69/0 20/0 4/0 2/0 24/0 1986 NS 24/0 90/0 57/0 10/0 NS 145/0 43/0 73/0 24/0 Total 144/13 164/95 291/157 354/199 139/92 224/162 165/0 113/34 110/3 108/59 NMFS 1988 55 Tl 36 ' 60-cm bongo net, 0.333-mm mesh, oblique-tow from depth. 2 1 x 2 m neuston net, 0.947-mm mesh, 10 min. surface-tow, unmetered. Abstract. — Dolphinfishes are highly prized commercial and rec- reational species of worldwide dis- tribution in tropical and subtropi- cal seas, but the development and distribution of their larvae are poorly understood. Common dol- phin eggs hatch in about 38 hours at 25°C based on a predictive re- lationship among egg diameter, water temperature, and develop- ment time. Morphometries are generally greater in pompano dol- phin than in common dolphin. Pompano dolphin are deeper-bod- ied and have a larger eye by 9 mm, and a larger mouth and longer pre-anal length by about 13 mm. Differences in pigment along the caudal peduncle and its finfold separate common dolphin from pompano dolphin <4. 0—4.5 mm SL; common dolphin lack pigment in these areas. Number of spines along the outer shelf of the pre- opercle also separate species al- though preopercle spines are often difficult to count on larvae not cleared and stained; common dol- phin have four spines along the outer preopercular shelf and pom- pano dolphin have five. Pigmented pelvic fins and bands of pigment laterally on both the body and me- dian fins of common dolphin are diagnostic for separating species >8 mm SL; pompano dolphin lack these characters. Both common dolphin and pompano dolphin lar- vae usually are found at >24°C, >33 ppt, and beyond the 50 m isobath. Preflexion larvae (<7.0- 7.5 mm SL) were primarily col- lected in oceanic waters. Both spe- cies may spawn year-round, at least in the southern part of the survey area. Larval common dol- phin are significantly more abun- dant than pompano dolphin. Larval development, distribution, and abundance of common dolphin, Coryphaena hippurus, and pompano dolphin, C. equiselis (family: Coryphaenidae), in the northern Gulf of Mexico* James G. Ditty Richard F. Shaw Coastal Fisheries Institute. Center for Coastal, Energy, and Environmental Resources Louisiana State University, Baton Rouge, LA. 70803 Churchill B. Grimes Southeast Fisheries Science Center, National Marine Fisheries Service, NOAA Panama City Laboratory, 3500 Delwood Beach Road Panama City, FL 32408 Joseph S. Cope Coastal Fisheries Institute, Center for Coastal . Energy, and Environmental Resources Louisiana State University, Baton Rouge, LA 70803 The dolphinfishes, Coryphaena hip- purus (common dolphin) and C. equiselis (pompano dolphin), are distributed worldwide in tropical and subtropical seas (Briggs, 1960). Highly prized as food, these fishes are important recreational and commercial species, but relatively little is known about their early life stages. Gibbs and Collette (1959) reviewed spawning and adult sea- sonal distribution for the western North Atlantic Ocean, and Palko et al. ( 1982) compiled dolphinfish bio- logical data. Aoki and Ueyanagi (1989) discussed larval and early juvenile distribution for the eastern Pacific, and similar information is available for the western Pacific and Indian oceans (Shcherbachev, 1973). Preliminary distribution maps are available for the Gulf of Mexico (Gulf), but associated envi- ronmental data are not included (Richards et al., 1984; Kelley et al., 1986). Embryonic development is described for common dolphin (Mito, 1960; Hassler and Rainville, 1975; Hagood and Rothwell 1 ) and osteological development for both species (Potthoff, 1980), but de- scriptive larval morphology is pri- marily limited to sizes >13 mm SL (Gibbs and Collette, 1959; Shcher- bachev, 1973). Okiyama (1988) and Aoki and Ueyanagi (1989) provide information on developmental mor- phology of Pacific specimens <13 mm SL, but their illustrations are 1 Hagood, R. W., and G. N. Rothwell. 1979. Sea Grant interim project report — 1979. Aquaculture in tropical ocean — Cory- phaena sp. Oceanic Inst., Makapuu Point, Waimanalo, HI 96795. Manuscript accepted: 20 October 1993 Fishery Bulletin 92:275-291 (1994). Contribution No. LSU-CFI-92-5 of Louisiana State University Coastal Fisheries Institute. 275 276 Fishery Bulletin 92(2). 1994 insufficient to examine important details, and Okiyama's study is a general overview of exisitng information. The utility of early life stages of Coryphaena in examining previous phylogenetic hypotheses and evolutionary interrelationships of echeneoids (i.e. Coryphaenidae-Rachycentridae- Echeneididae) is discussed by Johnson (1984). Our objectives are 1) to describe and compare early lar- val development of common dolphin and pompano dolphin using the dynamic approach to larval de- scription (Berry and Richards, 1973) and 2) to de- scribe the spatial and temporal distribution and abundance of early life stages of dolphinfishes in the northern Gulf. Materials and methods Seasonal occurrence, distribution, and abundance of dolphinfish larvae were determined primarily from 814 neuston net collections taken during Southeast Area Monitoring and Assessment Program (SEAMAP) ichthyoplankton surveys of the Gulf be- tween 1982 and 1984 (1982-276 stations, 1983-260, 1984-278). These years represent the first time in- terval for which a complete set of data was currently available. SEAMAP collections were made with an unmetered 1x2 m net (0.947-mm mesh) towed at the surface for 10 minutes at each station. The SEAMAP effort also involved the collection and pro- cessing of about 1,819 bongo net stations between 1982 and 1986 (1982-384 stations, 1983-288, 1984- 409, 1985-272, and 1986-466) (SEAMAP 1983- 1987) 2 . Bongo nets (60-cm net, 0.333-mm mesh) were towed obliquely to the surface from within 5 m of the bottom or from a maximum depth of 200 m. Sampling during April and May was primarily beyond the continental shelf, and that during March and from June to November was primarily over the shelf at stations <180 m depth. No samples were taken during January and February. Tows were made during both day and night depending on when the ship occupied the station. Latitude 24°30'N was the southern boundary of our survey area in the eastern Gulf and latitude 26°00'N the southern boundary of the central and western Gulf (Appen- dix Fig. 1). These coordinates approximate the U.S. Exclusive Economic Zone (EEZ)/Fishery Conserva- tion Zone (FCZ). Additional information on tempo- ral and spatial coverage of SEAMAP plankton sur- veys are found in Stuntz et al. ( 1985), Thompson and 2 SEAMAP. 1983-1987. (plankton i. ASCII characters. Data for 1982-1986. Fisheries-independent survey data/National Ma- rine Fisheries Service. Southeast Fisheries Center: Gulf States Marine Fisheries Commission, Ocean Springs, MS (producer!. Bane (1986, a and b), Thompson et al. (1988), and Sanders et al. (1990). Ichthyoplankton collections were also examined from riverine/oceanic frontal zones off the Missis- sippi River delta. These collections were from sur- face-towed 1x2 m neuston nets (0.947-mm mesh, 10- min. tows, sample rc=311) and were obtained from the National Marine Fisheries Service (NMFS), Panama City