■ .--.s .- ;v: ~ ;-.-:3: S' U.S. Department of Commerce Volume 95 Number 1 January 1997 Fishety Bulletin U.S. Department of Commerce Michael Kantor Secretary National Oceanic and Atmospheric Administration D. James Baker Under Secretary for Oceans and' Atmosphere National Marine Fisheries Service Rolland A. Schmittpn Assistant Administrator for Fisheries 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. Periodicals postage is paid at 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 en- tirely, reference to source is appreci- ated. The Secretary of Commerce has deter- mined that the publication of this peri- odical 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 ap- proved 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: $32.00 domestic and $40.00 foreign. Cost per single issue: $13.00 domestic and $16.25 foreign. See back page for order form. Scientific Editor Dr. John B. Pearce Editorial Assistant Kimberly T. Murray Northeast Fisheries Science Center National Marine Fisheries Service, NOAA 1 66 Water Street Woods Hole, Massachusetts 02543-1097 Editorial Committee Dr. Andrew E. Dizon National Marine Fisheries Service Dr. Harlyn O. Halvorson University of Massachusetts, Dartmouth Dr. Ronald W. Hardy 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. Fredric M. Serchuk National Marine Fisheries Service Dr. Tim D. Smith National Marine Fisheries Service Managing Editor Sharyn Matriotti National Marine Fisheries Service Scientific Publications Office 7600 Sand Point Way NE, BIN Cl 5700 Seattle, Washington 98115-0070 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. 1103. 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 95 Number 1 January 1997 Fishery Bulletin Contents The National Marine Fisheries Service (NMFS) does not approve, recommend, or endorse any proprietary product or proprietary material mentioned in this publication. No reference shall be made to NMFS, or to this publication furnished by NMFS, in any advertising or sales promotion which would indicate or imply that NMFS approves, recommends, or endorses any proprietary product or proprietary material mentioned herein, or which has as its purpose an intent to cause directly or indirectly the adver- tised product to be used or purchased because of this NMFS publication. Articles 1 Bertram, Douglas F., Thomas J. Miller, and William C. Leggett Individual variation in growth and development during the early life stages of winter flounder, Pleuronectes americanus 1 1 Crockford, Susan J. Archeological evidence of large northern bluefin tuna, Thunnus thynnus, in coastal waters of British Columbia and northern Washington 25 Fisher, Joseph R, and William G. Pearcy Dietary overlap of juvenile fall- and spring-run Chinook salmon, Oncorhynchus tshawytscha, in Coos Bay, Oregon 39 Goodyear, C. Phillip Fish age determined from length: an evaluation of three methods using simulated red snapper data 47 Griffiths, Marc H. The life history and stock separation of silver kob, Argyrosomus inodorus, in South African waters 68 Hampton, John Estimates of tag-reporting and tag-shedding rates in a large-scale tuna tagging experiment in the western tropical Pacific Ocean 80 Hinton, Michael G., Ronald G. Taylor, and Michael D. Murphy Use of gonad indices to estimate the status of reproductive activity of female swordfish, Xiphias gladius: a validated classification method 85 Kane, Joseph Persistent spatial and temporal abundance patterns for late- stage copepodites of Centropages hamatus (Copepoda: Calanoida) in the U.S. northeast continental shelf ecosystem Fishery Bulletin 95(1), 1997 ii 99 Livingston, Mary E., Marianne Vignaux, and Kathy A. Schofield Estimating the annual proportion of nonspawning adults in New Zealand hoki, Macruronus novaezelandiae 114 McKenna, James E., Jr. Structure and dynamics of the fishery harvest in Broward County, Florida, during 1 989 126 McQuinn, I an H. Year-class twinning in sympatric seasonal spawning populations of Atlantic herring, Clupea harengus 137 Parrish, Frank A., Edward E. DeMartini, and Denise M. Ellis Nursery habitat in relation to production of juvenile pink snapper, Pristipomoides fiiamentosus, in the Hawaiian Archipelago 1 49 Restrepo, Victor R., and Joseph E. Powers Application of high-breakdown robust regression to tuned stock assessment models 161 Shi, Yunbing, Donald R. Gunderson, and Patrick J. Sullivan Growth and survival of 0+ English sole, Pleuronectes vetulus, in estuaries and adjacent nearshore waters off Washington Notes 1 74 Richardson, Linda R., and John R. Gold Mitochondrial DNA diversity in and population structure of red grouper, Epinephelus morio, from the Gulf of Mexico 1 80 Zavala-Gonzalez, Alfredo, and Eric Mellink Entanglement of California sea lions, Zalophus californianus californianus, in fishing gear in the central-northern part of the Gulf of California, Mexico 185 Erratum 1 86 Subscription form Abstract .—We studied phenotypic variation in larval and juvenile growth and development, using laboratory- reared winter flounder, Pleuronectes americanus. Larvae were reared indi- vidually to metamorphosis and beyond and were measured at weekly intervals. Growth in length was rapid until 30 d but slowed thereafter until metamor- phosis. Standard length peaked and often declined as metamorphosis ap- proached, and notochord length de- creased during flexion. Length at 30 d (an index of larval growth rate) was inversely related to age at metamorpho- sis, confirming previous assertions that larvae that grow rapidly also develop most rapidly. The relation between growth rate and larval-period duration, however, was not straightforward. The time from the day of peak larval length until metamorphosis (7-35 d) appeared to be inversely related to larval growth rate. Juvenile growth rates during the first 3 weeks following metamorphosis were unrelated to length at 30 d. Addi- tional juveniles, reared in groups as larvae and tracked as individuals fol- lowing metamorphosis, showed no change in growth rates during the first 4 weeks of the juvenile period in rela- tion to increasing age at metamorpho- sis or larval growth rates. These results are consistent with earlier findings that size at age does not diverge continually throughout the larval and juvenile pe- riods. Compensatory juvenile growth among fish that grew slowly as larvae was observed but not to the same ex- tent as previously reported. We empha- size the utility of the individual-based approach for identifying patterns of phenotypic variability in growth and development during the early life stages in fishes. Manuscript accepted 13 August 1996. Fishery Bulletin 95:1-10 (1997). Individual variation in growth and development during the early life stages of winter flounder, Pleuronectes americanus Douglas F. Bertram* * Thomas J. Miller * William C. Leggett*** Department of Biology, McGill University I 205 Avenue Docteur Penfield, Montreal, QC, Canada H3A 1B1 Mechanisms controlling survival and recruitment of fishes operate at the level of the individual (Crowder et al., 1992). Further, small initial differences among individual larvae and juveniles within fish popula- tions may have disproportionate effects on the probability of their survival (Crowder et al., 1992; Rice et al., 1993). Consequently, research programs in fisheries have begun to focus on phenotypic variability within cohorts in an effort to iden- tify particular traits that may be unique to the small minority of sur- vivors (Fritz et al., 1990; Taggart and Frank, 1990). If survivors are not random subsets of the original cohort, interpretations of recruit- ment processes based upon analy- ses of population averages are likely to be misleading (Pepin and Miller, 1993). Consequently, individual- based approaches are increasingly favored (Crowder et al., 1992). How- ever, there have been few quantita- tive measurements of either indi- vidual variation in early life history traits of fishes or in their survival consequences (but see Rosenberg and Haugen, 1982; Rice et al., 1987; Chambers et al., 1989; Chambers and Leggett, 1992; D’Amours, 1992; Bertram and Leggett, 1994; Loch- mann et al., 1995; Miller et al., 1995). In theory, longitudinal data can be obtained from sequential measurements of individuals or from back calculations of size at age from otolith microstructure. Variation in larval growth rates is widely believed to be a central feature in year-class formation in fishes (Leggett and Deblois, 1994). Traditionally, growth parameters are estimated from a restricted number of samples of the popula- tion. Each sample includes a range of fish lengths and ages. Impor- tantly, each fish provides only a single estimate of length at age. Such data are termed cross-sec- tioned. The calculated growth pa- rameters represent composite pic- tures and cannot reveal variability among the growth patterns of indi- viduals simply because they aggre- gate data at a level higher than the individual. Chambers and Miller (1995) have discussed the effects of the level of aggregation of data on the inferences that can be made. In addition, composite growth curves Present address: Department of Biologi- cal Sciences, Simon Fraser University, Burnaby, British Columbia, Canada 156 V5A. E-mail address: dbertram@sfu.ca * Present address: The University of Mary- land System, Chesapeake Biological Laboratory, P.O. Box 38, Solomons, Mary- land 20688-0038. Present address: Queen’s University, 206 Richardson Hall, Kingston, Ontario, Canada K7L 3N6. 2 Fishery Bulletin 95(1 ), 1997 are subject to several potential biases (Chambers and Miller, 1995). For example, composite curves are not accurate in cases where mortality of individual age classes are biased towards small or large individu- als (Litvak and Leggett, 1992; Pepin et al., 1992). Hence, if the survival consequences of variability in growth rates are to be evaluated adequately, indi- vidual phenotypic variability in growth must be quantified (Lynch and Arnold, 1988; Chambers, 1993). This requires that longitudinal data based upon repeated measures of individuals be collected. These problems are illustrated in the following ex- ample. Consider a cohort of 220 larvae with an aver- age size of 5.5 mm and a uniform size distribution of 20 larvae in each of ten 0.1-mm size classes from 5 to 6 mm. In a hypothetical 7-d interval, no larval growth occurs, but a gape-limited predator consumes all larvae less than 5.5 mm, leaving 120 larvae with an average size of 5.75 mm. If only cross-sectional data were available, the larval growth rate was esti- mated as 0.11 mm/d for the 7-d interval. However, if longitudinal data were available (i.e. measurements of survivors at the beginning and end of the week), it would be clear that no growth had occurred. Bertram et al. ( 1993) have argued that the dynam- ics of larval and juvenile growth rates should be ex- amined in unison, rather than separately. Using labo- ratory-reared winter flounder, Pleuronectes ameri- canus, they have shown that size-at-age trajectories do not diverge continually during the larval and ju- venile periods. In fact, juvenile size-at-age trajecto- ries converge because fish that grew slowly as lar- vae compensated for their slow growth by growing rapidly as juveniles. However, Bertram et al. (1993) assumed that larval growth was linear; therefore juvenile fish used in their experiments were pooled into groups. This approach precluded the study of individual phenotypic variability. The objectives of the present study were 1 ) to provide estimates of the individual variability in growth rate in fish during early life stages because it is upon this individual variability that phenotypic selection acts and 2) to evaluate the validity of previous estimates of larval growth rate. Also, we explore how individual varia- tion in larval growth affects growth during the sub- sequent juvenile period. Methods Rearing protocoS The research in this study was conducted at the Huntsman Marine Science Centre, St. Andrews, New Brunswick, during summer 1991. In May, adult win- ter flounder were collected from Passamaquoddy Bay with a bottom trawl and held at ambient seawater temperature (7-8°C). When ripe, eggs from individual females were combined with sperm pooled from three males to create half-sibling families. Families were maintained separately throughout the study. Fertil- ized eggs were placed in a slurry of diatomateous earth for 12 h following fertilization to prevent clump- ing. Incubation temperature was 7 (±0.5)°C (mean ± SD). At approximately 24-h after fertilization, the eggs were immersed in solutions of penicillin (0.0158 g/L) and streptomycin (0.02 g/L) for 24 h. Filtered, UV-sterilized seawater was replaced every 2-3 d until hatching commenced at approximately 14 d after fer- tilization. Upon hatching, 118 larvae from two families (fami- lies 1 and 2) were individually stocked into black cylindrical 0.4-L containers (15 cm diameter x 6.5 cm high) with clear plexiglass bottoms. Water tem- perature was maintained at 10 (±0.5)°C in a tem- perature controlled room with a 16:8 day:night pho- toperiod. At weekly intervals, 75% of the water was removed from each container and replaced with UV- sterilized filtered seawater. Additional “reserve” lar- vae from the same families were reared in groups under identical conditions in 18-L cylindrical, black plastic containers. Individual larvae that died within the first 3 weeks were replaced with siblings from the appropriate reserve group. Larvae were also reared in groups in 38-L aquaria covered externally with black plastic. Five aquaria were each stocked with four-hundred 1-d-old larvae drawn from another half-sibling family (family 3). Temperatures in the aquaria were maintained at 10 (±0.5)°C. At weekly intervals, 3 liters of water were removed from each aquarium and replaced with UV- sterilized filtered seawater. Dead larvae were si- phoned regularly from the tank. All larvae were fed Brachionus sp. at 2 /(mL-d) until the end of week 7. Rotifers were cultured by using Isochrysis sp. and Chaetoceros sp. Twenty-four hours prior to being fed to larvae, rotifers were provided with Microfeast artificial plankton (Provesta Corpo- ration) to enhance their nutritional quality. From the end of week 5 onwards, larvae were also offered Artemia nauplii (0.25/[mL-d]). Nauplii were enriched with Microfeast 24 h prior to use. At metamorphosis, larvae from family 3, which had been reared in groups, were individually stocked into 0.4-L rearing containers (see above) to examine ju- venile growth. To standardize the developmental stage of individuals used in this study, we used only fish whose left eye had just crossed the midline on its migration to the right side of the body (stage H of Seikai et al., 1986). All fish that metamorphosed on Bertram et al.: Growth and development during the early life stages of Pleuronectes americanus 3 the same day were treated as a discrete cohort. The creation of these cohorts was repeated at intervals of 3-8 d until all fish had metamor- phosed. Table 1 summarizes the rearing condi- tions for the 3 families used in the study. At weekly intervals, length data on individual larvae were recorded by using a dissecting mi- croscope linked to a video system at 6x magni- fication. Larvae were filmed without being re- moved from their rearing containers. Fish were not anesthetized at any time. Larval movement was restricted by confining larvae within a 6- cm diameter plexiglass ring placed within the rearing container. Length data were collected only when fish were in the horizontal plane. To account for variation in the position of the larvae in the vertical plane, we constructed a small set of “stairs” with a plastic ruler segment attached at each level. After filming each larva, we immediately cali- brated the image against the ruler segment that was in focus. For fish that were close to, or past, meta- morphosis, the process of filming was simplified be- cause these fish generally remained motionless on the bottom of the container. Following metamorpho- sis, individual juveniles were filmed weekly for up to 4 weeks, when rearing was terminated. Standard lengths of all fish were obtained by using an image analysis system (Optimus vers. 3.11, Bioscan Corpo- ration, Seattle, WA). We used the image analysis system to “capture” two images for each fish for esti- mating standard length at age and used the largest value in all analyses. Analysis We constructed individual growth trajectories for larvae that survived until metamorphosis, using spline functions fitted to repeated measures of size at age. Individual larval growth trajectories were based on between 3 and 9 weekly observations per larva. Individual growth trajectories were examined quantitatively by using four indexes: 1) larval size at 30 ± 1 d (roughly midway through the development period, an index of larval growth rate [e.g. Travis, 1981]); 2) average larval growth rates, defined as the difference between the length at metamorphosis and the mean length at hatching for the family divided by the time elapsed between the two events; 3) la- tency period, defined as the time between the age at which maximum larval length was attained and the time of metamorphosis; and 4) larval-period dura- tion, defined as the age at metamorphosis. Correla- tion analyses (Pearson’s correlation coefficient) were used to examine the relationships among pairs of the above variables for individual larvae. Variables were Table 1 Summary of rearing conditions and filming schedules for the 3 families used in the study. Family 1 Family 2 Family 3 Larval container size (L) 0.4 0.4 38 Number of larvae/container 1 1 400 Weekly measures of larvae Yes Yes No Juvenile container size (L) 0.4 0.4 0.4 Number of juveniles/container 1 1 1 Weekly measures of juveniles Yes Yes Yes tested for normality with normal probability plots (Wilkinson, 1990). When heteroscedasticity was de- tected with techniques outlined in Zar (1984), vari- ables were log-transformed. For comparison with the individual growth trajectories, we also constructed a composite size-at-age plot by using data for all lar- vae used in the study. We checked for size-dependent mortality during the first part of the larval period by comparing the length of those fish that lived until their next weekly measurement with those that died during that time, using Ctests for independent samples. Size-depen- dent analyses were conducted for larvae after hatch- ing (1-2 d); week 1 (8-9 d); week 2 (15-16 d); and week 3 (22-23 d). Group-reared larvae that were used as replacements for fish that died during the first 3 weeks were not included in the analysis. Individual juvenile growth rates were estimated from the slope of a least squares fitted to weekly measures of individual size at age from metamor- phosis to week 3 of the juvenile period. Thus, growth estimates were based upon up to 4 size-at-age mea- surements. Growth parameters were not calculated when less than 3 size-at-age measurements were available. We examined the correlations between juvenile growth rates and both age at metamorpho- sis and length at 30 d. Juvenile growth rates were also examined in relation to Bertram et al.’s (1993) measure of average larval growth rate estimated as the difference between the mean length at metamor- phosis for fish that metamorphosed on the same day and the mean size at hatching for the family, divided by the number of days between the 2 events. For comparison with the work of Bertram et al. (1993), we restricted the analysis of juvenile growth to weeks 1 through 4 for fish that had been reared together as larvae. The relation between juvenile growth rates and age at metamorphosis was exam- ined by using regression and correlation analyses. Similar analyses were performed to examine the re- 4 Fishery Bulletin 95 ( 1 ), 1997 lationship between juvenile growth rates and aver- age larval growth rate. For fish that had metamor- phosed early, measurements of size at age were avail- able until week 7 of the juvenile period. For these individuals we compared growth rates during weeks 1-4 with those during weeks 5-7, using a paired £-test. Results Individually reared larvae Thirty-two individually reared fish, 31 of which were from a single family (family 1), survived until meta- morphosis. For 5 of these fish, weekly measures were available from hatching to metamorphosis. For the remaining 27 larvae weekly measures began at day 22. We could not detect size-dependent mortality in the weeks following: hatching (£=1.83, df=116), week 1 (£=0.18, df=87), week 2 (£=1.4, df=32), or week 3 (£=0.21, df=ll). Data from the 32 fish provided esti- mates of individual larval growth trajectories (Fig. 1A). The trajectories exhibited considerable pheno- typic variation in size at age, maximum larval size, size at metamorphosis, latency period, and the du- ration of the larval period (Fig. 1A). To demonstrate the loss of information introduced by basing growth parameters on cross-sectional data, we treated the original individual-level longitudinal data as cross- sectional. When individual larval sizes were depicted in this composite fashion (Fig. IB), mean larval length at age increased rapidly from day 1 until day 30 and then leveled off. Important information, how- ever, can be obtained only from cross-sectional data. For example, coefficients of variation (CV ) for length at age increased from 0.07 on day 1 to 0. 12 on day 30 but declined subsequently and leveled off at approxi- mately 0.08. The largest larvae at 30 d were also largest at 22 d (r=0.66, ti=18, P=0.03) and at 36 d (r=0.5, n=18, P=0.034), indicating positive covariance in size at age for individually reared larvae that were alive at each of those ages. There was a significant positive rela- tionship between larval length at 30 d and maximum larval size (r=0.6, ti=27, P=0.001). However, larval length at 30 d and age at metamorphosis were nega- tively correlated (n=27, r=-0.589, P=0.001; Fig. 2A). The negative correlation coefficient between size at 30 d and age at metamorphosis was larger than corre- lation coefficients calculated for all other age classes. Average larval growth rate and length at 30 d were positively correlated (n=25, r=0.49, P=0.01; Fig. 2B). The age of maximum larval size (log-transformed) was negatively correlated to length at 30 d (r=-0.71, n= 27, P<0.001; Fig. 3A). The latency period (range:7- 2 1 1 1 1 1 ; I 0 10 20 30 40 50 60 Figure I (A) Growth trajectories for individual laboratory- reared winter flounder, Pleuronectes americanus, larvae (n= 32) based upon spline functions fitted to repeated weekly measures of standard length. Lar- val standard length (mean ±SD) versus age. (B) The sample size and CV for each age class are given above and below the error bar (respectively). Also shown is the Gompertz curve based upon the equation re- ported by Jearld et al. (1993) for winter flounder. 35 d; 14.4 ±7.5 d, 7i=31)(log-transformed) was in- versely related to age of maximum larval size (r= -0.58, re=31, P=0. 001; Fig. 3B). In contrast, the latency period showed a positive trend with increasing length at 30 d, but the relationship was not significant (r= 0.2, n=26, P=0.38). Age at metamorphosis ranged from 44 d to 71 d (55.2 ±7.9 d). Length at metamorphosis ranged from 6.1 mm to 7.5 mm (6.6 ±0.3 mm). Length and age at metamorphosis were positively correlated for indi- vidually reared larvae (i'=0.46, n=29, P=0.012). Among individually reared larvae, subsequent ju- venile growth rate during the first 3 weeks of the juvenile stage bore no relation to age at metamor- Bertram et al.: Growth and development during the early life stages of Pleuronectes americanus 5 75 70 A 3 65 CO co o • mm mm • e- 60 o cd • S? 55 - "cd g> 50 < • • ## * tt 45 40 • • • • i i i 6 7 8 0.08 B • \ § 0.07 CD "cd i » • 1 • • • • •• • .c • •• 1 006 • "cd • • •* id • • • , a 0.05 CD < 0.04 . • i i i 5 6 7 8 Length at 30 d (mm) Figure 2 (A) Age at metamorphosis versus length at 30 d. (B) The relationship between average larval growth rate and length at 30 d. 60 55 50 45 40 35 - 30 - A 25 40 r 30 - Q- 20 10 - 6 7 Length at 30 d (mm) B • •• • •• • • •• 25 30 35 40 45 50 55 Age at maximum larval length (d) 60 Figure 3 (A) Age at maximum larval length (untransformed) versus length at 30 d. (B) Latency period (untrans- formed) versus the age at maximum larval length. phosis (r= 0.22, rc=ll, P=0.52; Fig. 4A). Similarly, ju- venile growth rate showed no relationship to length at 30 d (r=— 0.001, n=10, P-0.99, Fig. 4B). For com- parative purposes, juvenile growth rate was also re- gressed against the measure of larval growth rate used by Bertram et al. (1993). The slope of the rela- tionship, -0.18 mm/d, although not significantly dif- ferent from 0, was identical to that reported by Bertram et al. (1993). Group-reared larvae Length at metamorphosis was independent of age at metamorphosis among members of family 3 (r=-0.09, rc=205, P=0.23) reared in groups as larvae and indi- vidually as juveniles. (Note that 175 of these larvae came from a single rearing aquarium and that the remainder were also from a single tank.) Length at metamorphosis ranged from 5.6 mm to 7.36 mm (6.6 ±0.3 mm). Age at metamorphosis ranged from 32 d to 59 d (42.6 [(±6.7] d). Individual growth rates dur- ing weeks 1-4 of the juvenile period were unrelated to age at metamorphosis (r=0.055, n- 52, P=0.71; Fig. 5A). Juvenile growth rates were unrelated to larval growth rates (r=0.14, n= 52, P= 0.322; Fig 5B). Because juvenile growth rates were equivalent, we pooled co- horts that metamorphosed at different times on the basis of the number of days after metamorphosis. Co- efficients of variation for size-at-d postmetamorphosis were unrelated to postmetamorphic age (weeks 1-4) and never exceeded 0.08. Individual juveniles exhibited significantly faster growth rates during weeks 1-4 than during weeks 5-7 (f=9.45, df=17, P<0.0001). 6 Fishery Bulletin 95(1 ), 1997 0.20 0.15 =§ 0.10 - 0.05 40 0.20 r 0.18 - E g ® 0.16 - 0.14 - 0.12 - 45 50 55 Age at metamorphosis (d) 60 B 0.10 5.5 6.0 6,5 7.0 7.5 Length at 30 d (mm) 8.0 Figure 4 Larval and juvenile growth for fish reared individu- ally in both periods. (A) Growth rate from week 0 to week 3 of the juvenile period versus age at meta- morphosis. (B) Growth rate from week 0 to week 3 of the juvenile period versus length at 30 d, an in- dex of larval growth rate. 0.20 r f 0.10 - a) | 0.12 - z> 0.10 - 0.08 - 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 Average larval growth rate (mm/d) Figure 5 Larval and juvenile growth for fish reared individu- ally (n= 52) as juveniles. (A) Growth rate during weeks 1—4 of the juvenile period versus age at meta- morphosis. (B) Growth rate during weeks 1-4 of the juvenile period versus average larval growth rate. Discussion Our data on growth dynamics of larval fishes show that size at age is highly variable during the larval period. Patterns in CV’s for size at age demonstrate that most of the variation upon which selection can act is found during the early to mid phase of the lar- val period. Chambers et al. (1988), who analyzed average growth rates of larvae reared in groups, also found that CV’s for size at age increased from hatch- ing to a peak of 0.135 at 28 d and subsequently de- clined as metamorphosis approached. In this study, most larval growth occurred during the first 30 d. Individual larvae that grew most rapidly and reached the largest size at about day 30, midway through the larval period, metamorphosed at the youngest ages. Travis (1981), who reared anuran larvae individu- ally, also reported that age at metamorphosis was inversely related to size midway through the larval period. Despite the nonlinear growth observed, the length at 30 d and the average larval growth rate were positively correlated. Thus, the general conclu- sion (derived from those studies where average lar- val growth was used and larval growth was assumed to be linear) that rapid growth reduces the duration of the larval period is supported (Chambers and Leggett, 1987; Chambers et al., 1988; Bertram et al., 1993). Bertram et al.: Growth and development during the early life stages of Pleuronectes americanus 7 The relationship between larval growth rate and larval-period duration, however, may not be straight- forward. Although larval growth rate is the primary factor influencing larval-period duration, its effects appear to be modified by the duration of the latency period. Larvae that grow rapidly tend to reach maxi- mum larval length at an early age. However, indi- viduals that reach maximum larval length at an early age have a longer latency period than those larvae that reach maximum length late in the larval pe- riod. This finding suggests that rapid growth is as- sociated with a long latency period. Slower-growing larvae, in comparison, may reach metamorphosis at a later age but have a considerably shorter latency period. Moreover, this suggests that metamorphosis may require a minimum duration, independent of size. These findings are consistent with Ricklefs’ (1973, 1979) hypothesized tradeoff between growth rate and the acquisition of mature tissue function in birds. In this connection, it is noteworthy that a tradeoff between growth rate and the rate of protein turnover has been documented for the mussel Mytilus edulis (Hawkins et al., 1986). Our findings are also consistent with Balon’s (1990) suggestion that through epigenetic interactions, individuals within a clutch may form distinct developmental groups — some being more altricial and others more precocial. Growth rate estimates will be biased if mortality is size dependent. Biased growth-rate estimates will, in turn, reduce estimates of variation in growth rate. However, the extent of variability in larval growth rates reported here are not due to size-dependent mortality. Our analysis could not detect size-depen- dent mortality, and there was no reduced variability in growth until the end of week 4. Survival to meta- morphosis was relatively high (26 out of 53 [49%] from family 1) for individuals replaced on day 22. High survival from day 1 to metamorphosis (175 out of 400 [44%]) was also observed for group-reared lar- vae in one of the rearing aquaria for family 3. Impor- tantly, the CV for size and age at metamorphosis was similar for the individual and group-reared larvae. The CV for age at metamorphosis was 0.14 and 0.17 for individually reared and group reared larvae ( n =175), respectively. The CV for size at metamorphosis was 0.045 and 0.046 for individually reared and group- reared larvae (n=175), respectively. (Note that the CVs for age and size at metamorphosis for the full data set of group reared larvae [n=205] were indistinguishable from those reported above for the reduced data set [n = 175]). The similarity of CVs for age and size at metamorphosis implies similar scope for variation in growth rate despite differences in the rearing protocol. We used a small number of female broodstock. This small number of fish, however, did not preclude in- sight into the potential for variation in larval growth and development at the population and species level. Previous research on early life history traits in win- ter flounder has shown that most of the total varia- tion in metamorphic traits (age and size at meta- morphosis) occurred within rather than among ma- ternal families (Chambers and Leggett, 1992). Dif- ferences between families in the relationship between size and age at metamorphosis (Chambers and Leggett, 1987; see below) and length at metamor- phosis (Chambers and Leggett, 1992, Bertram et al., 1993), although detectable, appear small in compari- son with the similarities between families for varia- tion in age at metamorphosis. Indeed, Chambers and Leggett (1992) reported that most variation in age at metamorphosis resided within each rearing aquarium. The CV’s for age and size at metamor- phosis reported here are similar to those reported by Chambers and Leggett (1987) despite differences in rearing temperatures and origins of broodstock em- ployed in the two studies. Chambers and Leggett ( 1992) developed several qualitative expectations for parental influences on larval flatfishes. They sug- gested that parentage is likely to influence larval traits but that its contribution to the total pheno- typic variation in larvae is expected to diminish dur- ing the larval period. In addition, the degree of pa- rental influence is likely to be trait-specific. The ab- sence of parental effects on important traits such as larval-period duration (age at metamorphosis) sup- ports the potential generality of our results on early life history traits based on few parents. Moreover, in the absence of field data, laboratory-based research such as this represents the only basis for character- izing and predicting the dynamics of patterns of in- dividual larval growth and development. The survival consequences of individual variation in larval growth and development reported here are presently unknown. We do not know whether indi- viduals that grow rapidly and metamorphose at an early age have a survival advantage over those that grow more slowly and metamorphose at an older age. Despite the limited supporting evidence, there has been widespread acceptance of the hypothesis that rapid larval growth conveys a survival advantage because those individuals are large at age and often have a reduced larval-period duration (Bertram, 1993; Leggett and Deblois, 1994). D ’Amours (1992) tested directly the hypothesis that rapid larval growth increases survival by using wild 0-group ( 17— 47 d) Atlantic mackerel, Scomber scombrus. Compar- ing the otolith microstructure of larvae from one co- hort captured at two different intervals in time, he found no evidence of higher survivorship among faster-growing larvae. In addition, two studies have 8 Fishery Bulletin 95(1), 1997 found that larvae that are small at age may, under certain circumstances, be less vulnerable to preda- tion than are large members of a cohort (Litvak and Leggett, 1992; Pepin et ah, 1992; Bertram, 1996). In flatfishes and in other fishes that switch habitats at metamorphosis, the time of transition is likely to be associated with high mortality. In recent laboratory experiments, Whiting and Able ( 1995) demonstrated that mortality from shrimp (Crangon septemspinosa) predation on settled winter flounder (10.1-14.5 mm) was twice that of presettled individuals (<11 mm). Bertram and Leggett (1994), however, could detect no difference in shrimp-induced mortality for winter flounder that differed in either length or age at meta- morphosis. In the present study, there was a posi- tive relation between length and age at metamor- phosis for individually reared winter flounder, but this trend was not evident from the larger sample of group-reared fish (see also Bertram et al., 1993). However, the results may not be strictly comparable because different families were used for the indi- vidual and group rearing. Chambers and Leggett (1987) reported a positive relation between length and age at metamorphosis for 8 of 18 families of labo- ratory-reared winter flounder from Newfoundland. The appropriate experiments have not been con- ducted to determine whether both large size and old age at metamorphosis reduce mortality due to pre- dation. Therefore, to date, there is no firm basis for interpreting the survival consequences of the indi- vidual variation in larval growth and development patterns reported here. The results of this study are consistent with Bertram et al.’s (1993) finding that size at age does not diverge continuously during the larval and juve- nile periods. Overall, the results show that juvenile growth rates are parallel, despite differences in lar- val growth rates and age at metamorphosis. The parallel nature of juvenile growth rates shows that slow-growing larvae partially compensated for their small size at age by increasing their juvenile growth rates to a greater degree than did fish that grew rap- idly as larvae. However, the compensatory growth among slow-growing larvae was not sufficient to cause convergence in juvenile size at age. If these growth rates are maintained, differences in size at age of juveniles that metamorphosed early and late would remain. Previous work has shown that growth rates of group-reared fish were maintained from weeks 1-7 of the juvenile period (Bertram et al., 1993). In the present study, however, the growth rate of individu- ally reared juveniles was slower in weeks 5-7 than during weeks 1-4. There is reason to believe that food availability was a factor in this difference. Ju- veniles reared in groups in 7-L containers were ex- posed to concentrations of 292 prey/d per fish. Juve- niles reared individually in 0.4-L containers received 100 prey/d because rations were 0.25 Artemia nau- pliiAmL-d) for both container sizes. Consequently, food availability may have limited the growth rate of individually-reared juveniles during weeks 5-7 when fish were relatively large and food require- ments were maximal. An important conclusion from this study is that the insight into the dynamics of larval growth and development was gained only because the data were presented as individual-based observations. Although the CV’s of size at age would have been available if the weekly length measures of individuals had been pooled, the underlying growth dynamics and indi- vidual variability would have been concealed. More- over, a single “growth” curve fit to such size-at-age data would not accurately reflect the growth patterns of individual larvae. In this connection, we point out that a recent description of winter flounder larval “growth,” based upon reconstructions of size at age from otolith microstructure ( Jearld et al., 1993), bears little resemblance to the individual growth trajecto- ries shown here. Darwin (1859) wrote: “No one supposes that all individuals of the same species are cast in the very same mould”; but it is only recently that fishery sci- entists have begun to investigate the potential popu- lation consequences of phenotypic variability in early life history stages. Because mechanisms controlling survival and recruitment of fishes operate at the level of the individual (Crowder et al., 1992), baseline es- timates on phenotypic variability are required. This study clearly shows that rearing marine fish larvae individually in the laboratory can provide such esti- mates. We have shown that there is considerable variability in the dynamics of individual larval growth and development. Studies that examine the survival consequences of such variability represent a logical next step in research programs designed to provide a mechanistic understanding of the factors that affect survival during fish early life history. Acknowledgments We thank M. Litvak, F. Purton, and the staff of the Huntsman Marine Science Centre (HMSC) for their cooperation in this research. J. Farrell, S. Gutterman, and S. Whelan provided technical assistance. Finan- cial support was provided by Natural Sciences and Engineering Research Council (NSERC) of Canada operating and strategic grants to WCL. Additional financial support to DFB was provided by a NSERC Bertram et al.: Growth and development during the early life stages of Pleuronectes americanus 9 post graduate scholarship, the Anne Vallee Ecological Fund Scholarship, and a HMSC graduate fellowship. Literature cited Balon, E. K. 1990. 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K., and W. C. Leggett. 1992. Age and size-selective predation on larval fishes: the bigger-is-better hypothesis revisited. Mar. Ecol. Prog. Ser. 81:13-24. Lochmann, S. E., G. L. Maillet, K. T. Frank, and C. T. Taggart. 1995. Lipid class composition as a measure of nutritional condition in individual larval Atlantic cod ( Gadus morhua). Can. J. Fish. Aquat. Sci. 52:1294-1306. Lynch, M., and S. J. Arnold. 1988. The measurement of selection on size and growth. In B. Ebenman and L. Persson (eds.), Size-structured popu- lations, p. 47-59. Springer Verlag, Berlin. Miller, T. J., T. Herra, and W. C. Leggett. 1995. An individual-based analysis of the variability of eggs and their newly hatched larvae of Atlantic cod (Gadus morhua) on the Scotian Shelf. Can. J. Fish. Aquat. Sci. 52:1083-1093. Pepin, P., and T. J. Miller. 1993. Potential use and abuse of general empirical models of early life history processes in fish. Can. J. Fish. Aquat. Sci. 50:1343-1345. Pepin, P., T. H. 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Haugen. 1982. Individual growth and size-selective mortality of lar- val turbot ( Scopthalmus rnaximus) reared in enclosures. Mar. Biol. 72:73-77. Seikai, T., J. B. Tanangonan, and M. Tanaka. 1986. Temperature influence on larval growth and meta- morphosis of the Japanese flounder Paralichthys olivaceus in the laboratory. Bull. Jpn. Soc. Fish. 52:977-982. Taggart, C. T., and K. T. Frank. 1990. Perspectives on larval fish ecology and recruitment processes: probing the scales of relationships. In K. Sherman, L. M. Alexander, and B. D. Gold (eds.), Large marine ecosystem: patterns processes and yields, p. 151- 164. Am. Assoc. Adv. Sci. Publ., Washington, D.C. Travis, J. 1981. Control of larval growth variation in a population of Pseudoacris triseriata (Anura: Hylidae). Evolution 35:423-432. Whiting, D. A. and K. W. Able. 1995. Predation by sevenspine shrimp Crangon septem- spinosa on winter flounder Pleuronectes americanus dur- ing settlement: laboratory observations. Mar. Ecol. Prog. Ser. 123:23-31. Wilkinson, L. 1990. Systat: the system for statistics. Evanson, IL, 677 p. Zar, J. H. 1984. Biostatistical analysis, 2nd ed. Prentice-Hall, Int, Inc., Englewood Cliffs, NJ, 718 p. Archeological evidence of large northern bluefin tuna, Thunnus thynnus, in coastal waters of British Columbia and northern Washington Susan Janet Croekford Pacific Identifications 601 1 Oldfield Rd., RR #3, Victoria, British Columbia, Canada V8X 3X1 E-mail address: Vo anthuvic@uwm.uvic.ca Abstract .—This study presents ar- cheological evidence for the presence of adult bluefin tuna, Thunnus thynnus , in waters off the west coast of British Columbia and northern Washington State for the past 5,000 years. Skeletal remains of large bluefin tuna have been recovered from 13 archeological sites between the southern Queen Charlotte Islands, British Columbia, and Cape Flattery, Washington, the majority found on the west coast of Vancouver Island. Vertebrae from at least 45 fish from 8 sites were analyzed. Regression analy- sis (based on the measurement and analysis of modern skeletal specimens) was used to estimate fork lengths of the fish when alive; corresponding weight and age estimates were derived from published sources. Results indicate that bluefin tuna between at least 120 and 240 cm total length (TL) (45-290 kg) were successfully harvested by aborigi- nal hunters: 83% of these were 160 cm TL or longer. Archeological evidence is augmented by the oral accounts of na- tive aboriginal elders who have de- scribed strategies used until the late 19th century for hunting bluefin tuna. Despite this information, there are no 20th-century records of adult bluefin tuna in the northeastern Pacific. Ar- cheological evidence suggests that ei- ther perturbations in the distribution of Pacific bluefin have occurred rela- tively recently or the specific environ- mental conditions favoring the move- ment of large tuna into northeastern Pacific waters have not occurred in this century. Manuscript accepted 4 September 1996. Fishery Bulletin 95:11-24 (1997). Evidence is presented here for the occurrence of adult bluefin tuna, Thunnus thynnus , in waters of the northeastern Pacific, off the coast of British Columbia and northern Washington, for the past 5,000 years. The physical evidence con- sists of archeological remains of large bluefin tuna harvested by ab- original hunters. Aboriginal North Americans of this area (part of the so-called “Northwest Coast” culture region) were accomplished seamen and skilled hunters of marine mam- mals (Mitchell and Donald, 1988). Coastal archeological sites through- out this region often contain abun- dant skeletal remains of the many fish and marine mammal species that sustained human populations over thousands of years (Calvert, 1980; Huelsbeck, 1983; Mitchell, 1988). Skeletal remains of large bluefin tuna have been recovered from 13 archeological sites. The archeologi- cal deposits containing tuna date from at least 5,000 years ago until the early 20th century. The exist- ence of bluefin tuna remains from this region have been previously reported (McMillan, 1979), but none were systematically analyzed until now. For this study, 78 intact vertebrae from 8 archeological sites were mea- sured and the data compared with those from vertebrae of modern specimens (specimens from the re- maining 5 sites could not be exam- ined, owing largely to difficulties in retrieving archived specimens but, in one case, because all skeletal material had been discarded by museum staff). Tentative estimates of the size of the archeological speci- mens were made by comparing the size of vertebrae from modern speci- mens of known length with the size of vertebrae collected from archeo- logical deposits. The resulting length estimates were then used to calculate weight and age estimates by using length-weight algorithms derived from recent data. Data are presented in a manner that should facilitate the analysis of any addi- tional archeological specimens recovered. In addition to the results of the analysis of the archeological mate- rial, anecdotal evidence is presented from ethnographic accounts of tuna- fishing methods related by native elders of the Mowachaht tribe who live on the west coast of Vancouver Island. These recent oral accounts substantiate and augment the physi- cal evidence: they describe bluefin tuna ethology, pinpoint the time of year that bluefin tuna were present and confirm that large bluefin tuna were being harvested in the north- eastern Pacific until the late 19th century. The historic evidence for bluefin tuna occurrence in this area, although sparse, is also presented. 12 Fishery Bulletin 95( 1 ), 1997 The archeological evidence and ethnohistoric ac- counts are significant because of the absence of mod- ern records for adult bluefin tuna in the northeast- ern Pacific. Consequently, the distribution of north- ern bluefin tuna of all age classes in the Pacific and all modern records for adults in the eastern Pacific are reviewed. The addition of historical information presented here to our present state of knowledge of modern bluefin tuna distributions has important implications for our understanding of changing en- vironmental conditions over time and perhaps also for determining the impact of 20th-century fisheries on Pacific bluefin tuna populations. Distribution of Pacific bluefin tuna The distribution of northern bluefin tuna in the Pa- cific is somewhat enigmatic, especially that of the adult portion of the population (Foreman and Ishizuka, 1990; Bayliff, 1994; Smith et al., 1994). Sexual maturity in Pacific northern bluefin is reached at about 5 years, and most spawning is re- ported between April and July in waters off Japan and the Philippine Islands, and in August in the Sea of Japan (Bayliff, 1994). Northern bluefin tuna are transoceanic migrators in both the Atlantic and Pa- cific; the movements of these fish are largely deduced by tagging experiments and catches of various age classes at specific times and locations (Nakamura, 1969; Rivas, 1978; Bayliff, 1994). Some of the population of Pacific bluefin tuna mi- grate from the western to the eastern Pacific Ocean during their first or second year. The proportion of the population that undertakes this migration ap- pears to vary from year to year (Bayliff et al., 1991). These migrating fish spend a period of one to six years in the eastern Pacific, a sojourn which may or may not be interrupted by visits to the central or western Pacific before the survivors return to spawn in the west (Bayliff, 1994). Adult fish in the Pacific appear to follow a general pattern of being distributed far- ther to the west during the spring (when spawning occurs) and farther to the east in the fall (Bayliff, 1993). It is not known if all fish return to spawn every year after sexual maturity is reached. Tagging ex- periments indicate that although the journey from west to east may take 7 months or less, the journey from east to west takes nearly 2 years; therefore there does not appear to be enough time for mature adult fish migrating from the eastern Pacific to spawn in the west every year. In addition, because a few adult fish have been captured in the eastern Pacific either just before or after the spawning season, some adults probably do not return to the western Pacific every year but rather spend variable lengths of time in the eastern Pacific (Bayliff, 1994). Most harvested adult bluefin tuna are caught in the western Pacific, where they are known to range as far north as the Sea of Okhotsk at about 50°N (Bayliff, 1980). Catch records of large bluefin tuna are noted at feeding areas off northeastern Honshu, Japan (ca. 40°N), off eastern Taiwan (about 25°N), and in the central Pacific near the Emperor Sea- mount (40°N, 175°E) (Nakamura, 1969). Adult bluefin tuna are considered rare everywhere in the eastern Pacific; sporadic records have come from southern California and northern Mexico only. Although small bluefin tuna (less than 120 cm total length [TL] and 5—45 kg) are caught regularly off California and Mexico and somewhat larger fish (120-160 cm TL and 45-80 kg) occasionally, adults over 160 cm TL (80 kg) are seldom encountered (Fore- man and Ishizuka, 1990; Bayliff, 1994). In the northern portion of the eastern Pacific, few modern records exist for bluefin tuna. Neave (1959) mentioned three occurrences in British Columbia waters during August 1957 and 1958, but no sizes or numbers were given. These reports came from an area approximately 200-400 miles off the west coast of Vancouver Island (49°N, 134°24'W; 48°N, 131°06'W; 51°N, 130°W). A 7.5-kg bluefin tuna was caught in a salmon seine in July 1958, near Kodiak, Alaska, and on 1 October 1957, bluefin tuna were sighted 80- 100 miles off Cape Flattery, Washington (Radovich, 1961). Sea-surface temperatures off the British Co- lumbia coast were reported as being warmer than usual during both years. I presume (because no sizes are mentioned in the reports) that these recent northern records are for relatively small fish of 5-45 kg because this size range is the most common in the eastern Pacific. Bluefin tuna larger than 45 kg in the eastern Pacific are rare enough that they are noteworthy when en- countered. Although the earliest modern record of a very large bluefin tuna in southern California ap- pears to be that of 1899 (Holder, 1913), sporadic oc- currences of bluefin tuna over 50 kg have been re- ported since then (Dotson and Graves, 1984; Fore- man and Ishizuka, 1990). The largest reported catch of giant bluefin tuna in the eastern Pacific was made in 1988 (Foreman and Ishizuka, 1990). Seiners caught an estimated 987 adult bluefin tuna between November and early January off southern California, including many over 100 kg and some more than 250 kg, including one that broke California records at 458 kg and 271.2 cm TL. Seiner operators involved in this fishery re- ported that large bluefin tuna travelled in small Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 13 Figure 1 Map of the Pacific northwest coast of North America, showing the location of archeological sites from which bluefin tuna, Thunnus thynnus, remains have been recovered, ^indicates samples examined in this study. *1 = FaTt 9 Louscoone Point; 2 = ElSx 1 Namu; 3 = DjSp 1 Yuquot village; *4 = DjSp 3 Yuquot midden; *5 = DkSp 1 Kupti; *6 = DkSp 3 Tahsis midden; *7 = DiSo 1 Hesquiat; 8 = DfSi 4 Macoah; * 9 = DfSi 5 Ch’uumat’a; *10 = DfSj 23AT’ukw’aa village; *11 = DfSj 23B T’ukw’aa defensive site; 12 = DhSe 2 Shoemaker Bay; 13 = 45CA24 Ozette village. schools of less than 10 similar-size in- dividuals, often less than 5 for very large fish. Analysis of stomach contents of some of these fish indicated that they had been feeding at the surface on chub mackerel, Scomber japonicus, and the opalescent inshore squid, Loligo opal- escens, a strongly phototactic species (Recksiek and Frey, 1978). Bluefin tuna are also reported to be phototactic (Bayliff, 1980). When water tempera- tures were recorded for these 1988 catches, they indicated lower than av- erage sea-surface temperatures (mean 14.1°C) for southern California waters in the eastern Pacific. Bluefin tuna are generally found associated with water temperatures of 17-23°C (Bell, 1963). The commercial catch of such high numbers of large fish in 1988 has raised the possibility that adult bluefin tuna may occur regularly off California but are only occasionally recognized or ob- served. Foreman and Ishizuka (1990) have suggested that small schools of adult bluefin tuna may go unrecognized if mistaken for pods of marine mammals or go undetected if travelling or feeding at depth. If so, it may be that the condi- tions that govern their infrequent move- ment into inshore feeding areas are very specific and thus rarely occur. Analyses and results The archeological sample Vertebrae were examined from 8 of the 13 sites from which remains of bluefin tuna were found. As is typical for fau- nal remains recovered from archeological sites, chro- nological dates for tuna specimens are estimated in relation to the 14C-dated strata from which they were recovered: none of the bluefin tuna remains have yet been dated directly. The northernmost archeological evidence for the occurrence of bluefin tuna is from the southern Queen Charlotte Islands (Fig. 1), whereas Namu on the cen- tral British Columbia mainland is the oldest known deposit yielding bluefin tuna remains ( dated at 4050- 3050 BC). Bluefin tuna have also been recovered from sites at Hesquiat Harbour and Shoemaker Bay on the west coast of Vancouver Island and from the Ozette site near Cape Flattery, Washington (see Table 1 for more details). Vertebrae are the only traces of bluefin tuna recovered from the above sites, and only specimens from the Hesquiat Harbour and Queen Charlotte Islands were available for analysis. Archeological excavations at four sites each in both Nootka and Barkley Sounds on the west coast of Vancouver Island also yielded bluefin tuna remains and, in contrast to other area sites, both vertebral and nonvertebral skeletal remains are represented. Neither scales nor otoliths, however, were found. Tuna were reported from all strata of the 1966 exca- vation at the village of Yuquot on Nootka Sound 14 Fishery Bulletin 95(1 ), 1997 Table 1 Archeological sites from which bluefin tuna , Thunnus thynnus, remains have been recovered, with excavation information, dates, references and numbers of remains reported that could not be analyzed in this study. Area and site no. Description of site and excavated remains Queen Charlotte Islands and the North Coast, British Columbia 1 (FaTt 9) Louscoone Point village, Kunghit Haida territory; 52°08'N, 131°14'W; small test excavation 1985 (Wigen;;Acheson2); from deposits dated ca. AD 800-ca.l800. 2 (ElSx 1) Namu village, Bella Bella territory; 51°52'N, 127°52'W; major excavation 1969-71; from deposits dated 4050-3050 BC (Cannon, 1991); 1 vertebra reported. Vancouver Island, British Columbia Nookta Sound area sites, Mowachat territory; ca. 49°40'N, 126°37'W 3 (DjSp 1) Yuquot village; major 1966 excavation; from all deposits 2300 BC- AD 1880 (McMillan, 1979); 87 vertebral and nonvertebral specimens reported. 4 (DjSp 3) Yuquot fishing station; from surface collection 1968;no dates (Marshall3). 5 (DkSp 1) Kupti village; small 1968 excavation; from deposits ca. AD 1260-1460 (Marshall3). 6 (DkSp 3) Tahsis Inlet midden; from 1990 shovel test; no dates (Marshall3). 7 (DiSo 1) Hesquiat village, Hesquiat territory; 49°24'N, 126°28'W; major 1973-75 excavation; from deposits dated AD 1230-1430 (Calvert, 1980). Barkley Sound area sites, Toquat territory; ca. 49°N, 125°20'W; 1991-93 excavations (McMillan and St. Claire4) 8 (DfSi 4) 9 (DfSi 5) 10 (DfSj 23A) 11 (DfSj 23B) 12 (DhSe 2) Macoah village; bluefin from upper levels of deposits dated 2460 BC-ca.AD 1880. Ch’uumat’a village; bluefin from deposits dated ca. AD 1370. T’ukw’aa village; bluefin tuna from deposits dated AD 760-1310. T’ukw’aa defensive site; bluefin tuna from deposits dated AD 1175-1880 . Shoemaker Bay, Tseshaht territory; 49°15'N, 124°49'W; major 1973/74 excavation; from deposits dated AD 500-820 (Calvert and Crockford, 1982); 17 vertebrae reported. Olympic Peninsula, Washington State 13 (45CA24) Ozette village, Cape Alava; Makah (Nuu-chah-nulth subdivision) territory; 48°10’N; 124°44'W; major 1971-80 excavation; from house floor deposits dated AD 1510 (Huelsbeck, 1983); 2 vertebrae reported (one modified). 1 Wigen, R. J. 1990. Identification and analysis of vertebrae fauna from eighteen archaeological sites on the southern Queen Charlotte Islands. British Columbia Heritage Trust, 800 Johnson St. Victoria, British Columbia, Canada V8W 1N3. Unpubl. rep., 79 p. 2 Acheson, S. 1992. Archaeology Branch, British Columbia Ministry of Small Business, Tourism, and Culture, 800 Johnson St., Victoria, British Columbia, Canada V8W 1N3. Personal commun. 3 Marshall, Y. M. 1990. The Mowachaht archaeology project, phase 1, 1989. Archaeology Branch, British Columbia Ministry of Small Business, Tourism, and Culture, 800 Johnson St., Victoria, B.C., Canada V8W 1N3. 4 See Footnote 3 in the main text of this paper. (dated from about 2300 BC to ca. AD 1880), making this the longest continuous record of Thunnus oc- currence in the region (McMillan, 1979; Marshall, 1993). Unfortunately, these specimens are archived in Ottawa and could not be retrieved easily for analy- sis: three other small excavations undertaken dur- ing 1968 and 1990 at sites along Nootka Sound, how- ever, recovered remains of bluefin tuna and these speci- mens were available for inclusion in this analysis. It is pertinent to mention that all fish remains from the 1966 excavation of the village at Yuquot were identified to genus level only (McMillan, 1979), per- haps giving the impression that the tuna remains might be albacore (T. alalunga), a species that oc- curs regularly in the eastern Pacific (Hart, 1973). However, crew working on the excavation of Yuquot reported that remains of some very large fish were recovered (Dewhirst1). According to the literature (and in my own twenty years experience analyzing faunal remains from this area), albacore have never been reported from any archeological site in British Columbia. Moreover, albacore rarely, if ever, exceed 50 kg; it therefore seems unlikely that Thunnus remains from Yuquot are albacore rather than bluefin tuna. 1 Dewhirst, J. 1992. Archeo Tech Associates, 1114 Langley St., Victoria, British Columbia, Canada V8W 1W1. Personal commun. Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 15 Table 2 Calculated fork lengths (cm) and estimated weights (kg) of comparative specimens — USNM catalog numbers 269001, 269004, 268964, 269002 (Nankai collection numbers 1, 2, 3, 6) National Museum of Natural History (NMNH), Smithsonian Institution. Nankai 1 269001 Nankai 2 269004 Nankai 3 268964 Nankai 6 269002 Skull length (cm) 26.2 25.3 23.5 28.5 Total of vertebral lengths (1-39 cm)7 148.1 124.1 121.2 146.9 Total skeletal length (SL) (cm) 174.3 149.4 144.7 175.4 Estimate of intervertebral cartilage — 40 spaces 20.0 20.0 20.0 20.0 Estimate of snout and tail flesh (cm) 10.0 10.0 10.0 10.0 Estimated fork length (cm) 204.3 179.4 174.7 205.4 Estimated weight (kg)2 184 130 121 187 1 All measurements available from the author or NMNH, Smithsonian Institution. 2 Foreman and Ishizuka, 1990; 184. Recent excavations at four locations along Toquart Bay in Barkley Sound on the west coast of Vancouver Island have recovered relatively large numbers of both vertebral and nonvertebral bluefin tuna skel- etal remains. Full analysis of this material is still in progress: only a few of the nonvertebral remains have been examined thus far. All vertebrae, however, are included in this study. Modern skeletal samples In order to estimate the size of fish represented by isolated vertebrae from archeological samples, it was necessary to determine the size relationship between individual vertebrae and the corresponding fork length in modem samples of the fish. Measurements taken from the vertebrae of modem skeletal specimens of known-size fish of comparable size were used for this purpose (Casteel, 1976; Wheeler and Jones, 1989). Recent skeletal specimens of large ( 160 cm TL and over) Pacific bluefin tuna were found to be extremely rare, and the only known specimens had, unfortu- nately, no corresponding size data (length or weight); therefore fork lengths (snout to fork of the tail) had to be estimated for these specimens as well. Fortu- nately, these four recent specimens of bluefin tuna (loaned by B. Collette, Museum of Natural History, Smithsonian Institution, Washington, D.C.) have skulls that are still articulated, and it was possible to determine a “skeletal length” for these specimens (Table 2). The skeletal length is defined as the basal length of the skull plus the combined lengths of all 39 vertebrae. The vertebral column of the compara- tive specimens had been sawed into sections during skeletal preparation, sometimes by cutting through a centrum. Vertebra no. 30, either by itself or with portions of no. 29 and no. 31 attached, was appar- ently removed from the specimens at some point and not returned. Estimates of the length measurements of all three of these vertebrae were used in the re- gression equations. These four fish appear to be the only disarticulated skeletal specimens of large Pa- cific bluefin tuna available for analysis (however, several museums have reconstructed skeletal speci- mens of large individuals on display). All raw data for these specimens are available on request from the author and are also on file at the National Mu- seum of Natural History, Smithsonian Institution. In order to estimate a fork length from the skel- etal length for these comparative specimens, I as- signed a value of 0.5 cm to the intervertebral carti- lage (40 spaces, 20 cm total) and an additional 5 cm each for flesh on the snout and the tail. These values consistently added 30 cm to the measured skeletal length and yielded an estimated fork length. This method was chosen so that if a more accurate deter- mination of the “soft tissue” component of the fork length of bluefin tuna is subsequently developed, the estimates given in this report can be easily adjusted. The vertebral centrum length and breadth mea- surements from the four comparative specimens (Fig. 2) were used in single (least-squares) regres- sion equations for each of the 39 vertebrae in the spinal column by using logarithmic transformations of vertebral and skeletal length measurements to de- termine their linear relationship. Because the size and shape of vertebrae change (sometimes quite dra- matically) over the length of the fish, it was neces- sary to calculate a separate algorithm for each ver- tebra in the spinal column. 16 Fishery Bulletin 95(1 ), 1997 Table 3 presents the resulting values for the rela- tionship between the greatest centrum length, GL, and greatest proximal centrum breadth, GB(p), to the skeletal length, SL, for the recent specimens as calculated by regression analysis. In this table, some values are missing or are based on only 3 specimens owing to the original preparation of the comparative skeletons. Standard deviations and confidence lim- its are not given but are available on request. Size determination of archeological specimens The standard formula used to estimate the size of fish represented by the archeological specimens is given by Casteel (1976, p. 96) as: log (fork length) = a + b x log (GL or GB). The constant (a) and the slope — or x coefficient — (b) are taken from Table 3 (i.e. the values derived from the comparative speci- mens), and the logarithm of the greatest length, GL, or proximal breadth, GB(p), from each archeological specimen (Table 4). Definitions of vertebral measurements taken from both comparative and archeological specimens of bluefin tuna. Greatest length (GL): maximum length of the centrum, taken at the lateral midpoint with digital calipers and measured to the nearest mm. Greatest breadth (GB) = maximum breadth of the centrum, taken at the lateral mid- point of the proximal face, GB(p), and distal face, GB(d), taken with digital calipers and measured to the nearest mm. Radius (R) = the maximum distance from the center of the cone to the edge, of the proximal face, R(p), and dis- tal face, R(d), taken at the lateral midpoint. This measure- ment was taken with a plastic ruler cut diagonally to fit into the cone of the centrum; in this way the amount of growth from the center of the cone to the sharp raised ridge at the lip of the centrum was measured to the nearest 0.5 mm. Casteel (1976) noted that although the regression method is the most accurate way to estimate fish length from bone size, these length estimates always vary somewhat between vertebrae from the same individual, even when the predictive value (r) of the equation is high. When both length and breadth measurements were available for an archeological specimen, the measurement that produced the length estimate with the highest correlation coefficient (r) value for that individual was used to represent that fish. Alternatively, an average of all available mea- surements could have been made, although this method allowed both comparative and archeological specimens to be treated similarly. The method used in the present study required that vertebral specimens be identified to exact column position. This can be problematic for archeological specimens because several of the centra in the ver- tebral column are almost identical and because ar- cheological specimens may often lack diagnostic neu- ral or haemal arches and spines. However, an archeo- logical specimen can almost always be defined to a small range within the column (e.g. vertebrae num- bers 14-16). Vertebrae not identified to exact posi- tion were found to be so similar in size and propor- tion to adjacent vertebrae that they could be treated as interchangeable for the purpose of the estimations attempted here. Where the exact position of an ar- cheological specimen was uncertain (which occurred for less than one third of the specimens examined), the number of the vertebra used to calculate the size estimates is given in parentheses, e.g. (15). Table 4 presents all archeological vertebrae mea- sured (by vertebra number) and the length estimates derived from them. Where eroded edges prevented accurate measurement, an estimate was taken if it was likely to be accurate to within 1 mm. A total of 78 vertebrae were measured, representing at least 45 individuals. Several vertebrae were found at- tached (occasionally in articulated position) or could potentially have belonged to the same individual by virtue of similar size and proximity within the ar- cheological deposit (this is a standard assumption for determining the minimum number of individu- als represented by skeletal remains recovered from archeological contexts). Radius measurements of these specimens were also taken (because this di- mension is preferred by some researchers for ageing purposes) but are not reported or used in the calcu- lations. All measurements are available on request from the author. The fork-length estimates for the archeological sample listed in Table 4, as for the comparative skel- etons, are derived by adding 30 cm to the estimated skeletal length to yield a fork length (to account for Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 17 Table 3 Regression analysis values: log of vertebral lengths, GL, and proximal breadth, GB(p), vs. log of skeletal length, SL, of 4 modern bluefin specimens (USNM 269001, 269004, 268964, 269002). NA = not applicable. Number of observations=4 (*=3); Degrees of freedom=2 (*=1). Vertebra no. Constant GL X Coefficient GL r value GL Constant GB(p) X Coefficient GB(p) r value GB(p) 1 4.8553 0.7814 0.903 NA NA NA 2 4.6692 0.8328 0.881 3.8396 0.9154 0.892 3 4.4210 0.9155 0.917 4.0180 0.8634 0.945 4 4.6286 0.8486 0.901 4.4710 0.7433 0.965 5 5.2932 0.6351 0.908 4.5377 0.7295 0.997 6 4.9966 0.7257 0.933 4.6478 0.7096 0.981 7 4.2052 0.9593 0.992 4.3649 0.8043 0.970 8 4.1994 0.9492 0.932 4.0022 0.9111 0.970 9 4.6598 0.8064 0.955 3.9509 0.9272 0.985 10 4.7950 0.7543 0.928 3.9373 0.9300 0.994 11 3.9914 0.9810 0.993 3.8234 0.9561 0.996 12 5.3210 0.5964 0.908 4.2293 0.8463 0.985 13 4.6473 0.7785 0.971 4.3599 0.8081 0.994 14 4.4486 0.8287 0.983 4.0404 0.8918 0.992 15 4.5968 0.7815 0.954 4.4404 0.7807 0.988 16 4.6118 0.7746 0.991 4.4515 0.7768 0.986 17 4.2226 0.8808 0.972 4.3918 0.7903 0.989 18 3.5879 1.0526 0.962 4.3059* 0.8088* 1.000 19 4.3551 0.8328 0.997 4.2695 0.8175 0.988 20 4.3449 0.8360 0.994 4.4124 0.7797 0.983 21 4.1621 0.8809 0.985 4.3422 0.7963 0.987 22 4.2308 0.8604 0.987 4.2783 0.8129 0.991 23 5. 1630* 0.5922* 0.705 NA NA NA 24 4.3756 0.8189 0.987 4.4521 0.7663 0.989 25 4.2320 0.8561 0.989 4.1347 0.8494 0.988 26 4.0288 0.9085 0.997 4.2699 0.8127 0.975 27 3.8883 0.9387 0.995 4.4668 0.7626 0.984 28 3.8464 0.9475 0.996 3.9946 0.8854 0.993 29 3.4217 1.0551 0.976 4.1875 0.8305 0.979 30 4.0737 0.8741 0.937 4.1472 0.8386 0.999 31 3.6730 0.9715 0.886 NA NA NA 32 4.0061 0.8852 0.876 NA NA NA 33 4.5576 0.7398 0.859 3.7206 0.9430 0.995 34 5.1867 0.5850 0.598 3.9220 0.9076 0.998 35 5.6621 0.4777 0.595 4.3345 0.8317 0.976 36 5.3978 0.5952 0.942 5.8817* 0.4347* 0.713 37 5.7937 0.6068 0.921 3.5138 1.1630 0.990 38 6.2246 0.5138 0.862 3.1768 1.3458 0.988 39 3.7247 0.9270 0.901 4.5084 0.9313 1.000 intervertebral cartilage [20 cm] and flesh on the snout and tail [10 cm]). Weight estimates have been calcu- lated from the formula derived by Foreman and Ishizuka ( 1990) for large Pacific bluefin and are pre- sented in Table 5. Age estimations included in Tables 5 and 6 are compiled from data presented by Bayliff (1994) that was based on fork-length estimates. However, Hales and Reitz (1992) cautioned that age data determined from modern population samples may differ from prehistoric populations. They report a distinct change in growth rates over time for Atlantic croaker, Micropogonias undulatus (Perciformes: Sciaenidae), from Florida, a change determined from the analy- sis of otolith growth increments from prehistoric samples. Compared with modern populations, croak- ers from populations of several centuries ago grew more slowly and lived much longer. 18 Fishery Bulletin 95(1 ), 1997 Although the results of the croaker study suggest that modern data relating size to age may not accu- rately predict the age of prehistoric fish specimens, no data are currently available to address this phe- nomenon for any population of bluefin tuna. Should bluefin tuna be shown to exhibit the same pattern as croaker, the archeological specimens of bluefin tuna reported here would actually represent fish older than those predicted by this analysis. However, length measurements were converted to age and weight estimates in this study primarily so that com- parisons could be made with modern tuna distribu- tion data, which are often reported by age class or weight. The critical point was to establish whether adult, rather than juvenile, tuna were more abun- dant in the archeological sample, because these age classes display distinctive behaviors and, more im- portantly, have different ecological requirements. Size of bluefin tuna represented by the archeological sample Table 5 presents the final length, weight, and age estimates of bluefin tuna by geographic area. By far the majority of fish within the total sample (83%) were at least 6 years or older, ranging between 160 Table 4 Archaeological bluefin tuna vertebrae measurements and fork length estimates, by vertebrae number. All specimens. Measure- ments are defined in Figure 2. Vertebra no. Centrum GL (mm) Centrum GB(p) (mm) Estimated FL (cm) Vertebra no. Centrum GL (mm) Centrum GB(p) (mm) Estimated FL (cm) 01 27.0 44.4 198.7 21 38.3 45.1 189.6 02 26.3 48.2 191.5 22 39.2 44.8 189.6 04 22.5 39.6 164.7 22 39.2 46.5 188.6 04 31.6 65.1 224.9 24 39.4 47.5 193.5 05 27.1 58.7 212.4 24 45.1 195.3 06 30.8 58.9 218.2 25 40.8 48.3 209.8 09 25.6 34.4 168.2 26 40.1 47.6 194.8 (09) 48.7 220.8 (28) 30.2 35.9 190.7 (09) 22.3 30.5 153.6 29 33.7 36.3 159.3 (10) 33.4 45.0 206.7 29 45.3 49.5 155.3 11 24.4 34.6 165.5 29 50.3 58.2 201.1 (11) 31.0 37.8 177.5 30 28.1 32.0 221.1 (12) 35.0 200.5 30 36.7 138.5 (12) 36.7 46.5 206.9 30 38.3 44.0 167.1 (12) 34.9 41.9 192.0 30 46.7 172.3 14 33.5 187.0 30 47.3 50.0 199.2 (14) 33.9 40.9 185.4 30 47.5 51.2 201.1 (14) 34.1 42.0 189.3 31 48.6 52.8 201.7 (14) 32.5 42.1 189.7 31 52.7 53.8 201.3 (14) 27.7 32.8 157.8 32 41.7 45.6 215.3 15 35.4 42.5 188.4 32 43.7 179.3 (15) 31.4 42.5 188.4 32 49.7 54.4 185.6 16 35.9 45.6 191.2 33 26.6 27.0 204.4 16 36.1 42.9 191.9 33 43.8 44.8 122.4 (16) 31.3 35.4 175.0 33 44.5 47.2 178.9 (16) 33.8 40.9 183.9 33 44.7 45.6 186.5 (16) 40.2 47.2 206.0 33 47.9 53.9 181.4 (16) 41.2 51.8 209.4 33 52.2 53.7 207.3 17 36.0 42.8 187.3 33 53.7 55.2 206.7 17 37.5 47.3 200.2 34 48.1 211.3 (17) 37.0 43.9 190.4 34 48.2 202.4 (17) 38.2 44.2 191.3 35 41.2 42.0 202.6 18 37.2 46.5 195.4 36 30.5 200.8 (18! 26.8 32.4 153.5 38 11.9 28.2 198.9 19 39.4 47.2 196.0 38 16.5 31.8 210.3 (19) 27.2 32.7 151.9 39 23.0 243.2 20 35.5 44.5 182.4 39 19.5 198.3 (20) 38.0 45.0 191.3 (20) 44.3 54.0 213.4 Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 19 Table 5 Archeological bluefin tuna length and age estimates, per individual represented, listed by geographic area. The length estimate associated with the highest correlation coefficient (r) for individuals represented by several elements is used here. See Figure 1 for locations. Specimen no. Site no. Vertebra no. Estimated fork length7 (cm) Estimated weight2 (kg) Estimated age class3 (yr) Barkley Sound, Vancouver Island, n = : 36 52 DfiSi5 33 122.4 47 4 84 DfSj23a 30 138.5 65 5 51 DfSi5 (19) 151.9 83 5-6 54 DfSj23a (18) 153.5 86 5-6 59 DfSj23a 29 155.3 88 5-6 30 DfSj23a (14) 157.8 92 5-6 58 DfSj23a 11 165.5 105 6 25 DfSi5 30 167.1 107 6 55 DfSj23a 09 168.2 109 6 85 DfSj23a 30 172.3 117 6-7 83 DfSj23a (16) 175.0 122 6-7 49 DfSi4 33 178.9 129 6-7 56 DfSj23a 33 181.4 134 7 48 DfSj23a (16) 183.9 139 7 61 DfSj23a 33 186.5 144 7 39E DfSi4 17 187.3 146 7 36C DfSi4 22 188.6 149 7 47 DfSi4 (14) 189.3 150 7 60 DfSj23a (14) 189.7 151 7 69F DfSj23b 26 190.7 153 7 86 DfSi4 (20) 191.3 154 7 87 DfSj23a 02 191.5 155 7 57 DfSi5 01 198.7 171 7-8 26 DfSj23b (12) 200.5 175 7-8 32 DfSi4 30 201.1 176 7-8 24 DfSj23a (16) 206.0 188 8 44 DfSj23a 33 206.7 190 8 31 DfSj23a (12) 206.9 190 8 68 DfSj23a (16) 209.4 197 8 45A DfSi4 24 209.8 198 8 64 DfSj23a 05 212.4 204 8 67 DfSj23b 31 215.3 212 8 63 DfSi4 06 218.2 219 8 72 DfSi4 (09) 220.8 226 8-9 66 DfSi4 29 221.1 227 8-9 80 DfSj23a 38 243.2 293 9-10 Hesquiat Harbour, Vancouver Island, n = 2 21 DiSol 32 186.0 143 7 20 DiSol (20) 213.4 207 8 Nootka Sound, Vancouver Island, n = 6 NA DkSpl (28) 159.0 94 5-6 NA DkSpl (11) 177.0 125 6-7 1 DjSp3 39 198.3 170 7-8 3E DkSpl 36 198.9 171 7-8 4 DkSpl (10) 206.7 190 8 2 DkSpl 38 210.3 199 8 Queen Charlotte Islands, n = 1 15E FaTt9 19 196.0 165 7-8 1 All raw data and calculations available from the author. 2 Log (weight, kg) = (-9.02408) + 2.6767 x log (length, cm) (Foreman and Ishizuka, 1990). 3 After Bayliff 1994a: 246. 20 Fishery Bulletin 95( 1 ), 1997 Tabie 6 Distribution of estimated age and size classes of bluefin tuna harvested within the Barkley Sound area only, based on archeological remains from Barkley Sound area sites. Estimated fork length (cm) Number of individuals Estimated age class (yr); 120-129 1 4 130-159 5 5-6 160-179 8 6-7 180-199 11 7-8 200-219 8 8 220-239 2 8-9 240-260 1 Total = 36 9-10 1 After Bayliff, 1994a: 246 (data for vertebrae only). and 240 cm TL and between approximately 96 to 293 kg in weight. The youngest fish was estimated at 4 years (120 cm TL) and the oldest between 9 and 10 years (240 cm TL). Of the total sample of 45 indi- viduals, 36 were recovered from the Barkley Sound area on the southwest coast of Vancouver Island, and the range of sizes from that area is summarized in Table 6. The relative size range of the bluefin tuna vertebrae harvested from Barkley Sound is shown pictorally in Figure 3. Ethnographic and historic information Information from ethnographic sources substantiates and augments archeological evidence indicating that large bluefin tuna were present and harvested by Nuu-chah-nulth people of Vancouver Island well into the 19th century. Elders of the Mowachaht group from Nootka Sound on Vancouver Island have con- tributed invaluable details about tuna hunting strat- egies employed by their elders, some through inter- views with Richard Inglis of the Royal British Co- lumbia Museum, Victoria, British Columbia during 1991 and 1992 (Inglis2). These accounts represent the only ethnographic description of aboriginal tuna hunting on the northwest coast (McMillan, 1979). Pertinent details that substantiate the occurrence of adult bluefin tuna during the historic period are presented here. The month of August is said to have been the time when tuna could be found feeding at the surface in inshore waters (sea-surface temperatures during Au- gust usually average about 14°C [Sharp, 1978]). The occurrences of large tuna were apparently preceded and accompanied by recognizable changes in water and weather conditions and by a unique set of asso- ciated fauna. Tuna traveled well inside Nootka Sound into protected inlets and were harpooned at night as 2 Inglis, R. 1993. British Columbia Ministry of Aboriginal Af- fairs, #100-1810 Blanchard St., Victoria, British Columbia, Canada V8V 1X4. Personal commun. A BC Figure 3 Selected vertebrae from Barkley Sound archeological deposits. See Figure 1 for locations. (A) Specimen no. 44, vertebra no. 33; site DfSj 23a; length ca. 210 cm; (B) Specimen no. 61, vertebra no. 33; site DfSj 23a; length ca. 190 cm; and (C) Specimen no. 52, vertebra no. 33; site DfSi 5; length ca. 120 cm. Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 21 they fed at the surface in shallow inshore waters (lo- cated by spotters positioned on nearby cliffs). Biolu- minescent plankton present in the water made the big fish especially visible at night, even from a distance. A fire was sometimes built in the bow of the hunter’s canoe to attract the fish to within spearing distance, a strategy called “pit-lamping.” Another method was to paddle the canoe quickly away from an area where tuna were spotted: the canoe created a path of light as it moved through the biolumines- cence. The tuna would follow the light, right up to and under the canoe, and were harpooned as they emerged at the bow. The word for tuna (“silthkwa”) means “like the bow wave made by a boat,” and un- doubtedly reflects their surface-feeding behavior. These tuna were always referred to as “big fish, 6 to 8 feet (ca. 180-244 cm TL) long.” George Louis of the Ahousat Band was about 80 years old when interviewed in 1992. He said that his father told a story about the tuna hunting he ob- served as a small boy (perhaps when about 10 years old) sometime between 1880 and 1890. No official records or unofficial accounts have been found which indicate that large tuna have been observed in Brit- ish Columbia waters since that time. Large bluefin tuna were captured, however, by sport anglers dur- ing the 1890’s in southern California (Holder, 1913). The only written reference to tuna found to date in the historic record is a footnote in the account of a meeting between George Vancouver and Bodega y Quadra at Nootka Sound in 1792. Mention is made of a porpoise and tuna stew (“large Tunny and a Porpus”) being served during a feast given in their honor by Nuu-chah-nulth chief Maquinna on 4 Sep- tember 1792 (Lamb, 1984, p. 304). There is, of course, no way of knowing if the “tuna” was bluefin tuna, some other tuna species, or some other taxon alto- gether. The capture of porpoise, however, would have required similar hunting skills and equipment as those described above for bluefin, and both could have been caught during a single hunting expedition. “Por- poise” remains are reported from a number of coastal shell middens (Mitchell, 1988) and are most likely to be either harbour porpoise, Phocoena phocoena, white-sided dolphin, Lagenorhyncus obliquidens, or Dali’s porpoise, Phocoenoides dalli (Leatherwood et al., 1988). Moreover, bluefin tuna (even very large ones) would have been quite familiar to the Europe- ans exploring the coastal waters of British Colum- bia because the similar Atlantic subspecies occurs in European coastal waters. In marked contrast to the many unknown species regularly encountered by explorers in the north Pacific, large bluefin tuna might have been so familiar that they did not war- rant special comment. Discussion Archeological evidence and potential sampling bias The archeological remains described above represent a size class of bluefin tuna previously unknown in the northern portion of the eastern Pacific and con- stitute a small but valuable biological sample of the ancient population. However, some of the cultural and taphonomic (postdepositional) influences that affected the sample must be considered before eco- logical or zoogeographic interpretations can be made. The archeological shell middens from which the bluefin tuna specimens have been recovered are es- sentially garbage dumps created over many centu- ries by the disposal of food and other household waste. The calcium carbonate leaching from abun- dant shellfish remains in these midden deposits ef- fectively neutralizes acids in the soils that would oth- erwise rapidly destroy bone. Preservation of verte- brate skeletal remains is often excellent under these conditions, even after several thousand years. The bones of animals recovered during archeologi- cal excavation of a shell midden represent a very small portion of the animals harvested by aboriginal people. Many processes operate on the carcass of a harvested animal to reduce the number of bones that might eventually be discarded into a midden ( Davis, 1987; Lyman, 1994). These include butchering meth- ods, distribution of edible parts (sharing), cooking procedures, and consumption of the edible portions. Some bones may have been set aside for tool or orna- ment manufacture (only one piece of altered bluefin tuna has been recovered: a vertebra fashioned into a spool, from the Ozette Village site in Washington). Moreover, scavengers, especially dogs and birds, may have removed or destroyed parts of a carcass so that in the end only a few bones from any given animal are represented in the midden. Finally, only small portions of most large midden deposits are actually excavated by archeologists, further reducing the sample of harvested animals available for archeo- zoological analysis (Ringrose, 1993, for detailed dis- cussions of these issues; Lyman, 1994). For example, the remains of the 36 individual bluefin tuna recov- ered from Barkley Sound (Table 6) represent an em- pirically undeterminable fraction of what was actu- ally harvested and consumed by the aboriginal people in that area. In addition, the number of fish success- fully landed constituted a very small proportion of the available population of bluefin tuna. Presumably only a few bluefin tuna would have been actively pursued and some of these would invariably have been lost during the hunt. Thus, even if only one gi- 22 Fishery Bulletin 95(1 ), 1997 ant bluefin tuna was successful harvested every few seasons by native hunters, this could still constitute evidence of a significant population of tuna available as a local resource. Unfortunately, the time interval between catches of bluefin is not precisely determinable from the dated archeological deposits; it is impossible at this time to determine if catches were made annually, every 10 years, or every 100 years. Although expen- sive, the use of accelerator 14C-dating methods on small samples of bluefin tuna remains is the only way to determine a more precise time frame. The remains of bluefin tuna recovered from Barkley Sound during several recent field seasons are per- haps the best candidates for future analysis because there are many vertebral and nonvertebral skeletal elements and the remains appear to represent less than 2,000 years of harvesting activities (McMillan and St. Claire3). Geographic range of prehistoric bluefin tuna remains As discussed at greater length previously (Crockford, 1994), it appears probable that the ability to hunt large Pacific bluefin tuna was strongly correlated with native groups who were capable of active whal- ing. This possible correlation with active whaling rather than with the use of so-called “drift” whales (which die naturally and are fortuitously encountered at sea or as beached carcasses) is important. No other archeological sites in western North America or northeastern Asia appear to contain remains of large bluefin tuna. No large bluefin tuna have been re- ported from sites in southern California where adult tuna are occasionally taken today, although the re- mains of other large fish, such as marlin, have been identified and large marine mammals, such as sea lions, were clearly taken (Moratto, 1984; Raab4). We cannot assume, however, that large bluefin tuna were not present in southern California waters during prehistoric times because a lack of whaling technol- ogy may have prevented aboriginal Californians from harvesting such a resource. In northern Japan, active whaling is not clearly indicated by the archeological record although hunt- ing of sea lions and other large marine mammals was practiced. Large bluefin tuna remains have not been 3 McMillan, A. D., and D. E. St. Claire. 1992. The Toquart ar- chaeology project: report on the 1992 excavations. Archaeology Branch, British Columbia Ministry of Small Business, Tourism and Culture, 800 Johnson St. Victoria, British Columbia, Canada V8W 1N3: permit 1991-46. Unpubl. rep., 100 p. 4 Raab, M. 1994. Anthropology Department, California State University, Northridge, CA 91330. Personal commun. reported from archeological sites bordering the Sea of Okhotsk and the Sea of Japan where large bluefin tuna occur today (Niimi, 1994; Otaishi, 1994), but it appears that not many large sites in these areas have been excavated. As in the case for California, it would be inappropriate, given the absence of evidence for an active whaling technology, to suggest that adult bluefin tuna were absent in Japanese waters during prehistoric times. In contrast, the recovery of large bluefin tuna among dated archeological deposits that span almost 5,000 years is evidence that the occurrence of adult bluefin tuna off the British Columbia coast was longstanding. Clearly, large bluefin tuna were a re- source consistently (if sporadically) available to ab- original people on the central northwest coast until relatively recently. The Nuu-chah-nulth people, in particular, were especially adept at using this re- source, and their material culture included large sea- going canoes, detachable harpoon heads, braided ropes, and floats required for the successful hunting of both whales and large tuna (Huelsbeck, 1983; Mitchell and Donald, 1988). Archeological remains are, by inference, invaluable indicators that the en- vironmental conditions that favored the presence (i.e. the inshore surface-feeding behavior ) of bluefin tuna must have existed off the coast of British Columbia as a recurring pattern for at least 5,000 years. Implications The lack of reports of adult bluefin tuna off the Brit- ish Columbia coast since the late 19th century may be due to several factors, including the impact of 20th- century fisheries in both the eastern and western Pacific, the association of large bluefin in northern waters of the eastern Pacific with very specific envi- ronmental conditions that have not recurred since the late 19th century, and the misidentification of small schools of large bluefin tuna as marine mammals. Although relative abundance records over the past 100-150 years are not available for Pacific bluefin tuna, it has been shown for other species that when abundance decreases, the range of a species often contracts (Kawasaki, 1991). In order to investigate how 20th-century fisheries may have impacted abun- dance and thus the distribution of bluefin tuna, a comprehensive record of the history of the bluefin tuna fishery as conducted by all nations throughout the north Pacific would be needed. This is especially true for Japanese waters because of the use there of large-scale harvesting methods. It is also possible, however, that short- or long-term (or both) changes in environmental conditions may be affecting bluefin tuna distributions in the east- Crockford: Archeological evidence of Thunnus thynnus off British Columbia and northern Washington 23 era Pacific (Rothschild, 1991). Hubbs (1948) partially addressed this issue in his presentation of evidence that mean water temperatures in southern Califor- nia were warmer in the mid-to-late 1800’s ( 1850-80). Such water temperatures appeared to be associated with distinctly tropical fauna that no longer occur so far north. This period corresponds roughly to that mentioned in the northwest coast ethnographic ac- counts as the last time when large tuna were hunted and may reflect a recurring pattern of occasional warm periods along the whole coast of North America. Because the surface-feeding behavior of large blue- fin tuna makes them very conspicuous in inshore waters, it would be extremely unlikely for adult tuna to go totally unnoticed for the last 100 years in Brit- ish Columbia waters (even if they could not be caught or were indeed mistaken for marine mammals in deeper waters). It seems reasonable to assume un- der the circumstances that modern records are cor- rect: large adult bluefin tuna have not frequented the northern waters of the eastern Pacific during the last 100 years. The reasons for their absence, how- ever, remain to be determined. Clearly, more investigation into the history of the distribution and harvesting of all age classes of blue- fin tuna within the entire north Pacific will be nec- essary before we really understand the implications of the archeological remains reported in this study. Complex interactions of changes in ecological condi- tions and harvesting pressures on various age classes over the last 100 years probably have affected and may have had unexpected repercussions on the popu- lation structure of Pacific bluefin tuna. A better un- derstanding of the distribution of adult tuna in the north Pacific through inclusion of archeological records may help document perturbations in the modern fishery. Acknowledgments The author would like to thank Richard Inglis, for- merly of the Royal British Columbia Museum, Victoria, British Columbia, for his collection of eth- nographic information and Bruce Collette at the Smithsonian Institution, Washington D.C., for con- firmation of the taxonomic identity of the archeologi- cal material and for the loan of comparative speci- mens. Terry Foreman, a former associate of the In- ter-American Tropical Tuna Commission, La Jolla, California, and Skip McKinnel, of the Department of Fisheries and Oceans Canada, Nanaimo, British Columbia, both supplied unbridled enthusiasm, en- couragement and advice. The Royal British Colum- bia Museum in Victoria provided assistance in han- dling loaned material and photography. Greg Monks of the University of Manitoba, Winnipeg, helped enor- mously by setting aside tuna vertebrae as they were excavated from archeological deposits in Barkley Sound and allowed me immediate access to the ma- terial. This paper has been greatly improved through revision of earlier drafts by several anonymous re- viewers; their efforts are much appreciated. Raw data are available on request from the author. Literature cited Bayliff, W. H. 1980. Synopsis of biological data on the northern bluefin tuna, Thunnus thynnus (Linnaeus, 1758), in the Pacific Ocean. Inter-Am. Trop. Tuna Comm., Spec. Rep. 2:262-293. 1993. Annual report of the Inter-American Tropical Tuna Commission, 1993, W. H. Bayliff (ed). IATTC, La Jolla, CA, 91 p. 1994. A review of the biology and fisheries for northern blue- fin tuna, Thunnus thynnus , in the Pacific Ocean. In R. S. Shomura, J. Majkowski, and S. Langi (eds.), Interactions of Pacific tuna fisheries, p. 244-295. FAO Fish. Tech. Pa- per 336(2). Bayliff, W. H., Y. Ishizuka, and R. B. Deriso. 1991. Growth, movement, and attrition of northern blue- fin tuna, Thunnus thynnus, in the Pacific Ocean, as deter- mined by tagging. Inter-American Tropical Tuna Comm. Bull. 20(11:1-94. Bell, R. R. 1963. Synopsis ofbiological data on California bluefin tuna Thunnus saliens Jordan and Evermann 1926. FAO Fish. Rep. 6<2):380-421. Calvert, S. G. 1980. A cultural analysis of faunal remains from three archaeological sites in Hesquiat Harbour, British Colum- bia. Ph.D. diss., Univ. British Columbia, Vancouver, 336 p. Calvert, S. G., and S. J. Crockford. 1982. Appendix IV. In A. D. McMillan and D. E. St. Claire (eds.), Alberni prehistory: archaeological and ethnographic investigations on western Vancouver Island, p.174- 219. Theytus Books, Penticton, British Columbia. Cannon, A. 1991. The economic prehistory of Namu. Department of Archaeology Publ. 19, Simon Fraser Univ., Burnaby, Brit- ish Columbia, 107 p. Casteel, R. W. 1976. Fish remains in archaeology and palaeoenviron- mental studies. Academic Press, San Francisco, CA, 180 p. Crockford, S. J. 1994. New archaeological and ethnographic evidence of an extinct fishery for giant bluefin tuna (Thunnus thynnus orientalis) on the Pacific Northwest coast of North America. In W. Van Neer (ed.), Ann. Mus. R. Afri. Cent. Ser. Quarto Zool., p. 163-168. Sciences Zoologiques 274, Tervuren, Belgium. Davis, S. J. M. 1987. The archaeology of animals. Yale Univ. Press, New Haven, CT, 224 p. Dotson, R. C. and J. E. Graves. 1984. Biochemical identification of a bluefin establishes a new California size record. Calif. Fish Game 70( 1 ):62-64. 24 Fishery Bulletin 95( 1 ), 1997 Foreman, T. J. and Y. Ishizuka. 1990. Giant bluefin tuna off southern California, with a new California size record. Calif. Fish Game 76(3): 181-186. Hales, L. S., Jr., and E. J. Reitz. 1992. Historical changes in age and growth of Atlantic croaker, Micropogonias undulatus (Perciformes: Sciaen- ldae). J. Archaeol. Sci. 12:73-99. Hart, J. L. 1973. Pacific fishes of Canada. Fish. Res. Board Can. Bull. 180, 740 p. Holder, C. F. 1913. The game fishes of the world. Hodder and Stough- ton, New York, NY. Hubbs, C. L. 1948. Changes in the fish fauna of western North America correlated with changes in ocean temperature. J. Mar. Res. 7:459-482. Huelsbeck, D. R. 1983. Mammals and fish in the subsistence economy of Ozette. Ph.D. diss., Washington State Univ., Pullman, WA, 166 p. Kawasaki, T. 1991. Long-term variability in the pelagic fish popu- lations. In T. Kawasaki, S. Tanaka, Y. Toba, and A. Taniguchi (eds), Long-term variability of pelagic fish populations and their environment, p. 47-60. Pergamon Press, Oxford. Lamb, W. K. (ed.). 1984. A voyage of discovery to the North Pacific Ocean and round the world: 1791-1795, vol. II. The Hakluyt Soc., London, 437 p. Leatherwood, S., R. R. Reeves, W. F. Perrin, and W. E. Evans. 1988. Whales, dolphins, and porpoises of the Eastern North Pacific and adjacent arctic waters: a guide to their identi- fication. Dover Publications, Inc., New York, NY, 245 p. Lyman, R. L. 1994. Vertebrate taphonomy. Cambridge Manuals in Ar- chaeology, Cambridge Univ. Press. Marshall, Y. M. 1993. A political history of the Nuu-Chah-nulth people: a case study of the Mowachaht and Muchalaht tribes. Ph.D. diss., Simon Fraser Univ., Burnaby, B.C., 467 p. McMillan, A. D. 1979. Archaeological evidence for aboriginal tuna fishing on western Vancouver Island. Syesis 12:117-119. Mitchell, D. 1988. Changing patterns of resource use in the prehistory of Queen Charlotte Strait, British Columbia. In B. L. Isaac (ed.), Research in economic anthropology, supplement 3: prehistoric economies of the Pacific Northwest coast, p. 245-290. JAI Press, Inc., Greenwich. Mitchell, D., and L. Donald. 1988. Archaeology and the study of Northwest coast eco- nomies. In B. L. Isaac (ed). Research in economic anthro- pology, supplement 3: prehistoric economies of the Pacific Northwest coast, p. 293-351. JAI Press, Inc., Greenwich. Moratto, M. J. 1984. California archaeology. Academic Press, San Fran- cisco, CA, 756 p. Nakamura, H. 1969. Tuna distribution and migration. Whitefriars Press, London, 76 p. Neave, F. 1959. Records of fishes from waters of the British Colum- bian coast. J. Fish. Res. Board Can. 16(31:383-384. Niimi, M. 1994. Sea mammal hunting in northern Japan during the Jomon period. Archeozoologia VI/2:37-56. Otaishi, N. 1994. The Okhotsk culture: prehistoric hunting and fish- ing reconstructed from the Sea of Okhotsk paleofauna. Archaeozoologia VI/2:71-78. Radovich, J. 1961. Relationships of some marine organisms of the north- east Pacific to water temperatures, particularly during 1957 through 1959. Calif. Fish Game, Fish. Bull. 112, 62 p. Recksiek, C. W., and H. W. Frey. 1978. Biological, oceanographic, and acoustical aspects of the market squid (Loligo opalescens Berry). Calif. Fish Game, Fish. Bull. 169, 185 p. Ringrose, T. J. 1993. Bone counts and statistics: a critique. J. Archaeol. Sci. 20:121-157. Rivas, L. R. 1978. Preliminary models of annual life history cycles of the north Altantic bluefin tuna. In G. D. Sharp and A. E. Dizon (eds.), The physiological ecology of tunas, p. 369- 393. Academic Press, New York, NY. Rothschild, B. J. 1991. On the causes for variability of fish population — the linkage between large and small scales. In T. Kawasaki, S. Tanaka, Y. Toba, and A. Taniguchi (eds). Long-term vari- ability of pelagic fish populations and their environment, p. 367-376. Pergamon Press, Oxford. Sharp, G. D. 1978. Behavioral and physiological properties of tuna and their effects on vulnerability to fishing gear. In G. D. Sharp and A. E. Dizon (eds), The physiological ecology of tunas, p. 397-450. Academic Press, New York, NY. Smith, P. J., A. M. Conroy, and P. R. Taylor. 1994. Biochemical-genetic identification of northern blue- fin tuna Thunnus thynnus in the New Zealand fishery. N.Z. J. Mar. Freshwater Res. 28:113-118. Wheeler, A., and A. K. G. Jones. 1989. Fishes. Cambridge Univ. Press, Cambridge, 210 p. 25 Abstract— During the summer of 1987 in Coos Bay, Oregon, dietary over- lap (Schoener index) between juvenile fall-run chinook salmon, Oncorhynchus tshawytscha, and an introduced stock of juvenile hatchery-reared spring-run chinook salmon was high (0.82), indi- cating the potential for competition for food between these two groups in times of food scarcity. Both groups consumed a variety of prey, including fishes, adult insects, algae, barnacle molts, gam- marid and caprellid amphipods, and juvenile decapods. Diets of both salmon groups varied with fish size and cap- ture location. Overlap was low (0.25- 0.55) between the smallest juvenile fall chinook salmon (<80 mm FL), for which insects were the predominant prey (26% by weight), and all other length groups of both fall and spring chinook salmon, for which fish were the predom- inant prey (49%-94% by weight). Dietary overlap between both salmon groups was high in the lower bay (0.82), where fish prey predominated in the diets, and was also high in the mid bay (0.75), where algae and barnacle molts pre- dominated in the diets. Three pieces of evidence suggest that the introduced hatchery-reared spring chinook salmon did not outcompete fall chinook salmon for food: 1) both the median stomach fullness and the percentage of stomachs containing food was higher for fall chinook salmon than for spring chinook salmon, 2) the median stomach fullness of fall chinook salmon was as high in the period following releases of spring chinook salmon into the bay as in the period prior to the releases, and 3 ) food of high caloric density (i.e. fish prey) formed an equally high proportion of the diets of both salmon groups, indi- cating that the quality of food eaten by both was similar. Manuscript accepted 31 July 1996. Fishery Bulletin 95:25-38 (1996). Dietary overlap of juvenile fall- and spring-run chinook salmon, Oncorhynchus tshawytscha, in Coos Bay, Oregon Joseph R Fisher William G. Pearcy College of Oceanic and Atmospheric Sciences Oregon State University, Corvallis, Oregon 97331-5503 E-mail address: fisherjo@ucs.orst.edu Estuaries serve as rich feeding grounds and as refuges from preda- tion for many juvenile subyearling fall-run (hereafter referred to as “fall”) chinook salmon, Oncorhyn- chus tshawytscha, that reside in them for weeks or months before entering the ocean (Healey, 1980a, 1982, 1991; Myers, 1980; Kjelson et al., 1982; Myers and Horton, 1982; Simenstad et al., 1982). The sur- vival of subyearling fall chinook salmon may be enhanced by ex- tended residence in estuaries. Reimers (1973) reported that sur- vival was greater among juvenile fall chinook salmon that resided in the Sixes River estuary from early summer through early fall than among those that quickly migrated through the estuary to the ocean in early summer. Although there is much concern about the interaction between hatchery and wild stocks of salmon (Hilborn and Winton, 1993; Thomas and Mathisen, 1993; Winton and Hilborn, 1994), few reports docu- ment possible competition between groups of salmon for food in estuar- ies or the ocean. Peterman (1984) and Rogers and Ruggerone (1993) found negative correlations between size of sockeye salmon at different ages and their population and sug- gested that growth of sockeye salmon in the ocean was density dependent. Reimers (1973) and Neilson et al. ( 1985) found that the average growth rate of juvenile fall chinook salmon in the Sixes River estuary decreased during mid-sum- mer when the population of juvenile salmon was high. Reimers (1973) attributed this drop in growth rate to intraspecific competition for lim- ited food resources, leading to den- sity-dependent growth, whereas Neilson et al. (1985), noting that the decrease in growth rate occurred dur- ing a period of increased abundance of the principal prey (Corophium sp. ), suggested that lowered conversion efficiencies due to high temperatures in the estuary as well as intraspecific competition may have contributed to the drop in growth rate. These studies suggest that re- leases of large numbers of hatchery salmon smolts into an estuarine basin could affect the native salmon in the system through competition for food in the estuary. The effect of competition on growth and survival of native fish would depend on sev- eral factors, among them the inten- sity and duration of the competition between the two groups. If the hatchery-reared fish eat different prey from that eaten by the wild fish, or if they move quickly through the estuary, their impact on the native fish may be relatively small. On the other hand, if the two groups 26 Fishery Bulletin 95 ( I ), 1997 have similar feeding behaviors and if hatchery fish re- side in the estuary for a substantial period, then the effect of hatchery fish on the wild fish may be great. Anadromous, Inc. operated a salmon-rearing and release facility on the North Spit of Coos Bay, Or- egon in the 1980’s. From this facility millions of smolts are released into the bay annually, principally large subyearling spring-run (“spring”) chinook salmon, thus creating the potential for competition between these hatchery-produced spring chinook salmon and the native runs of fall chinook salmon in the Coos Bay drainage. During the late spring and summer of 1987 we undertook a sampling program in the lower half of Coos Bay to study the use of the estuary by different groups of juvenile chinook salmon. In 1987 two groups of juvenile chinook salmon were present in Coos Bay: fall chinook salmon from the Coos and Millacoma River drainages (both wild fish and fish released by the Salmon and Trout Enhancement Pro- gram | STEP]) and spring chinook salmon released from the saltwater rearing pens of the Anadromous, Inc. facility, North Spit of Coos Bay (Fig. 1). About 400,000 STEP fall chinook salmon were released in tributaries of the Coos River between 30 April and 28 June at average fork lengths (FL) of between 48 and 94 mm, and over five million spring chinook salmon (123-156 mm FL) were released from the Anadromous, Inc. release facility on North Spit be- tween 19 June and 1 October. In an earlier paper (Fisher and Pearcy, 1990) we reported on the distri- butions and residence times of juvenile spring and fall chinook salmon in the bay. In this paper we de- scribe the food habits of these two groups, overlap in their diets, and the potential for competition for food between them. Fisher and Pearcy: Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 27 Fall Chinook salmon Stomachs 225 Prey identified from 116 Spring Chinook salmon Stomachs 155 Prey identified from 65 90 100 110 120 130 140 150 160 170 180 Fork length (mm) + Stomachs sampled Stomach contents identified Fin-marked (fall chmook) or adipose-clipped (spring chmook) Figure 2 Length-frequency distributions of juvenile fish classified as fall and spring chinook salmon from which stomach samples were taken. Dark shading represents those stomach samples in which prey species were examined and identified. White bars in the up- per and lower graphs represent numbers of fin-clipped STEP- reared fall chinook salmon and adipose clipped Anadromous, Inc.- reared spring chinook salmon, respectively. Methods Juvenile chinook salmon were caught by beach seine (60 m x 2.5 m with 19- and 13-mm mesh in the wings and bunt, respectively) at five locations on the margins of channels in the lower half of Coos Bay, Oregon, between late May and early October 1987 (Fig. 1). The sub- stratum was sand at all but station 5, where it was a mixture of gravel fill and mud. At sta- tions 2, 3, and 4, portions of eel grass beds were sampled during low tide. The area of Coos Bay we sampled was influenced strongly by the ocean and was highly marine in char- acter with high salinities at all sampling sites, usually greater than 29 psu after mid-June. Water temperature (at 0.3 m depth) was fairly constant between May and October but in- creased with distance from the mouth, averag- ing 12.3°C at station 1 and 16.8°C at station 5 (Fisher and Pearcy, 1990). Subsamples of juvenile chinook salmon caught in beach-seine sets were preserved in approximately 4% formaldehyde solution. Later, these were measured to the nearest mm FL and weighed to the nearest 0.01 g after excess moisture was removed by blotting. Stomachs were removed from 380 juvenile chinook salmon caught between 31 May and 4 September 1987 (Fig. 2). Stomach-content boluses were weighed to the nearest milligram after removing excess moisture by blotting. After weighing, they were preserved in 50% ethanol, then transferred to 75% ethanol. At the time the stomach samples were ob- tained, the fish were examined for fin marks or for external parasites that could help to determine their origin. Most fish with clipped adipose fins also contained coded wire tags (CWT’s) that identified them as spring chinook salmon produced at Anadromous, Inc. Fish with other fin clips were mainly STEP-reared fall chinook salmon released in freshwater tributaries of Coos Bay (Fisher and Pearcy, 1990). The encysted metacercarial stage of a strigeoid trematode parasite is common in the skin of juvenile salmonids found in freshwater tributaries of Coos Bay. These cysts are surrounded by a black pigment that can be seen easily without magnification (Amandi1). The presence of metacercarial cysts on the skin or fins of juvenile chinook salmon caught in 1 Amandi, T. 1995. Oregon Dep. Fish and Wildl., 516 Nash Hall, Oregon State Univ., Corvallis, OR 97331. Personal commun. Coos Bay appeared to be a reliable indicator that the fish originated in the freshwater tributaries of the bay. Cysts were present on 43% of known fall chinook salmon (fin-marked STEP fish or fish caught before the first release of spring chinook salmon) and on 71% of small fish <101 mm FL (>2SD below the mean FL of most release groups of spring chinook salmon by Anadromous, Inc.). Conversely, cysts were absent on adipose-clipped spring chinook salmon and found on only 13% of fish in the size range of the spring chinook salmon released by Anadromous, Inc. (>101 mm FL). Fish >100 mm FL with cysts were probably native salmon or STEP-reared fall chinook salmon that attained these greater lengths through growth. On the basis of this evidence, we classified fish caught in Coos Bay as fall chinook salmon if they met any of 28 Fishery Bulletin 95(1), 1997 the following criteria: 1) they were caught before the first release of Anadromous, Inc. spring chinook salmon on 19 June; 2) metacercarial cysts were present on their skin or fins; 3) they had one of the STEP fin clips; or 4) they were <100 mm FL. Fish were classified as spring chinook salmon if they were >101 mm FL and did not meet any of the criteria for fall chinook salmon. Stomach contents were examined and prey items identified to the lowest possible taxon from 116 fall chinook salmon and 65 spring chinook salmon col- lected between 29 June and 13 August 1987, the pe- riod of greatest overlap in the bay of the two groups (Fig. 2). Stomach contents from a single fall chinook salmon caught on 7 June were also examined. Individual prey taxa in each stomach were weighed to the nearest 0.001 g after removing excess mois- ture by blotting. Those taxa that were too light to register on the scale (weight <0.0005 g), were as- signed a weight of 0.0004 g. The estimated total weight of all food assigned this arbitrarily small value was only 0.05 g out of a total weight of 60.2 g for all taxa from all stomachs. In the analyses of stomach contents, juvenile fall and spring chinook salmon were grouped by FL, by two collection areas (“lower bay,” stations 1-3, and “mid-bay,” stations 4-5) and by two sampling peri- ods: 29 June to 17 July and 3-13 August. Within each class, the percent frequency of occurrence (FO) and percent by weight of each prey category in the diet was calculated. The percent by weight (p.xlOO) of each prey category in each class was calculated as 100(p,-)= 100 N n V <7=11=1 (1) where wiq is the weight of food category i in fish q, n is the number of food categories, and N is the num- ber of fish in the class. Dietary overlap between classes was calculated by using the Schoener overlap index (ro; Schoener, 1970; Wallace, 1981; Linton et al., 1981): where ptj and pik are the proportions by weight of food category i (Eq. 1) in the diets of fish in classes j and k, respectively, and n is the number of food cat- egories. Dietary overlap was calculated by using 14 categories of major prey and, because the overlap index is sensitive to the taxonomic resolution (Brodeur and Pearcy, 1992), it was also calculated by using the 86 lowest taxonomic levels identified (to genus or species in some cases). An overlap of >0.60 was considered significant (Zaret and Rand, 1971; Brodeur and Pearcy, 1992). Results Stomach fullness The frequency distribution of stomach-content weight as a percentage of body weight (“stomach fullness”) was skewed for both fall and spring chinook salmon (Fig. 3); therefore, nonparametric ranks tests were used to compare stomach fullness among different classes of fish. The median stomach fullness was higher for fall chinook salmon than for spring chinook salmon (2.4% vs. 1.2%, respectively; Mann Whitney (Wilcoxon) W test, W=ll,630, P<0.0001). Stomachs were empty in a higher percentage of spring chinook salmon than of fall chinook salmon (16% vs. 1%), contributing to the difference in median stomach fullness of these two groups (Fig. 3). Fisher and Pearcy. Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 29 Range in stomach fullness was similar among fish of different lengths, and stomach contents weights of 8% of body weight or higher occurred in fish from 69 mm to 145 mm FL (Fig. 4). No significant differ- ence in median stomach fullness was found among four FL classes (<80 mm, 81-100 mm, 101-120 mm, and 121-140 mm) of fall chinook salmon (Kruskal- Wallace test, P=0.09). However, a significant differ- ence in median stomach fullness was found among the three FL classes (101-120 mm, 121-140 mm, and >141 mm) of spring chinook salmon (Kruskal-Wallace test, P=0.03). Median stomach fullness was lowest (0.4%) for the largest spring chinook salmon (>141 mm FL). Median stomach fullness of fall chinook salmon was fairly constant during the study period, both before and after spring chinook salmon were released into the bay. No short-term decreases in stomach full- ness of fall chinook salmon were associated with in- dividual releases of spring chinook salmon, except for the 4 August release (Fig. 5). Conversely, median stomach fullness of spring chinook salmon was low immediately following releases of large numbers of spring chinook salmon from the Anadromous, Inc. C 0) Fork length (mm) Figure 4 Weight of stomach contents as a percentage of body weight versus fish length for juvenile fall chinook salmon (top graph) and spring chinook salmon (bottom graph). facility, especially the 4 August and the August 31-3 September releases (Fig 5). Diets of fall and spring chinook salmon Percent FO and percent by weight of fourteen major prey categories from stomachs of juvenile fall and spring chinook salmon are summarized in Table 1. By weight, juvenile or larval fish were dominant prey of both fall and spring chinook salmon, representing 64% and 65% of the total weight of stomach contents, respectively. The fish prey of fall chinook salmon were juvenile smelt, unidentified fish remains, Ammodytes hexapterus, juvenile Sebastes sp., and an unidenti- fied cottid, representing 41%, 10%, 8%, 6%, and <1% of stomach-content weight, respectively. Fish prey of spring chinook salmon were similar: juvenile smelt, Ammodytes hexapterus , unidentified fish remains, and Sebastes sp., accounted for 49%, 13%, 3%, and <1% of stomach-content weight, respectively. Other prey categories accounted for much smaller fractions of stomach-content weights of the two groups of juvenile chinook salmon. Of the nonfish prey, insects and plants (mainly algae) composed the largest fractions by weight in stomachs of fall chinook salmon (8% and 7%, respectively), whereas plants (mainly the algae Ulva sp. and Enteromorpha sp.) and barnacle molts composed the largest fractions by weight in stomachs of spring chinook salmon ( 16% and 12%, respectively; Table 1). The most numerous insects2 in fall chinook salmon stomachs were adults of terrestrial taxa (61% of the total) and adults of taxa having aquatic or semi- aquatic larvae (36% of the total). Larvae and pupae composed only 3% of the total number of individu- als. Adults in the orders Diptera, Hemiptera, Homoptera, Pscoptera, Hymenoptera, Coleoptera, and Trichoptera accounted for 33%, 23%, 15%, 10%, 7%, 6%, and 2% of the total number of insects in fall chinook salmon stomachs, respectively. The most numerous taxa in these insect orders (and their per- centages of total insect numbers) were midges (Chironomidae; 25%), plant bugs (Miridae; 22%), aphids (Aphididae; 11%), book and bark lice (10%), parisitoid wasps (5%), rove beetles (Staphylinidae; 4%), and caddis flies (2%), respectively. Although insects were a much larger fraction by weight of the diet of fall chinook salmon than of the diet of spring chinook salmon ( 8% vs. 1%, respectively, Table 1 ), they occurred frequently in stomachs of both salmon groups (80% and 60%, respectively). Many of the same insect taxa were consumed by both fall 2 The different insect taxa were not weighed separately, but in- dividuals of each taxon were counted. 30 Fishery Bulletin 95 ( 1 ), 1997 and spring chinook salmon. The most numerous in- sects from spring chinook salmon stomachs were chironomids (31%), book and bark lice (23%), aphids (9%), tipulids (crane flies, 8%), and plant bugs (4%). Other prey categories that occurred frequently in stomachs of both fall and spring chinook salmon were barnacle molts (47% and 51%, respectively), algae and other plant material (46% and 68%), gammarid amphipods (41% and 43%), fishes (40% and 37%), and crab larvae (27% and 35%). Isopods, caprellid amphi- pods, nonanomuran or nonbrachyuran decapod larvae, spiders, unidentified arthropods, and molluscs were less common, occurring in 14% or fewer of stomachs Gammarid amphipods were a moderately impor- tant component of the diet of fall chinook salmon (4% by weight), but were less important in the diet of spring chinook salmon (only 1% by weight). A vari- ety of gammarid species were eaten by fall chinook salmon, the most abundant were Jassa spp. uniden- tified gammarids, Megalorchestia pugettensis, Ischy- rocerus spp., Atylus tridens, and Corophium spp. (2.0%, 0.6%, 0.3%, 0.2%, 0.2%, and 0.1% of total food weight respectively). Dietary overlap between juvenile fall and spring chinook salmon, according to the relative weights (Eq. 2) of the 14 major food categories (Table 1), was high (0.82), owing largely to the predominance of fish prey in diets of both groups. Diet overlap based on relative weights of prey identified to the lowest pos- sible taxonomic level (86 categories of varying taxo- nomic level) was lower but still relatively high (0.66). Diets by fish length Insect prey were relatively more important and fish prey were relatively less important in the diet of the Fisher and Pearcy: Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 31 Table 1 Percentage by weight and frequency of occurence (in parentheses) of fourteen major food categories in stomachs of juvenile fall- run and spring-run chinook salmon caught in 1987 in Coos Bay. Numbers in brackets are sample sizes. Fall chinook salmon Spring chinook salmon Food category [116] [65] Cirripedia molts 5 (47) 12 (51) Isopods <1 (9) <1 (11) Caprellid amphipods 1 (14) <1 (11) Gammarid amphipods 4 (41) 1 (43) Brachyuran, anomuran larvae 2 (27) 2 (35) Other decapod larvae <1 (8) <1 (2) Crustacean fragments 5 (20) 1 (25) Araneae <1 (14) <1 (5) Insects 8 (80) 1 (60) Other arthropods <1 (6) <1 (2) Molluscs <1 (4) <1 (12) Teleosts 64 (40) 65 (37) Algae, plants 7 (46) 16 (68) Other material 4 (43) 2 (55) Table 2 Percentage by weight and frequency of occurence (in parentheses) of fourteen major food categories in stomachs of different size groups of fall and spring chinook salmon caught in 1987 in Coos Bay. Numbers in brackets are sample sizes. Food category Fall chinook salmon FL (mm) Spring chinook salmon FL (mm) <80 [32] 81-100 [73] >101 [11] 101-120 [39] 121-140 [19] >141 [7] Cirripedia molts 9(63) 5 (45) 3(18) 22 (62) 9 (42) <1 (14) Isopods <1 (9) <1 (10) 0 <1 (10) <1 (16) 0 Caprellid amphipods 1 (9) 1 (18) 0 <1 (8) <1 (16) <1 (14) Gammarid amphipods 4 (44) 5 (42) <1 (18) 1 (51) 1 (37) <1 (14) Brachyuran, anomuran larvae 7 (25) 2 (32) 0 1 (28) 2 (42) 3 (57) Other decapod larvae <1 (13) <1 (7) 0 0 <1 (5) 0 Crustacean fragments 11 (22) 6 (22) 0 2 (23) 1 (32) <1 (14) Araneae <1 (13) <1 (16) 0 <1 (5) <1 (5) 0 Insects 26 (94) 7 (81) 1 (36) 1 (69) <1 (63) 0 Other arthropods <1 (13) <1 (4) 0 0 <1 (5) 0 Molluscs 1 (6) <1 (4) 0 <1 (5) <1 (26) <1 (14) Teleosts 18 (9) 62 (45) 94(91) 49 (31) 68 (32) 91 (86) Algae, plants 12(56) 7 (42) 2(36) 20 (72) 18 (63) 4(57) Other material 11 (59) 4 (38) <1 (27) 4 (59) <1 (42) 2(71) smallest fall chinook salmon ( <80 mm FL) than in the diets of the other length groups of both fall and spring chinook salmon. Insect prey made up 26% of food by weight in stomachs of the smallest fall chinook salmon (Table 2). The insect fraction of the diet dropped to 7% and 1% for larger fall chinook salmon 81-100 mm FL and >101 mm FL, respec- tively, and was <1% for all length groups of spring chinook salmon. Fish made up only 18% by weight of the diet of fall chinook salmon <80 mm FL, but 62% and 94% by weight of the diet of fall chinook salmon 81-100 mm FL and >101 mm FL, respec- 32 Fishery Bulletin 95(1), 1997 tively, and between 49% and 91% by weight of the diet of spring chinook salmon. Larval crab (anomuran and brachyuran) prey were a moderately important component of the diet of the two smallest length classes of fall chinook salmon (<80 mm FL and 81-100 mm FL) and of the two larg- est length classes of spring chinook salmon ( 121-141 mm FL and >141 mm FL), representing 7% and 2%, and 2% and 3% of total food by weight, respectively (Table 2). However, crab larvae of various taxa and at different developmental stages were consumed by chinook salmon of different stock and length groups. The smallest fall chinook salmon (<80 mm FL) fed mainly on porcellanid, pinnotherid, callianassid, and unidentified brachyuran zoea rather than on megalopae (95% zoea and 5% megalopae by weight), whereas larger fall chinook salmon (81-100 mm FL) fed more on megalopae than on zoea (69% vs. 31% by weight), and the large spring chinook salmon (>121 mm FL) fed exclusively on megalopae, mainly large Cancer magister (72%), Cancer oregonensis (19%), and Cancer sp. (4%). Gammarid amphipods were also a fairly important component of the diet of the small fall chinook salmon, representing 4% and 5% by weight for fish <80 mm FL and 81-100 mm FL, respectively, but were a less important component of the diets of the largest fall chinook salmon (>101 mm FL) and spring chinook salmon (Table 2). Dietary overlap, based on the 14 major prey cat- egories was low (<0.55) between the smallest fall chinook salmon (<80 mm FL) and all other length categories of fall and spring chinook salmon (Table 3) . This reflects the reduced relative importance of fish and the greater relative importance of insects and crab larvae in the diet of the smallest fall chinook salmon than in the diet of larger fish (Table 2). Exclud- ing the smallest fall chinook salmon, diet overlap was high for eight of ten comparisons among length groups of fall and spring chinook salmon (Table 3). Dietary overlap, in respect to the lowest identified taxa (86 categories) was also low between fall chinook salmon <80 mm FL and all other groups. Overlap among the other groups was generally higher than that with the small fall chinook salmon but was >0.60 for only four of the ten comparisons. Diets by location Dramatic differences in the diets of fall and spring chinook salmon were associated with where the fishes were caught in the bay. For both salmon groups, fish were a much more important component of the diet at lower-bay stations 1-3 than at mid-bay stations 4-5 (Table 4). Conversely, barnacle molts and algae made up a much larger fraction of stomach contents at mid-bay stations than at lower-bay stations (Table 4) . Dietary overlap based on the 14 major food cat- egories was high between fall and spring chinook salmon caught in the same areas of the bay but was low for all comparisons of salmon caught in the two different areas of the bay (Table 5). Dietary overlap based on the 86 lower taxonomic categories was also highest for fall and spring chinook salmon caught in the same area of the bay, but only for fish caught in the lower bay was the overlap value >0.60 (Table 5). Table 3 Dietary overlap of different length groups of fall and spring chinook salmon. Overlap values based on 14 major food categories are in normal type and those based on 86 lower taxonomic categories are in italics. High overlap values (>0.60) are in bold type. Fall chinook salmon FL (mm) Spring chinook salmon FL (mm) 81-100 >101 101-120 121-140 >141 Fall chinook salmon FL (mm) <80 0.55 0.25 0.48 0.44 0.27 0.36 0.13 0.25 0.21 0.07 81-100 — 0.68 0.70 0.79 0.70 0.56 0.57 0.67 0.46 >101 — — 0.55 0.74 0.94 0.35 0.63 0.72 Spring chinook salmon FL (mm) 101-120 0.80 0.56 0.63 0.32 121-140 — — — — 0.74 0.59 Fisher and Pearcy. Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 33 Diets by sampling period Between two sampling periods (29 June-17 July and 3-13 August) moderate changes occurred in the pro- portions of the 14 major food categories in stomachs of both fall and spring chinook salmon. In stomachs of fall chinook salmon, the percentage by weight of insects, gammarid amphipods, and crab larvae was higher in the earlier than in the later period, whereas the percentage by weight of fish prey was higher in the later than in the earlier period (Table 6). In spring chinook salmon stomachs, barnacle molts and fish were more abundant in the earlier period than in the later period, whereas algae and crab larvae were more abundant in the later period than in the ear- lier period. Despite these shifts in prey composition, diet over- lap based on the 14 major prey categories was high for all comparisons of fall and spring chinook salmon caught in the two time periods (Table 7). However, Table 4 Percentage by weight and frequency of occurrence (in parentheses) of fourteen major food categories in stomachs of fall and spring chinook salmon caught in 1987 in the lower (stations 1-3) and mid (stations 4-5) sections of Coos Bay. Numbers in brackets are sample sizes. Mean fork lengths (FL) of fish in each area are also shown. Fall chinook salmon Spring chinook salmon Food category Sta. 1-3 87 mm FL [90] Sta. 4-5 88 mm FL [261 Sta. 1-3 123 mm FL [39] Sta. 4-5 118 mm FL [26] Cirripedia molts 2 (39) 22 (77) 4 (33) 35 (77) Isopods <1 (4) <1 (23) <1 (8) <1 (15) Caprellid amphipods 1 (14) 1 (12) <1 (15) <1 (4) Gammarid amphipods 4 (42) 2 (35) <1 (33) 2 (58) Brachyuran, anomuran larvae 2 (28) 6 (23) 2 (44) 1 (23) Other decapod larvae <1 (10) 0 <1 (3) 0 Crustacean fragments 6 (22) 3 (12) 1 (26) 1 (23) Araneae <1 (12) <1 (19) <1 (3) <1 (8) Insects 6 (76) 17 (96) <1 (49) 1 (77) Other arthropods <1 (6) 1 (8) 0 <1 (4) Molluscs <1 (2) 1 (12) <1(13) <1 (12) Teleosts 71 (47) 14 (15) 79(54) 24(12) Algae, plants 3 (37) 31 (77) 11 (56) 32 (84) Other material 4 (44) 3 (38) 1 (54) 3 (58) Table 5 Dietary overlap of fall and spring chinook salmon caught in the lower (stations 1- 3) and mid (stations 4- 5) sections of Coos Bay. Overlap values based on 14 major food categories are in normal type and those based on 86 lower taxonomic categories are in italics. High overlap values (>0.60) are in bold type. Fall chinook salmon Spring chinook salmon Sta. 4-5 Sta. 1-3 Sta. 4-5 Fall chinook salmon Sta. 1-3 0.37 0.82 0.38 0.28 0.68 0.35 Sta. 4-5 — 0.35 0.75 0.22 0.47 Spring chinook salmon Sta. 1-3 0.43 0.41 34 Fishery Bulletin 95(1 ), 1997 Table 6 Percentage by weight and frequency of occurrence (in parentheses) of fourteen major food categories in stomach of juvenile fall and spring chinook salmon caught during two time periods in 1987 in Coos Bay. Numbers in brackets are sample sizes. Mean fork lengths (FL) of fish caught during each time are also shown. Fall chinook salmon Spring chinook salmon Food category 29 Jun-17 Jul 85 mm FL [89] 3-13 Aug 95 mm FL [26] 29 Jun-17 Jul 117 mm FL [26] 3-13 Aug 123 mm FL [39] Cirripedia molts 6 (54) 3 (23) 17(65) 8 (41) Isopods <1 (11) 0 <1 (4) <1 (15) Caprellid amphipods 1 (17) <1 (4) <1 (12) <1 (10) Gammarid amphipods 6 (47) <1 (19) 1 (38) 1 (46) Brachyuran, anomuran larvae 2 (29) <1 (15) 1 (27) 2 (41) Other decapod larvae <1 (10) 0 0 <1 (3) Crustacean fragments 9 (24) <1 (8) <1 (15) 2 (31) Araneae <1 (17) <1 (4) <1 (8) <1 (3) Insects 11 (84) 2 (65) <1 (65) 1 (56) Other arthropods <1 (8) 0 <1 (4) 0 Molluscs <1 (3) <1 (8) 0 <1 (21) Teleosts 51 (36) 84 (54) 73 (38) 58(36) Algae, plants 6 (45) 7 (50) 6(62) 24(72) Other material 5 (47) 3 (31) 1 (38) 3 (67) diet overlap based on the lowest identified taxa (86 categories) was low for all comparisons except that between fall and spring chinook salmon caught in the period 3-13 August. Although a variety of fish prey were eaten by both salmon groups during the earlier period, during the later period fish prey were nearly all juvenile osmerids. Discussion Potential for competition The high dietary overlap values (Tables 3, 5, 7) be- tween juvenile fall chinook salmon >81 mm FL and hatchery spring chinook salmon suggest that there is the potential for competition for food between these two groups in Coos Bay under conditions of food limi- tation. However, whether or not the two groups were competing for food in 1987 cannot be determined from dietary overlap alone. In fact, high dietary overlap may sometimes indicate a condition in which abun- dant food resources are shared between potential competitors rather than a condition in which there is competition for a resource in short supply (Zaret and Rand, 1971; Myers, 1980). Zaret and Rand (1971), in a study of tropical stream fishes, found that dietary overlap between species was high dur- ing the rainy season, when food resources were abun- dant, and low during the dry season, when food re- sources were scarce and when the different fish spe- cies targeted different prey. We found little evidence in this study that the in- troduced hatchery-reared spring chinook salmon outcompeted native and STEP-reared fall chinook salmon for food. One potential result of competition for food between groups is a shift to less desirable prey in the diet of the weaker competitors (Hanson and Leggett, 1986). However, during the period when both fall and spring chinook salmon were in Coos Bay, calorically dense (high-quality) fish prey made up an equally large fraction by weight of the diets of both salmon groups (Table 1); i.e. fall chinook salmon were eating just as nutritious prey as that eaten by spring chinook salmon. Another potential result of competition is a decrease in growth rate (or average stomach fullness) of one or all of the competing groups (Reimers, 1973; Nielson et al., 1985; Hanson and Leggett, 1986). If spring chinook salmon outcompeted fall chinook salmon for food, the average stomach fullness of fall chinook salmon might be expected to drop following releases of the spring chinook salmon; this, however, did not occur. Stomach fullness of fall chinook salmon was equally high in the periods be- Fisher and Pearcy: Dietary overlap of juvenile fall- and spring-run Oncorhynchus tshawytscha 35 Table 7 Dietary overlap of fall and spring chinook salmon during two time periods. Overlap values based on 14 major food categories are in normal type and those based on 86 lower taxonomic categories are in italics. High overlap values (>0.60) are in bold type. Fall chinook salmon Spring chinook salmon 3-13 Aug 29 Jun-17 Jul 3- -13 Aug Fall chinook salmon 29 Jun-17 Jul 0.66 0.67 0.73 0.26 0.44 0.32 3-13 Aug ~ 0.83 0.49 0.73 0.70 Spring chinook salmon 29 Jun-17 Jul — — 0.75 0.56 fore and after spring chinook salmon were released into the bay (Fig. 5). In fact, stomach fullness of fall chinook salmon was usually higher than that of spring chinook salmon throughout the study period (Figs. 3 and 5). The low stomach fullness among spring chinook salmon following releases from the Anadromous, Inc. facility (Fig. 5) may reflect a delay in the start of feeding on natural prey by these hatch- ery fish. Paszkowski and Olla (1985) suggested that the inability of some hatchery fish to adapt to the natural environment may contribute to the poor survival of some groups of hatchery salmon. We con- clude that the high dietary overlap between juvenile fall and spring chinook salmon indicates the poten- tial for competition for food between these salmon groups in Coos Bay, but that in the summer of 1987 there was little evidence of actual food limitation or competition. Differences between smaller fall chinook salmon and larger hatchery spring chinook salmon in spa- tial distribution and duration of residence within estuaries may tend to minimize their competition for food. Small fish tend to occur in shallow, nearshore areas or in salt marshes, whereas large fish tend to occur in deeper channel areas (Healey, 1980a, 1991; Kjelson et al., 1982; Levings, 1982; Simenstad et al., 1982; McCabe et ah, 1986; Macdonald et ah, 1987). Larger juvenile chinook salmon also tend to spend less time in estuaries than do smaller fish (Myers, 1980; Simenstad and Wissmar, 1984; Fisher and Pearcy, 1990). Both these differences may tend to decrease competition for food between hatchery- reared and wild chinook salmon in estuaries if there is a large difference in their size. However, large re- leases of hatchery salmon smolts into an estuary may affect wild smolts detrimentally by attracting birds and other predators that prey on juvenile salmon (Emlen et ah, 1990). We did not investigate rates of secondary produc- tion in the bay, rates of exchange of prey between the adjacent ocean and the bay, the rations required by juvenile salmon to maintain optimum growth rates, or the fractions of available prey in the bay eaten by juvenile salmon and other potential com- peting species. Without such information it is diffi- cult to assess the likelihood that the growth and sur- vival of juvenile salmon was limited by food in Coos Bay in 1987. The lower half of Coos Bay is strongly influenced by the adjacent ocean (Burt and McAlister, 1959; Fisher and Pearcy, 1990). In a study ofYaquina Bay, an Oregon estuary with physical characteris- tics similar to Coos Bay, Myers ( 1980) suggested that much of the food for juvenile salmon residing in the bay was supplied by tidal exchange with the ocean. Undoubtedly, the productivity of the adjacent ocean has a strong influence on the capacity of Coos Bay to support juvenile chinook salmon. Upper-bay and Jower-bay gradients in diet Between the mid and lower sections of Coos Bay the diet of juvenile fall chinook salmon shifted from pre- dominantly drift insects, barnacle molts, and drift algae to predominantly marine fishes (Table 4). A similar increase in piscivory in the lower bay also occurred among spring chinook salmon (Table 4). Shifts in the diet of juvenile chinook salmon as they move from the river, through the estuary, and to the ocean appear to be related to the changes in habitat and foraging behavior which occur as a consequence of growth and development. Macdonald et al. ( 1987) observed that large hatchery-reared chinook salmon were often found in deeper, more saline waters of the salt-wedge of the Campbell River estuary, whereas smaller wild chinook salmon were often found in the freshwater layer near the surface. Small 36 Fishery Bulletin 95( I ), 1997 fry and subyearling chinook salmon often use tidal marshes where they eat drift and emergent insects and epibenthic crustaceans (Kjelson et al., 1982; Simenstad et al., 1982; Levings et al., 1991; Shreffler et al., 1992), whereas, larger, yearling chinook salmon spend little time in salt marshes but quickly move to neritic habitats (Simenstad et al., 1982). When subyearling fish move to neritic habitats their diet shifts to fishes, decapod larvae, euphausiids, and drift insects (Simenstad et al., 1982). McCabe et al. (1986) observed that, in the Columbia River estuary, subyearling chinook salmon in pelagic areas were significantly larger than those caught in shallow in- tertidal habitats and that the prey of juvenile chinook salmon varied with season, habitat, and position in the estuary. Feeding behavior is also influenced by environmental factors, for example turbidity (Gre- gory and Northcote, 1993). Diets of juvenile chinook salmon in freshwater reaches of river systems often are dominated by lar- val, pupal, or adult insects that are captured mainly in the drift at the surface or in the water column (Becker, 1973; Craddock et al., 1976; Sagar and Glova, 1987, 1988; Rondorf et al., 1990; Healey, 1991; Levings and Lauzier, 1991; Smirnov et al., 1994). Depending on season and habitat, both terrestrial insects as well as different developmental stages of aquatic insects can be important prey for chinook salmon in rivers (Rondorf et al., 1990; Levings and Lauzier, 1991). Insects are also important constitu- ents of the diets of juvenile chinook salmon in many estuaries (Healey, 1980, a and b, 1982, 1991; Levings, 1982; McCabe et al., 1986; Kask et al., 1988; this study), particularly in fresh or brackish water tidal marshes (Kjelson et al., 1982; Levings et al., 1991; Shreffler et al., 1992). Whereas insects are important prey in freshwater and upper estuaries, fishes are important prey of juvenile chinook salmon constituents in the lower reaches of estuaries as well as in marine, neritic or subtidal areas (Healey, 1980a; Myers, 1980; Kjelson et al., 1982; Simenstad et al., 1982; Argue et al., 1986; McCabe et al., 1986; Levings et al., 1991; Reimers et al.3; Nicholas and Lorz4). Fish prey are also predomi- nant in the diets of juvenile chinook salmon in ma- rine waters off Oregon and Washington (Peterson et 3 Reimers, P. E., J. W. Nicholas, T. W. Downey, R. E. Halliburton, and J. D. Rogers. 1978. Fall chinook ecology project, AFC- 76-2. Federal Aid Progress Reports, Fisheries. Oregon Dep. Fish and Wild]., 2501 S.W. First Ave., PO. Box 59, Portland, OR 97207. 4 Nicholas, J. W., and H. V. Lorz. 1984. Stomach contents of juvenile wild chinook and juvenile hatchery coho salmon in sev- eral Oregon estuaries. Oregon Dep. Fish and Wildl., 2501 S.W. First Ave., P.O. Box 59, Portland, OR 97207. Progress Rep. 84-2, 9 p. al., 1982; Emmett et al., 1986; Brodeur and Pearcy, 1990, 1992; Brodeur et al., 1992), in the Gulf Islands area of the Strait of Georgia (Healey, 1980b), and in the Fraser River plume (St. John et al., 1992). In Coos Bay, the increase in importance of marine fish in the diets of juvenile fall and spring chinook salmon at the lower-bay stations may reflect an up- per-bay, lower-bay gradient in the abundance of fish prey. Juvenile osmerids, sandlance, and rockfish were the predominant fish prey of juvenile chinook salmon in Coos Bay. In Yaquina Bay, larval and juvenile stages of these species were present in peak abun- dances in plankton samples from the extreme lower- bay and offshore stations (Pearcy and Myers, 1974). Myers (1980) caught more species of fishes in the lower than in the upper section of Yaquina Bay and suggested that much of the food for juvenile chinook salmon residing in the bay was supplied by tidal ex- change with the ocean. She also suggested that high temperatures in the upper bay inhibited movement of predominantly marine species into the upper bay. A similar mechanism may be operating in Coos Bay. In our beach-seine samples large juvenile and adult surf smelt were much more abundant at lower than at mid-bay stations (average catch per set was 2,290, 237, 108, 30, and 12 at stations 1, 2, 3, 4, and 5, re- spectively). If, as was the case in Yaquina Bay, smaller larval and juvenile smelt also are more abundant in lower Coos Bay, the increased consumption by juve- nile chinook salmon of these fish prey in the lower bay may be a consequence of their greater density there. Acknowledgments We thank John Chapman for assistance in the iden- tification of gammarid amphipods. Jim Digiulio of Aquatic Biology Associates identified the insect prey. Carl Brookins, Alton Chung, Matt Wilson, Ann Raich, and Karen Young assisted in the field, and Terrin Ricehill assisted in the laboratory; their help is greatly appreciated. Three anonymous reviewers provided very helpful comments on earlier drafts of this manuscript. This study was supported by NOAA, Northwest Fisheries Science Center, Grant NA47FE0182-02. Literature cited Argue, A. W., B. Hillaby, and C. D. Shepard. 1986. 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Surface, planktonic and benthic foraging by juvenile chinook salmon (Oncorhynchus tshawytscha 1 in turbid labo- ratory conditions. Can. J. Fish. Aquat. Sci. 50:233-240. Hanson, J. M., and W. C. Leggett. 1986. Effect of competition between two freshwater fishes on prey consumption and abundance. Can. J. Fish. Aquat. Sci. 43:1363-1372. Healey, M. C. 1980a. Utilization of the Nanaimo River estuary by juve- nile chinook salmon, Oncorhynchus tshawytscha. Fish. Bull. 77:653-668. 1980b. 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, OR. 1982. Juvenile Pacific salmon in estuaries: the life support system. In V. S. Kennedy (ed. ), Estuarine comparisons, p. 315-341. Academic Press, New York, NY. 1991. Life history of chinook salmon (Oncorhynchus tshawytscha). In C. Groot and L. Margolis (eds.), Pacific salmon life histories, p. 313-393. 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Intertidal habitats used by juvenile chinook salmon (Oncorhynchus tshawytscha) rearing in the north arm of the Fraser River estuary. Mar. Pollut. Bull. 22:20-26. Levings, C. D„ and R. B. Lauzier. 1991. Extensive use of the Fraser River basin as winter habitat by juvenile chinook salmon (Oncorhynchus tshawy- tscha). Can. J. Zool. 69:1759-1767. Linton, L. R., R. W. Davies, and F. J. Wrona. 1981. Resource utilization indices: an assessment. J. Ani- mal Ecol. 50:283-292. Macdonald, J. S., I. K. Birtwell, and G. M. Kruzynski. 1987. Food and habitat utilization by juvenile salmonids in the Campbell River estuary. Can. J. Fish. Aquat. Sci. 44:1233-1246. McCabe, G. T., Jr., R. L. Emmett, W. D. Muir, and T. H. Blahm. 1986. Utilization of the Columbia River estuary by sub- yearling chinook salmon. Northwest Sci. 60:113-124. Myers, K. W. 1980. An investigation of the utilization of four study areas in Yaquina Bay, Oregon, by hatchery and wild juvenile salmon- ids. M.S. thesis, Oregon State Univ., Corvallis, OR, 234 p. Myers, K. W., and H. F. Horton. 1982. Temporal use of an Oregon estuary by hatchery and wild juvenile salmon. In V. S. Kennedy (ed.). Estuarine comparisons, p. 377-392. Academic Press, New York, NY. Neilson, J. D., G. H. Geen, and D. Bottom. 1985. Estuarine growth of juvenile chinook salmon ( Oncorhynchus tshawytscha) as inferred from otolith microstructure. Can. J. Fish. Aquat. Sci. 42:899-908. Paszkowski, C. A., and B. L. Olla. 1985. Foraging behavior of hatchery-produced coho salmon (Oncorhynchus kisutch ) smolts on live prey. Can. J. Fish. Aquat. Sci. 42:1915-1921. Pearcy, W. G., and S. S. Myers. 1974. Larval fishes of Yaquina Bay, Oregon: a nursery ground for marine fishes? Fish. Bull. 72:201-213. Peterman, R. 1984. Density-dependent growth in early ocean life of sock- eye salmon ( Oncorhynchus nerka). Can. J. Fish. Aquat. Sci. 41:1825-1829. Peterson, W. T., R. D. Brodeur, and W. P. Pearcy. 1982. Food habits of juvenile salmon in the Oregon coastal zone, June 1979. Fish. Bull. 80:841-851. Reimers, P. E. 1973. The length of residence ofjuvenile fall chinook salmon in Sixes River, Oregon. Oreg. Fish. Comm. Res. Rep. 4(2), Oreg. Dep. Fish. Wild]., Portland, OR, 43 p. Rogers, D. E., and G. T. Ruggerone. 1993. Factors affecting marine growth of Bristol Bay sock- eye salmon. Fish. Res. 18:89-103. Rondorf, D. W., and G. A. Gray, and R. B. Fairley. 1990. Feeding ecology of subyearling chinook salmon in riv- erine and reservoir habitats of the Columbia River. Trans. Am. Fish. Soc. 119:16-24. 38 Fishery Bulletin 95(1 ), 1997 Sagar, P. M., and G. J. Glova. 1987. Prey preferences of a riverine population of juvenile chinook salmon, Oncorhynchus tshawytsch. J. Fish Biol. 31:661-673. 1988. Diel feeding periodicity, daily ration and prey selec- tion of a riverine population of juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum). J. Fish Biol. 33:643-653. St John, M. A., J. S. Macdonald, P. J. Harrison, R. J. Beamish, and E. Choromanski. 1992. The Fraser River plume: some preliminary observa- tions on the distribution of juvenile salmon, herring, and their prey. Fish. Oceanogr. 1:153-162. Schoener, T. W. 1970. Nonsynchronous spatial overlap of lizards in patchy habitats. Ecology 51:408-418. Shreffler, D. K., C. A. Simenstad, and R. M. Thom. 1992. Foraging by juvenile salmon in a restored estuarine wetland. Estuaries 15:204—213. Simenstad, C. A., and R. C. Wissmar. 1984. Variability of estuarine food webs and production may limit our ability to enhance Pacific salmon (Oncorhynchus spp.). In. W. G. Pearcy ed.), The influence of ocean condi- tions on the production of salmonids in the North Pacific, p. 274—286. Sea Grant College Program, Oregon State Univ., Corvallis, OR. Simenstad, C. A., K. L. Fresh, and E. O. Salo. 1982. The role of Puget Sound and Washington coastal es- tuaries in the life history of Pacific salmon: an unappreci- ated function. In V. S. Kennedy (ed.), Estuarine compari- sons, p. 343-364. Academic Press, New York, NY. Smirnov, B. P., V. V. Chebanova, and T. V. Vvedenskaya. 1994. Adaptation of hatchery-raised chum salmon, Onco- rhynchus keta, and chinook salmon, O. tshawytscha , to natural feeding and effects of starvation. J. Ichthyol. 34:96-106. Thomas, G. L., and O. A. Mathisen. 1993. Biological interactions of natural and enhanced stock of salmon in Alaska. Fish. Res. 18:1-17. Wallace, R. K., Jr. 1981. An assessment of diet-overlap indexes. Trans. Am. Fish. Soc. 110:72-76. Zaret, T. M., and A. S. Rand. 1971. Competition in tropical stream fishes: support for the competitive exclusion principle. Ecology 52:336-342. 39 Abstract .—The accuracy and pre- cision of estimates of catch at age from sampled lengths were evaluated for three different methods with simulated red snapper, Lutjanus campechanus, data for 1984-94. The methods in- cluded a growth curve, age-length keys, and a probabilistic method to classify a known total number of fish into ages from samples of the length frequency of the catch. In the first method, ages were estimated from sample lengths directly from the growth curve. The second method involved expanding the sample length frequency to age fre- quency by using age-length keys. The probabilistic method incorporated the cumulative frequency distributions of length at age, year-class strength, and estimates of prior survival to build age probability distributions from sampled lengths. The evaluation was based on the error in the assigned catch at age and on the resulting estimates of num- bers at age and fishing mortality aris- ing from sequential population analy- sis. The probabilistic method was the best of the three for the situation evalu- ated here, and application of the age- length key was better than that of the growth model. However, the probabilis- tic method requires knowledge of growth, the distributions of size at age, and recruitment that may not be known, or only poorly so. Age-length keys require no such ancillary informa- tion and may be more practical in most situations, but the probabilistic method is superior if the data requirements can be met. Manuscript accepted 31 July 1996. Fishery Bulletin 95:39-46 (1997). Fish age determined from length: an evaluation of three methods using simulated red snapper data* C. Phillip Goodyear Miami Laboratory, Southeast Fisheries Science Center National Marine Fisheries Service, NOAA 75 Virginia Beach Drive, Miami, Florida 33149 E-mail address: Phil_Goodyear@msn.com Age-structured stock-assessment methods require estimates of the age composition of the catch. In stock assessments for Gulf of Mexico red snapper, Lutjanus campechanus, age compositions are used that are estimated from the sampled size distribution of the catch with growth models (Goodyear* 1). The application of age-length keys de- veloped from age determinations of length-stratified samples of the catch is a superior method (Ketchen, 1950; Hoenig and Heisey, 1987 ) that has been recently incorporated into the data collection program for this stock. However, it cannot be readily applied retroactively to improve the estimates of the age composition of historical catch, and it requires sig- nificantly more resources than the former method. In this paper, I com- pare the precision of the estimates of the age composition of the catch from these two methods with an al- ternative, using simulated red snapper data. The comparisons in- clude both accuracy and precision of the estimates of the age composi- tion of the catch and the consequent estimates of numbers at age and fishing mortality arising from their application to sequential population analysis following the methods of Gavaris 2 and Powers and Restrepo (1992). Methods Simulated data The population simulation model used in this analysis (Goodyear, 1989) employed 30 discrete ages with an instantaneous annual natu- ral mortality (M) of 0.2 for all ages in the fishery. Each year class was further partitioned into growth pla- toons (cohorts with identical age but different mean lengths). The posi- tion of a growth platoon in the dis- tribution of size at age was fixed so that the larger individuals of a year class at age 1 remained larger throughout their lifetime. Mean lengths (L) at age (A) at the begin- ning of January were assumed to be equal to the estimates in the 1994 stock assessment for Gulf of Mexico red snapper ( Goodyear1 ) and to cor- respond to the von Bertalanffy equation, L=88.24( l-exp(-0. 159 Miami Laboratory Contribution MIA-94/ 95-42. 1 Goodyear, P. 1994. Red snapper in U.S. waters of the Gulf of Mexico. National Marine Fisheries Service, Southeast Fish- eries Science Center, Miami Laboratory, Miami, Admin, rep. MIA 93/94-63, 150 p. 2 Gavaris, S. 1988. An adaptive frame- work for the estimation of population size. Canadian Atlantic Fisheries Scien- tific Advisory Committee Research Docu- ment 88/29, 4 p. 40 Fishery Bulletin 95(1), 1997 (A+0.458)), where L is total length in cm and A is age in years. Size at age in the absence of fishing mortality was assumed to be normally distributed with a coefficient of variation of length at age (v) of 0.10 based on the observed variability in size at age for red snapper (Goodyear1). The mean length of in- dividuals of age a in growth platoon p, l , was de- termined from mean size at age (L ) by using the normal distribution and the coefficient of variation of length at age as ^ ap La “FT'a^pV, where: 2 is the standard normal deviate for the pth percentife of the distribution. The simulation con- sidered 101 growth platoons in each age class. The resulting distributions of lengths at the beginning of the year for ages 1-10 are shown in Figure 1. Within- year growth was evaluated as Wap = Wa_i,pexp(Gap), where Wap is the weight (kg) of an individual in growth platoon p at age a, and GQp - instantaneous growth rate of growth platoon p at age a. The Gap were estimated from lengths at age predicted from the von Bertalanffy growth equation. The weight of a fish at capture VF, was evaluated as Wc = WapZap(exp(Ga -Zap)~ l)/ (( Gap ~ Zap > C 1 - exP< ~Zap )), where Z is the total instantaneous mortality for growth platoon p at age a during the time period. Weight was converted to length with the length- weight equation (W = 1.158 x L3 056, r2=0.985, n =25,375) Growth, mortality, and catch were evalu- ated monthly. The period simulated was from 1954 to 1994, but catch and sample data were retained and analyzed for 1984-94, which corresponds to the time span of actual data from the fishery. Recruitment in the model was specified by year class from 1954 to 1994 (Fig. 2). The values for 1972-94 follow the recruit- ment pattern observed in trawl surveys (Goodyear1). Earlier values were arbitrarily varied around the level observed at the beginning of the time series because landings from shrimp trawlers (predomi- nantly juvenile fish) during these years were higher than those after 1972. The value of fishing mortality in the model is the product of a maximum potential value for the year and a selectivity value based on the fish’s age (Figs. 3 and 4). A dome-shaped selectivity schedule was (ill Jk A3e2 jiT Ttu A ge 3 A ge 4 Age 5 : A96 6 Tbw_ : A96 7 J ; a„8 ^ ilW ; Age 9 I A ge 1 0 ' -r^T dfll-JfttW. 0 10 20 30 40 50 60 70 80 90 100 Total length (cm) Figure 1 Simulated length-frequency distributions of red snapper, Lutjanus campechanus, at the begin- ning of the year for ages 1-10. selected on the basis of age distribution of the catch (predominantly from handlines) in the 1994 assess- ment (Goodyear1). The value of the annual maximum for 1984-94 also follows the trend in the best estimates from the 1994 assessment, whereas earlier values were arbitrarily varied around the level observed at the be- ginning of the time series. The reduction in fishing mortality after 1990 was a response to management actions. The selectivity schedule (Fig. 4) was selected to produce a sample length frequency similar to that observed in the fishery (Fig. 5). Samples were trun- cated below 33 cm after 1990 to include the effects of changes in minimum size regulations at that time. The simulated observations of length (and age) were obtained from the simulated catch. The catch from a growth platoon in the population structure was picked at random. It was evaluated for inclu- sion as an observation on the basis of the ratio of its magnitude (Np) to the maximum catch from any other growth platoon (N ). This was accomplished by drawing a uniform random number (R) between 0 and 1.0. If the ratio Np/Nmax > R, the length and age attributes of the cell were included as an observation; otherwise, they were discarded. This convention caused the sampled growth platoon to be proportional to their magnitude in the simulated catch. The process was repeated for each month of the simulation until 1,000 samples had been drawn. This provided 12,000 length Goodyear: Fish age from length: an evaluation of three methods 41 samples per year. No error was added to either age or length to simulate measurement error. The ages of the first two fish sampled in each 1-cm length stratum each month were retained with their lengths to build the age-length key for that month. This provided a maximum sample size of 24 fish per length stratum or about 4,000 fish per year to con- struct the age-length key, but sample size varied slightly because of the stochastic nature of the sam- pling process. 42 Fishery Bulletin 95(1], 1997 Age (yr) Figure 4 Selectivity schedule used in the simulation. The fishing mortality rate is the prod- uct of the maximum fishing mortality rate ( F ) and the selectivity value correspond- ing to fish age. Total length (cm) Figure 5 Observed and simulated length frequencies of red snapper harvested in 1993. Goodyear: Fish age from length: an evaluation of three methods 43 Age-estimation methods For all three ageing methods evaluated, the number of fish in the catch at age a, N , was estimated as K=Cfa, where C, the total catch in numbers of fish, was the known value from the simulation, and fa was the es- timated fraction of catch at age a. Age frequencies were estimated separately for each year between 1984 and 1994. With the first method, the von-Bertalanffy growth equation was rearranged to predict age from length, A = - loge(l- L/88. 24)/0.159 - 0.458, and the f were estimated as the ratios of the num- ber of sampled fish assigned age A to the total num- ber of fish in the sample. For this method any sampled fish larger than the asympotic size was dis- carded. With the second method typical age-length keys (Ketchen, 1950; Westrheim and Ricker, 1978) were constructed annually from the monthly age-fre- quency samples. In this case the fa were estimated by multiplying the observed age frequencies for each length stratum by the ratio of length samples in each length stratum to the total number of length samples and by summing over ages. The third (probabilistic) method is a proposed al- ternative and requires estimates of prior survival of year classes in the population and independent esti- mates of year-class strength. In this method j n SS'. r _ i= 1 q=0 la j where j is the number of length samples, n is the number of ages, and _ WaRy_aSa a n a= 1 and where Da is the cumulative probability distribution of length for age a, L; is the observed length of fish i, is the recruitment strength in year y-a, y is the year of observation, and Sa is survival probability from recruitment to age a and is given by a-1 sa =exp ^-(Ff+Mf), i= 0 where F is the fishing mortality of the year class at age a when it was age i, and M( is the natural mor- tality of the year class at age a when it was age i. Inspection of the data used in this method reveals that the method requires values for nearly everything one would wish to estimate from the age composi- tion of the catch and consequently seems to place the cart before the horse. However, in many cases ancillary data on year-class strength may be avail- able from research surveys, and estimates of natu- ral and fishing mortalities may be available from earlier assessments. In this investigation, this method was applied in two ways. The first assumed preexisting accurate knowledge of year-class strengths and mortality. The second application as- sumed knowledge of year-class strengths and natu- ral mortality and proceeded iteratively. In the first iteration, age composition was estimated with the assumption that there was no fishing mortality. This led to a set of estimates of catch at age that were then used through sequential population analysis to estimate fishing mortality at age. With the second iteration the resulting estimates of fishing mortal- ity were added and catch at age was reestimated. This process was repeated several additional times. Overall, the three methods provided 4 sets of esti- mates of catch at age that could be compared with the true values from the simulation: those from the growth model, those from the age-length key, those from the probabilistic method given knowledge of survival, and those from the iterated probabilistic method. In addition, numbers at age and fishing mortality for each year were estimated from the catches at age for each set by using sequential popu- lation analysis (Powers and Restrepo, 1992). For the purpose of this exercise, the selectivities for the ter- minal year of the population analysis were the known values from the simulation, and the tuning index was the known number of age-4 individuals alive at the beginning of the year. The methods were compared by correlating the known true values from the simu- lation to the values estimated by each method. Be- cause there were 31 ages in the model (0-30) and 11 years, these provided a total maximum sample size of 341; however, year-age combinations where the true catch at age was below 100 were dropped. Thus sample sizes for most analyses were reduced to 331. In addition, scattergrams of the logs of the ratios of estimated to true values were constructed for each comparison. The r2 values for the correlations be- tween true and estimated values are presented with 44 Fishery Bulletin 95( 1 ), 1997 each of the scattergrams. Although the scattergrams involve transformations to reflect the error more ac- curately, the correlations themselves are based on the untransformed data. Results The estimates of catch at age from each of the meth- ods were highly correlated with the true values (Fig. 6). But the error in catch at age was clearly highest for the ages assigned with the growth model (Fig. 6A). Catch at age from the age-length key was con- siderably better than that from the growth model, particularly for the younger more abundant ages in the catch (Fig. 6B). The younger ages in these fig- ures tend to be to the right side of the scattergrams and the older, less abundant ages are on the left. The probabilistic method, given prior knowledge of fishing mortality and recruitment, provided the best result, with very little difference between true and estimated age compositions except at the oldest ages (Fig. 6C). The bias in the estimates obtained with this method with only natural mortality is evi- dent in Figure 6D, but even so, the estimates for the youngest ages are better than the estimates from the growth model. The bias was reduced by the fifth it- eration (Fig. 6E) and almost completely removed by the tenth iteration (Fig. 6F). The estimates of number at age derived from each set of catch at age by using an age-sequenced analy- sis are presented in Figure 7. Again the results were least favorable for the catch-at-age matrix developed from the growth model (Fig. 7 A), followed by the age- length key (Fig. 7B) and the probabilistic method (Fig. 7C). The bias in estimated number at age from the probabilistic method, where fishing mortality is not used, is even more pronounced than it was for the catch-at-age matrix (Figs. 5D and 6D). However, the bias was reduced by the fifth iteration by using the fishing mortality rates derived from prior itera- tions and almost completely removed by the tenth iteration (Fig. 7, E-F). The similarity of r2 values for the correlations between observed and predicted val- ues for the age-length key and probabilistic methods in Figures 6 and 7 are somewhat misleading because of the very large dynamic range of the numbers and corresponding catches at age used in the analysis. In actuality, the precision of the estimates arising from applicaton of the age-length key was much lower than that for the probabilistic method for age classes that were infrequent in the catch. The estimates of fishing mortality at age, derived from each set of catch at age by using age-sequenced 2 0- A B 1 0- CD CD - - - - - - - - A~vr —v- • • - -1.0- -2.0- r2 = 0.845 r2 = 0.994 2.0- c ■ 1) 1.0- o.o- _ _^r. _ . ' ~ “ -1.0- -2.0- r2 = 0.997 r2 = 0.992 2.0- E F 1.0- 00- - - -1.0- -2.0- r2 = 0.996 r2 = 0.996 2 3 4 5 6 2 3 4 5 6 Log (true catch at age) Figure 6 Ratios of estimated to true catch at age from the growth model (A), age-length key (B), and from the probabilistic method with knowledge of prior survival (C), and probabi- listic iterations 1, 5 and 10 (D-F). Log (true number at age) Figure 7 Ratios of estimated to true number at age from analysis of catch at age from the growth model (A), age-length key (B) , probabilistic method with knowledge of prior survival (C) , and probabilistic iterations 1, 5 and 10 (D-F). Goodyear: Fish age from length: an evaluation of three methods 45 analysis, are presented in Figure 8 for ages up to 10. Again, the results were least favorable for the catch- at-age matrix developed from the growth model (Fig. 8A), followed by the age-length key (Fig. 8B), and the probabilistic method (Fig. 80. The upward bias in estimated number at age from the probabilistic method in the absence of fishing mortality in Figure 7D led to an underestimate of fishing mortality of Figure 8D. However, the bias was reduced by the fifth iteration and almost completely removed by the tenth iteration (Fig. 8, E-F). The relatively higher error in the catch at age for older ages estimated by using the growth model and age-length key (Fig. 6, A-B) led to relatively higher error in the estimates of numbers at age from their analysis. This resulted in poor estimation of fishing mortality for the oldest ages in the simulated catch which caused the correlation between true and esti- mated fishing mortalities to decline when fish older than 10 years were included in the analysis (Fig. 9, A-B). The results from the probabilistic approach also showed a similar trend but were much less sen- sitive than those for the other two methods (Fig. 9, D-F). Discussion These results indicate that for the situation evalu- ated here the probabilistic method is superior to age assignment from either a growth model or an age- length key. Factors leading to this conclusion include knowledge of the actual history of year-class strengths and perfect knowledge of growth, natural mortality, and the distribution of size at age. Imper- fect knowledge of any of these elements would de- grade the performance of the probabilistic method. If there are sufficient data to develop a growth curve then it should be possible to characterize the distri- bution of size at age, at least for the more abundant ages in the population. Poor knowledge of the growth curve itself would also adversely affect the estimates obtained directly from the growth curve. The results from the age-length key would be un- affected by poor knowledge of growth, past recruit- ment, and natural mortality. However, the compari- sons among methods in the current analysis assumed no error in age assignments for the age-length key. Experience suggests that there is uncertainty in age assignment from hard-part analysis, an uncertainty that increases with fish age (Beamish and Fournier, 1981). Including such error would have added to the difference between the results of this method and those obtained with the probabilistic method. None- theless, the construction and application of age- A B ^ • c D - E F - 0 O-1 ■ ■ ' ' 1 r-1 , , , . T r- 3 8 13 18 23 28 3 8 13 18 23 28 Oldest age in analysis (yr) Figure 9 Precision of fishing mortality estimates ( r 2 from correlations of true and estimated rates) from the growth model (A), age- length key (B), probabilistic method with knowledge of prior survival (C), and probabilistic iterations 1, 5, and 10 (D-F). 46 Fishery Bulletin 95 ( I ), 1997 length keys involves the fewest assumptions. Where almost certain knowledge of growth and year-class strengths is lacking, and the method for ageing the fish is robust, this method is probably the best choice for monitoring the age composition of a catch. Where time-series data for year-class strengths are available, where growth and natural mortality are reasonably known, and where there are insufficient age determinations to construct age-length keys, the probabilistic method is clearly superior to age assign- ments from inverted growth models and also might be as good as, or better than, the age-length keys if they were available. Where growth and year-class strengths are well characterized and natural mor- tality is reasonably known, the probabilistic method should outperform all the other alternatives. Addi- tionally, this method should be very useful for esti- mating the age composition of catches for the most recent year of a time series for which sample-age analysis may not yet be complete. It should also pro- vide a reliable method to estimate the age composi- tions of catches for intermediate years of a time se- ries, for which insufficient age determinations are available to construct age-length keys. Acknowledgments I thank M. Schirripa, C. Porch, and two anonymous reviewers for helpful comments on the manuscript. Literature cited Beamish, R. J., and D. A. Fournier. 1981. A method for comparing the precision of a set of age determinations. Can. J. Fish. Aquat. Sci. 38:982-983. Goodyear, C. P. 1989. LSIM-A length-based fish population simulation model. U.S. Dep. Commer, NOAATech. Memo. NMFS-SEFC-219. Hoenig, J. M., and D. M. Heisey. 1987. Use of log-linear model with the EM algorithm to correct estimates of stock composition and to convert length to age. Trans. Am. Fish. Soc. 116:232-243. Ketchen, K. S. 1950. Stratified subsampling for determining age- distributions. Trans. Am. Fish. Soc. 79:205-212. Powers, J. E., and V. R. Restrepo. 1992. Additional options for age-sequenced analysis. Int. Comm. Conserv. Atl. Tunas Coll. Vol. Scientific Papers 39(21:540-553. Westrheim, S. J., and W. E. Ricker. 1978. Bias in using an age-length key to estimate age-fre- quency distributions. J. Fish. Res. Board Can. 35:184—189. 47 AbStraCt.-Fishery dependent and fishery independent distribution analy- ses together reveal that there are three discrete areas of Argyrosomus inodorus abundance between Cape Point and the Kei River: one in the southeastern Cape, one in the southern Cape, and one in the southwestern Cape. On the ba- sis of migratory patterns determined from tagging and catch data, differences in growth rates, otolith-dimension and fish-length relationships, growth zone structure, sizes at maturity and sex ra- tios, and on the fact that each region has nursery and spawning areas, the conclusion has been drawn that these areas of abundance represent three separate stocks. Each stock apparently disperses offshore in winter (to ca. 100 m depth) and concentrates nearshore in summer (<60 m depth) in response to oceanographic patterns. Although there is evidence of spawning activity through- out the year, the main spawning season for silver kob is from August to Decem- ber, with a peak in spring (Sep-Nov). Size at sexual maturity for silver kob was smaller in the southeastern Cape than in the southern Cape, and in both regions males matured before females. Median sizes at maturity (Lg0) for fe- males and males were 310 mm TL (1.3 yr) and 290 mm TL (1 yr) respectively in the southeastern Cape and 375 mm TL (2.4 yr) and 325 mm TL (1.5 yr) re- spectively in the southern Cape. East of Cape Agulhas, A. inodorus are found just beyond the surf zone to depths of 120 m. Adults occur predominantly on reefs (>20 m), whereas juveniles are found mainly over soft substrata of sand or mud (5-120 m depth). Young juve- niles recruit to nurseries immediately seaward of the surf zone (5-10 m depth) but move deeper with growth. Because of lower water temperatures west of Cape Agulhas, the adults in this area are found from the surf zone to depths of only 20 m in summer. Manuscript accepted 22 August 1996. Fishery Bulletin 95:47-67 ( 1997). The life history and stock separation of silver kob, Argyrosomus inodorus, in South African waters Marc H. Griffiths Sea Fisheries Research Institute Private Bag X2, Roggebaai 80 1 2, Cape Town, South Africa and Department of Ichthyology and Fisheries Science, Rhodes University PO. Box 94, Grahamstown 6140, South Africa E-mail address, mgriffith@sfri.sfri.ac.za Silver kob, Argyrosomus inodorus , is an important commercial and rec- reational sciaenid fish (max. size 34 kg) that is known from northern Namibia on the west coast of south- ern Africa to the Kei River on the east coast of South Africa (Griffiths and Heemstra, 1995). It is not com- mon between Cape Point and cen- tral Namibia; therefore it is likely that the Namibian populations are not continuous with those off the eastern seaboard of South Africa (Griffiths and Heemstra, 1995). Until recently A. inodorus was misidentified as A. hololepidotus throughout its distribution; off South Africa it was also confused with a sympatric species, A. japoni- cus (Griffiths and Heemstra, 1995). The South African line fishery consists of about 2,900 commercial (Kroon1) and some 4,000 club-affili- ated recreational (Ferreira, 1993) vessels. These vary from 5 to 15 m in length and operate on both east and west coasts. Silver kob is prob- ably the most valuable species caught by the line fishery between Cape Point and East London if mar- ket value and annual catch are com- bined; A. inodorus is also landed as a bycatch of the sole- and hake-di- rected inshore trawl fishery be- tween Cape Agulhas and Port Alfred (Japp et al., 1994) and is caught by rock and surf anglers and commer- cial beach-seine fishermen in the southwestern Cape. Although an important species, trawl and line catch per unit of effort for this spe- cies has declined substantially dur- ing the last three decades, and con- cern has been expressed over the large contribution of recruits to line catches in the southeastern Cape (Smale, 1985; Hecht and Tilney, 1989). Knowledge of the life history of fishes “is an almost essential pre- requisite to successful identification of stocks” (Pawson and Jennings, 1996) and is fundamental to stock assessment and to the formulation of effective management strategies for their sustainable use. Despite the importance of A. inodorus and evidence for declining catches, little has been published on its life his- tory; wise management has there- fore not been possible. Smale ( 1985) investigated the sex ratio and spawning seasonality of “A. holo- lepidotus" based on the catches of lineboat fishermen in Algoa Bay but inadvertently included both A. inodorus and A. japotiicus in his study (established via voucher specimens and otoliths). Griffiths (in press, a) recently described the growth of A. inodorus from three geographical regions between Cape 1 Kroon, W. 1995. Sea Fisheries, Permit Division, P. BagX2, Roggebaai, 8012, Cape Town, South Africa. Personal commun. 48 Fishery Bulletin 95(1 ), 1997 Point and the Kei River. On the basis of grow rate, fish-length and otolith-dimension relationships, and the appearance of growth zones, he concluded that silver kob within this area comprise at least three separate stocks. The objective of the present study was to provide information on the life history of A. inodorus occur- ring between Cape Point and the Kei River, includ- ing reproductive seasonality, spawning grounds, size at maturity, juvenile and adult distribution, and migration. Because the identification of discrete stocks or “management units” is essential for effec- tive management (Pawson and Jennings, 1996), the multiple stock concept is further developed with in- formation on distribution and abundance, life his- tory parameters, and mark-recapture data. Materials and methods The study area (from Cape Point to Kei River) was divided into three regions for sampling purposes (Fig. 1). These regions were identical to those used by Griffiths (in press, a); they were not divided ac- cording to political boundaries but rather generated to increase analytical resolution. Biological (March 1990-January 1992) and length-frequency (January 1990-December 1994) data were collected in each region from fish caught 1) by the line fishery, 2) by the inshore trawl fishery, 3) by trawlers during South Coast Biomass Surveys conducted by the Sea Fish- eries Research Institute, and 4) by research linefishing operations. Biological data were also ob- tained from silver kob caught by beach seines in False Bay (Oct 1991). Trawled fish were generally caught over sand or mud substrata, and line-caught fish over reef. Owing to the high relief rocky nature of the in- shore habitat west of Cape Agulhas, this species is not trawled in the southwestern Cape. Fish sampled for biological purposes were mea- sured (to the nearest 1 mm [total length]), weighed (to the nearest gram [fish <500 g], the nearest 20 g [fish 500 g-5 kg], or the nearest 100 g [fish 5 kg-25 kg]), cut open, and sexed. Gonads were removed, as- signed a visual index of maturity (see Table 1), and weighed to the nearest 0.1 g. Males were assigned an index of drumming muscle development ( l=none, 2=partially developed, 3=fully developed). Owing to logistical constraints, monthly biological data were obtained only for the southeastern Cape; in the other two regions biological sampling was limited to the spawning season. Areas of silver kob abundance were delineated by using returns from the commercial line fishery and data from South Coast Biomass Surveys (SCBS’s). Line catches consisted predominantly of adult fish, whereas trawl catches from SCBS’s comprised mostly juveniles and young adults (see below). Annual catch- per-unit-of-effort data (catch per outing) were plot- ted on a subregional basis for the commercial line fishery (an outing did not exceed one day), and the data from 14 SCBS’s (Table 2) were used to calculate Figure 1 Map of the three coastal regions where silver kob were sampled, and localities mentioned in the text. Griffiths: Life history and stock separation of silver kob, Argyrosomus modorus 49 Classification Table 1 and description of the macroscopic gonad maturity stages of Argyrosomus inodorus. Stage Description 1 Juvenile This stage is generally only found in fish < 200 mm TL. Testes are threadlike, and the ovaries appear as transparent pinkish flaccid sacs, about half the length of those at stage 2. 2 Immature or resting Testes are extremely thin, flat, and pinkish white. Ovaries appear as translucent orange tubes. Eggs are not visible to the naked eye. 3 Active Testes are wider, triangular in cross section and beige. Sperm are visible if the gonad is cut and gently squeezed. Eggs become visible to the naked eye as tiny yellow granules in a gelatinous orange matrix. There is very little increase in the diameter of the ovary. 4 Developing Testes become wider and deeper and are mottled and creamy beige. They are also softer in texture, rupturing when lightly pinched. Besides the obvious presence of sperm in the main sperm duct, some sperm are also present in the tissue. Ovaries become larger in diameter and opaque yellow in color. Clearly discernible eggs occupy the entire ovary. 5 Ripe Testes still larger in cross section and softer in texture. They become creamier in color owing to considerable quantities of sperm. The ovaries are larger in diameter as a result of an increase in egg size. 6 Ripe and running Testes even larger in cross section and uniformly cream in color. They are extremely delicate at this stage and rupture easily when handled. Sperm are freely extruded when pressure is applied to the abdomen of the whole fish. Ovaries amber in color and have a substantial proportion of hydrated eggs. 7 Spent Testes are shrivelled and a mottled beige and cream. A little viscous semen may still ooze from the genital pore when pressure is applied to the abdomen. Ovaries are reduced in size, similar in appearance to those at stage 2 and have a few remaining yolked oocytes. These yolked oocytes are generally aspherical and appear to be undergoing resorption. mean numbers of silver kob per 30-min trawl per grid block. The SCBS methods are fully described by Badenhorst and Smale ( 1991); therefore only a sum- mary is given here. The survey area extended from Cape Agulhas to Port Alfred and seawards to a depth of 500 m. This area was divided into four depth zones (0-50 m, 51-100 m, 101-200 m, and 200-500 m), which were in turn subdivided into blocks of 5 x 5 nautical miles. The blocks trawled during each sur- vey were determined semirandomly according to the ratio of blocks per stratum. Bobbins were not used; therefore trawling was limited to nonreef substrata. The shallowest depth over which the research vessel ( F.R.S . Africana ) could operate was 20 m. A 180-ft German trawler with a 25-mm-mesh (bar) liner at- tached to the trawl bag was used. Trawl duration was limited to 30 min, and the results of shorter trawls (owing to technical reasons or to hitting the reef) were standardized to that time. Bottom temperature was recorded immediately after most trawls with a Neil Brown MK III-B conductivity, temperature, and depth probe (CTD). Mean numbers of A. modorus per trawl for each 1°C of bottom temperature were plotted to ob- tain the preferred temperature range of this species. Migration of A. inodorus was studied by using tag- ging and catch data. A tagging program was initi- ated in February 1994. Silver kob captured with hook Table 2 Number of trawls in which juvenile A. inodorus were caught during South Coast Biomass Surveys between Cape Agulhas (20°E) and Port Alfred (27°E) during the period 1987-95. Total Trawls with Cruise trawls silver kob 9 Sep-4 Oct 1987 88 24 11 May-2 Jun 1988 93 8 11 May-28 May 1989 62 12 24 May-12 Jun 1990 58 12 8 Sep-26 Sep 1990 73 21 8 Jun-1 Jul 1991 91 24 14 Sep-2 Oct 1991 75 30 1 Apr-20 Apr 1992 82 6 3 Sep-20 Sep 1992 87 32 19 Apr-10 May 1993 109 9 2 Sep-28 Sep 1993 106 30 8 Jun-3 Jul 1994 89 11 22 Sep- 16 Oct 1994 92 18 23 Apr-15 May 1995 95 9 All cruises 1,200 246 and line were tagged with plastic T-bar tags in False Bay (n = l,034), off Struis Bay (/?=750), and off Stil Bay (t?=291). The data for recaptured silver kob 50 Fishery Bulletin 95( 1 ), 1997 (n = 157), predominantly adults, were analyzed ac- cording to tagging locality, days free, and the mini- mum aquatic distance travelled. Owners of commercial line boats and inshore trawl- ers are required to submit daily catch returns to the Sea Fisheries Research Institute. The monthly catches of A. inodorus made by commercial line- fishermen in each of the three regions and the monthly catches made by the inshore trawl fishery in the southern Cape and the southeastern Cape for the period 1986-94 were expressed as percentages of the respective annual totals. The median size at first maturity (L50) for males and females was estimated by fitting a logistical func- tion (LOGIT) to the fractions of mature fish (gonad stage 3+) per 50-mm length class (midpoint) that were sampled in the southern Cape and the south- eastern Cape during the breeding season. Many of the smaller males with active testes lacked drum- ming muscles. Logistical functions were therefore also fitted to the fractions of males (per 50-mm length class) with fully developed drumming muscles. Be- cause A. inodorus are not trawled in the southwest- ern Cape, few juveniles were sampled and L50 val- ues could not be calculated for that region. Reproductive seasonality was established in the southeastern Cape by calculating both gonado- somatic indices (GSI’s) and the monthly percent fre- quency of each maturity stage for fish >L50. GSI = gonad weight/ (fish weight - gonad weight) x 100. The extent of the spawning area was determined by computing the percent frequency of each maturity stage for fish ( >L50 ) that were sampled during peak spawning (Oct and Nov 1991) off East London, Port Alfred, Mossel Bay, St Sebastian Bay, and False Bay. Sex ratios were tested statistically for significant deviations from unity with a chi-square test (P<0.05). Nursery areas were delineated by comparing the length-frequency distributions of silver kob caught 1) during South Coast Biomass Surveys (SCBS), 2) during experimental linefishing expeditions (no mini- mum size) and 3) by the line fishery (1990-94) as well as by analyzing the catch and effort distributions generated for silver kob during SCBS’s (1987-95). Results Catch distribution and migration Geographically related catch and CPUE trends for the line fishery consisted of three modal groups (Fig. 2), indicating that there are three areas of adult abundance between Cape Point and the Kei River (one in each region). Data from SCBS’s showed that adult abundance trends were reflected in juvenile distribution, at least for the east coast (Fig. 3). Sub- stantial differences in growth rates, otolith-dimen- sion and fish-length relationships, and growth zone structure (Griffiths, in press, a) suggest that these areas of abundance represent three allopatric stocks. Tag returns from the present study revealed that South African silver kob are capable of migrations of 240 km in six months but that most fish (84%) did not move more than 50 km from their tagging local- ity (Fig. 4). Only one fish tagged in False Bay was recaptured outside of that bay. Of the silver kob tagged in the Struis Bay vicinity, five (5.3%) had migrated westwards to False Bay, and the rest were recaptured either within 50 km of the tagging local- ity (77.3%) or had moved eastwards (17%), but only as far as Mossel Bay. None of the tagged fish were recaptured in the southeastern Cape. Tagging data therefore support the three-stock concept but sug- gest that there is limited exchange between silver kob in the southern Cape and those in the south- western Cape. Based on catch data, the foci of each stock are apparently False Bay, Stil Bay, and Port Alfred, which are separated by distances of 396 and 630 km, respectively. Struis Bay is situated towards the westerly extreme of the area occupied by the southern Cape stock: therefore it is not surprising that of the recaptured silver kob that had moved substantial distances (>50 km) from this tagging lo- cality, most had moved to the east. Interviews with commercial linefishermen ([n=36]; also confirmed by author’s personal experience) in- dicated that their silver kob catch was made on reefs at depths of 20-60 m to the east and 5-20 m to the west of Cape Agulhas. Inshore trawling between Cape Agulhas and Port Alfred occurs on soft ground at depths of 50-120 m (Japp et al., 1994). Decreases in the line catches of all three stocks during winter (Fig. 5) and corresponding increases in the catches made by inshore trawlers (Fig. 6) suggest that silver kob move farther offshore at this time of the year. Because inshore trawlers fish over substrata that are different from those over which linefishermen fish and since they land mostly juvenile and young adult A. inodorus (Fig. 7), it could be argued that trawl catch data do not reflect the winter locality of the adult population. The offshore movement of adults is supported, however, by the recapture of four speci- mens (435-720 mm) tagged in 30 m of water off Struis Bay in summer 1995 by inshore trawlers operating in 80 m off Stil Bay and off Cape Infanta in the winter and early spring of that year. Presumably, large adults Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 51 Figure 2 Annual catch per unit of effort and mean annual catch for commercial linefishermen operating in 12 subregions between Cape Point and the Kei River, 1986-94. See Figure 1 for localities. are also found on predominantly untrawlable rocky sub- strata during their offshore winter distribution. Size at maturity Silver kob were found to mature at a smaller size in the southeastern Cape than in the southern Cape, and in both regions males matured at a smaller size than did females. Females began to mature at about 250 mm in both regions, but the percentages of ma- ture fish in consecutive size classes increased more rapidly in the southeastern Cape than in the south- ern Cape (Fig. 8, Aand B). Estimated median lengths at maturity (L50) were 310 mm and 375 mm for the two regions respectively. All females in the south- eastern Cape larger than 450 mm and all females in the southern Cape larger than 550 mm were mature (Fig. 8, A and B). A comparison of the testes method with the drum- ming muscle method for estimating male maturity indicated that, within each region, the two methods produced similar estimates for length at total matu- rity but that the testes method produced higher es- timates for the proportions of mature fish in size classes below this length. In the southeastern Cape, males began to mature at 150 mm (testes method) and at 200 mm (drumming muscle method), L50 was calculated at 205 mm (testes method) and 290 mm (drumming muscle method), and total maturity was attained at 400 mm (both methodsXFig. 8, C and E). In the southern Cape, males began to mature at 200 mm (testes method) and at 250 mm (drumming muscle method), Lr)0 was calculated at 270 mm (tes- tes method) and 325 mm (drumming muscle method), and total maturity was attained at 450 mm (both methodsXFig. 8, B and F). Many of the smaller males (<300 mm) classified as mature (i.e. testes contained sperm), had dispro- portionately smaller gonads and also lacked drum- ming muscles. Because male drumming plays an important role in sciaenid spawning behavior (Takemura et al., 1978; Saucier and Baltz, 1993; Connaughton and Taylor, 1995; Connaughton, 1996), it is not known whether these fish would actually spawn. Even if the small males (without drumming muscles) managed to spawn with a communal spawn- 52 Fishery Bulletin 95(1 ), 1 997 ing aggregation (doubtful as this may be), their con- tribution to the total reproductive output (of the ag- gregation), in relation to the small size of their tes- tes, would likely be extremely low. Therefore, from a management view point, the Lf)0 estimates based on drumming muscle development were regarded as more useful than those based on gonad staging. According to Griffiths (in press, a), there was no difference between the growth rates of A. inodorus in the southeastern Cape and those in the southern Cape during 1990-91. The smaller sizes at maturity in the former region were therefore due to earlier ma- turity and not to slower growth. Female L50 and total maturity are attained at about 1.3 and 3.5 yr in the southeastern Cape and at about 2.4 and 4.7 30" in the southern Cape. Male L50, based on testes staging and on drumming muscle development, was attained at <1 yr and at 1 yr for silver kob in the southeastern Cape, and at <1 yr (testes staging) and at 1.5 yr (drumming muscle development) in the southern Cape. Total male maturity was attained at about 2.8 yr in the south- eastern Cape and at about 3.4 yr in the southern Cape. Spawning Gonadosomatic indices (Fig. 9) and gonad maturity indices (Fig. 10) for silver kob in the southeastern Cape showed that although some spawning occurred throughout the year, there was a clearly defined breeding season from August to December and that peak spawning occurred in spring (Sep-Nov). These results are in general agreement with those of Smale (1985) for Algoa Bay, but his spawning season ap- pears to have been “extended” by about one month, through the inclusion of A. japonicus, which spawns from October to January (Griffiths, in press, b) in the southeastern Cape. The low proportion of ripe and running (stage-6) females sampled during the spawning season (Fig. 10; and Smale, 1985) suggests that females feed less and are therefore less prone to capture (with hook and line) after oocyte hydra- tion. This inference is supported by a much higher proportion of stage-6 females in catches of silver kob caught by beach seines in False Bay than in catches made by using hook and line in four other localities Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 53 (during similar months and times of dayXFig. 11). Very low numbers of females with hydrated oocytes have also been reported in line catches of other sciaenids, e.g. Sciaenops ocellatus (Fitzhugh et ah, 1988), Micropogonias undulatus (Barbieri et al., 1994), and Atractoscion aequidens (Griffiths and Hecht, 1995a); no hydrated oocytes were detected from line catches of Argyrosomus japonicus (Griffiths, in press, b). Most silver kob caught during SCBS’s were juveniles. Nevertheless, none of the adult fe- males that were trawled had hydrated oocytes, per- haps because these fish were captured on the nurs- ery grounds and not on adult habitat where spawn- ing is expected to occur (see below). The large proportion of ripe and ripe and running (stages 5 and 6) males and females at each of the five sites between Cape Point and the Kei River ( Fig. 11) during October-November suggests that spawn- ing occurs throughout the study area and that peak spawning occurs during spring for all three stocks. The inshore distribution of the adults during spring and summer, the absence of Argyrosomus eggs and larvae in the Agulhas Current (ca. 200 mXBeckley, 1993), and the occurrence of significant numbers of early stages of A. inodorus larvae (identified as “A. holo- lepidotus “) in 5-7 m in Algoa Bay (Beckley, 1986) sug- gest that spawning occurs in less than 50 m depth of water. However, even though early life stages of lar- vae and juvenile recruits (see “nursery areas” below) are found just seaward of the surf zone (5-7 m), it is not certain whether spawning occurs in this area or whether it occurs in slightly deeper water and the eggs and larvae are transported shorewards by currents. Although spawning in other sciaenids, including A. japonicus, occurs at night (Fish and Cummings, 1972; Takemura et al., 1978; Holt et al., 1985; Saucier and Baltz, 1993; Connaughton and Taylor, 1995; Griffiths, in press, b), the fact that large proportions of ripe and running females caught in seine nets in False Bay (Fig. 11) were caught between 11:30 h and 14:30 h, suggests that spawning in A. inodorus may occur during the day. The water temperature in which ripe and running females were captured was 18- 19°C, but as indicated for other sciaenids (Saucier 54 Fishery Bulletin 95(1 ), 1997 01994 □ 1993 0 1992 □ 1991 □ 1990 □ 1989 □ 1988 □ 1987 ■ 1986 J FMAMJ JASOND □ 1994 □ 1993 H 1992 □ 1991 □ 1990 □ 1989 □ 1988 □ 1987 11986 Figure 5 Cumulative monthly catches of Argyrosomus inodorus made by South African commercial linefishermen in each region, expressed as percentages of the re- spective annual catches, 1986-94. and Baltz, 1993; Connaughton and Taylor, 1995), a range of spawning temperatures is expected. Hydrophonic monitoring of drumming levels (Takemura et al., 1978; Saucier and Baltz, 1993; Connaughton and Taylor, 1995) would provide bet- ter information on the times, sites, and oceanographic conditions necessary for spawning of silver kob. Although the ovaries of A. inodorus were not ex- amined microscopically, substantial increases in the number of spent gonads towards the end of, and im- mediately after, the five-month spawning season (Fig. 10), as opposed to throughout the season, suggest that they are multiple spawners. Unfortunately par- tially spawned fish could not be identified macro- Griffiths. Life history and stock separation of silver kob, Argyrosomus inodorus 55 Southeastern Cape Trawl Total catch = 376. El 994 □ 1993 01992 □ 1991 □ 1990 □ 1989 01988 □ 1987 ■ 1986 0 N D 01994 □ 1993 01992 □ 1991 □ 1990 □ 1989 01988 01987 ■ 1986 Month Figure 6 Cumulative monthly landings of Argyrosomus inodorus made by the inshore trawl fish- ery in the southeastern Cape and the southern Cape, expressed as percentages of the respective annual totals, 1986-94. scopically. Multiple-batch spawning has been de- scribed for several other species of sciaenids, e.g. Sciaenops ocellatus (Fitzhugh et al., 1988), Seriphus politus (DeMartini and Fountain, 1981), Cheilotrema saturnum (Goldberg, 1981), Genyonemus lineatus (Love et al., 1984), Cynoscion nebulosis (Brown- Peterson et al., 1988), Pogonias cromis (Fitzhugh et al., 1993; Nieland and Wilson, 1993), and Micro- pogonias undulatus (Barbieri et al., 1994). Sex ratios From the total numbers of silver kob sampled, it was evident that there were significantly more females (1.6x) in the southeastern Cape, more males (1.2x) in the southern Cape, and more females (2. lx) in the southwestern Cape (Table 3). Except for the small- est size class sampled in the southeastern Cape (where males predominated), all other size classes sampled in the southeastern Cape and in the south- western Cape contained significantly more females. Smale (1985) also recorded consistently higher pro- portions of female “A. hololepidotus ” per 100-mm length class for fish sampled in the southeastern Cape (1978-81). Although he included both A. inodorus and A. japonicus in his study, the latter species recruits to the line fishery only at about 1,000 mm TL (Griffiths, in press, b), therefore his speci- 56 Fishery Bulletin 95(1), 1997 Southeastern Cape Trawl n = 1,322 0 10 20 30 40 50 60 70 80 90 100 110 120 Length class (5cm) Figure 7 Total length distributions of Argyrosomus inodorus landed by the inshore trawl fishery in the southeastern Cape and in the southern Cape, 1990-94. The proportions of immature fish were based on re- gional averages of the values for males (drumming muscle method) and females presented in Figure 8. Sex ratios of Argyrosomus P< 0.05. Table 3 nodorus from three regions along the South African eastern seaboard. * = significant difference at Total length (mm) Southeastern Cape Southern Cape Southwestern Cape M : F n X2 M : F n X2 M : F n X2 100-199 1.4 : 1 188 4.2* 1.2 : 1 114 0.9 200-299 1 : 1.2 562 4.4* 1.1 : 1 320 1.3 300-399 1 : 1.7 1,194 72.4* 1.2 : 1 289 2.5 1 : 1.3 72 0.9 400-499 1 : 2.0 541 61.9* 1.3 : 1 544 10.6* 1 : 3.1 49 12.8* 500-599 1 : 2.9 212 49.1* 1.5 : 1 212 9.1* 1 : 1.7 57 4.0* 600-699 1 : 2.0 119 7.1* 1 : 1.2 76 0.8 1 : 2.6 36 7.1* 700-799 1 : 1.8 58 4.4* 1 : 1.2 30 0.1 1 : 1.8 61 13.8* 800-899 1 : 1.5 56 2.6* 1.1 : 1 25 0.0 1 : 2.0 77 8.1* 900-999 1 : 3.2 25 6.8* 1.9 : 1 43 3.9* 1 : 2.8 57 12.8* 1000-1099 1 : 6.3 21 11.6* 1.3 : 1 80 1.0 1 : 2.6 29 5.8* 1100-1199 1 : 1.5 5 0.2 1 : 1.1 30 0.1 All sizes 1 : 1.6 2,982 158.7* 1.2 : 1 1,764 20.1* 1 : 2.1 446 57.4* Griffiths: Life history and stock separation of silver kob, Argyrosomus modorus 57 mens below this length were mostly A. inodorus. Of the 11 size classes sampled in the southern Cape, eight contained more males and three contained more females (Table 3). However, the ratios of only three of these size classes (each with more males) were sig- nificantly different from the expected 1:1. Because more males were sampled within most size classes in the southern Cape, it would appear that there are more males than females in this region and that the lack of significance for several of the size classes could be due to limitations of the statistical test. The chi- square test is based on absolute differences (between observed and expected) and does not take into ac- count sample size, e.g. although a ratio of 1.2:1 0 200 400 600 800 1000 1200 Figure 8 Percentage of mature female (gonad stage 3+) and male (gonad stage 3+ and drumming muscle stage 3) Argyrosomus inodorus by 50-mm total-length intervals, sampled during the spawning period in the southeastern Cape and in the south- ern Cape. The solid line describes the fitted logistical function. L50= median length at first maturity (mm total length); n = sample size. 58 Fishery Bulletin 95 ( 1 ), 1997 O X5 CO c o 0 Figure 9 Mean monthly (circles) and monthly (crosses) gonadosomatic indices for mature female and male Argyrosomus inodorus sampled in the southeastern Cape, April 1990-January 1992. n=sample size. (rc=l,764) was significant (even at PcO.OOl), one of 1.3:1 (n= 80) was not (Table 3). Thus researchers studying sex ratios should make every effort to ob- tain large samples, particularly if the chi-square method is to be used as a test for significant difference. Nursery areas Argysomus inodorus landed by the line fishery were mostly adult fish between 40 and 120 cm TL (Fig. 12). Although fish <40 cm TL were not represented in the present study owing to the minimum size limit imposed on linefishermen, experimental linefishing indicated that silver kob on the linefishing grounds (reef substrata) were mostly >30 cm TL (Fig. 12) and that in the southeastern Cape and in the southern Cape they were generally larger than the female L50 estimates. Argysomus inodorus trawled between Cape Agulhas and Port Alfred during SCBS’s (nonreef sub- strata) ranged between 5 and 120 cm TL but were generally <45 cm TL, and largely immature (Fig. 13). The modal length class increased from 20-25 cm at depths of 25-50 m, to 25—30 cm at 50-100 m and 30-35 cm at 100-150 m; and the proportion of ma- ture fish increased accordingly. Although depths <25 m were not sampled during SCBS’s, silver kob (iden- tified as “A. hololepidotus ”) trawled in <9 m during an earlier survey of the bays between Mossel Bay and Algoa Bay were mostly 9-18 cm TL (Smale, 1984). Beckley ( 1984) recorded A. inodorus (also iden- Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 59 100% 80% 60% 40% 20% Females n = 1 ,434 □ Stage 7 ■ Stage 6 □ Stage 5 m Stage 4 □ Stage 3 □ Stage 2 AMJ JASONDJ FMAMJ JASONDJ Males n = 784 □ Stage 7 ■ Stage 6 □ Stage 5 □ Stage 4 □ Stage 3 □ Stage 2 AMJ JASONDJ FMAMJ JASOND J 1 990 1 1 991 h- Figure 10 Monthly percentage of gonad stages for mature female and male Argyrosomus inodorus in the southeastern Cape, April 1990-January 1992. n=sample size. tified as “A. hololepidotus”) as small as 1.3 cm TL just behind the breakers (5-7 m depth) in Algoa Bay. Voucher specimens (including otoliths) from both of these studies were identified as A. inodorus. There is therefore a trend of increasing length with increas- ing depth and distance from the shore for juvenile silver kob occurring between Cape Agulhas and Port Alfred. This finding suggests that juveniles are re- cruited to the nursery grounds just seaward of the surf zone and that they move farther offshore as they grow. Silver kob do not enter estuaries, and between Cape Agulhas and the Kei River, they do not occur in the surf zone (Griffiths and Heemstra, 1995). The SCBS CPUE-analyses revealed that juvenile A. inodorus were not homogenously distributed over the survey area but were found mostly in <120 m depth and comprised two disjunct distributional ranges, i.e. Cape Agulhas to Mossel Bay and Cape St Francis to Port Alfred (Fig. 3). Although the inshore areas of the southwestern Cape are not suitable for trawl- ing, analysis of commercial beach-seine catches (Lamberth et al., 1994) revealed that juvenile A. inodorus (identified as “A. hololepidotus ”) are also found in False Bay. Discussion The results of this study strongly suggest that silver kob between Cape Point and the Kei River comprise three discrete stocks. The foci of each of these stocks are False Bay in the southwestern Cape, Stil Bay in the southern Cape, and Port Alfred in the southeast- ern Cape. Tagging evidence indicates that there is lim- 60 Fishery Bulletin 95(1 J, 1997 EL PA MB SSB FB Nov-91 Oct/Nov-91 Oct-91 Oct-91 Oct-91 Males m Stage 7 ■ Stage 6 □ Stage 5 □ Stage 4 a Stage 3 □ Stage 2 EL PA MB SSB FB Nov-91 Oct/Nov-91 Oct-91 Oct-91 Oct-91 Sampling site Figure 1 1 Percentage of gonad stages observed for male and female Argyrosom us inodorus at five localities during the peak spawning season in 1991. EL=East London, PA=Port Alfred, MB=Mossel Bay, SSB= St Sebastian Bay, FB=False Bay, and n=sample size. Fish sampled at the first four localities were caught by hook and line, whereas those in False Bay were caught with beach-seine nets. ited exchange between the southwestern Cape and the southern Cape stocks but that there is no exchange between either of these two stocks and the one in the southeastern Cape. Analysis of catch and tagging data shows that each of the three stocks is concentrated in- shore in summer but disperses seawards in winter. The distribution of silver kob on the South African eastern seaboard, including the existence of the three stocks and their onshore-offshore movement, is also supported by regional oceanographic patterns. Dur- ing spring, summer, and autumn, the east coast be- tween Cape Agulhas and the Kei River is character- ized by three zones: 1) a warm inshore band (0-20 m) with an average temperature of 21°C (although in certain areas temperatures can drop to <12°C for brief periods following coastal upwelling); 2) a zone of intermediate temperature (12-19°C) between 20 and 50 m; and 3) a bottom mixed layer of <12°C found below 50 m (Eagle and Orren, 1985; Swart and Largier, 1987; Goschen and Schumann, 1988; Boyd and Shillington, 1994; Greenwood and Taunton- Clark2). Silver kob prefer temperatures of 13-16°C (Fig. 14) and are therefore mainly confined to the 2 Greenwood, C., and J. Taunton-Clark. 1992. An atlas of mean monthly and yearly average sea surface temperatures around the southern African coast. Sea Fisheries Research Institute, Private BagX2, Roggebaai 8012, Cape Town, South Africa. In- ternal report 124, 112 p. Griffiths. Life history and stock separation of silver kob, Argyrosomus inodorus 61 Length class (5 cm) Figure 1 2 Regional total-length distributions of Argyrosomus inodorus 1) landed by the South African line fishery, 1990-94 (hatched lines) and 2) caught on linefish grounds during experimental fishing operations and which were below the minimum size limit (40 cm) (shaded area). Arrows indicate female median length at matu- rity (L50). L50 was not estimated for the southwestern Cape. intermediate zone. During spring, summer, and au- tumn, the intermediate zone is restricted to an area within a few kilometres of the coast and is within easy range of line boats (see 50-m isobath in Fig. 1). In winter the bottom mixed layer retreats down the shelf to about 100 m (Schumann and Beekman, 1984; Eagle and Orren, 1985; Swart and Largier, 1987). As the intermediate zone expands, I propose that A. inodorus stocks disperse seaward, moving beyond the grounds of the linefishery and onto the inshore trawl- ing grounds. Because Agulhas Bank is much nar- rower in the southeastern Cape than in the south- ern Cape (Fig. 1), offshore movement would have been more constrained than in the latter region and hence would explain the higher winter line catches in the southeastern Cape (Fig. 5). Owing to a higher degree of coastal upwelling off the southwestern Cape, the bottom mixed layer 62 Fishery Bulletin 95( 1 ), 1997 (<12°C) is shallower (20 vs. 50 m on the east coast) from spring to autumn, and the temperatures above 20 m are generally 13-19°C during this period (Atkins, 1970; Boyd et ah, 1985; Largier et al., 1992; Greenwood and Taunton-Clark2). Because inshore temperatures are lower than those on the east coast, silver kob are caught by linefishermen from the surf zone to depths of 20 m. As along the east coast, the bottom mixed layer deepens to about 100 m in win- ter (Atkins, 1970; Boyd et ah, 1985; Largier et al., 1992), and silver kob are expected to move offshore (as indicated by catch trends [Fig. 5]). Because the depth contours broaden east of Cape Hangklip, it is possible that there is also an easterly component to the offshore dispersal of silver kob. A seaward and eastward winter migration has also been postulated Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 63 for subadult Atractoscion aequidens (Sciaenidae), occurring in the southwestern Cape (Griffiths and Hecht, 1995a). Data from all 14 SCBS’s revealed that the bottom mixed layer (<12°C) extends farther up the shelf (and is closer to the coast) in the area between Knysna and Cape St Francis (Fig. 15; see also Le Clus and Roberts, 1995), thus inhibiting exchange between the silver kob stock in the southern Cape and that in the southeastern Cape. Along the eastern and western sides of False Bay, the 20-m isobath is found <500 m from the shore (van Ballegooyen, 1991). Because suit- able temperatures for silver kob are found at depths shallower than 20 m in the southwestern Cape dur- ing spring to autumn, the movement of silver kob into or out of False Bay (the focus of the southwest- ern Cape stock) during this period is therefore re- stricted. In addition, the upwelled bottom mixed layer frequently extends to the shore between Cape Hangklip and Cape Agulhas, particularly from De- cember to April (Boyd et al., 1985; Largier et al., 1992), thus further limiting exchange between in False Bay and in the southern Cape. Spawning occurred throughout the distributional ranges of all three stocks and peaked in spring ( Sept- Nov) in all regions. Sizes and ages at maturity were, however, substantially smaller in the southeastern Cape than in the southern Cape. Female L50 was 310 mm TL (1.3 yr) in the former and 375 mm TL (2.4 yr) in the latter region. Changes in ages and sizes at maturity have been correlated with exploitation rate for several fish species (Healey, 1975; Borisov, 1978; Ricker, 1981; Beacham, 1983; Wysokinski 1984; Armstrong et al., 1989). Because fishing mortality is significantly higher in the southeastern Cape than in the southern Cape (F= 0.67 vs. 0. 42)( Griffiths, in press, c), the smaller sizes at maturity recorded for the southeastern Cape are possibly due to fishing pressure. The mechanisms accounting for the de- creases in the size and age at maturity in the south- eastern Cape, however, remain to be identified. One explanation is that the younger ages and smaller sizes at maturity for silver kob in the southeastern Cape could be the result of density-dependent effects; higher mortality results in more food for surviving fish, in additional energy for gonad growth, and con- sequently in earlier maturity. In several other spe- cies, ages or sizes (or both) at maturity have been correlated with the amount of accumulated surplus energy within a fish (Armstrong et al., 1989; Rowe et al., 1991; Berglund, 1992; Kerstan3). On the other hand, Ricker (1981) stated that “If a fish matures before it is large enough to be vulnerable to fishing, its expectation of contributing to future generations will be greater than that of a sibling of the same size that does not mature until a year later. The result can be a gradual decrease in the mean size at matu- rity.” Female silver kob attain the minimum size limit for the line fishery (400 mm TL) at ca. 2.8 yr in both the southeastern Cape and the southern Cape (Griffiths, in press, a). Approximately 95% of these 3 Kerstan, M. 1995. Sex ratios and maturation patterns of horse mackerel (Trachurus trachurus) from the NE-and SE- Atlantic and the Indian Ocean — a comparison. ICES council meeting H:6, 20 p. (Mimeo.) 64 Fishery Bulletin 95( 1 ), 1997 Figure 1 5 Bottom temperature structure on the central and eastern Agulhas Bank during the October 1987 South Coast Biomass Survey. This temperature pattern, with cold water (<12°C) at shallower depths and closer to the coast between Knysna and Cape St Francis, was typical of SCBS’s be- tween 1987 and 1995. See Figure 1 for additional localities. new recruits are mature in the southeastern Cape, but only 69% in the southern Cape. Assuming that size at maturity for silver kob in the southeastern Cape was at one time similar to that in the southern Cape, the removal of late-maturing fish, before they had spawned for the first time, could have reduced the sizes and ages at maturity to those recorded in this study. The large contribution of recruits (400- 450 mm TL) to the southeastern Cape line catch (ca. 50% by numberXFig. 12), supports this hypothesis. Investigation of sex ratios showed that there are substantially more females in the southeastern Cape (1.6x) and southwestern Cape (2. lx) stocks, but more males in the southern Cape (1.2x). Most natural populations tend to stabilize at sex ratios of 1:1 (Conover and Van Voorhees, 1990), including those of other sciaenids (Shepherd and Grimes, 1984; Murphy and Taylor, 1989; Wilson and Nieland, 1994; Ross et al., 1995; Griffiths and Hecht, 1995a). The deviations from this ratio observed for South Afri- can silver kob are not easily explained. Basically, the reasons for an observed sex ratio that deviates from unity may be grouped into three categories: 1) more individuals of either sex are produced (e.g. environ- mental sex determination); 2) equal numbers of both sexes are produced, but those of one sex are dimin- ished through either emigration or mortality; and 3) sampling methods are biased towards one of the sexes. Although environmental sex determination can temporarily result in skewed sex ratios in some species (Conover and Heins, 1987), frequency-depen- dent selection is expected to return the ratios of such populations to equality through future generations (Conover and Van Voorhees, 1990). Higher propor- tions of either sex over most size classes (therefore spanning several age classes) in each region, with consistency over two periods (1978-81 and 1990-91) in the southeastern Cape, render environmental sex determination an unlikely cause of the observed sex ratios. Male emigration from the southeastern Cape and the southwestern Cape to the southern Cape is also unlikely. Because the smallest size class sampled in the southeastern Cape consisted of males without drumming muscles and also consisted of more males than females, it is tempting to suggest that male drumming during the protracted spawning season may have resulted in sex-selective predation in this region and in the southwestern Cape. However, there is no reason to believe that predation rates should be higher in the southeastern Cape and the south- Griffiths: Life history and stock separation of silver kob, Argyrosomus inodorus 65 western Cape than in the southern Cape. Because capture methods were the same in all three regions, increased vulnerability of either sex to capture is not a plausible explanation either. Thus additional re- search is required before the regionally specific sex ratios observed for A. inodorus can be adequately explained. Silver kob use inshore (<120 m depth) sand and mud substrata as nursery areas. They apparently recruit (ca.1.5 cm TL) just seawards of the surf zone (5-7 m depth) but move offshore with growth. Upon attaining maturity they recruit to adult populations that are found on reefs. Distributional analyses have revealed that juvenile A. inodorus between Cape Agulhas and the Kei River comprise two disjunct distributional ranges, one in the southern Cape and the other in the southeastern Cape. Although the inshore areas of the southwestern Cape are not suit- able for trawling, analysis of commercial beach-seine catches (Lamberth et al., 1994) has revealed that juvenile A. inodorus (identified as “A. hololepidotus ”) are also found in False Bay. The existence of nursery areas and spawning grounds in each of the three sampling regions, and the differences in size at ma- turity and sex ratio, lend further credence to the sepa- rate stock concept. Conclusion Distributional analyses based on fishery dependant and fishery independent data revealed that there are three areas of silver kob abundance between Cape Point and the Kei River. The fact that each of these “populations” has its own spawning grounds and nursery areas and the fact that there are observed differences in growth rates, otolith-dimension and fish-length relationships, growth zone structure, sizes at maturity, and sex ratios, together indicate that these “populations” represent separate stocks. This three-stock concept is further supported by mi- gratory patterns indicated from catch and tagging data and by the oceanography between Cape Point and the Kei River. Although genetic differentiation should ideally form the basis of inferences concerning stock distinc- tion, analyses based on protein electrophoresis and mitochondrial DNA have generally been unsuccess- ful in differentiating between marine stocks (see Campana and Casselman, 1992; Pawson and Jennings, 1996), including those of sciaenids ( Ramsey and Wakeman, 1987; Graves et al., 1992; King and Pate, 1992). Although none of the data used to infer separate silver kob stocks necessarily reflect genetic differences (Ihssen et al., 1981), the identification of three allopatric units of fish with different popula- tion parameters indicates that each may respond differently to fishing and that the exploitation of one unit will not affect the size or composition of the other two, thereby supporting separate management of the three units and their stock status (Spangler et al., 1981; Brown and Darcy, 1987; Campana and Gagne, 1995; Edmonds et al., 1995; Pawson and Jennings, 1996). Recent application of per-recruit models to South African silver kob (based on the results presented in this study) indicates that, owing to their different population parameters, each stock requires a differ- ent combination of fishing mortality and age at first capture for optimal exploitation (Griffiths, in press, c). Therefore A. inodorus should ideally be managed on a regional and not on a national basis. Studies of the life histories of two other South African sciaenids, Atractoscion aequidens (Griffiths and Hecht, 1995a) and Argyrosomus japonicus (Griffiths and Hecht, 1995b; Griffiths, in press, b), have revealed that they consist of single stocks with allopatric age or size- determined subpopulations, even though they occur from Cape Point to southern Mozambique and are therefore more widely distributed on the eastern sea- board than are A. inodorus. Inferences from stock struc- ture, based on closely related taxa, are therefore not desirable because they could result in erroneous con- clusions and consequently in poor management. Acknowledgments The author thanks Chris Wilke, Peter Sims, and John Prinsloo (Sea Fisheries Research Institute) for pro- viding CPUE and length-frequency summaries for the line fishery, the inshore trawl fishery, and South Coast Biomass Surveys, respectively; M. Roberts, A. Boyd, and G. Nelson for oceanographic data and for helpful discussions on the oceanography of the study area; Malcolm Smale and Larry Hutchings for valu- able comments on an earlier draft of the manuscript; and all those commercial and recreational fishermen who made their catches available for sampling. The project was partially funded by the Sea Fisheries Fund. Literature cited Armstrong, M. J., B. A. Roel, and R. M. Prosch. 1989. 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In A. I. L Payne., J. A. Gulland, and K. H. Brink (eds.), The Benguela and comparable ecosystems. S. Afr. J. Mar. Sci. 5: 243-253. Takemura, A., T. Takita, and K. Mizue. 1978. Studies on the underwater sound — VII. Underwater calls of the Japanese marine drum fishes (Sciaenidae). Bull. Jpn. Soc. Fish. 44:121-125. Van Ballegooyen, R. C. 1991. The dynamics relevant to the modelling of synoptic scale circulations within False Bay. Trans. R. Soc. S. Afr. 47:419-431. Wilson, C. A., and D. L. Nieland. 1994. Reproductive biology of red drum, Sciaenops ocellatus, from the neritic waters of the northern Gulf of Mexico. Fish. Bull. 92:841-850. Wysokinski, A. 1984. Length structure of the Cape horse mackerel popu- lation and changes in sexual maturity length in the Namibian region. Colin. Scient. Pap. Int. Comm. SE. Atl. Fish 11:91-98. 68 Abstract .—Estimates of tag-shed- ding and tag-reporting rates are re- quired for an estimation of fishing and natural mortality rates from tagging data. For this purpose, double-tagging and tag-seeding experiments were un- dertaken by the South Pacific Commis- sion, in conjunction with a large-scale tuna tagging program, in the western tropical Pacific Ocean during 1989- 1992. Estimates of tag-shedding rates indicated that 89% (95% confidence in- terval of 82%-94%) of tagged tuna still retained their tags after two years at liberty. Differences in shedding rates among skipjack, yellowfin, and bigeye tuna, and differences in shedding rates among taggers were found not to be sta- tistically significant. Tag seeding car- ried out on board purse seiners by ob- servers resulted in 342 returns of the 532 tags seeded, for a return rate of 64% (60%-68%). The return rate of seeded tags varied significantly by unloading location (most tags were recovered dur- ing unloading), but not by species. The highest return rates of seeded tags oc- curred from American Samoa, Philip- pines, and Solomon Islands, whereas Korea and Thailand had the lowest re- turn rates. The overall average report- ing rate, weighted by the estimated numbers of tags recovered at each lo- cation, was 0.59. A bootstrap procedure was used to estimate a 95% confidence interval of 0.49-0.67. These results implied that, of the 146,581 tags re- leased during the large-scale tagging program, 31,166 (27,208-37,264) were recaptured, of which 18,266 were re- turned to the South Pacific Commission. Manuscript accepted 17 July 1996. Fishery Bulletin 95:68-79 (1997). Estimates of tag-reporting and tag-shedding rates in a large-scale tuna tagging experiment in the western tropical Pacific Ocean John Hampton South Pacific Commission B.R D5 Noumea Cedex, New Caledonia E-mail address: wjh@spc.org. nc Tag release-recapture experiments are commonly used to estimate pa- rameters, such as growth, mortal- ity, and population size, of exploited fish stocks (Beverton and Holt, 1957; Seber, 1973). One method used to estimate mortality rates is to fit a tag-attrition model to a time series of tag-return data (Seber, 1973; Kleiber et al., 1987). In its simplest form, the tag attrition model can be expressed as j =(1 -a)T exp [-(F + M + X)(j- D] — ~ — - F + M + A [l- exp(-F - M - A)], (1) where (f), is the predicted number of tag returns in time period j, a rep- resents all type-1 tag losses, T is the number of tag releases, F is the in- stantaneous rate of fishing mortal- ity (assumed constant), M is the in- stantaneous rate of natural mortal- ity (assumed constant), and A rep- resents all continuous type-2 tag losses. Type-1 tag losses include im- mediate tag shedding, immediate tagging-induced mortality, and fail- ure to report recovered tags. Type- 2 tag losses include continuous tag shedding, continuous mortality di- rectly attributable to the tag, and emigration of tagged fish away from the area of the fishery. For unbiased estimates of F and M to be obtained, it is clear from Equation 1 that these tag losses must be estimated and included in the tag-attrition model. In general, type-1 and type-2 loss rates cannot be estimated directly from tag-return data, although es- timation of type-1 losses may be possible under circumstances where fishing intensity is highly variable (Beverton and Holt, 1957). More commonly, loss rates are estimated from independent experiments car- ried out in conjunction with a tag- ging program. Tag-shedding rates may be estimated from double-tag- ging (two tags per fish) experiments (Wetherall, 1982) or from direct ob- servation of tagged fish held in cap- tivity. Tag-reporting rates may be estimated from tag-seeding experi- ments (Youngs, 1974; Green et al., 1983; Campbell et al., 1992), from sequential observations of recover- ies at different stages of catch han- dling and processing (Hilborn, 1988), and by comparing tag return rates from the fishery with those from a control group (such as ves- sels carrying fisheries observers) assumed a priori to report all tag recoveries (Paulik, 1961; Seber, 1973). Type-1 and type-2 tagging mortality rates may, for some spe- cies, be estimated from observations of tagged and untagged fish held in captivity. The South Pacific Commission (SPC) recently conducted a large- Hampton: Estimates of tag-reporting and tag-shedding rates for tuna in the tropical Pacific Ocean 69 scale tuna tagging program, the Regional Tuna Tag- ging Project (RTTP), in the western tropical Pacific. From 1989 to 1992, 146,581 tagged skipjack tuna, Katsuwonus pelamis, yellowfin tuna, Thunnus albacares, and bigeye tuna, Thunnus obesus, were released throughout the western tropical Pacific from the Philippines and eastern Indonesia to approxi- mately 170°W. This area is fished by purse-seine, pole-and-line, longline, handline, and troll vessels, which have collectively harvested more than one million metric tons of tuna per year since 1989 (Lawson, 1994). As of 31 May 1995, 18,266 tagged fish had been recaptured and the tags and accompa- nying recapture information returned to SPC. Tagged tuna were recaptured by all of the fishing methods of the western Pacific fishery. Most tag returns (76%) originated from purse seiners, consistent with the pro- portion of total catch attributed to that gear (67% for 1990-1993). Few additional tag recoveries are expected. One of the major objectives of the tagging program was to estimate the rates of fishing-induced and natu- ral mortality by using models similar to Equation 1, so that the impacts of the fishery on the stocks could be assessed. It was therefore necessary to obtain es- timates of type-1 and type-2 tag losses. In this pa- per, I focus on the estimation of tag-shedding rates and tag-reporting rates. Tag-shedding rates were estimated from double-tagging experiments carried out in conjunction with the tag-release program. Dif- ferences in shedding rates among species and differ- ences among individual taggers were evaluated. Tag- reporting rates were estimated from tag-seeding ex- periments in which tuna caught by purse seiners were surreptitiously tagged by fisheries observers prior to the fish being placed in the fish wells. Dif- ferences in the rates of reporting seeded tags by spe- cies, time, and port of unloading were investigated. An estimate of the overall reporting rate of recov- ered RTTP tags and its variability, which takes into account the variability in tag reporting among un- loading ports, was obtained. Materials and methods Double-tagging experiments Field operations Tagging was carried out on a pole- and-line vessel from which tuna were captured with standard commercial gear. Only uninjured fish that were cleanly hooked in the jaw were selected for tag- ging. Fish with excessive mouth damage, eye dam- age, or gill damage were not tagged. Selected fish were placed in a vinyl tagging cradle and their fork lengths measured to the nearest centimeter. For single-tagged fish, a Hallprint '' 13 cm dart tag was inserted by using a sharpened stainless steel appli- cator, into the musculature at an angle of about 45°, 1-2 cm below the posterior insertion of the second dorsal fin. Smaller ( 10-cm) tags were used for tuna less than 35 cm FL. Ideally, the tag barb was anchored be- hind the pterygiophores of the second dorsal fin. Throughout the three-year tag release program, a small sample (approximately 3%) of the tagged tuna were double tagged. Double tagging occurred on par- ticular days chosen in advance by the cruise leader and on such days, most fish were double tagged. The objectives were for each principle tagger to double tag at least 400 tuna, and for the double-tag releases to be as representative as possible of the species and size composition of the single-tag releases. These objectives were largely accomplished (Fig. 1). The technique for double tagging was identical to that of single tagging, with the exception that a sec- ond tag was inserted on the opposite side of the fish, 1-2 cm anterior to the first tag to avoid damaging it with the applicator. For single and double tagging, fish were generally out of the water for less than ten seconds. Data analysis Observations of the numbers of tags retained by double-tagged tuna at recapture can be used to estimate tag-shedding rates. I used a simple tag- shedding model (Beverton and Holt, 1957; Hampton and Kirkwood, 1990), which defines the probability, Q(t), of a tag being retained at time t after release as Q(t) = ( 1- p)exp(-Lt), (2) where p is the immediate type-1 shedding rate and L is the continuous type-2 shedding rate. These pa- rameters can be estimated from a double-tagging experiment under the assumption that all tags not immediately shed have identical shedding probabili- ties that are independent of the status of the com- panion tag. Given this assumption, the probabilities of two, one, and no tags being retained at time t af- ter release are, respectively, P2(t) = Q(t)2, P1(t) = 2Q(t)[l-Q(t)] (3) P0(t) = [l-Q(t)f. Consider a double-tagging experiment resulting in m recaptures of fish bearing two tags at times t2i ( i = 1, ..., m) and in n recaptures bearing one tag at times ty (J = 1, ..., n). The negative log likelihood of the data (t.,, tj) given the model parameters p and L is 70 Fishery Bulletin 95( 1 ), 1997 Tagger 100,000 ■ 90.000 - 80.000 ■ 70.000 - 60.000 ■ 50.000 - 40.000 - 30.000 ■ 20.000 - 10,000 - 0 ■ □ Single tagging □ Double tagging Skipjack Yellowfin Species Bigeye 3.000 2,500 2.000 • 1,500 - 1,000 - 500 0 C 10,000 Double tagging T l i — i — T — i — i — r — i — i — i — i — i — i 15 25 35 45 55 65 75 85 95 105 115 125 135 Length (cm) Figure 1 Distributions of single-tag and double-tag releases by (A) tagger, (B) species and (C) size. £^(t2 , t x I p,L) = -y,ln i= 1 ^2 ^2; ) 1- P0(^2; ) X'" 7=1 L 1 — Pq (ty ) (4) where the terms in square brackets represent the probabilities of two tags and one tag being observed for each recapture, given that at least one tag is ob- served. Maximum-likelihood estimates of p andL can therefore be obtained by minimizing with respect to the parameters. The model was fit to pooled recapture data, to data classified by species, and to data classi- fied by tagger. As an approximate indication of the overall losses due to tag shedding for each data set, the proportion of tags retained after two years (99% of RTTP tag returns were re- captured within two years of release), Q2yr, was calculated from Equation 2 by using the esti- mated parameters. Approximate 95% confi- dence intervals for Q2 were obtained by the percentile method (Efron, 1982) applied to dis- tributions of Q,,v;. generated from 1,000 paramet- ric bootstrap (or Monte Carlo) replicates of each data set. The replicates were constructed by using the observed distributions of times at lib- erty, and the numbers of tags observed for each pseudo-return were determined randomly with the conditional probabilities of a recaptured tuna bearing two tags or one tag, i.e. and respectively, given the estimated parameters. The statistical significance of improvements in fit of models that included species-specific and tagger-specific shedding parameters was determined by using likelihood-ratio tests (Kendall and Stuart, 1961). Tag-seeding experiments Rationale Tag seeding was carried out by ob- servers placed on board purse-seine vessels as part of regional and national observer pro- grams. The purse-seine fleet was targeted for tag-seeding experiments for several reasons. First, purse seiners account for most of the tuna catch in the western Pacific (and also recovered most tags); the estimation of reporting rates for this gear type in particular was therefore of critical importance. Second, the large, modern purse seiners typical of the western Pacific fleet handle large quantities of tuna very rapidly, with little opportunity for onboard inspection of individual fish for tags. As a result, tagged tuna recaptured by purse seiners were mostly detected during unloading (when individual fish are handled) or during the initial stages of pro- cessing in canneries. The efficacy of tag detec- tion during these periods was unknown prior to the commencement of the tagging experi- ment; it was feared that delayed detection of tags might result in significant losses which, if ignored, would compromise the objectives of the tagging experiment. Third, the very fact that most tagged tuna recaptured by purse seiners Hampton. Estimates of tag-reporting and tag-shedding rates for tuna in the tropical Pacific Ocean 71 would be detected during or after unloading of the catch in port offered the opportunity for tagged tuna to be planted in the catches before these detection processes began. Furthermore, the layout of purse- seine vessels and the method of onboard handling of the catch facilitated the opportunity for planting tagged tuna surreptitiously, out of sight of the vessel’s crew. Such tag-seeding operations would be more difficult on other types of vessels, e.g. pole-and-lin- ers and longliners, operating in the fishery. Field operations Selected observers on purse sein- ers were asked to plant up to five tagged tuna in the catch during a voyage. The number of tagged tuna was limited to five so as not to attract undue atten- tion during unloading; it was not unusual during the RTTP for five (and sometimes more) tagged tuna to be recovered from a single unloading. The exact tim- ing of tagging individual fish depended on the cir- cumstances encountered during a cruise, particularly the frequency of successful sets. Therefore, the pe- riod over which the five tags were seeded ranged from a few days to several weeks. Fish were tagged discretely, usually on the well deck (one level below the work deck where the fish are landed), as they passed down the chute just prior to entering the well. The tags and manner of attach- ment were identical to those used in the tagging pro- gram proper. Tag numbers, dates, species, sizes, and well numbers were recorded and the information sent to SPC at the completion of the voyage. Upon recov- ery, seeded tags were processed in the same fashion as genuine tag recoveries. Tag finders were paid the standard reward for seeded tags and were not informed that the tags were part of a seeding experiment. Estimation of return rates of seeded tags Return rates of seeded tags were calculated for the overall data set, for the three species (skipjack, yellowfin, and bigeye tuna ) and for the seven unloading loca- tions represented in the data (American Samoa, Ja- pan, Korea, Philippines, Puerto Rico, Solomon Is- lands, and Thailand). For one unloading location (American Samoa), there were sufficient returns to estimate reporting rates by time period (year). Dif- ferences in seeded tag-return rates among species, unloading locations, and time periods were assessed by using chi-square tests (Sokal and Rohlf, 1981). Return rates were estimated by assuming that the number of returns, r, in a given category was a bino- mial variate. Given the number of tags seeded, N, the estimated return rate is given by p-r/N. Under these conditions, 95% confidence limits for return rates were also obtained. Lower and upper confidence limits, pA and pB, for p were determined by solving the equations where l-2a is the confidence level (0.95 in this in- stance). Solutions for pA and pB can be easily obtained using an optimization program, such as the Microsoft Excel Solver. Estimation of overall reporting rate for the RTTP An unbiased estimate of the overall return rate of recovered tags (i.e. the total number of tags returned divided by the total number of tags recap- tured) is required for the estimation of fishing and natural mortality rates from the RTTP data. The return rates of seeded tags can be considered as sample means of the overall (population) mean re- porting rate. It transpired that seeded tag-return rates varied greatly by unloading location, requir- ing that the data be stratified by unloading location in the estimation procedure. The parametric boot- strap (or Monte Carlo) approach was used to obtain approximate 95% confidence intervals for the over- all reporting rate and its components (with the per- centile method), taking account of the different prob- ability distributions of reporting rate by unloading location. One thousand simulations (or bootstrap replicates) were run. In each, the weighted average reporting rate across locations is given by where R is the number of tags returned from loca- tion j and p'j is the bootstrap (or pseudo) reporting rate for location j. For each replicate, the p', were randomly sampled from probability distributions. For recoveries in lo- cations covered by tag-seeding experiments, beta dis- tributions B(x , y , ajt b) were used to represent the probability distributions of the true reporting rates. These continuous distributions are related to the bi- nomial distributions defined by the tag-seeding data by x =r and y^N-r + 1 (Mendenhall and Scheaffer, 197^). 'the limits of the distributions, a and b, would normally be 0 and 1, respectively. In this case, we assumed b- 1 and set the lower limit of reporting rate for location j, a-, to the local tag-return rate (i.e. num- ber of local returns divided by the number of local releases), so as to avoid the possibility of estimated recoveries out-numbering releases for any replicate. For two locations, Solomon Islands and Philippines, 72 Fishery Bulletin 95 ( I ), 1997 there were local tag releases that resulted in most of the tag returns from those locations. These local tag- return rates (0.126 and 0.223, respectively) were used as the lower bounds for the reporting rate distribu- tions for Solomon Islands and Philippines. For the other locations, it was not possible to identify sets of local releases to calculate local tag-return rates be- cause the release locations of the local returns were widely distributed throughout the tag release area, not just in the vicinity of the unloading ports. In these cases, I made the minimal assumption that the “lo- cal” releases comprised all tag releases except those returned from other locations. Thus, the shapes of the reporting-rate probability distributions are de- termined by the tag-seeding data and by this notional minimum possible return rate. Note that the means and medians of such distributions could be quite dif- ferent from the tag-seeding sample means pr In general, the differences will be greatest where is close to 0 or 1 and n is small. For returns from locations where no tag-seeding data were available or for tag returns that could not reasonably be pooled with purse-seine returns be- cause the recovery processes were different (14% of all returns), values of p' were sampled from uni- form distributions [7(0.5, 1.0). While somewhat arbi- trary, this procedure is meant only to reflect some knowledge of the minimum possible reporting rate from these locations. In fact, this assumption prob- ably understates the likelihood of tags being reported; almost all of these tags were recovered in Indonesia and in Pacific Island countries, where widespread publicity and the attractiveness of cash rewards are likely to have resulted in high reporting rates. Results Tag shedding In all, 4,541 tuna (2,557 skipjack, 1,493 yellowfin, and 491 bigeye) were double-tagged during the RTTP. Return rates of double-tagged tuna were comparable to those of single-tagged tuna. Returns from 525 double-tagged tuna were avail- able for analysis. Fitting the model specified in Equa- tion 2 to the pooled data provided estimates of p and L that, according to the model, would result in 89% (95% confidence interval of 82%-94%) of the origi- nal tags being retained after two years at large (Table 1) . Both p and L were significantly different from zero (P<0.001). Fitting the model to the three species separately, although yielding somewhat different tag-retention rates (Table 1), did not result in an overall statisti- cally significant improvement in fit (P=0.334, Table 2) . Similarly, there were differences in tag-shedding estimates for the different taggers (Table 1), but over- Table 1 Double-tagging results (m is the number of returns bearing two tags and n is the number of returns bearing one tag), tag- shedding parameter estimates, the estimated proportion of tags retained after two years at liberty (Q2yr), an Ld ) = PQtt * Pd'Ld)2 - 2 pQ(t \ pd,Ld)+ 1 (3a) and Pl2(t\p„Li) = p2Q(t\pl,Lf Pil(t\pi,Li) = 2pQ(t\pi,Li)[l-pQ(t\ pi , L )] Pioit I Pi, Li) = [1- pQ(t I Pi,Li)f, (3b) where the d and i subscripts indicate the dependent and independent hypotheses, respectively. It can be shown that substitution of the right-hand sides of Equations 3a into the log-likelihood Equa- tion 4 produces an identical result to substitution of Equation 3; the p’s cancel out and reporting rate has no influence on the estimates pd and Ld when the dependent hypothesis is true. This is therefore equivalent to using Equation 2 as the tag-shedding and reporting model, as I have done in this study. Under the independent hypothesis, substitution of the right-hand sides of Equations 3b into the log- likelihood Equation 4 does not result in a canceling out of p terms, and therefore p must be included in the tag-shedding and reporting model as shown in Equation 2a. However, p is totally confounded with 1 -p., and cannot be estimated from the double-tag- ging data. If an independent estimate of p is avail- able (for example, from a tag-seeding experiment), Equation 2a can be applied and p; estimated free of the effects of p. For most double-tagging experiments, it will not be known with any certainty whether the dependent or independent hypothesis is more appropriate. The following procedure may provide some insight in this regard: 1 Obtain tag-shedding parameter estimates pd and Ld, assuming that the dependent hypothesis is true (using Equations 2 and 3). 2 If an independent estimate of the reporting rate, p , is available, obtain tag-shedding parameter estimates p, and L, , assuming that the indepen- dent hypothesis is true (using Equations 2a and 3b). Small values of p (less than 1 - pd ) will usu- ally result in p, entering an unreasonable (nega- tive) domain. Alternatively, if p, is constrained to be nonnegative, Lt will differ from Ld and the fit to the data will degrade (i.e. >Qd). In either case, this indicates inconsistency between the re- porting rate estimate p and the independent hy- pothesis. In the present study, the estimated re- porting rate (0.586) was much smaller than 1 - pd (0.941, see Table 1). If p is applicable to the double-tagged tuna, this implies that the indepen- dent hypothesis is inappropriate for these data. In reality, it is likely that the actual situation with respect to the reporting of tag pairs will lie some- where between completely dependent and completely independent reporting. It is possible to generalize the tag-shedding model with respect to these hypoth- eses by defining a coefficient of independence, c, such that U[t)_ p( 1- p)exp(-Lt) c(l— p) + p Setting c=0 is equivalent to the dependent hypothesis, c=l is equivalent to the independent hypothesis, while 0 1.375. Classification methods for species with comparable reproductive habits and characteristics may be alike, and it is speculated that results for other billfishes would be similar to those described for swordfish. Manuscript accepted 12 August 1996. Fishery Bulletin 95:80-84(1997). Use of gonad indices to estimate the status of reproductive activity of female swordfish, Xiphias gladius : a validated classification method Michael G. Hinton Inter-American Tropical Tuna Commission 8604 La Jolla Shores Drive, La Jolla, California 92037-1508 Ronald G. Taylor Michael D. Murphy Florida Marine Research Institute, Florida Department of Environmental Protection 100 Eighth Avenue SE, St. Petersburg, Florida 33701-5095 We describe a validated classifica- tion method that uses gonad indi- ces (GI’s) to determine accurately the reproductive condition of female swordfish, Xiphias gladius. This study uses previously unpublished data as well as histological analy- ses detailed in Taylor and Murphy’s (1992) study of the reproductive bi- ology of swordfish captured in the Straits of Florida. It is standard practice to use GI’s to identify re- gions and times of active spawning in studies of the distribution and structure of stocks of many species of fish, including swordfish (e.g. Kume and Joseph, 1969; Shingu et al., 1974; Miyabe and Bayliff, 1987; Sosa-Nishizaki, 1990; Nakano and Bayliff, 1992; Arocha and Lee, 1993; Arocha et ah, 1994; Gouveia and Mejuto, 1994; Arocha and Lee, 1995; Hinton and Deriso, in press). Data on the reproductive activity of swordfish are costly and difficult to obtain but essential to studies such as those noted; full use should be made of all available information. Our classification method over- comes problems of published meth- ods (e.g. Miyabe and Bayliff, 1987), which have the potential to reduce the information database of the re- searcher by over 50%. To our knowl- edge, it is the first method appli- cable to female swordfish to have been validated with data obtained from histological analyses, which provide a verifiable measure of the reproductive status of individual swordfish. The standard practice (Gouveia and Mejuto, 1994) in studies using values of GI for female swordfish has been to estimate GI = 104 x GW/ EFL3, where GW = gonad weight in grams, and EFL = length from the posterior edge of the orbit to the fork of the tail in centimeters (we note that without loss of generality, other length measurements, such as lower-jaw fork length (LJFL), have been used) (Kume and Joseph, 1969). The latter assumed that “[fe- males] with gonad indices equal to or greater than 3 are about to spawn.” Miyabe and Bayliff (1987) modified their method by assuming that “only females with gonad indi- ces of 7.0 or greater were [about to spawn].” Arocha and Lee (1995) modified the method of Kume and Joseph ( 1969) when they noted that females with GI’s greater than 4.0 were in prespawning condition. In certain applications of these meth- ods, e.g. comparison of average GI’s for different regions or time periods, Hinton et at: Use of gonad indices to estimate the reproductive status of Xiphias gladius it is necessary to ensure that the averages being com- pared are for individuals with comparable reproduc- tive potential or maturity. Thus, it is standard prac- tice to use a minimum length (e.g. Miyabe and Bayliff, 1987; Sosa-Nishizaki, 1990; Nakano and Bayliff, 1992, Arocha et al., 1994; Arocha and Lee, 1995) to decide which data to include in estimates of average values of GI, making it important to document that minimum-length criteria have no impact on meth- ods used to estimate reproductive status (Cayre and Laloe, 1986). Data and methods Details on data collection and histological analyses, other than estimation of the values of individual go- nad indices, may be found in Taylor and Murphy (1992). Female swordfish were assigned to eight de- velopmental classes (Murphy and Taylor, 1990) based on the appearance of histological features (Wallace and Selman, 1981). These classes and mean observed oocyte diameters were 1) immature, < 20 pmm; 2) developing, 71 pmm; 3) maturing, 160 pmm; 4) ma- ture, 434 pmm, 5) gravid, 723 pmm; 6) spawning or partially spent, 823 pmm; and 7) spent, 181 pmm. Individuals in class 8 (recovering) were observed but not described in Taylor and Murphy (1992). Gonads of swordfish in class 8 exhibited signs of having spawned in the previous season and were undergo- ing maturational, prespawning development for sub- sequent reproductive efforts. The preferred formulation for GI may be deter- mined by examining the relation of gonad weight to measures of body size (de Vlaming et al., 1982). The formulation chosen should meet the underlying as- sumptions (de Vlaming et al., 1982) for use of GI as an index of reproductive status. In addition to exam- ining the previously described “standard” expression of GI (hereafter referred to as GI(1)), we examined GI - \n(GW)I\n(EFL), hereafter referred to as GI(2), and GI = GW/EFL. Stepwise analysis of covariance (ANCOVA) was used to examine how well these for- mulations for GI met the underlying assumptions (de Vlaming et al., 1982) for use of GI as an index of the reproductive status of female swordfish. Values of GI were determined for the fish for which histo- logical data had been obtained. For those individu- als for which measurements of EFL were not ob- tained, measurements of LJFL were used to estimate EFL as follows: EFL = -8.259 + 0.930 x LJFL [n= 316, r2=0.996,P<0.001] (Taylor and Murphy, 1992). Of the over 400 fish examined by Taylor and Murphy ( 1992), there were 85 individuals (Table 1) with measure- ments (40) or estimates (45) of EFL ranging from 73 to 253 cm, for which there were GW’s and data from Table 1 The status of reproductive activity (R) of swordfish determined by histological analyses [Taylor and Murphy, 1992]), EFL = eye fork length (cm), and GW = gonad weight (gm). R GW EFL R GW EFL R GW EFL R GW EFL 2 3 77 2 100 113 3 752 169 6 8,740 221 2 13 86 2 110 147 4 530 161 6 8,840 208 2 14 88 2 135 115 4 780 182 6 9,920 206 2 15 94 2 140 118 4 800 187 6 10,180 188 2 30 96 2 140 128 4 1,320 186 6 10,430 227 2 30 95 2 150 121 4 2,140 219 6 11,340 223 2 30 92 2 220 93 4 2,888 201 6 15,140 253 2 35 97 2 225 73 5 2,690 169 8 446 167 2 35 99 2 255 148 5 3,950 184 8 540 172 2 35 106 3 100 121 6 1,240 181 8 600 166 2 35 106 3 105 130 6 1,540 202 8 640 164 2 37 101 3 no 123 6 1,760 174 8 730 183 2 45 114 3 200 137 6 2,270 182 8 800 190 2 50 99 3 240 139 6 3,650 181 8 980 179 2 50 96 3 300 168 6 3,780 171 8 995 197 2 55 104 3 353 166 6 3,850 208 8 1,325 200 2 70 103 3 437 120 6 3,900 155 8 1,340 222 2 70 105 3 470 184 6 4,000 180 8 1,360 249 2 80 104 3 500 181 6 4,700 191 8 1,500 211 2 80 112 3 620 185 6 4,720 197 8 1,790 191 2 90 113 3 680 186 6 6,034 154 2 100 128 3 680 174 82 Fishery Bulletin 95(1 ), 1997 histological analyses. According to the results of the histological analyses, swordfish were considered to be in spawning condition, i.e. spawning was in progress or imminent in the region in which the fish was captured, if they were either “gravid” or “spawn- ing or partially spent” (classes 5 and 6 of Taylor and Murphy [1992] ). Individuals in these conditions were classified as in an active (A) reproductive status. All others were classified as quiescent (Q). In our study the results of histological analyses represent the known reproductive status of indi- vidual swordfish, and the H(i) are various hypoth- esized classification methods (Table 2) for placing swordfish into categories A or Q. In some cases, these H(i) indicate minimum-length criteria used to deter- mine which data from individual swordfish should be included in estimates of average values of GI. To facilitate the examination of impacts of minimum- length criteria on classification methods, these cri- teria were treated as a component of the H(i) in our analyses. We do not attempt to define minimum- length criteria for size at maturity in this study (cf. Taylor and Murphy [1992]); our concern was to de- termine if classification methods based on GI were independent of such criteria. The hypotheses identi- fied in Table 2 as “Present study” are representative of a multitude of hypotheses we examined. The optimum value (OV) and confidence intervals for the value of GI to be used as criteria, GI*, to esti- mate the reproductive condition of individual female swordfish were determined by using maximum-like- lihood estimation procedures and the following model: erwise R = 0. Then for individual fish, i, selected at random, P (a swordfish is reproductively given its gonad in- dex) = KiGlY* x (1 - n(GI))(1~R\ Maximum-likelihood estimates of niGI ) are given by the number of successes in the series of trials deter- mined by the value of GI* in the data, divided by the number of trials. Thus, for our data set: [Glv ..., GIk*, GIn ] and [i?p ..., Rk, I?;j] = [the observed values of GI] and [the respective measure of reproductive status determined by histological analyses] for indi- vidual fish, these estimates are given by 7T{GI) = (number of individuals with R = 1 and GI GI^) n-k + 1 for individuals with GI < GI*k for all others It follows that the optimum value, GI* , is that which maximizes the following log-likelihood function ( LKLHD ): LKLHD = II* x ln(7r( GI)) + ( 1 - R) x ln( 1 - /r(G/))] Results and discussion Let R - 1 if a swordfish is reproductively active (classes 5 and 6 of Taylor and Murphy [1992]), oth- Table 2 Levels of gonad indices (GI) used to classify the reproduc- tive activity of female swordfish and minimum eye fork length (EFL) criteria used to standardize statistics for com- parison among areas and times, as employed by various researchers. Formulations for GI(1) and GI(2) are given in the text. H(i) Classification method Author 1 GI(1) >3.0 Kume and Joseph 2 GI( 1) > 7.0 and EFL > 150 cm Miyabe and Bayliff 3 GIG) > 7.0 and EFL > 160 cm Sosa-Nishizaki 4 GIG) > 4.0 and EFL >131 cm' Arocha and Lee 5 GIG) >6.0 Present study 6 GI(2)> 1.37 Present study 1 Authors used lower-jaw fork length >150 cm. Results of classifying individuals as either A or Q based on the results of the histological analyses and from application of the various H(i) (treated in each test as the null hypothesis) are given in Table 3. It is clear that H(2) and H(3) fail to classify individuals correctly according to reproductive status as deter- mined by histological analyses. Under H(2) and H(3), individuals below the minimum-length criteria are not classified. About 75% of the individuals whose lengths were above the size restrictions stated in these hypotheses were classified correctly, but only 48% of the individuals in this group that were repro- ductively active were correctly classified which rep- resents a significant type-1 error that may be ex- tremely costly in terms of loss of information on the distributions of reproductively active swordfish. How- ever, this is a result of the value of GI included in the hypotheses and not a result of restrictions placed on lengths of individuals included in the analyses, as is evidenced by the results for the other H(i). We also note that length was not found to be a signifl- Hinton et al. : Use of gonad indices to estimate the reproductive status of Xiphias gladius 83 Table 3 Comparison between the correct (from histological analy- ses [HA] of the ovaries) and estimated (from GI’s) classifi- cation of reproductive status of female swordfish. Individu- als were classified as reproductively active (A) or quies- cent (Q). Asterisks designate incorrect classifications based on GI’s, IC is the percentage of all individuals [ n deter- mined by H(i)] classified correctly, and AC is the percent- age of reproductively active individuals, within the n indi- viduals, that were classified correctly. H(i) n HA GI A GI Q IC AC 1 85 A 19 2* 95.3 90.5 Q 2* 62 2 48 A 10 11* 77.1 47.6 Q 0* 27 3 46 A 10 11* 76.1 47.6 Q 0* 25 4 52 A 17 4* 92.3 81.0 Q 0* 31 5 85 A 15 6* 92.9 71.4 Q 0* 64 6 85 A 21 0* 95.3 100.0 Q 4* 60 cant term in logistic regressions that included EFL as a classification variable. Under H(l) and H(6), 95% of the 85 individuals were classified correctly (Table 3); further, under H(6) all individuals that were reproductively active were correctly classified, which was significantly (see fol- lowing discussion) more than the 91% of the repro- ductively-active individuals correctly classified un- der H( 1) and the 71% to 81% correctly classified un- der H(5) and H(4), respectively. Hypothesis H(6) placed about 4.7% of the individuals that were qui- escent in the active category, and H(l), about 2.5%, both of which represent relatively low rates of type- 2 error. Although length-cubed is often the choice to stan- dardize GW, as in the “standard” expression for GI, length is also frequently used. Further, GW may be exponentially related to body size (de Vlaming et al., 1982), in which case log transformation as in GI(2) is indicated. We examined these hypotheses in for- mulations of GI for female swordfish. The results of ANCOVA revealed significant (P<0.01) heterogene- ity among slopes and intercepts of the regressions of GW on EFL and on EFL3 for reproductive classes (2, 3, 4, [5, 6] and 8) of Taylor and Murphy (1992). At the same time, ANCOVA on the log-transformed data yielded only one significant (P<0.01) coefficient, that for the intercept of class (5, 6); however this coeffi- cient was only about 28% of the estimated intercept of the overall regression. Thus, the formulation of the gonad index that best conformed to the underly- ing assumptions (de Vlaming et al., 1982) was GK2). In addition, the maximum-likelihood test based on the values of LKLHD for the difference between methods showed that model GI(2) provided a signifi- cant ( j2) 1), P=0.033) improvement over model GI(l). The estimate of OV obtained from maximum-likeli- hood analyses for GI( 2) was (1.366 < OV < 1.375). Note that although GK2) has a continuous distribu- tion, the interval estimate of OV is a function of the distribution of values of GI(2) in the sample data, and thus any hypothesized value in this range would yield LKLHD and tabled results identical to those shown for H(6). Further, because the solution for the function LKLHD is so knife-edged, the estimate of OV includes the 90% confidence interval, and the 95% confidence interval for the estimate of OV, (1.357 < OV < 1.375), differs only in the lower bound. Two points should now be clear. First, the classifi- cation methods that are based on GI(1) that have been used and published in studies requiring esti- mates of the reproductive status of female swordfish do not meet the underlying assumptions (de Vlaming et al., 1982) for use as an indicator of reproductive status, and they may also be viewed in some in- stances as overly restrictive, in that they may have excluded significant amounts of usable data from analyses that were already hampered by limited in- formation. This resulted, at least in part, from using values of GI that corresponded to a fully ripe and running condition of the gonad. Second, the classifi- cation methods, both previously published and de- scribed herein, were not impacted by minimum- length criteria which may be required to standard- ize comparisons of statistics that are based on gonad indices. Our results are conservative in the following re- spect. We have no knowledge of the frequency of spawning of female swordfish. Thus, because hydra- tion of eggs may occur over a very short period of time, by not including individuals in class 4 (“ma- ture ovaries” of Taylor and Murphy [1992]), some individuals that might be expected to spawn within a short period of time, and thus presumably within the general area of capture, may be excluded from consideration. The question of whether to include these individuals as reproductively active could be addressed by conducting a study of spawning fre- quency of female swordfish based on the condition of yolk development in the eggs, as has been done for yellowfin tuna, Thunnus albacares (Schaefer, 1996). Alternatively, it may be possible to determine whether or not class-4 individuals should be included 84 Fishery Bulletin 95 ( 1 ), 1997 by estimating the distribution of spawning using both a classification scheme that considers these individu- als as reproductively active, and the classification developed herein, with subsequent testing by com- parison of these distributions to other measures of spawning activity, such as distributions of larval fish or of male:female ratios. Given the interval estimate for OV, and taking a conservative approach with respect to including in- dividuals that are not reproductively active in esti- mates of the spatial and temporal distributions of spawning, we recommend that researchers requir- ing an estimate of the reproductive status of female swordfish adopt a method that classifies reproduc- tively active female swordfish as those for which GI = ln(GW)/ln(EFL) > 1.375, the upper limit of the in- terval. When additional information becomes avail- able, further analyses should be undertaken. The need to develop species-specific classification methodologies has been clearly documented (de Vlaming et al., 1982). However, methods for species with similar characteristics and reproductive habits may be similar (Cayre and Laloe, 1986). Merrett (1970) found that for sailfish ( Istiophorus platy- pterus)', striped ( Tetrapturus audax), blue (T. nigricans), and black ( T. indica) marlin; and spearfish (T. angustirostris), changes in ovaries through oogenic cycles were similar in all species, as was the shape of the gonads (with the exception of the shape of the spearfish gonad, which was Y-shaped rather than bilaterally symmetrical). These changes are similar to those observed in swordfish (cf. Taylor and Murphy, 1992). Thus, while we concur with de Vlaming et al. (1982) and strongly recommend that classifica- tion methods be developed and validated for each spe- cies of billfish, we would speculate that results for these species would be comparable to those shown herein. Acknowledgments We thank Kurt M. Schaefer and George M. Watters for their helpful discussions of various aspects of this manuscript during its development. We also thank William H. Bayliff and Richard B. Deriso for review- ing this manuscript and for their useful suggestions for its improvement. Helpful remarks were also re- ceived from three anonymous reviewers. Literature cited Arocha, F., and D. W. Lee. 1993. Preliminary observations on sex ratio and maturity stages of the swordfish, Xiphias gladius, in the northwest Atlantic. Int. Comm. Conserv. Atl. Tunas, Coll. Vol. Sci. Pap. 40( l):425-432. 1995. The spawning of swordfish from the northwest Atlantic. Int. Comm. Conserv. Atl. Tunas, Coll. Vol. Sci. Pap. 44( 3 ): 179—186. Arocha, F., D. W. Lee and J. R. Grubich. 1994. Observations on sex ratio, maturity stages, and fe- cundity estimates of the swordfish, Xiphias gladius, in the northwest Atlantic Ocean. Int. Comm. Conserv. Atl. Tu- nas, Coll. Vol. Sci. Pap. 42( 1 ):309— 3 18. Cayre, P., and F. Laloe. 1986. Review of the gonad index (GI) and an introduction to the concept of its “critical value”: application to the skip- jack tuna Katsuwonus pelamis in the Atlantic Ocean. Mar. Biol. 90:345-351. de Vlaming, V., G. Grossman, and F. Chapman. 1982. On the use of the gonosomatic index. Comp. Biochem. Physiol. 73A(l):31-39. Gouveia, L., and J. Mejuto. 1994. Notes on biological and biometric data of the sword- fish (Xiphias gladius L.) in areas off Maderia. Int. Comm. Conserv. Atl. Tunas, Coll. Vol. Sci. Pap. 42(l):274-280. Hinton, M. G., and R. B. Deriso. In press. Distribution and stock assessment of swordfish in the eastern Pacific Ocean from catch and effort data standardized using biological and environmental param- eters. In I. Barrett (ed ), Proceedings of the international symposium on Pacific swordfish; Ensenada, B.C., Mexico, 1994. U.S. Dep. Commer., NOAATech. Rep. NMFS. Kume, S., and J. Joseph. 1969. Size composition and sexual maturity of billfish caught by the Japanese longline fishery in the Pacific Ocean east of 130° W. Bull. Jpn. Far Seas Fish. Res. Lab. 2:115-162. Merrett, N. R. 1970. Gonad development in billfish (Istiophoridae) from the Indian Ocean. J. Zool., Lond. 160:355-370. Miyabe, N., and W. H. Bayliff. 1987. A review of the Japanese longline fishery for tunas and billfishes in the eastern Pacific Ocean, 1971-1980. In- ter-Am. Trop. Tuna Comm. Bull. 19(1): 1-163. Murphy, M. D., and R. G. Taylor. 1990. Reproduction, growth, and mortality of red drum Sciae- nops ocellatus in Florida waters. Fish. Bull. 88:531-542. Nakano, H., and W. H. Bayliff. 1992. A review of the Japanese longline fishery for tunas and billfishes in the eastern Pacific Ocean, 1981-1987. In- ter-Am. Trop. Tuna Comm. Bull. 20(5):184-355. Schaefer, K. M. 1996. Spawning time, frequency, and batch fecundity of yellowfin tuna, Thunnus albacares , near Clipperton Atoll in the eastern Pacific Ocean. Fish. Bull. 94:98-112. Shingu, C., P. K. Tomlinson, and C. L. Peterson. 1974. A review of the Japanese longline fishery for tunas and billfishes in the eastern Pacific Ocean, 1967-1970. In- ter-Am. Trop. Tuna Comm. Bull. 16(2):65-230. Sosa-Nishizaki, O. 1990. A Study on the Swordfish Xiphias gladius Stocks in the Pacific Ocean. Ph.D. diss., Department of Fisheries, Faculty of Agriculture, Univ. Tokyo, Tokyo, 246 p. Taylor, R. G., and M. D. Murphy. 1992. Reproductive biology ofthe swordfish Xiphias gladius in the Straits of Florida and adjacent waters. Fish. Bull. 90:809-816. Wallace, R. A., and K. Selman. 1981. Cellular and dynamic aspects of oocyte growth in teleosts. Am. Zool. 21:325-343. 85 Abstract .—The annual cycle of abundance and the monthly distribu- tions of the copepod Centropages hamatus are described for U.S. north- east continental shelf waters from plankton samples collected approxi- mately bimonthly from 1977 to 1987. The copepod was found distributed throughout the study area with a strong onshore-offshore abundance gradient. After its annual low, C. hamatus was found to increase in abun- dance slowly along the coast and to ex- pand offshore following the northward progression of spring conditions. The highest monthly mean abundance es- timates of C. hamatus were found on Georges Bank during the month of July. Distribution begins to constrict inshore following peak abundance periods. Examination of environmental vari- ables revealed that in general Centro- pages hamatus was prevalent when surface temperatures ranged from 12 to 17°C, when water-column chloro- phyll levels were high, and where sa- linity was low on the shelf. The popu- lation in the Middle Atlantic Bight sub- area declines sharply as water tem- peratures rise in summer and does not begin to recover until temperatures decline in the fall. In contrast, popula- tions in the more northern regions de- crease slowly from peak abundance and do not increase from their annual low until water temperatures rise in early spring. The pelagic population that sur- vives through low abundance periods is concentrated in shoal or inshore (or both) waters where temperature is low and phytoplankton biomass high. There was no evidence from survey data that predation by ctenophores, chaetognaths, or the copepod Centropages typicus has a major effect on C. hamatus abundance. Manuscript accepted 10 July 1996. Fishery Bulletin 95:85-98 ( 1997). Persistent spatial and temporal abundance patterns for late-stage copepodites of Centropages hamatus (Copepoda: Calanoida) in the U.S. northeast continental shelf ecosystem Joseph Kane National Marine Fisheries Service, NOAA 28 Tarzwell Drive, Narragansett, Rhode Island 02882-1 199 E-mail address: jkane@whsun 1 .wh.whoi.edu The calanoid copepod Centropages hamatus (Lilljeborg, 1853) is one of the dominant members of the zoop- lankton assemblage found within North Atlantic shelf waters (Davis, 1987; Sherman et al., 1987 ). The species has a wide latitudinal range that is reported to be as far north as Labrador (Pinhey, 1926) and southward to coastal waters off Florida in the Gulf of Mexico (Marcus, 1989). It occurs primarily in shel- tered, coastal, and shoal regions of the continental shelf. This omni- vorous copepod produces subitan- eous eggs during the breeding sea- son and also can produce diapausal ones in response to an environmen- tal trigger (Pertzova, 1974; Marcus, 1989). McLaren (1978) estimated that generation period is compara- tively short, 21-25 days at 12-13°C, and describes C. hamatus as a highly productive and ecologically efficient component of the zooplank- ton community. Sherman et al. (1987) reported that it is a major prey item of larval, juvenile, and adult fish stocks within continental shelf waters. The National Marine Fisheries Service has monitored the zooplank- ton populations of the U.S. north- east shelf ecosystem with broad- scale surveys since 1977 as part of the MARMAP (Marine Resources Monitoring, Assessment, and Pre- diction) program (Sherman, 1980). The resulting historical data set provides the information needed to form a baseline for detection of fu- ture changes to the ecosystem. Pre- vious reports on the annual abun- dance cycle of Centropages hamatus within the ecosystem have been lim- ited to specific areas or to compara- tively short periods (or both) (Bige- low, 1926; Deevey, 1956, 1960; Judkins et al., 1980; Davis, 1987; Sherman et al., 1987; Grant, 1988; Kane, 1993). No description of the monthly distribution of the copepod in this region has been published from collected data. This report uses information collected during MARMAP surveys from 1977 to 1987 to describe the persistent dis- tribution and abundance patterns of C. hamatus throughout the eco- system. Measurements of salinity, temperature, bottom depth, chloro- phyll, and potential predator abun- dance were considered to gain in- sight into factors affecting the dis- tribution and annual abundance cycle of C. hamatus. Methods Sample collection and analysis The U.S. northeast shelf ecosystem extends from the Gulf of Maine to 86 Fishery Bulletin 95(1 ), 1997 Locations of standard MARMAP stations (•) in the U.S. northeast shelf ecosystem and subarea boundaries (MAB=Middle Atlantic Bight; SNE=Southern New England; GBK=Georges Bank; and GOM=Gulf of Maine). Cape Hatteras (Sherman, 1994). Plankton samples were collected within the ecosystem at monthly or bimonthly inter- vals from 1977 to 1987. Plank- ton surveys occupied approxi- mately 184 standard station lo- cations that were relatively un- changed during the 11-yr period (Fig. 1). Samples were also col- lected on trawl and dredge cruises at randomly selected lo- cations that varied yearly. Ar- eal coverage and station spac- ing on these surveys were simi- lar to broadscale plankton cruises. Zooplankton were collected at each station from one side of a 61 -cm bongo frame fitted with a 0.333-mm mesh net. The gear was lowered at 50 m/min to within 5 m of the bottom, or to a depth of 200 m maximum, and retrieved at 20 m/min. Ship speed was adjusted to maintain a 45° angle to the towing wire. A digital flowmeter was posi- tioned in the center of the bongo frame to measure the volume of water filtered. All collections were preserved in 5% formalin. Samples were reduced to ap- proximately 500 organisms in the laboratory by subsampling with a modified box splitter. Zooplankton were sorted, identified, and counted at the Plankton Sorting Cen- ter, Szczecin, Poland. The total number of samples analyzed for this report was 10,715. The abundance of Centropages hamatus is expressed here as num- bers/100 m3 of water filtered and includes only ad- vanced copepodite stages CV and CVI. Earlier copepodite stages were excluded because other cope- pods of similar size are undersampled by 0.333-mm mesh nets (Anderson and Warren, 1991). The seasonal abundance cycles of known preda- tors of copepods captured with the nets used during the surveys were examined to determine which might affect Centropages hamatus population levels. The three copepod predators examined in this study are: 1) ctenophores, 2) the copepod Centropages typicus, and 3) chaetognaths. Sea-surface temperature was measured at each station to the nearest 0.1°C with a stem thermom- eter. During plankton surveys from 1977 to 1986, water bottles with reversing thermometers were used to collect water samples at standard depths in order to measure salinity and temperature. Measurements of bottom temperature were determined by means of the deepest bottle or by means of a special bottom- tripped water-bottle sampler in water less than 75 m. Temperature and salinity data in 1987 were col- lected with a CTD (conductivity-temperature-depth) probe. Phytoplankton biomass was determined by measuring the concentration of chlorophyll a in the netplankton (>20 pmm) and the nanoplankton (<20 pmm) size fractions from water samples down to 100 m on plankton surveys from 1977 to 1984. These size fractions were summed to generate an estimate of total chlorophyll. The average water-column value of a variable for each station was calculated by ar- ithmetically integrating measurements over depth. More detailed accounts of sampling procedures and individual cruise tracks are given by Sibunka and Silverman (1984, 1989). Kane: Spatial and temporal abundance patterns of Centropages hamatus 87 Statistical analysis Estimates of Centropages hamatus and predator abundance were log transformed [log10(no./10Gm3 + 1)] prior to contouring and data analysis. Contoured C. hamatus distribution maps were made by using Surface III software (Sampson, 1988) on station abundance data from the 11-yr data set grouped by monthly intervals. Evaluation of species interannual abundance vari- ability was facilitated by subdividing the ecosystem into four subareas: Middle Atlantic Bight (MAB), Southern New England (SNE), Georges Bank (GBK), and Gulf of Maine (GOM) (Fig. 1). Each subarea is characterized by distinct patterns of circulation and bathymetry (Sherman et a!., 1983). The average an- nual cycle of abundance and its variation was por- trayed for each subarea by plotting the monthly mean abundance of all samples with its 95% confidence interval bar. Individual survey mean abundance and its 95% confidence interval bar were then superim- posed on the latter plot. Surveys where the error bar did not overlap the one from the average cycle were judged to be situations where abundance departed substantially from the average cycle. Only surveys, except the one noted below, that covered 75% or more of a subarea were included in the analysis of interannual variability. Statistical analyses, compar- ing individual survey means with the time series monthly mean were not undertaken because they re- quire the assumption of independence. Several surveys (see Table 1) prior to 1981 were conducted by foreign vessels that did not have per- mission to sample east of the U.S. -Canada maritime boundary line in the GBK and GOM subareas. Al- though areal coverage in these surveys was reduced approximately 40% in relation to complete surveys, I included them in the analysis of this study because the area undersampled was consistent and our sur- veys still provided adequate coverage of the depth strata found within the two subareas. Spearman’s rank correlation coefficients were cal- culated for monthly subsets of station data to mea- sure the strength of the relationship within indi- vidual months between Centropages hamatus abun- dance and the following variables: surface tempera- ture, bottom depth, and the average water-column values of temperature, salinity, and total chlorophyll. Initial distribution plots of C. hamatus revealed that species abundance has a strong onshore-offshore gradient. Thus, to control the effect of depth on the calculation, Spearman’s partial correlation coeffi- cients were calculated for monthly subsets where both abundance and the other variable were signifi- cantly (,P<0.05) correlated to depth. Results Distribution and abundance The time-series mean distribution charts by month for Centropages hamatus are presented in Figure 2, A and B. Immediately apparent is the persistent on- shore-offshore abundance gradient throughout the study area. There are high concentrations of the cope- pod inshore and within the shoal waters of GBK. Abundance in offshore waters is always much lower. Centropages hamatus is found throughout most of the ecosystem at some time during the year, the only exception being certain areas of the eastern offshore waters of the GOM where it is absent year round. The timing of the annual abundance cycle of Centropages hamatus was not consistent through- out the ecosystem. The population in southern reaches of the study area declines through the sum- mer, nearly disappearing from the water column during early autumn (Fig. 3). In December dense concentrations of C. hamatus begin to appear close to shore in the MAB subarea. These inshore centers of abundance slowly enlarge and expand along the coast and over the central shoals of GBK with the northward progression of spring (Fig. 2, A and B). Thus, peak times of abundance in the designated subareas vary with latitude (Fig. 3): May in the MAB, June in SNE, July on GBK, and September in the GOM. The population becomes distributed over nearly the entire shelf of each subarea during the annual peak period of abundance. The distribution and monthly abundance figures clearly show that GBK is the area of highest abundance for C. hamatus within the northeast shelf ecosystem. Distribution begins to constrict towards the shore in each subarea during the months approaching the annual period of low abundance (Fig. 2). Abundance estimates in the SNE, GBK, and GOM subareas de- cline slowly through the autumn and, unlike the MAB region, do not reach the annual low until win- ter (Fig. 3). Interannual variation in abundance of Centropages hamatus is shown in Figure 4 and individual survey statistics are given in Table 1. Although no long-term temporal trends in abundance were evident within any of the subareas, population estimates in certain years were exceptional. For example, the copepod’s abundance in both the MAB and SNE subareas was high for an extended period in 1984 (Fig. 4). Both of these areas also had high abundance during the spring of 1987 and low population estimates in 1982. Departures from the average annual cycle of abun- dance were not always continuous across these sub- areas; C. hamatus density was low during early 88 Fishery Bulletin 95( I ), 1997 Table 1 Centropages hamatus abundance data for each subarea by survey. The asterisk indicates where survey operations were not completed past the US-Canadian maritime boundary. Abbreviation Key: MAB = Middle Atlantic Bight; GBK = Georges Bank; SNE = Southern New England; GOM = Gulf of Maine; Yr = year; no. = number of samples, Mid-day = survey midpoint (jday ), Log mean =log ( 10) mean abundance, SE = standard error of the mean. MAB SNE GBK GOM Mid- Log Mid- Log Mid- Log Mid- Log Yr no. day mean SE Yr no. day mean SE Yr no. day mean SE Yr no. day mean SE 77 30 86 2.59 0.33 77 29 74 1.95 0.29 77 19 49 0.16 0.11 77 30 123 0.00 0.00 77 30 140 2.90 0.32 77 46 133 1.89 0.24 77 32 80 0.46 0.15 77 25 308 0.87 0.24 77 30 238 0.16 0.11 77 36 242 1.29 0.27 77 23 114 1.71 0.30 77 27 315 0.18 0.12 77 30 293 0.09 0.09 77 30 300 1.34 0.25 77 31 147 1.18 0.28 78* 25 139 0.09 0.09 78 29 49 1.16 0.27 78 31 60 1.07 0.20 77* 24 219 3.38 0.25 78* 31 193 0.44 0.17 78 28 112 2.00 0.29 78 30 131 1.38 0.25 77 19 307 2.58 0.45 78 29 240 0.46 0.18 78 29 177 2.43 0.30 78 34 188 2.12 0.27 77 22 333 2.47 0.32 78* 31 286 1.49 0.23 78 31 227 0.72 0.25 78 31 233 1.25 0.25 78 28 49 1.33 0.20 78* 31 322 0.44 0.17 79 46 59 1.74 0.24 78 31 294 1.62 0.28 78 29 137 0.46 0.19 79 40 114 0.06 0.06 79 30 129 3.06 0.29 79 40 64 1.64 0.20 78 19 241 3.04 0.42 79 32 147 0.19 0.11 79 49 172 2.74 0.22 79 27 107 1.12 0.29 78 32 287 2.84 0.25 79* 37 240 1.13 0.22 79 46 226 0.59 0.18 79 27 134 2.30 0.29 79 30 94 0.80 0.23 79 32 297 0.51 0.20 79 31 280 0.17 0.12 79 44 188 2.13 0.23 79 20 143 0.80 0.29 79 47 331 0.17 0.10 80 49 64 1.35 0.23 79 37 232 0.59 0.21 79* 18 192 2.38 0.50 80* 34 54 0.29 0.14 80 47 111 2.66 0.23 79 27 290 1.41 0.28 79* 17 238 3.15 0.43 80* 33 178 0.38 0.17 80 48 147 2.52 0.23 80 43 70 1.98 0.18 79 29 296 2.66 0.31 80* 37 217 1.06 0.23 80 45 201 0.97 0.23 80 41 117 2.21 0.21 79 33 349 2.19 0.32 80 51 296 0.34 0.12 80 47 273 0.11 0.08 80 43 157 2.28 0.26 80* 20 62 1.49 0.35 81 53 52 0.13 0.07 80 40 327 0.66 0.22 80 40 207 2.56 0.22 80 29 88 1.32 0.30 81 46 146 0.44 0.14 81 48 82 1.82 0.24 80 43 282 0.38 0.15 80 28 123 1.46 0.34 81 40 339 0.19 0.11 81 43 90 2.08 0.30 80 44 341 1.08 0.23 80* 21 163 3.05 0.40 82 35 49 0.04 0.04 81 42 222 0.99 0.25 81 43 77 1.25 0.18 80* 20 215 3.13 0.36 82 48 124 0.13 0.08 81 43 271 0.00 0.00 81 44 103 1.39 0.23 80 30 293 1.68 0.33 82 37 156 0 0 82 35 80 1.75 0.27 81 35 162 2.07 0.25 80 30 353 1.49 0.29 82 49 302 0.50 0.17 82 44 81 2.60 0.26 81 33 191 2.75 0.29 81 26 66 0.29 0.14 82 52 334 0.44 0.15 82 29 157 1.62 0.31 81 30 228 1.80 0.35 81 20 96 0.92 0.30 83 53 26 0.25 0.09 82 34 214 1.29 0.28 81 38 284 1.43 0.26 81 24 115 1.63 0.32 83 38 116 0.46 0.13 82 38 268 0.55 0.19 82 40 75 1.62 0.19 81 24 157 1.57 0.40 83 55 167 0.68 0.15 83 36 53 1.33 0.28 82 34 100 1.25 0.25 81 31 196 3.34 0.32 83 46 306 0.22 0.11 83 39 78 2.05 0.27 82 44 146 1.44 0.21 81 52 296 2.36 0.25 83 31 349 0 0 83 46 149 2.60 0.18 82 39 198 2.58 0.20 81 32 335 1.71 0.32 84 47 14 0.20 0.10 83 33 212 0.31 0.13 82 24 285 1.86 0.38 82 29 64 0.47 0.18 84 40 112 0.20 0.12 83 43 268 0.07 0.07 82 43 348 1.20 0.21 82 36 109 1.22 0.22 84 54 151 0.36 0.11 continued on next page spring 1979 in SNE (Fig. 4) and above average in the MAB (Table 1). There were no substantial upward abundance departures recorded on surveys of GBK and only one in the GOM (1978). This is probably due to the limited coverage the areas received during the peak periods of abundance (Fig. 4). There were several years in three of the subareas where survey mean abundance had substantial downward departures from the aver- age annual cycle when C. hamatus was at or near its annual low (Fig. 4). These anomalies are probably not significant because log transformation increases the amplitude of low values. Plots of untransformed data show little interannual variation between survey means during low periods of abundance. Correlation of abundance with other variables Bottom depth Centropages hamatus abundance is negatively correlated to depth in all the subareas for most or all of the entire year (Table 2). Exceptions occur and correlations weaken during low periods of abundance in the MAB and GOM subareas when the copepod is present only at a few inshore locations. Kane: Spatial and temporal abundance patterns of Centropages hamatus 89 Table 1 (continued) MAB SNE GBK GOM Mid- Log Mid- Log Mid- Log Mid- Log Yr no. day mean SE Yr no. day mean SE Yr no. day mean SE Yr no. day mean SE 83 48 323 0.75 0.19 83 29 41 1.44 0.27 82 29 140 0.66 0.23 84 50 298 0.59 0.15 84 40 37 1.31 0.26 83 29 91 2.14 0.24 82 34 208 2.82 0.30 85 29 99 0.21 0.13 84 41 71 1.96 0.30 83 41 158 2.84 0.26 82 31 295 2.57 0.29 85 44 260 0.97 0.20 84 48 133 3.32 0.16 83 38 222 2.74 0.28 82 29 323 2.52 0.30 85 37 306 0.29 0.14 84 38 193 2.12 0.31 83 38 278 0.96 0.27 83 28 21 1.55 0.32 85 56 340 0.21 0.09 84 51 198 2.39 0.28 83 42 334 1.28 0.20 83 32 104 1.70 0.33 86 50 39 0.18 0.08 84 31 211 1.24 0.29 84 43 26 1.07 0.20 83 30 163 2.37 0.32 86 44 112 0.23 0.11 84 37 264 0.51 0.20 84 38 83 1.55 0.23 83 36 233 2.69 0.36 86 39 154 0.49 0.17 84 47 309 0.46 0.17 84 42 138 2.75 0.19 83 37 292 1.95 0.29 86 32 262 1.21 0.27 85 38 35 1.22 0.28 84 31 189 3.19 0.23 83 28 340 2.10 0.26 86 45 303 0.59 0.16 85 36 66 1.69 0.30 84 31 205 3.28 0.25 84 29 21 1.88 0.24 87 42 115 0.48 0.14 85 51 110 2.50 0.26 84 35 221 2.07 0.25 84 37 95 0.90 0.21 87 56 155 0.57 0.15 85 51 142 2.64 0.26 84 34 272 0.83 0.26 84 32 146 1.73 0.32 87 55 259 0.92 0.18 85 32 209 0.34 0.14 84 42 318 0.48 0.16 84 25 210 4.95 0.11 87 40 295 0.85 0.21 85 51 245 0.30 0.09 85 50 29 1.00 0.20 84 37 227 3.33 0.27 85 26 277 0.17 0.12 85 29 79 1.57 0.27 84 35 284 2.37 0.28 85 47 314 0.21 0.12 85 42 100 1.55 0.24 84 31 334 2.12 0.30 86 46 12 0.60 0.18 85 43 137 1.93 0.26 85 31 14 2.08 0.25 86 42 68 1.99 0.27 85 48 214 2.37 0.26 85 27 86 1.56 0.29 86 46 133 2.37 0.29 85 44 254 1.32 0.24 85 31 94 1.20 0.29 86 45 175 2.40 0.28 85 33 289 0.97 0.24 85 32 132 2.01 0.35 86 41 217 0.73 0.20 85 42 323 0.79 0.18 85 45 235 2.78 0.29 86 47 243 0.15 0.08 86 43 22 0.85 0.17 85 36 258 2.02 0.35 86 40 263 0.06 0.06 86 31 88 2.41 0.27 85 32 297 1.76 0.34 86 47 311 0.11 0.08 86 41 138 2.48 0.24 85 29 328 2.00 0.35 87 47 10 1.45 0.24 86 31 189 2.88 0.31 86 31 35 2.17 0.28 87 46 87 2.74 0.25 86 37 213 1.51 0.27 86 25 102 1.75 0.37 87 51 105 3.53 0.15 86 42 252 1.29 0.24 86 31 150 1.95 0.30 87 58 129 2.90 0.20 86 36 278 0.96 0.24 86 24 197 4.63 0.20 87 29 193 1.27 0.31 86 43 316 1.56 0.23 86 36 237 3.70 0.23 87 48 234 0.79 0.19 87 42 28 1.68 0.20 86 31 260 2.62 0.31 87 37 261 0.27 0.11 87 37 100 2.45 0.24 86 26 293 2.14 0.32 87 46 311 0.35 0.13 87 38 110 2.97 0.19 86 31 328 1.68 0.29 87 53 134 1.55 0.24 87 30 37 1.74 0.24 87 46 149 1.84 0.23 87 26 113 1.31 0.29 87 37 199 1.23 0.24 87 30 140 1.48 0.30 87 43 239 0.98 0.21 87 37 217 2.70 0.31 87 36 273 1.39 0.27 87 29 248 3.05 0.35 87 43 323 0.75 0.18 87 31 280 1.86 0.30 87 29 342 1.17 0.27 Temperature Centropages hamatus was found at station locations where surface temperatures ranged from -0.5°C to 28.7°C and where the average water column temperatures were between 0.2 and 24.6°C. Although the copepod can tolerate a wide range of tem- peratures, abundance was greatest at stations where surface temperature ranged from 12 to 17°C (Fig. 5A). The relationship between surface temperature and the annual abundance cycle is shown in Figure 3. Ris- ing temperatures in the MAB during summer may be responsible for a rapid decline of Centropages hamatus there. The population nearly disappears during late summer as surface temperature reaches annual maxi- mums. The July correlation coefficient between vari- ables indicates a strong inverse relationship (P<0.01). Centropages hamatus density remains low until the mean surface temperature falls below 15°C in Decem- ber. Abundance in the more northern subareas slowly declines after the annual temperature high is reached. Unlike that for the population in the MAB, abundance in these subareas does not increase as temperatures de- cline in the fall, but only with spring warming (Fig. 3). Monthly correlations between Centropages hama- tus station abundance and temperature variables 90 Fishery Bulletin 95(1 ), 1997 A Figure 2 (A) Monthly composite distribution and abundance of Centropages hamatus in the U.S. northeast shelf ecosystem: 1977-1987. (B) Monthly composite distribution and abundance of Centropages hamatus in the U.S. northeast shelf ecosystem: 1977-1987. Kane: Spatial and temporal abundance patterns of Centropages hamatus 91 B Figure 2 {continued! 92 Fishery Bulletin 95( 1 ), 1997 were significant (P<0.05) during certain months in each of the subareas (Table 2). Significant relation- ships persisted between C. hamatus density and a temperature variable for several extended periods. Surface temperature was negatively correlated with abundance from November to March in the MAB subarea and also from June to February in SNE Month Figure 3 Bar graph of the monthly log mean abundance of Centropages hamatus with a line graph of the monthly mean surface water temperature for each subarea (MAB=Middle Atlantic Bight; SNE=Southern New En- gland; GBK=Georges Bank; and GOM=Gulf of Maine) . waters. Abundance in the GBK subarea was posi- tively correlated to average water-column tempera- tures from May to July and with bottom tempera- tures from September to December. In the GOM sub- Month Figure 4 Time series monthly log mean abundance (solid line) of Centropages hamatus and the 95% confidence interval (dashed line) of the mean for each subarea. Single points are the log mean abundance of individual surveys. Sur- veys that departed substantially from the time series mean are labeled and the 95% confidence interval of the mean indicated with a error bar (MAB=Middle Atlantic Bight; SNE=Southern New England; GBK=Georges Bank; and GOM=Gulf of Maine). Kane: Spatial and temporal abundance patterns of Centropages hamatus 93 area there were no strong correlations for extended periods between variables. Chlorophyll Estimates of the abundance of Centro- pages hamatus were highest at locations where chloro- phyll biomass was also high (Fig. 5B). Total chlorophyll and abundance measures at stations were significantly (PcO.Ol) correlated during certain times of the year in all subareas (Table 2). In the MAB, variables were posi- <7 7 8 9 10 11 12 13 14 15 16 17 18 19 20 ;>21 Surface temperature (°C) <0.2 0.4 0.6 0.8 1.0 1.4 1.8 2.2 3.5 ;>3.5 Chlorophyll (mg/m3) <31 31 31.5 32 32.5 33 33.5 34 34.5 *35 Salinity (psu) Figure 5 Mean abundance of Centropages hamatus by (A) surface temperature, (B) chlorophyll, and (C) salinity interval. All time series data from the entire survey area was used. tively correlated from May through July and, in SNE waters, during October and February. Variables on GBK were positively correlated from May through January, except for October. GOM correlations were significantly positive in August, November, and December. Partitioning of total chlorophyll values into net- plankton and nanoplankton size fractions did not typically change the correlation coefficients between Centropages hamatus and phytoplankton abundance listed in Table 2. There were a few scattered months in the subareas where coefficients with netplankton were 0. 1-0.2 units higher. The most substantial change occurred during October on GBK. The correlation coef- ficient with netplankton was 0.27 units above the value in Table 2 and was positively correlated (P=0.02). Salinity Centropages hamatus was present at sta- tions where integrated water-column salinity ranged from 27.09 to 36.00 psu. Maximum abundance oc- curred in the lower region of this range (Fig. 5C). Monthly correlation coefficients between station abundance and salinity were usually negative and oftentimes significant during the year (Table 2). No- table were the comparatively high negative correla- tions found during January in both the MAB and SNE subareas. Values in the MAB were also nega- tively correlated in February and again in August and September. SNE correlations were also signifi- cantly negatively correlated during April, July, and from September through December. GBK correla- tions, though not always significant, were positive from February through July and negative in the re- maining six months. GOM coefficients were gener- ally weak throughout the year. Predation Pressure On average, Centropages hamatus and ctenophores both reach peak abundance during June in the SNE subarea (Figs. 3 and 6). During June and July of 1981 a large patch (9-12 stations) of ctenophores occupied inshore waters in the southern region of the subarea offshore of Long Island, New York. This concentration pushed over- all mean abundance in the subarea to an 11-year high (Fig. 6). Predation on C. hamatus was apparently minimal; its mean abundance in late spring 1981 was slightly above the 11-year average (Table 1; Fig. 3). However, the abundance of C. hamatus in June within a ctenophore patch was much lower (611/ 100m3) than outside (2,712/lOOm3) it. Evidence for predation pressure was also found in the July survey; C. hamatus density was 8,138/lQQm3 where it co- occured with ctenophores, 22,87 l/100m3 where cteno- phores were absent. In the SNE subarea, the omnivorous copepod Centropages typicus is present at relatively high lev- 94 Fishery Bulletin 95(1 ), 1997 Table 2 Summary of correlation analysis between abundance and the different environmental variables. An asterisk indicates where partial correlation coefficients were used. Abbreviation key: temp. = temperature; chi. = chlorophyll; no. = number of observa- tions; r = spearman correlation coefficient; P = probability that correlation is zero; MAB = Middle Atlantic Bight; SNE = Southern New England; GBK = Georges Bank; GOM = Gulf of Maine. Area Month Bottom Depth Surface temp. Column temp. Bottom temp. Column salinity Total chi. no. r P no. r P no. r P no. r P no. r P no. r P MAB 1 93 -0.54 <0.01 89 -0.45 <0.01* 93 -0.47 <0.01* 90 -0.48 <0.01* 93 -0.54 <0.01* 0 2 190 -0.63 <0.01 190 -0.23 <0.01* 145 -0.33 <0.01* 139 -0.33 <0.01* 146 -0.40 <0.01* 146 -0.01 0.89* 3 434 -0.65 <0.01 432 -0.11 0.03* 148 0.05 0.55* 139 0.05 0.60* 148 -0.18 0.03* 161 0.16 0.04 4 223 -0.65 <0.01 218 -0.02 0.77 67 0.12 0.35 66 0.05 0.68 67 0.09 0.45* 75 0.05 0.64 5 352 -0.68 <0.01 350 0.01 0.82 278 -0.05 0.41* 266 -0.02 0.78* 278 -0.21 <0.01* 197 0.24 <0.01* 6 162 -0.64 <0.01 161 -0.03 0.75 75 -0.12 0.29* 73 -0.12 0.31* 75 -0.17 0.16* 79 0.25 0.03* 7 290 -0.50 <0.01 287 -0.42 <0.01* 19 0.30 0.23* 19 0.30 0.21 19 -0.41 0.09* 70 0.38 <0.01* 8 354 -0.22 <0.01 352 -0.01 0.98* 146 -0.18 0.04* 143 -0.18 0.04* 146 -0.30 <0.01* 77 0.18 0.12* 9 313 -0.20 <0.01 306 0.01 0.89 93 -0.09 0.37* 86 -0.24 0.03* 93 -0.28 <0.01* 38 0.20 0.24* 10 128 -0.04 0.70 127 0.12 0.19 73 0.18 0.14 69 0.17 0.16 73 -0.04 0.76 80 -0.02 0.84 11 278 -0.36 <0.01 278 -0.24 <0.01 266 -0.23 <0.01 257 -0.20 <0.01 266 -0.08 0.17* 117 0.08 0.38* 12 27 -0.69 <0.01 27 -0.31 0.12* 26 -0.21 0.32* 25 -0.22 0.31* 26 -0.23 0.27* 27 -0.12 0.56* SNE 1 146 -0.34 <0.01 146 -0.41 <0.01* 145 -0.44 <0.01* 132 -0.43 0.01* 145 -0.52 <0.01* 64 -0.29 0.02* 2 92 -0.39 <0.01 92 -0.29 <0.01* 63 0.08 0.56* 57 0.02 0.90* 63 -0.03 0.82* 66 0.25 0.05* 3 339 -0.33 <0.01 336 -0.01 0.98* 175 -0.11 0.14* 165 -0.09 0.27* 175 -0.09 0.22* 205 -0.04 0.59* 4 314 -0.42 <0.01 282 0.07 0.24* 61 -0.35 <0.01* 56 -0.35 0.01* 61 -0.49 <0.01* 58 -0.23 0.09* 5 371 -0.51 <0.01 365 0.08 0.15 245 0.14 0.03* 230 0.23 <0.01* 241 -0.02 0.81* 167 0.17 0.03* 6 146 -0.56 <0.01 146 -0.30 <0.01* 119 -0.07 0.46 110 0.06 0.55 119 -0.16 0.08* 123 0.01 0.95* 7 343 -0.66 <0.01 343 -0.27 <0.01* 66 0.22 0.08* 63 0.24 0.06 66 -0.34 <0.01* 107 -0.05 0.63* 8 289 -0.32 <0.01 286 -0.36 <0.01 89 -0.01 0.90* 89 0.09 0.39* 89 -0.14 0.21* 68 0.09 0.48* 9 188 -0.33 <0.01 176 -0.44 <0.01* 104 -0.28 <0.01* 101 0.14 0.18* 104 0.28 <0.01* 14 0.52 0.06 10 354 -0.27 <0.01 327 -0.58 <0.01* 113 -0.33 <0.01* 106 0.10 0.34* 113 -0.24 <0.01* 133 0.20 0.02* 11 225 -0.15 0.03 224 -0.36 <0.01* 205 -0.31 <0.01* 191 -0.22 <0.01 205 -0.14 0.05* 76 0.11 0.37* 12 160 -0.22 <0.01 158 -0.22 <0.01* 146 -0.16 0.06* 139 -0.02 0.78* 146 -0.37 <0.01* 141 0.01 0.91* GBK 1 100 -0.69 <0.01 100 -0.14 0.17 71 0.24 0.05* 61 0.24 0.07* 71 -0.04 0.77* 72 0.28 0.02* 2 104 -0.42 <0.01 103 0.38 <0.01* 54 0.52 <0.01* 45 0.50 <0.01* 54 0.24 0.08* 12 0 0 3 152 -0.51 <0.01 148 0.03 0.70* 75 0.01 0.92* 66 -0.16 0.21 75 0.22 0.06* 92 -0.10 0.34* 4 292 -0.53 <0.01 287 0.13 0.03 53 0.19 0.18* 46 0.22 0.15* 53 0.23 0.10* 28 0.31 0.10 5 250 -0.61 <0.01 241 0.16 <0.01* 171 0.33 <0.01 153 0.30 <0.01 171 0.27 <0.01* 134 0.45 <0.01* 6 97 -0.64 <0.01 93 -0.02 0.88* 69 0.40 <0.01* 58 0.45 <0.01* 69 0.14 0.26* 63 0.30 0.02* 7 163 -0.64 <0.01 161 -0.29 <0.01* 31 0.47 <0.01* 30 0.37 0.05* 31 0.18 0.35* 42 0.36 0.02* 8 250 -0.67 <0.01 242 -0.19 <0.01* 28 0.04 0.83* 25 0.03 0.91* 28 -0.55 <0.01* 48 0.33 0.03* 9 124 -0.73 <0.01 108 -0.21 0.03 96 0.19 0.07* 87 0.33 <0.01* 96 -0.16 0.12* 23 0.47 0.03* 10 393 -0.62 <0.01 388 0.21 <0.01 63 0.12 0.37* 55 0.42 <0.01* 63 -0.41 <0.01* 78 -0.01 0.99* 11 194 -0.73 <0.01 194 0.11 0.14* 151 0.09 0.29 138 0.21 <0.01 151 -0.13 0.13* 94 0.21 0.04* 12 165 -0.62 <0.01 152 0.36 <0.01 135 0.31 <0.01 114 0.30 <0.01 135 -0.12 0.16* 95 0.38 <0.01* GOM 1 95 -0.12 0.26 95 0.01 0.95 59 -0.05 0.70 42 -0.13 0.40 59 -0.06 0.65 8 -0.25 0.55 2 204 -0.23 <0.01 193 -0.01 0.95* 136 -0.04 0.62* 79 -0.05 0.64* 136 0.06 0.49* 99 0.18 0.07* 3 70 -0.24 0.05 70 0.06 0.64 48 0.04 0.81 29 -0.05 0.80 48 -0.01 0.96 63 0.12 0.35 4 288 -0.10 0.08 257 -0.01 0.97 19 -0.13 0.58 10 -0.12 0.74 19 -0.26 0.28 17 -0.22 0.40 5 278 -0.22 <0.01 251 0.04 0.52* 136 0.17 0.54 84 0.36 <0.01* 136 0.06 0.46* 113 -0.01 0.91* 6 245 -0.35 <0.01 244 0.10 0.11 189 0.10 0.17 105 -0.31 <0.01 149 -0.13 0.11* 52 -0.16 0.25 7 75 -0.19 0.10 74 0.07 0.54 37 0.07 0.69 30 0.06 0.75 37 -0.11 0.52 45 0.19 0.20 8 172 -0.40 <0.01 162 -0.06 0.45* 48 0.25 0.09* 46 0.21 0.17 48 -0.22 0.14* 93 0.25 0.01* 9 155 -0.28 <0.01 145 <0.01 0.99* 135 -0.09 0.29* 114 -0.20 0.03* 135 -0.12 0.16* 23 -0.06 0.80 10 307 -0.18 <0.01 302 0.10 0.09* 100 -0.01 0.89* 54 -0.02 0.91* 100 -0.15 0.13* 118 0.08 0.40 11 274 -0.42 <0.01 272 0.18 0.01* 93 0.20 0.06* 65 0.10 0.42* 93 -0.27 <0.01* 93 0.29 <0.01 12 274 -0.29 <0.01 269 0.05 0.42* 251 0.05 0.44 167 -0.07 0.38* 251 -0.02 0.72* 107 0.24 0.01 Kane: Spatial and temporal abundance patterns of Centropages hamatus 95 els year round and begins to increase inshore from its annual low in late spring-early summer (Fig. 6) when Centropages hamatus is at peak abundance. MARMAP data indicate that it is unlikely that the summer decline or the abundance levels reached by C. hamatus are controlled substantially by C. typicus predation. There was no strong inverse relationship Figure 6 Time series monthly log mean abundance (solid line) and the 95% confidence interval (dashed line) of the mean for the following copepod predators in the Southern New En- gland subarea: ctenophores, the copepod Centropages typicus, and chaetognaths. Single points are the log mean abundance of the taxon for individual surveys during cer- tain years. The error bars indicate the 95% confidence in- terval of the mean. between the abundance trends of the two species. For example, in 1987 C. hamatus reached peak abun- dance earlier than usual, in late April, and declined rapidly to below average levels (Table 1). The abun- dance of C. typicus was average in late April 1987 and also declined through the summer to below av- erage levels (Fig. 6). High levels of C. hamatus re- corded in 1984 (Fig. 4) were not due to the absence of C. typicus predators; abundance was close to av- erage for the copepod during spring and summer (Fig. 6). Monthly partial correlation coefficients between station abundance values of the two species during the time series were positive (0.07-0.24) from April through August, further evidence that predation by C. typicus is minimal. Peaks of Centropages hama tus abundance and the presence of chaetognaths do coincide in the SNE sub- area (Figs. 3 and 6). However, evidence that chaetog- nathan predation impacts C. hamatus abundance could not be found. All of the surveys that had ex- ceptional high or low C. hamatus abundance, 1979, 1984, and 1987 (Fig. 4), had near average chaetog- nath density (Fig. 6). Conversely, C. hamatus abun- dance was close to average when chaetognath den- sity was high in 1977 and low in 1985 (Fig. 6). Monthly partial correlation coefficients between sta- tion abundance values of the two species were not significant and very low (-0.10-0.25) throughout the year, indicating that chaetognath predation has little effect on C. hamatus abundance. Discussion Temperature affects most processes in marine eco- systems and the life cycle of Centropages hamatus is no exception. Opposite extremes in temperature ap- pear to limit the seasonal occurrence of the popula- tion at the southern and northern ends of the eco- system. Warm summer temperatures in the MAB were correlated with the rapid decline of the cope- pod in this area as values approach or surpass the critical upper thermal level for the species. Similar relationships between temperature and C. hamatus were found by Deevey (1960) for the population present near and within Delaware Bay. She reported that the copepod disappears as temperatures rise in summer but is present year round in small numbers during cool summers. Grant ( 1988) also reported that C. hamatus abundance in the MAB declines with increasing temperature and is absent in some years during summer and fall seasons. The MAB popula- tion begins to reappear or increase close inshore in late autumn where waters cool faster than those off- shore. Populations farther north decline slowly as 96 Fishery Bulletin 95( 1 ), 1997 winter approaches until only small aggregations of cold-adapted individuals overwinter in the far east- ern waters along the SNE coast, on the central shoals of GBK, and within inshore waters in the GOM. Abundance in these areas increase as temperatures rise in spring. The life cycle of many marine copepods involves the production of resting eggs that allow the species to repopulate areas when environmental conditions again become favorable (Uye, 1985). Evidence that Centropages hamatus produce resting eggs has been found in the western North Atlantic (Lindley, 1990), the Gulf coast of Florida (Marcus, 1989), and in the MARMAP survey area on GBK (Davis, 1987). Al- though this report provides no direct evidence that C. hamatus produces resting eggs, it seems unlikely that the small pelagic population that overwinters, or oversummers, could produce the great abundance of the next generation without recruitment from benthic resting eggs. Marcus (1989) found that a C. hamatus population residing in a subtropical embayment area produces diapause eggs that allow the species to survive warm summer temperatures. This also likely occurs in the MAB when the popula- tion rapidly declines to a few individuals, or disap- pears entirely during summer, and begins to increase as temperatures decline in winter. Lower maximum temperatures observed on GBK are apparently not sufficiently high to impact populations there dramati- cally; abundance declines slowly during autumn af- ter peak abundance is reached in summer and does not increase until temperatures rise in early spring. This slow decline in abundance may occur because success of egg hatching decreases as females gradu- ally switch from subitaneous egg production to rest- ing egg production owing to decreasing temperatures and daylengths, as was found for the copepod Labidocera aestiva in nearby waters (Marcus, 1982). The resting eggs hatch in the spring to supplement the production of overwintering late-stage copepodites and to ensure the success of the population. Such variation in egg production between well-separated populations has been reported for other species (Marcus, 1984; Uye, 1985). Somewhere in the SNE subarea there is prob- ably a transition zone between adults that are “tem- perature shocked” to release quiescent eggs and those that slowly change their egg-laying strategy as autumn progresses. Egg-production strategy in the GOM is probably similar to that found in the GBK. The strongly negative correlation of Centropages hamatus abundance to depth and its well-defined inshore-offshore abundance gradient confirm the importance of resting eggs in the life history of this species. Environmental conditions probably do not trigger the release of diapause eggs until after the population constricts inshore after peak abundance is reached. Evidence for this was found by Lindley (1990) in southern waters of Great Britain where C. hamatus eggs were found to be abundant only in depths of less than 50 m. When the eggs hatch, the prevailing westerly winds in the northwest Atlantic slowly spread the pelagic population and the new recruits offshore to establish the characteristic abun- dance gradient of this species. Abundance of Centropages hamatus appears to be related strongly to the availability of phytoplankton. The copepod’s abundance was highest at stations where chlorophyll values were high, and its distri- bution is similar to phytoplankton gradients in the study area (O’Reilly and Busch, 1984). However, cor- relation coefficients between variables were weak and inconsistent among subareas, indicating that the species is not particularly sensitive to phytoplank- ton availability. The low correlation may be because average water-column chlorophyll measurements are static measures that may not reflect the actual food concentrations that are, or were, available to the copepod over the previous 24 hours. Furthermore, it is also possible that late-stage copepodites of this om- nivorous species may be more sensitive to zooplank- ton prey concentrations. Nonetheless, food availabil- ity is a key limiting factor throughout nature and certainly has a major role in shaping the life history of this copepod. The maximum mean abundance of C. hamatus is greatest on GBK, the ecosystem subarea with the largest estimate of annual primary produc- tion (O’Reilly et al., 1987). Conversely, population den- sity is lowest in the GOM where average chlorophyll concentrations are also lowest. Monthly correlation coefficients between salinity and abundance of Centropages hamatus were also weak even though both variables have a strong off- shore gradient. Unlike chlorophyll correlations, these coefficients portray accurately the relationship be- tween variables. Centropages hamatus is a coastal species with a wide latitudinal range and must tol- erate wide environmental fluctuations. It has been reported in areas with salinity as low as 6 psu (Hernroth and Ackefors, 1977), as well as in Medi- terranean waters where salinity exceeds 36 psu (Gaudy, 1971). The large numbers of C. hamatus as- sociated with low salinity found in this study is prob- ably an artifact of the high phytoplankton concen- trations found in a narrow inshore band along the MAB and SNE coasts (O’Reilly et al., 1987). The an- nual spring increase in precipitation and subsequent river runoff that leads to lowered salinity in the MAB and SNE subareas (Manning, 1991) also introduces nutrient-enriched water that stimulates phytoplank- ton growth and zooplankton production. Further- Kane: Spatial and temporal abundance patterns of Centropages hamatus 97 more, the highest mean abundance of C. hamatus is found over the central shoals of GBK where salinity usually ranges from 32.2 to 32.7 psu during peak abundance, well above the coastal areas where abun- dance, on average, is much lower. High salinity off- shore may effect C. hamatus production there and restrict its distribution, but it is more likely that low offshore abundances are caused by low phytoplank- ton food stocks that cannot support an overwinter- ing population or the generation that produces rest- ing eggs after peak abundance is reached. There was no strong evidence from survey data that predation affects interannual variability or causes the seasonal decline of the population in the SNE subarea. Ctenophores appear to lower Centro- pages hamatus abundance when they are plentiful, but this occurred only during one year and in a re- stricted area. Chaetognaths and the copepod Centropages typicus also appear to have little affect on C. hamatus density. Clearly, however, a dedicated study analyzing stomach contents and the vertical distribution of the predator-prey field is needed to define the actual food web. Potential predators such as squid, juvenile fish, and populations of plank- tiverous adult fish must also be considered in order to fully define the role predation has in controlling C. hamatus population levels. Lindley and Hunt (1989) examined the distribu- tion of Centropages hamatus to the north and across the Atlantic to the North Sea. They described a life cycle similar to the one reported in this paper and speculated that the autumn decline in abundance is caused by the pressure of competition with Centro- pages typicus for food resources. Dagg and Turner (1982) studied copepod populations in the SNE and GBK subareas during autumn and calculated that copepod grazers may consume entire phytoplankton stocks. If true, high abundance of C. typicus could impact population levels of C. hamatus. However, MARMAP survey data indicate that high C. typicus abundance does not lead to an early decline of C. hamatus in either subarea. For example, in 1985 on GBK, median C. typicus abundance was 2-3 orders of magnitude above the ten-year average, but C. hamatus was also above average and increased in late autumn (Kane, 1993). Data presented in this report also show that the abundance of the two spe- cies are not related in the SNE subarea. Although competition pressure between the two species does not appear to cause the decline of C. hamatus, labo- ratory feeding experiments are needed to measure the effect of low food levels on species abundance. The copepod Centropages hamatus has evolved a unique life history to survive and reproduce within the waters of the northwestern Atlantic continental shelf. The population has a distinct seasonal cycle with peak abundance occurring in shallow areas where phytoplankton food stocks are rich and sur- face temperature ranges from 12 to 17°C. Predation pressure appears minimal, and C. hamatus abun- dance peaks between the annual maximum of early spring and autumn dominant copepod species (Sherman et al., 1983), thus reducing competition pressure for food resources. Centropages hamatus likely produces resting eggs that hatch and help re- populate the ecosystem when environmental condi- tions are favorable. Comprehensive laboratory and shipboard experiments are needed to distinguish how the above biotic and abiotic factors interact to deter- mine the annual success of the population. Acknowledgments The author acknowledges the individuals involved with the collection, analysis, and processing of MARMAP data. Special thanks go to Jay OReilly for his help with the distribution plots and to those who critically reviewed early drafts of the manuscript. Literature cited Anderson, J. T., and W. G. Warren. 1991. Comparison of catch rates among small and large bongo samplers for Calanus finmarchicus copepodite stages. Can. J. Fish. Aquat. Sci. 48:303-308. Bigelow, H. B. 1926. Plankton of the offshore waters of the Gulf of Maine. Bull, of the Bureau of Fish. XL, part 2. Gov’t Printing Office, Washington D.C., 509 p. Dagg, M. J., and J. T. Turner. 1982. The impact of copepod grazing on the phytoplankton of Georges Bank and the New York Bight. Can. J. Fish. Aquat. Sci. 39:979-990. Davis, C. S. 1987. Zooplankton life cycles. In R. A.Bakus and D. W. Bourne (eds.), Georges Bank, p. 256-267. MIT Press, Cambridge, MA. Deevey, G. B. 1956. Oceanography of Long Island Sound, 1952- 1954. Bull. Bingham Oceanogr. Collect., Yale Univ. 15:113-155. 1960. The zooplankton of the surface waters of the Dela- ware Bay region. Bull. Bingham Oceanogr. Collect., Yale Univ. 17(2 ):5— 53. Gaudy, R. 1971. Contribution a fetude du cycle biologique des copepodes pelagiques du Golf de Marseille. I: L’environment physique et biotique et la composition de la population de copepodes. Tethys 3(4):921-942. Grant, G. C. 1988. Seasonal occurrence and dominance of Centropages congeners in the Middle Atlantic Bight, USA. Hydrobiol. 167/168:227-237. 98 Fishery Bulletin 95 ( 1 ), 1997 Hernroth, L„, and H. Ackefors. 1977. The zooplankton of the Baltic proper: a long term investigation of the fauna, its biology and ecology. Insti- tute of Mar. Res., Lysekil, Sweden, 58 p. Judkins, D. C., C. D. Wirick, and W. E. Esaias. 1980. Composition, abundance, and distribution of zoop- lankton in the New York Bight, September 1974-Septem- ber 1975. Fish. Bull. 77:669-683. Kane, J. 1993. Variability of zooplankton biomass and dominant species abundance on Georges Bank, 1977-1986. Fish. Bull. 91(3):464— 474. Lindley, J. A. 1990. Distribution of overwintering calanoid copepod eggs in sea-bed sediments around southern Britain. Mar. Biol. 104:209-217. Lindley, J. A., and H. G. Hunt. 1989. The distributions of Labidocera wollastoni and Centropages hamatus in the north Atlantic Ocean and the North Sea in relation to the role of resting eggs in the sediment. In J. S. Ryland and P. A. Tyler (eds.), Repro- duction, genetics, and distribution of marine organ- isms, p. 407-413. Olson and Olson Press, Fredensborg, Denmark. Manning, J. 1991. Middle Atlantic Bight salinity: interannual varia- bility. Continental Shelf Res. 11(2):123— 137. Marcus, N. H. 1982. Photoperiodic and temperature regulation of diapause in Labidocera aestiva (Copepoda: Calanoida). Biol. Bull. 162:45-52. 1984. Variation in the diapause response of Labidocera aestiva (Copepoda: Calanoida) from different latitudes and its importance in the evolutionary process. Biol. Bull. 166:127-139. 1989. Abundance in bottom sediments and hatching re- quirements of eggs of Centropages hamatus (Copepoda: Calanoida) from the Alligator Harbor region, Florida. Biol. Bull. 176:142-146. McLaren, I. A. 1978. Generation lengths of some temperate marine cope- pods: estimation, prediction, and implications. J. Fish. Res. Board Can. 35:1330-1342. O’Reilly, J. E., and D. A. Busch. 1984. Phytoplankton primary production on the northwest- ern Atlantic shelf. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 183:255-268. O’Reilly, J. E., C. Evans-Zeitland, and D. A. Busch. 1987. Primary production. In R. A. Bakus and D. W. Bourne (eds.), Georges Bank, p. 220-233. MIT Press, Cambridge, MA. Pertzova, N. M. 1974. Life cycle and ecology of a thermophilous copepod Centropages hamatus in the White Sea. Zool. Zh. 52:1013- 1022. Pinhey, K. 1926. Entomostraca of the Belle Isle Strait expedition, 1926, with notes on other planktonic species, part I. Contrib. Can. Biol., N.S. 3:1-55. Sampson, R. J. 1988. SURFACE III users manual. Kansas Geological Survey, Lawrence, KS, 277 p. Sherman, K. 1980. MARMAP, a fisheries ecosystem study in the NW At- lantic: fluctuations in the ichthyoplankton-zooplankton com- ponents and their potential for impact on the system. In F. P.Diemer, F. J. Vernberg, and D. Z. Mirkes, (eds.), Advanced concepts on ocean measurements for marine biology, p. 9- 37. Belle W. Baruch Institute for Marine Biology and Coastal Res., Univ. South Carolina Press, Columbia, SC. 1994. Sustainability, biomass yields, and health of coastal ecosystems: an ecological perspective. Mar. Ecol. Prog. Ser. 112(3):277-301. Sherman, K., J. R. Green, J. R. Goulet, and L. Ejsymont. 1983. Coherence in zooplankton of a large Northwest At- lantic ecosystem. Fish. Bull. 81(4):855-862. Sherman, K., W. G. Smith, J. R. Green, E. B. Cohen, M. S. Berman, K. A. Marti, and J. R. Goulet. 1987. Zooplankton production and the fisheries of the Northeastern Shelf. In R. A. Bakus and D. W. Bourne (eds.), Georges Bank, p. 268-282. MIT Press, Cambridge, MA. Sibunka, J. D., and M. J. Silverman. 1984. MARMAP surveys of the continental shelf from Cape Hatteras, North Carolina, to Cape Sable, Nova Scotia (1977-1983). Atlas No. 1: summary of operations. U.S. Dep. Commer., NOAATech. Memo. NMFS-F/NEC-33. 1989. MARMAP surveys of the continental shelf from Cape Hatteras, North Carolina, to Cape Sable, Nova Scotia (1984-1987). Atlas No. 3: summary of operations. U.S. Dep. Commer., NOAATech. Memo. NMFS-F/NEC-68. Uye, Shin-ichi. 1985. Resting egg production as a life history strategy of marine planktonic copepods. Bull. Mar. Sci. 37(2): 440-449. 99 Estimating the annual proportion of nonspawning adults in New Zealand hoki, Macruronus novaezelandiae Mary E. Livingston Marianne Vignaux Kathy A. Schofield National Institute of Water and Atmospheric Research RO Box 14-901 Kilbirnie, Wellington, New Zealand E-mail address: m.livingston@niwa.cri.nz Abstract .—Trawl surveys of hoki, Macruronus novaezelandiae (Hector) in the Southland and subantarctic areas (Southern Plateau) of New Zealand’s Exclusive Economic Zone were carried out in May 1992 and 1993. The propor- tion of females of each age that would spawn in the coming spawning season (July-August) was estimated on the ba- sis of histological analysis of gonad samples and ageing data. Comparisons were made between numbers of fish at age in these surveys and numbers of fish at age in surveys in November-De- cember 1991 and 1992 to estimate mi- gration before May. The results indicate that 66% (stan- dard error [SE] of 3%) of females age 7 and over that were on the Southern Plateau in May 1992 would spawn in winter 1992, compared with 65% (SE 2%) in 1993. If the number of hoki esti- mated to have already migrated out of the survey area in May are included as prespawners, then up to 67% (SE 5%) of adult females were predicted to spawn in winter 1992 and 82% (SE 3%) in winter 1993. This study confirms that the propor- tion of adult hoki that spawn in a given year is substantially less than 1. It is not known how much this varies, whether it is with or without trend, or whether it is correlated with any envi- ronmental variables. Fishery indicators such as stock and fishery risk are par- ticularly sensitive to the annual propor- tion of adult hoki that spawn, and it is possible that its variation could ob- scure any underlying stock-recruitment relationship. Manuscript accepted 4 September 1996. Fishery Bulletin 95:99-113 (1997). Hoki ( Macruronus novaezelandiae Hector) form New Zealand’s largest commercial fishery with an annual catch of about 200,000 metric tons (t). The fish are widely distributed throughout New Zealand’s 200-mile Exclusive Economic Zone in depths of 50-800 m, but most commercial fishing is at depths of 200-800 m around the South Island (Fig. 1). Fishing effort is greatest during the July-August spawning season off the west coast and in Cook Strait but also occurs on the Chatham Rise and management areas south of Puysegur Point (hereafter referred to as the Southern Plateau) (Fig. 1) throughout the rest of the year. Although managed as a single stock, hoki are assessed annually as two stocks (Sullivan and Cordue1; Sullivan et al.2). There is no genetic evidence for a split, but because morphometric and growth rate dif- ferences have been found between the two spawning grounds (Horn and Sullivan, 1996; Livingston and Schofield, 1996), a cautious ap- proach in determining yield has been taken. Hoki are assessed as two stocks by using stock reduction models (Sullivan et al.2). Abundance indices estimated from acoustic sur- veys, trawl surveys, and catch-per- unit-of-effort data on the spawning grounds have been the main inputs to the models (Sullivan et al.2). Of the two stocks, the western stock, which resides primarily on the Southern Plateau and spawns off the west coast of the South Is- land, is substantially larger than the eastern stock, which resides primarily on the Chatham Rise and spawns in Cook Strait. Juvenile hoki (2-5 yr) of both stocks appear to reside and mix together on the Chatham Rise in relatively shallow water. As the fish reach maturity, it is assumed that they recruit to their respective stocks. Winter surveys of the Southern Plateau and Chatham Rise have shown that significant numbers of mature-size hoki, both males and females, do not partake in the spawning migration in a given year (Livingston et al., 1991; Hurst and Schofield, 1995). From trawl surveys of the South- ern Plateau in July-August and November-December 1990, it was estimated that the ratio of recruited 1 Sullivan, K. J., and P. L. Cordue. 1992. Stock assessment of hoki 1992. New Zealand Fisheries Assessment Res. Docu- ment 92/12, NIWA Greta Point library, Wellington, New Zealand, 43 p. 2 Sullivan, K. J., P. L. Cordue, and S. L. Ballara. 1995. A review of the 1992-93 hoki fishery and assessment of hoki stocks for 1994. New Zealand Fisheries Assess- ment Research Document 95/5, NIWA Greta Point library, Wellington, New Zea- land, 45 p. 100 Fishery Bulletin 95( 1 ), 1997 Figure 1 Map of New Zealand showing spawning and feeding grounds of hoki, Macruronus novaezelandiae. biomass of western stock hoki present in winter to the recruited biomass of western stock hoki present in summer was 1:2.05 (Hurst and Schofield, 1995). Hurst and Schofield concluded that the total propor- tion of adult hoki that spawned in 1990 was between 60% and 75%. The annual proportion of adult hoki that spawn was incorporated into the stock reduction analysis of hoki as a model parameter for the first time in 1992 (Sullivan and Cordue, 1992). A sensitivity analysis of the response of fishery indicators, such as stock and fishery risk to changes in various model parameters, found that they were particularly sensitive to the proportion of hoki that spawn in a given year (Sullivan and Cordue1). In view of this sensitivity, a research program, to estimate more accurately the annual proportion of hoki that spawn and the maturity ogive of hoki at age in the western stock, was initiated. The spawning season for hoki begins in late June and can extend into mid-September (Sullivan et al.2). Large fish tend to spawn earlier than smaller fish, and on the west coast, spawning extends northwards as the season progresses (Langley, 1993). Hoki are not caught in quantity in the vicin- ity of the west coast outside the spawn- ing season, and there is some evidence from commercial data and trawl survey data to suggest that hoki migrate to the west coast from the Southern Plateau during May-June (Ballara3). Female hoki gain up to 40% of their total body weight as their ovaries ripen (Kuo and Tanaka, 1984), but for the remainder of the year, the ovaries are small, weigh- ing less than 1% of total body weight. Ovaries begin to ripen in April before female hoki migrate to their spawning grounds (van den Broek et al., 1981; Kuo and Tanaka, 1984). Evidence of spawning at other times of the year has not been reported (Kuo and Tanaka, 1984). Preliminary histo- logical work on hoki females collected throughout the spawning season indi- cates that hoki are either synchronous or group synchronous spawners. That is, their ovaries develop a single set of oocytes in a given season, and these ooocytes are released in a single event (synchronous) or over several spawning events (group synchronous) (West, 1990). The same species in Tasmania develops a single set of oocytes in each season (Gunn et al., 1989). It is un- known whether the proportion of hoki that spawn in a given year is determined by environmental condi- tions or whether it relates to a shallow recruitment curve or even some nonannual endogenous rhythm. Our hypothesis was that if hoki develop their oo- cytes synchronously within each ovary, it should be possible to distinguish developing prespawners from nonspawners prior to the onset of the spawning sea- son and thereby estimate both the proportion of fish that would spawn and a maturity ogive based on his- tological characterization. In this study we collected monthly samples to moni- tor seasonal changes in the ovarian development of hoki prior to spawning. We also used random trawl surveys of the Southern Plateau in December and May in two consecutive years to determine 1) the 3 Ballara. S. 1995. Natl. Inst. Water and Atmospheric Res., P.O. Box 14-901 Kilbirnie, Wellington, New Zealand. Personal commun. Livingston et al.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae 101 proportion of female hoki on the Southern Plateau in May that would spawn in the coming season and 2) the proportion of females that already have begun their spawning migration by May. In this paper, we detail 1) the histological basis for classifying hoki as either nonspawners or prespawners and 2) the analy- ses used to estimate the maturity ogive and the to- tal proportion of fish that would spawn in July or August. Materials and methods Trawl surveys Two sets of surveys were completed: 1) 12 Novem- ber-23 December 1991 and 17 April-21 May 1992; 2) 14 November-22 December 1992 and 1 May-4 June 1993. The survey area (275,356 km2) incorpo- rated depths of 300-800 m south of Puysegur Point, excluding rough ground and the Bounty Platform (Fig. 1). The surveys used a two-phase random strati- fication design (Francis, 1984), and because hoki tend to be near the seabed during daylight, coming up off the bottom to feed at night (Kerstan and Sarhrage, 1980), trawling was carried out during daylight hours only. Trawling procedure and standardization of gear, station, and stratum details are reported for each survey individually by Chatterton and Hanchet (1994), Schofield and Livingston (1994, a and b), and Ingerson et al. (1995). Length-frequency samples of about 200 female hoki were collected from each tow on a random basis. The length-frequency distribution and the total numbers of fish were scaled up to the total stratum area by using the Trawlsurvey Analysis Program, as de- scribed by Vignaux.4 The scaling was done by assum- ing a catchability and vulnerability of 1.0 in all sur- veys. Because these assumptions were unlikely to be valid, the numbers of fish were used only in a rela- tive sense. Additional samples of 20 female hoki were collected from each tow to measure gonad and total body weight, to identify macroscopic gonad stage, and to obtain histological samples from ovaries. Otoliths were also collected from these fish for ageing. Ma- turing hoki in the later stages of vitellogenesis can be macroscopically distinguished from resting hoki. The ovaries at that point are swollen, and the indi- vidual oocytes, visible to the naked eye, are opaque and creamy pink. Resting and immature ovaries are small and translucent and no oocytes are visible. Previous surveys of hoki in April-May (e.g. van den Broek et al., 1981) reported few hoki that were suffi- ciently developed to be identified macroscopically as maturing. We therefore obtained histological samples of ovaries as well as recorded their macroscopic ap- pearance. The central portion of the ovary from each sample was preserved at sea in 8%-10% buffered formalin. Samples were later processed to produce thin sections that were stained with standard haemotoxylin and eosin preparations. Histological staging In developing a method to distinguish prespawning hoki from nonspawning hoki, we classified ovaries into the stages given by West (1990). Ovaries were classified according to the most advanced oocyte present in the ovary. A summary of these stages (as given by West) is as follows: Chromatin nucleolar stage Perinucleolar stage Yolk vesicle (cortical alveoli) stage Vitellogenic (yolk) stage Ripe (mature) stage The oocyte is surrounded by a few follicle cells and contains a large nucleus surrounded by a thin layer of cytoplasm. The nucleus has one large nucleolus and several small nucleoli. The nucleus has multiple nucleoli at its periphery. Late perinucleolar oo- cytes may have vacuoles in the cytoplasm. This stage is charact- erised by the appearance of large numbers of yolk vesicles in the cytoplasm. They increase in size and number to form several pe- ripheral rows. The chorion is visible at this stage. Small yolk granules which gradually enlarge until they form fluid-filled spheres are typical. The spheres may eventually fuse to form a continuous mass of fluid yolk. The nucleus may be pe- ripheral or may have dis- integrated completely. 4 Vignaux, M. 1994. Documentation of trawlsurvey analysis program. NIWA Greta Point Internal Report 225, NIWA Greta Point library, Wellington, New Zealand, 44 p. Criteria to distinguish prespawners from non- spawners were developed from a subsample of ovary sections from each survey and from some commer- I 02 Fishery Bulletin 95( 1 ), 1997 cial samples collected from January through August (see below). These criteria (described in the Results section) were then used by an independent reader to classify May 1992 and May 1993 female hoki as prespawners or nonspawners. Monthly samples were also collected from commer- cial vessels to monitor hoki development between January and September. Up to 40 ovary samples from a range of adult-size fish were preserved in 8% buff- ered formalin at the time of capture. Samples were processed in the laboratory, sectioned, and stained with standard haemotoxylin and eosin preparations. Samples were collected mostly from Chatham Rise (Jan-May) and the west coast of the South Island (Jun-Jul) because these were the areas where com- mercial vessels were operating at that time. Each fish was staged histologically (as described above) to determine the earliest month in which de- velopment began. Five of the most developed fish from each month were then selected for oocyte mea- surement to confirm that New Zealand hoki are syn- chronous or group synchronous like their Australian counterparts (Gunn et al., 1989). The mean diam- eter of 200 oocytes was measured (after Foucher and Beamish, 1980) from each of the five fish. Ageing It is unknown whether recruitment to the spawning fisheries is length-driven or age-driven. Observers from the west coast hoki fishery have found spawn- ing hoki as young as 2 years and as small as 42 cm total length, in some years (Sullivan et al.2). Because the model used for stock assessment is an age-struc- tured one (Sullivan et al.2), fish were aged as part of this study, and all analyses were carried out by us- ing age data. It was also important to age fish so that comparisons of the numbers of fish in each co- hort in December and May could be made. Otoliths from each fish in the histological samples from the May surveys and from the biological samples in December were aged by using the validated age- ing method described by Horn and Sullivan (1996). These data were also used to develop age-length keys. Where there were no fish in the sample of a given length, the age-length key was interpolated with nearby values of age. Proportion of each age class developing to spawn The proportion of fish in each age class that were classifed as prespawners was estimated for each age stratum from the aged histological samples. The number of fish at each age in each stratum was esti- mated from the age-length key and the length-fre- quency distribution in the stratum. For each age class the total number of prespawners was therefore esti- mated from the proportion of fish that were develop- ing to spawn and from the total number of fish in each stratum. The standard error of these estimates was estimated by using a resampling technique, whereby in each stratum, a sample, the same size as the original sample, was selected (with replacement) from the original sample. The age-length key and proportion of prespawners were calculated from the combined sample of the 15 strata. This process was repeated 1,000 times. The standard error of the esti- mates was estimated from the standard deviation of the values in 1,000 replicates. Proportion of adult spawning fish An estimate p+ of the proportion of prespawners in the plus group of adult fish (p+) can be obtained by using the method described above but by consider- ing all adult fish as a single plus group. However, if some fish had already left the Southern Plateau to spawn before the survey in May, they also should be counted as prespawning fish. This means that the proportion of adult prespawners present on the Southern Plateau in May ( p+ ) is an underestimate of the total proportion of fish that will spawn (p) as s + ns s + ns + g where s is the number of prespawners on the South- ern Plateau in May, ns is the number of nonspawners on the Southern Plateau in May, and g is the num- ber of fish that had left the Southern Plateau before May, presumably to spawn. If we define a migration ratio, x = — — — > (2) s + ns to be the ratio of the number of fish which have gone to spawn to the number of fish (both prespawners and nonspawners) that are still in the survey area when the second survey is done, then p= S + § =P±±±. (3) s + ns + g l + x Thus p equals p+ when x is zero and increases to- wards an asymptote of 1 when x is very large. Livingston et ai.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae 103 The migration ratio, x, cannot be estimated very precisely because both trawl surveys will be subject to measurement error and because an unknown num- ber of fish will have died naturally or have been caught between December and May. But to obtain the best estimate of x, the numbers of adult fish on the Southern Plateau in December and May were estimated from the total numbers of fish in the sur- veys and the proportion that were in the adult age group. The age distributions were estimated from the length-frequency distributions and the age-length keys were calculated from the samples collected for age determination. In determining the number of fish that moved out of the survey area before May, it is necessary to ac- count for fish that died between December and May. Five months of natural mortality was applied to the number of fish observed in the December surveys to estimate the number of fish that would be expected in the May surveys. The catch taken on the South- ern Plateau between December and May in these two years was 10,595 t in 1992 and 8,339 t in 1993. Be- cause the estimated size of the stock was 860,000 t in May 1992 and 1.3 million t in May 1993 (Cordue5), fishing mortality was considered to be negligible. The discrepancy between the number of fish ex- pected in the May survey and the number observed was the maximum number of fish that could be con- sidered to have left the Southern Plateau to spawn. This number was used in Equation 2 to obtain an estimate x of the migration ratio x. An estimate of the total proportion of fish that will spawn (p) wras calculated by using x and an estimate of p. of p+ in Equation 3. Standard errors of these numbers were calculated by a resampling procedure that included uncertainty regarding the total number of fish in the December and May surveys. Procedure for estimating the total proportion of adult fish spawning The estimation procedure for the proportion of adult spawning fish was as follows: 1 The total number of fish on the Southern Plateau in December, N1 was selected from a normal dis- tribution with mean equal to the estimated value for this survey and with standard deviation equal to the standard error of this estimate. This number of fish was then distributed over the length-fre- quency distribution (assumed to be known exactly). 5 Cordue, P. 1996. Natl. Inst. Water and Atmospheric Res. P.O. Box 14-901 Kilbirnie, Wellington, New Zealand. Personal commun. 2 An age-length key (including ageing error) was generated by sampling with replacement from the fish in the age-length sample from the December survey and was applied to the December length- frequency distribution to estimate the number of adult fish in December, nr The number of adult fish expected to be alive in May was calculated by applying the natural mortality M to nv as nxe-M5112. 3 The same procedures described in 1 and 2 above were applied to the May surveys to obtain the total number of fish on the Southern Plateau in May (N2) and the number of adult fish in May (n2) 4 The number of fish apparently missing (nm) was estimated as 5 Taken as a fraction of the number n2 on the South- ern Plateau in May, x (the migration ratio) was estimated by n2 6 p+ was calculated by using the simulated age-length key and the histological sample as de- scribed above. Hence p was estimated as - _ P+ + x This process was repeated 1,000 times. The standard errors of each of the values was calculated from the standard deviation of the distribution of the 1,000 values. Results Trawl surveys The four surveys were successfully completed with a combined total of 495 stations sampled. Gear param- eters were within the range necessary for survey standardization (Hurst et al.6), thereby permitting the direct comparison of survey results used for data analysis (Chatterton and Hanchet, 1994; Schofield and Livingston, 1994, a and b; Ingerson et al., 1995). 6 Hurst, R. J., N. Bagley, T. Chatterton, S. Hanchet, K. A. Schofield, and M. Vignaux. 1992. Standardisation of hoki/ middle depth time series trawl surveys. NIWA Greta Point Internal Report 194, NIWA Greta Point library, Wellington, New Zealand, 87 p. 104 Fishery Bulletin 95( I ), 1997 The numbers of hoki observed in the May surveys (26.8 million [1992], 24.4 million [1993]) were con- siderably less than in the December surveys (38.6 million [1991], 34.8 million [1992]) (Fig. 2). In addi- tion, the length-frequency histograms show a decline in bimodality in the adult part of the distribution between May and December (Fig. 2). In December 1991, 56% of females over 50 cm were in strata west of 170°E. In May 1992, 59% were west of this line. In December 1992, 49% were in the west and in May 1993, 68% were in the west. There were 541 fish in the histological sample in May 1992 and 1,136 fish in the sample in May 1993 (Table 1). In 1992, stratum 1 (300-600 m depth at Puysegur) was not sampled and female fish from every second station in the other strata were sampled. In 1993 female fish from every station in all 15 strata were sampled. Ageing There were very few young fish in most strata, and limited numbers of fish in the 1986 cohort, which appeared as age-6 fish in 1992 and as age-7 fish in 1993. Although fish were aged to a maximum age of 19 years, we combined them into a group of age 10+ and above. The ageing data were also used to de- velop age-length keys. Livingston et al.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae 105 Table 1 Percentages of each histological stage observed in female hoki sampled from each survey. Histological stage Dec 1991 May 1992 Dec 1992 May 1993 Chromatin nucleolar 0 0 0 0.1 Perinucleolar 100 45.4 100 39.7 Yolk vesicle 0 27.1 0 13.6 Vitellogenic 0 27.5 0 43.3 Ripe 0 0 0 3.3 Number in sample 452 541 1,039 1,136 Table 2 Numbers of hoki monthly samples showing ovarian development on the Chatham Rise and west coast spawning grounds. Region and Histological stage month Perinucleolar Yolk vesicle Vitellogenic Ripe Chatham Rise January 22 — — — F ebruary 10 — — — March no sample April 6 9 1 - May 10 9 1 — West coast June 4 3 13 — July — — 10 5 August — — 9 5 Histology Monthly samples of hoki from the Cha- tham Rise and west coast of the South Is- land showed little change in oocyte stage in January and February, all being classi- fied as perinucleolar (Table 2). In April, May, and June, larger oocytes of the yolk vesicle and vitellogenic yolk stages were evident. By July and August, females sampled on the west coast of the South Is- land were vitellogenic or ripe and had hya- line oocytes (Table 2). Oocytes clearly devel- oped as a synchronous group, evidenced by the separation in size of the developing clutch from the reserve fund of chromatin nucleolar and perinucleolar oocytes ( Fig. 3 ). During the December trawl surveys, most ovaries contained oocytes that could be classified as late perinucleolar (Fig. 4). By May, however, a significant change in oocyte stage had occurred; many ovaries contained cortical alveoli organized into a ring structure and showed increased oocyte size and oil droplets forming around the nucleus (Fig. 5). Because the oocyte stage observed in summer appeared to be a natural holding point in development, we classified a fish with such a stage as perinucleolar. When we saw the same develop- ment in fish in the autumn surveys they were classi- fied as nonspawners. Only those fish with a prolif- eration of cortical alveoli and oil droplets that had begun to form around the nucleus were classified as being at or beyond the yolk vesicle stage and there- fore counted as spawners for the coming season. Table 1 shows the proportion of fish at each stage in each of the surveys. In both December samples, most or all fish were classified as perinucleolar. In contrast, in the May surveys, only 45.4% and 39.7% were in the perinucleolar stages in 1992 and 1993 respectively. For fish caught during the trawl survey in May 1993, stage of development of the ovary of each fish in the histological sample was also evaluated mac- roscopically . The number of fish at each histological stage and at each stage of macroscopic gonad devel- opment are presented in Table 3. In total, of the 237 ovaries classified as maturing, only two were classi- fied histologically as nonspawners. However, of the 899 ovaries classified as resting, 450 were classified histologically as nonspawners and 449 as pre- spawners. This finding confirms that physical devel- opment for spawning begins before it is apparent macroscopically in the ovaries and reinforces the re- quirement for histological methods of analysis. The proportion of prespawners in each stratum was estimated from the aged histological samples from the May surveys (Table 4). If there were not at least two fish in a stratum of a particular age, the propor- tion was not estimated. Although there were many age-stratum combinations where the proportion of prespawners in a particular age class could not be estimated (mainly for the young fish and for the 1986 cohort), these were not generally found in strata that supported the greatest numbers of hoki of that age class (Table 4). 106 Fishery Bulletin 95(1), 1997 600 400 200 January o.o 0.2 0.4 0.6 0.8 1.0 600 February 400 200 0.0 0.2 0.4 0.6 0.8 1.0 600 April 400 200 0.0 0.2 0.4 0.6 0.8 1.0 600 400 200 May o.o 0.2 0.4 0.6 0.8 1.0 600 400 200 0 June 0.0 0.2 0.4 0.6 0.8 1.0 600 ju|y 400 200 0 0.0 0.2 0.4 0.6 0.8 1.0 600 August 400 200 0 0.0 0.2 0.4 0.6 0.8 1.0 Oocyte diameter (mm) Figure 3 Histograms of oocyte diameter of hoki from samples collected monthly on the Chatham Rise ( Jan-Jun 1994) and west coast of the South Island ( Jul-Aug 1994) by the commercial fleet. Proportion of each age class developing to spawn Table 4 shows the estimated proportion of pre- spawners in each age class in the 1992 and 1993 May surveys. Table 4 also shows the percentage of fish at each age that were in strata where the proportion of developing fish could be measured (i.e. had at least two fish in the sample). Where this is less than 50%, the estimate is based on fish from only a small frac- tion of the population, and should not be used. Where it is less than 66%, the estimate might be considered unreliable. In 1992 there were enough data to make a reliable estimate of the proportion of prespawners Livingston et al.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae 107 Figure 4 Resting ovary with oocytes in the late perinucleolar stage, December 1991. (ep = early perinucleolar stage oocyte, lp = late perinucleolar stage oocyte (some yolk vesicles present)). for ages 5, 7, 8, 9, and 10+, but age 6 (the 1986 cohort) and ages 1,2,3, and 4 could not be estimated because the strata with samples contained less than half of the total number of fish in the survey area. In 1993 there were enough data to make a reliable estimate of the proportion of prespawners for ages 5, 6, 8, 9, and 10+, but ages 3 and 7 (again, the 1986 cohort) were unreli- able and ages 1, 2, and 4 could not be estimated. The estimates of prespawners in each age class for the two surveys are shown in Figure 6. Although there were too few samples to obtain reliable estimates for fish of age 4 and under, it is clear that this propor- tion would be small. Only 3 of 42 (7%) fish in the sample that were age 4 or younger were classified as prespawners. It is therefore likely that the ogive in- creases steeply below age 5 before levelling off. Proportion of adult fish spawning Figure 6 suggests that there may have been some increase in the proportion of prespawners up to age 8 in 1992 but that in 1993 the numbers of pre- spawners did not increase after age 5. If we assume that any increase after age 7 is not significant, then the asymptotic values of the ogives Table 3 Numbers of hoki classified in each histological stage com- pared with numbers of hoki classifed in each macroscopic stage (May 1993). Macroscopic stage Histological stage Resting Maturing Total Nonspawners 450 2 452 Chromatin Nucleolar 1 0 1 Perinucleolar 449 2 451 Prespawners 449 235 684 Yolk Vesicle 154 1 155 Vitellogenic 295 197 492 Ripe 0 37 37 Grand total 899 237 1,136 in Figure 6 represent the measured proportion of adult prespawners (p+) in the years 1992 and 1993. Following a procedure identical to that above, but con- sidering only fish aged 7 and over, the proportion of adult prespawners in the survey area was estimated as 66% in 1992 (SE 3%) and 65% in 1993 (SE 2%). 108 Fishery Bulletin 95 ( 1 ), 1997 0-1 mm. Figure 5 Histological sample from May 1993 showing some oocytes that had developed into the yolk vesicle stage. Note the circumnuclear distribution of the oil droplets that are beginning to form, (c = chorion, p = primary oocytes at chromatin nucleolar stage, rf = nondeveloping re- serve fund oocytes at perinucleolar stage, v = vesicle ring around nucleus of yolk vesicle stage oocyte). Table 4 Estimated total numbers of female fish, numbers of female fish in sampled strata, percentage of female fish in strata covered by sampling, numbers of prespawners, proportion of prespawners with standard error (SE), for each age class in the May surveys (NA indicates that the value could not be estimated). SE = standard error. Age class 1 2 3 4 5 6 7 8 9 10+ May 1992 Total in survey (x 1,000) 54 1,182 0 1,648 7,645 557 2,101 6,025 3,407 4,150 In sampled strata 0.00 511 0 691 7,458 130 1,478 5,973 3,387 4,132 % in sampled strata 0 43 NA 42 98 23 70 99 99 100 Prespawners (x 1,000) 0.00 0.00 0.00 132 3,333 35 844 4,065 2,323 2,652 Proportion spawning NA 0.00 NA 0.19 0.45 0.27 0.57 0.68 0.69 0.64 SE NA NA NA 0.10 0.05 0.26 0.08 0.05 0.05 0.05 May 1993 Total in survey (x 1,000) 95 3,573 142 135 2,614 6,986 338 1,777 4,340 4,447 In sampled strata 0 0 83 29 2,586 6,986 183 1,739 4,304 4,418 % in sampled strata 0 0 59 21 99 100 54 98 99 99 Prespawners (x 1,000) 0 0 0 0 1,649 3,928 123 1,143 2,967 2,782 Proportion spawning NA NA 0 0 0.64 0.56 0.67 0.66 0.69 0.63 SE NA NA NA NA 0.05 0.03 0.18 0.06 0.03 0.03 Livingston et al.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae 109 Table 5 Estimation of total proportion spawning ( p ) of age-7+ hoki based on (p+ and on the number of hoki in the plus group on the Southern Plateau in of hoki x 103, SE = standard error; M = natural mortality). the proportion of prespawners on the Southern Plateau December 1991, 1992, and May 1992, 1993. ( n = number M = 0 M = 0.25 M = 0.3 Year n (SE) n (SE) n (SE) 1992 Observed, December 1991 17, 945 (1,700) 17, 945 (1,700) 17, 945 (1,700) Observed, May 1992 15, 682 (1,400) 15, 682 (1,400) 15, 682 (1,400) Expected, May 1992 17, 945 (1,700) 16, 170 (1,500) 15, 837 (1,500) Missing, May 1992 2, 263 (2,200) 488 (2,100) 155 (2,100) Migration ratio 0.14 (0.16) 0.03 (0.14) 0.01 (0.14) Proportion spawning 0.70(0.05) 0.67 (0.05) 0.66 (0.05) 1993 Observed, December 1992 23, 250 (1,800) 23, 250 (1,800) 23, 250 (1,800) Observed, May 1993 10, 902 (1,700) 10, 902 (1,700) 10, 902(1,700) Expected, May 1993 23, 250(1,800) 20, 950(1,600) 20, 518 (1,600) Missing, May 1993 12, 348 (2,400) 10, 048 (2,300) 9, 616 (2,300) Migration ratio 1.13 (0.43) 0.92 (0.39) 0.88 (0.38) Proportion spawning 0.84(0.03) 0.82 (0.03) 0.81 (0.04) Age Figure 6 Proportion of prespawners at age from 1992 and 1993 surveys. The error bars indicate two standard errors. Table 5 shows the estimates of the total propor- tion of adult fish that will spawn (i.e. including those fish that have already left the area) calculated as above. The calculations were done with three esti- mates of natural mortality M, including the best es- timate M = 0.25, and two bounding values M - 0 and M = 0.3 (Sullivan et al.2). Table 5 shows that this makes little difference to the estimate of p. Stan- dard errors of the estimates were calculated by us- ing the resampling technique. The best estimate of the total proportion of adult fish that would have spawned in the 1992 winter sea- son was 0.67 (SE 0.05, with M - 0.25). If M is as high as 0.3 or as low as 0, the estimate of p decreases to 0.66 or increases to 0.70 respectively. The best esti- mate of the total proportion of adult fish that will spawn in the 1993 winter season was 0.82 (SE 0.03). If M is as high as 0.3 or as low as 0, the estimate of p is 0.81 or 0.84 respectively. Figure 7 shows the effect of the estimate of x on the estimate of p for 1992 and 1993. In each plot there are three curves for p as a function of x. The solid curve is the function given the estimated value of p+ (0.66 in 1992 and 0.65 in 1993). The two dot- Fishery Bulletin 95 ( 1 ), 1997 I 10 0.0 0.5 1.0 1.5 2.0 Ratio of fish migrated to fish in survey area (x) Figure 7 Estimated total proportion spawning (p) among age-7+ fish as a function of the migration ratio ( x ) in (A) 1992 and (B) 1993. ted curves are the functions at plus and minus two standard errors of this value. The solid vertical lines show the value of x with M = 0.25. The two dashed vertical lines shown are at plus or minus two standard errors of this value. Clearly the value of x is not at all well known. How- ever, the function is changing slowly over this range; therefore it is still possible to obtain a useful esti- mate of p. Discussion The number of studies that have attempted to mea- sure the level of nonspawning in adult fish and to determine its effect on population estimates used for stock assessment appears to be few. It is often as- sumed that although the steepness of the maturity ogive varies among species, it will always level out at or near 100% spawning (e.g. Hislop, 1984). Spe- cies documented to reach less than 100% spawning include orange roughy, Hoplostethus atlanticus, off southeast Australia at 55% (Bell et al., 1992), the brackish water burbot Lota lota (L) in the Baltic sea at 70% (Pulliainen and Korhonen, 1990), and the es- tuarine yellow-fin (surf) bream, Acanthopagrus aus- tralis at 50% (Pollock, 1984). Although the annual proportion of hoki that spawn is similar to that of these other species, the other studies did not adjust for population size or take migratory movements into account. Our estimates for hoki are close to the range indicated by Hurst and Schofield (1995) who did ad- just for population size. There were some potential sources of error that we could not measure. First, the number of undevel- oped fish surveyed on the Southern Plateau in May that were classified as nonspawners, which could have developed late and gone on to spawn, is un- known. This would lead to an underestimate of the proportion of prespawners. Second, if a number of fish remained undeveloped but migrated to the spawning ground anyway, the number of fish that Livingston et a I.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae would leave the Southern Plateau after May would be underestimated. Third, the number of developing fish in May that could have resorbed their eggs and not gone on to spawn after all could lead to an over- estimation of the total proportion spawning. Other sources of error concern changes in catchability and vulnerability between surveys and the difficulty of detecting any bias or size selectivity when sampling a population with the trawl. With regard to the first source of error above, it was encouraging that hoki collected in April showed significant signs of development compared with those collected in February (Table 2). The most likely fish to be affected by late development are the younger, smaller fish because they spawn later in the season compared with the older, larger fish which spawn at the beginning of the season (Langley, 1993). Because we estimated the proportion of hoki age 7 years and above that were spawning, the problem was minimized. With regard to the second source of potential er- ror, undeveloped hoki of age 4 and greater are not caught on the spawning grounds during the spawn- ing season (Langley, 1993), suggesting that there may be 100% spawning among hoki that migrate to the west coast spawning grounds. We have no data on the number of fish that could resorb their eggs be- fore the spawning season, but none were found in the May samples in this study. With regard to the trawl survey technique, it is possible that there are systematic changes in catchability or vulnerability between December and May. We considered it more likely, however, that the changes in fish numbers were real, and that some fish had already migrated away from the Southern Plateau before the May survey, particularly in 1993 when the survey took place later than in 1992. The ratio of the number of missing fish to the num- ber of fish present on the Southern Plateau (the mi- gration ratio, x) can also be expressed in terms of the proportion of fish that have already migrated (p ) x = -P *- !-Pm or, equivalently x Both x and pm change as fish leave the Southern Pla- teau. In December, when no fish have migrated, both x = 0 and pm = 0. If a survey were to be done at a point when 33% of the fish had migrated, or pm = 0.33, there would be one fish missing for every two fish still in the survey area, and x would be 0.5. By July 1993, when an estimated 82% of fish have gone to spawn, pm is 0.82 and x has increased to 4.6. Therefore, in May 1993, when x - 0.92 and Pm = 0.48, we can estimate that more than half (58.5%) of the fish that were going to spawn had already gone. This is, of course, poorly estimated, as is x, but it is higher than was expected before the surveys were done. If the survey results are correct, they suggest that in May 1993, fish had already started to mi- grate in large numbers. The survey in May 1993 be- gan two weeks later in the year than that in 1992, indicating that May is a critical time for the spawn- ing migration of hoki. This interpretation is sup- ported by the change in distribution of fish from the east to the west between December and May in 1993, but not in May 1992. If, however, our estimates of the numbers of fish that have migrated away from the Southern Plateau by May are incorrect (e.g. be- cause of changes in catchability or vertical availabil- ity between December and May, or because not all fish have begun to develop by May), then the propor- tion of prespawners on the Southern Plateau in May could be used as a lower limit of the proportion spawning of the total population. Given that a stan- dardized trawl survey technique was the best method available to us to sample the adult hoki population, we believe that it would be difficult to improve on the estimates of proportion spawning obtained. The 4-6 yr age classes show different proportions of prespawners in each year, with more 4 year olds but with fewer 5 and 6 year olds developing to spawn in 1992 than those in 1993 (Fig. 6). Differences in the proportion of spawning fish in the younger age classes could also relate to the preceding spawning history of a particular age class. Although every year many hoki spawn on the west coast of the South Island, it is clear that a large num- ber of individuals do not. Species that exhibit such behavior usually have a major accessory activity that requires a significant amount of energy in addition to spawning itself (Bull and Shine, 1979). Hoki mi- grate over vast distances of about 1,500 km from the Southern Plateau to the west coast spawning grounds. The energy cost incurred during migration may be so high that there is not enough left for egg production the following year. Lack of food and migration distance have been suggested as reasons for lack of spawning among orange roughy (Bell et al., 1992) and yellow fin bream (Pollock, 1984). However, Pulliainen and Korhonen (1990) found that nonspawning burbot maintained a condition similar to that of spawning burbot and ruled out low food supplies as an explanation for nonspawning adults. Within species where different populations show different levels of nonspawning, it has been found Fishery Bulletin 95( 1 ), 1997 1 12 that the lowest frequencies are usually associated with increased stress, such as poorer quality habi- tat, food shortage, or a shorter growing season (Bull and Shine, 1979). Nonspawning condition has been induced experimentally for several species by reduc- ing their food supply (e.g. haddock [Hislop et al., 1978]; Newfoundland winter flounder [Burton and Idler, 1987]; and plaice [Norwood et al., 1989]). Nutrients are in good supply and do not limit pri- mary production on the Campbell Plateau (which forms a major portion of the Southern Plateau sur- vey area); however, chlorophyll concentrations over depths of 450 m or greater are generally low (Heath and Bradford, 1980). Heath and Bradford suggest that because of this and other characteristics of the area, there never will be a well-developed zooplank- ton community with a high biomass on the Campbell Plateau. Areas of higher productivity are found on the island shelves and shallow rises in the area, or downstream from the Campbell Plateau itself. The energetics of the food chain in the study area are not known. It is possible that the lack of high primary and secondary productivity in the area contributes toward nonreproduction in some hoki from year to year. Whatever the cause, nonspawning among adult hoki has important implications for stock assessment and risk estimation in the management of New Zealand hoki stocks. The proportion of fish that migrate to spawn is a scaling factor that relates the number of fish observed during the spawning season (using tools such as CPUE and acoustics) to the total population. It also provides a buffer between the total stock and the population vulnerable in any one year to the greatest fishing effort that is applied during the spawning season. Further, it reduces the stock size, which is needed for calculating the stock-recruit rela- tionship used in predicting future recruitment. The effect of these factors may be minimal if the level of nonspawning fish is constant. If, however, the proportion spawning varies from year to year, as suggested by our study, the implications for model- ling may be both complex and important. It is likely that there are other species not neces- sarily related to hoki that could also have signifi- cant and variable proportions of nonspawning fish. There may therefore be major implications for the stock assessment of those species as well. Any stock assessment tool that is used to obtain an estimate of absolute abundance from a spawning population (e.g. acoustics, egg-production method) should take nonspawning into account. It is also important that the effect of nonspawning on any stock-recruitment relationship (assumed or measured) be taken into account because one of the more serious difficulties in determining the stock-recruitment relationship of any species is obtaining a reliable measure of the spawning stock size (Hilborn and Walters, 1992). Further, the stock-recruitment dynamics of a popula- tion could be masked, particularly if the level of nonspawning is correlated to some environmental fac- tor or autocorrelated because of some inherent life strat- egy, such as improved longevity or increased egg size. We have shown that nonspawning among adult hoki is substantial, and it has important conse- quences for stock assessment. For these reasons, it is clear that a better understanding of its variabil- ity, and how widespread its occurrence might be among other species, would be useful for fisheries management worldwide. Acknowledgments We thank Peter Horn for ageing the samples; Kevin Sullivan, Patrick Cordue, and Len Tong for useful discussions; the scientific staff who participated in the trawl surveys; the officers and crew of GRV Tangaroa\ and the scientific observers for collecting samples from commercial vessels. We also thank three anonymous reviewers for their comments and suggestions. Literature cited Bell, J. D., J. M. Lyle, C. M. Bulman, K. J. Graham, G. M. Newton, and D. C. Smith. 1992. Spatial variation in reproduction and occurrence of non-reproductive adults in orange roughy, Hoplostethus atlanticus Collett (Trachichthyidae), from south-eastern Australia. J. Fish Biol. 40:107—122. Bull J. J., and R. Shine. 1979. Iteroparous animals that skip opportunities for reproduction. Am. Nat. 114(2):296-316. Burton, M. P., and D. R. Idler. 1987. An experimental investigation on the non-reproduc- tive, post-mature state in winter flounder. J. Fish Biol. 30:643-650. Chatterton, T. D., and S. M. Hanchet. 1994. Trawl survey of hoki and associated species in the Southland and Sub-Antarctic areas, November-December 1991 (TAN9105). N.Z. Fish. Data Rep. 41, 55 p. Foucher, R. P., and R. J. Beamish. 1980. Production of non-viable oocytes by Pacific hake (Merluccius productus). Can. J. Fish. Aquat. Sci. 37:41-48. Francis, R. I. C. C. 1984. An adaptive strategy for stratified random trawl surveys. N.Z. J. Mar. Freshwater Res. 18:59-71. Gunn, J. S., B. D. Bruce, D. M. Furlani, R. E. Thresher, and S. J. M. Blaber. 1989. Timing and location of spawning of blue grenadier Macruronus novaezelandiae (Teleosti: Merlucciidae) in Australian coastal waters. Aust. J. Mar. Freshwater Res. 40:97-112. Livingston et al.: Estimating the annual proportion of nonspawning adult Macruronus novaezelandiae I 13 Heath, R. A., and J. M. Bradford. 1980. Factors affecting phytoplankton production over the Campbell Plateau, New Zealand. J. Plankton Res. 2:169- 181. Hilborn, R., and C. J. Walters. 1992. Quantitative fisheries stock assessment: choice, dynam- ics and uncertainty. Chapman and Hall, London, 570 p. Hislop, J. R. G. 1984. A comparison of the reproductive tactics and strate- gies of cod, haddock, whiting and Norway punt in the North sea. In G. W. Potts, and R.J. Wootton, (eds.), Fish repro- duction strategies and tactics, p. 311-329. Academic Press, London. Hislop, J. R. G., A. P. Robb, and J. H. Gauld. 1978. Observations on effects of feeding level on growth and reproduction in haddock Melanogrammus aeglefinus (L) in captivity. J. Fish Biol. 13:85-98. Horn, P. L., and K. J. Sullivan. 1996. Validated aging methodology using otoliths, and growth parameters for hoki ( Macruronus novaezelandiae ) in New Zealand waters. N.Z. J. Mar. Freshwater Res. 31:161-174. Horwood, J. W., M. G. Walker, and P. Witthames. 1989. The effect of feeding levels on the fecundity of plaice {Pleuronectes platessa). J. Mar. Biol. Assoc. U.K. 69:81-92. Hurst, R. J., and K. A. Schofield. 1995. Winter and summer trawl surveys of hoki south of New Zealand, 1990. N. Z. Fish. Tech. Rep. 43, 55 p. Ingerson, J. K. V., S. M. Hanchet, and T. D. Chatterton. 1995. Trawl survey of hoki and associated species in the Southland and Sub-Antarctic areas, November-December 1992 (TAN9211). N.Z. Fish. Data Rep. 66, 44 p. Kerstan, M., and D. Sahrhage. 1980. Biological investigations on fish stocks in waters off New Zealand. Mitteilungen Inst. Seefischerei Bundes- forschungsanstalt Fischerei 29, Hamburg, 187 p. Kuo, C. L., and S. Tanaka. 1984. Maturation and spawning of Hoki Macruronus novaezelandiae (Hector) in waters around New Zea- land. Bull. Jpn. Soc. Sci. Fish. 50:397-402. Langley, A. D. 1993. Spawning dynamics of hoki in the Hokitika Canyon. N.Z. Fish. Tech. Rep. 34, 29 p. Livingston, M. E., and K. A. Schofield. 1996. The discrimination of hoki groups ( Macruronus novaezelandiae) in New Zealand waters using morph- ometries. N.Z. J. Mar. Freshwater Res. 31:197-208 Livingston, M. E., Y. Uozumi, and P. H. Berben. 1991. Abundance, distribution and spawning condition of hoki and other mid-slope fishes on the Chatham Rise July 1986. N.Z. Fish. Tech. Rep. 25, 47 p. Pollock, B. R. 1984. Relations between migration, reproduction and nu- trition in yellowfin bream Acanthopagrus australis. Mar. Ecol. Prog. Ser. 19:17—23. Pulliainen, E., and K. Korhonen. 1990. Seasonal changes in condition indices in adult ma- ture and non-maturing burbot Lota lota ( L. ), in the north eastern Bothnian Bay, northern Finland. J. Fish Biol. 36:251-259. Schofield, K. A., and M. E. Livingston. 1994a Trawl survey of hoki and associated species in the Southland and Sub-Antarctic areas, April-May 1992 (TAN9204). N.Z. Fish. Data Rep. 45, 38 p. 1994b Trawl survey of hoki and associated species in the Southland and Sub-Antarctic areas, May-June 1993 (TAN9304). N.Z. Fish. Data Rep. 47, 39 p. van den Broek, W. L. F., K. Tokusa, and H. Kono 1981. A survey of demersal fish stocks in waters south of New Zealand, March-May 1982. Fish. Res. Div. Occas. Publ. 44, 51 p. West, G. 1990. Methods of assessing ovarian development in fishes: a review. Aust. J. Mar. Freshwater Res. 41:199-222. Structure and dynamics of the fishery harvest in Broward County, Florida, during 1 989 James E. McKenna Jr. Florida Marine Research institute, Florida Department of Environmental Protection 100 Eighth Ave. S E , St, Petersburg, FL 33701 Present Address: Tunison Laboratory of Aquatic Science National Biological Service, 3075 Grade Road, Cortland, New York 13045 E-mail address: 76743. 1 403@compuserve.com Abstract .—Florida’s rich fisheries are among the state’s most valuable resources, attracting the interest of fishermen, divers, and others. Commer- cial and recreational exploitation of these resources has altered the abun- dances of some valuable species; con- sequently fishery regulations and a sys- tem to monitor landings have evolved in response. Until now, the biological structure of the multispecies harvest has not been examined. Landings from commercial trips in Broward County during 1989 were used to describe the structure and seasonal dynamics of that harvest. Cluster analysis classified fishing trips into distinct groups associated with dif- ferent habitats and gear. Swordfish landings dominated this low-diversity harvest. There were significant sea- sonal changes in the species assem- blages landed. However, most species associations were weak and negative. The observed structure of the Broward County harvest reflects the selectivity inherent in commercial fishing. It is a balance between the differential avail- ability of various species to the gear used and the market values driving the fishermen to select some species and discard others. Seasonal changes in the harvest structure reflect changes in the availability of various species and in the fishermen’s ability to adapt to these changes by switching to alternate tar- get species. The strong biases intro- duced by the selectivity of this system can obscure events in the natural sys- tem and provide little insight into the changes in the natural fish community. Manuscript accepted 4 September 1996. Fishery Bulletin 95:114—125 (1997). Florida waters are rich in fish and shellfish. Although the greatest di- versity is found in coral reef habi- tats (Starck, 1968), hundreds of spe- cies are found throughout Florida’s marine waters (Anderson and Geh- ringer, 1965; Herrema, 1974; Gil- more, 1977). Statewide, commercial landings are reported to the Depart- ment of Environmental Protection by using 531 different species codes, some of which represent groups of species. This assemblage is two to three times larger than any other state’s marine fishery resource. Historically, fish communities containing commercially valuable species have been strongly influ- enced by human activities, espe- cially fishing (e.g. Cushing, 1961; Idyll, 1973; Beddington and May, 1982; Gulland, 1983; Beddington, 1986; Sissenwine, 1986; Laevastu and Favorite1). Florida’s fishery re- source has been intensely exploited both commercially and recrea- tionally for many years (e.g. Naka- mura and Bullis, 1979; Newlin, 1991). Bobnsack et al. (1994) have provided a good description of the complexity of Florida fisheries, ex- plored the effects of exploitation, and discussed the difficulties in in- terpreting available landings data. The effects of exploiting this multi- species resource have been demon- strated for a few valuable species (Spanish mackerel: Williams et al.2; king mackerel: Fable, 1990; spiny lobster: Moe, 1991; red drum: Goodyear3; billfishes: Anonymous4; swordfish: Anonymous5; red grouper: Goodyear and Schirripa6). However, 1 Laevastu, T., and F. Favorite. 1978. The control of pelagic fishery resources in the eastern Bering Sea. Northwest and Alaska Fisheries Science Center, National Marine Fisheries Service, 7600 Sand Point Way NE, Seattle, WA98115. Proceedings report. (Manuscript.) 2 Williams, R. O., M. D. Murphy, and R. G. Muller. 1985. A stock assessment of the Spanish mackerel, Scomberomorus macula- tus, in Florida. Unpublished, third draft. Prepared for the Florida Marine Fisheries Commission, 2540 Executive Center, Circle West, Tallahassee, FL 32301, 65 p. 3 Goodyear, C. P. 1987. Status of red drum stocks in the Gulf of Mexico. Contribution report CRD 86/87-34, Southeast Fisheries Science Center, National Marine Fisheries Service, NOAA, 75 Virginia Beach Dr., Mi- ami, FL 33149, 49 p. 4 Anonymous. 1982. Draft fishery man- agement plan, draft environmental impact statement, and regulatory impact review for the Atlantic billfishes: white marlin, blue marlin, sailfish and spearfish. South Atlantic Fishery Management Council, 1 South Park Circle, Suite 306, Charleston, SC, report G#27 BF Fmv/k 8/82, 64 p. 5 Anonymous. 1991. Reference paper on 1991 swordfish stock assessments by SCRS swordfish assessment group. Miami Laboratory, Southeast Fisheries Science Center, NMFS, NOAA, Miami, FL, rep. SCRS/91/16, 193 p. 6 Goodyear, C. P., and M. J. Schirripa. 1991. The red grouper fishery of the Gulf of Mexico. Miami Laboratory, Southeast Fisheries Science Center, NMFS, NOAA, 75 Virginia Beach Dr., Miami, FL 33149. Contribution rep. MIA-90/91-86, 79 p. McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida 1 15 only anecdotal reports of associations among harvested species exist. Concern about the effects of harvesting this re- source has stimulated research on the structure and dynamics of Florida’s marine fish community. Struc- ture of the harvest is related to the structure of the natural community and to the economics influenc- ing fishermen’s behavior. It integrates the differen- tial fishing mortality affecting the natural commu- nity. In 1984, the Florida Department of Environ- mental Protection (FDEP) established a trip-ticket reporting system (Marine Fisheries Information Sys- tem) to monitor the fishery harvest and provide a database of basic information on commercial land- ings from this resource. All dealers buying fish from fishermen or fishing for themselves must report the amounts of all species landed on each fishing trip. These records normally represent those species brought to shore on a single fishing trip and sold (landed). They do not include species or individuals caught and subsequently discarded, those used as bait, or those brought to shore but not sold. Harvest- ing occurs in many different ways (e.g. traps, nets, hook-and-line) and can have a variety of effects on the fishery resource and the natural community as a whole. A better understanding of the multispecies resource and the potential effects of the harvest can be gained by relating the structure and its variabil- ity to what is known about the natural fish assem- blage and the harvesting behavior of fishermen. This study uses commercial landings data collected by the Marine Fisheries Information System (MFIS) to ex- amine the structure and temporal dynamics of the har- vest in Broward County, Florida (Fig. 1), during 1989. Methods The MFIS database includes, but is not limited to, information on the weight of each species landed from each commercial fishing trip, the date on which those landings occurred, and the time spent fishing. Infor- mation on depth and fishing area were provided on a voluntary basis in 1989 but the spaces for report- ing such information were often left blank by report- ing dealers. Information on gear used was not provided. All commercial landings reported from Broward County during 1989 were used in this analysis. This subset of the MFIS database was chosen for this study because Broward County fisheries landings are some of the most valuable in the state. Furthermore, because 3,246 landings records (out of more than 2.5 million) exist, it is computationally one of the most manageable data sets available. Each month of data was analyzed separately in an attempt to detect sea- sonal trends in species assemblage structure. Monthly assemblages were constructed on the basis of total monthly landings of each species. Fishery Bulletin 95( 1 ), 1997 I 16 Diversity was described by using the Shannon- Wiener Information Index ( H' ) (Shannon and Weaver, 1949) and its component parts, richness (number of species) and evenness (V') (Pielou, 1977). H' = ~YJPil°Se Pi V' = H'/ loge s, where p; is the proportion of species i and s’ is the number of species in the entire community (Pielou, 1977). In this study, the value of s* was set to the total number of species landed in a given month when calculating evenness per ticket, and to the total num- ber of species (76) landed over the entire year when calculating evenness per month. Heterogeneity ratios (HR) (HR actually measures beta diversity, which is an index of dissimilarity, Kobayashi, 1987) were calculated to measure the similarity between all pair-wise comparisons of monthly assemblages. All pair-wise combinations of assemblages were tested for significant differences (a=0.05) by using a Monte Carlo simulation tech- nique that compares the observed number of species common to the two assemblages of interest with that expected from randomly extracting two assemblages (each having the same number of species as one of the observed assemblages) from the community as a whole (FAUNSIM) (Raup and Crick, 1979; McKenna and Saila, 1991). A nonhierarchical cluster analysis (SAS FASTCLUS, SAS, 1985) was applied to classify the trips according to the species assemblage landed each month. A maximum of 3 iterations and 20 clusters were specified. No minimum radius was specified. The REPLACE = option was set to RANDOM so that a simple pseudorandom sample of observations was chosen as initial cluster seeds. The DRIFT option was specified to adjust cluster seeds to their cluster mean each time an observation was added. Spearman’s rank correlation analysis was per- formed on every pair-wise combination of monthly species landings, on a trip-by-trip basis, to test for significant associations. A Z-test was applied to de- termine the significance of each correlation at the 0.01 level (Freund, 1970, p. 311-313). Results A total of 1,355,421 kg (2,981,926 pounds) of finfish and shellfish were landed in Broward County during 1989 according to the 3,246 commercial fishing trips reported (Table 1). The monthly average was 112,840 kg (248,247 pounds) ranging from 41,889 kg (92,156 pounds) to 178,489 kg (392,675 pounds) (Fig. 2). Geography of the harvest Florida’s commercial fishing fleet is, in general, artisanal ( small boats operating near shore). In 1989, 907 saltwater products licenses, 61 wholesale dealer licenses, and 408 retail dealer licenses were issued to residents of Broward County. Most fishermen worked in the waters immediately adjacent to the Broward County coast. The fishing area (Fig. 1) was reported for 76% of the trips landing fish in Broward County. According to those trip-tickets that included fishing area, fishermen harvested from areas 741 (45%) or 744 (37%) on 82% of fishing trips. The num- ber of trips diminished as one moved away from the Broward County coast. Fish were caught from areas as far north as the waters off Indian River County (area 736), as far south and west as the Tortugas (area 2). Rarely, landings were reported from waters of Florida Bay off mainland Monroe County (area 3). Diversity Diversity of landed species was low in comparison with the natural diversity of this subtropical com- munity. A total of 76 species (or groups of species) were landed in Broward County in 1989. Diversity (H’) of the total harvest was 1.86; evenness (V') was 0.43. Mean monthly diversity (1.88) was almost iden- tical to that for the total harvest (Table 2). Monthly evenness values varied between 0.31 and 0.62, a mean of 0.43. Diversity and evenness followed roughly sinusoidal patterns throughout the year, with peaks in September (H'= 2.69, V'=0.62) approxi- mately double the minimum value in May (H'=1.36, V'=0.31). Monthly richness approached 50 species most of the year; June (36) and July (44) had the lowest values, April (54) and November (55) had the highest values. Despite the fact that the waters off Broward County contained a relatively rich (at least 76 species commercially harvested) multispecies fish community, as many as half of all trips in any given month landed only a single species. Mean alpha di- versity (defined here as diversity based on landings from a single fishing trip) was low (0.44) and showed little variability (Fig. 3). It was greatest in January and February and dropped to about 60% of those values for the rest of the year. Richness displayed a very small range (2. 5-3. 5 species per trip). Similarity Beta diversity (HR) among pair-wise comparisons of monthly assemblages ranged from 1.05 to 1.26 (Table 3). The species assemblages landed in March and September were most similar and those landed in McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida I 17 Table 1 Fish and shellfish landed in Broward County, Florida, during 1989. Species groups are identified by the following: BC = blue crab, BF = bait fishes, GS = grouper-snappers, ID = inshore demersals, IP = inshore pelagics, LB = lobsters, OD = offshore demersals, OP = offshore pelagics, SC = stone crab, SH = shrimps, and UM = unidentified miscellaneous fishes. Golden crab landings are included in the category “misc. invertebrates.” Species or complex Weight (lb) Group Species or complex Weight (lb) Group Amberjack 4,315 OP Sea bass, mixed 327 ID Bait Fish 246 BF Shark 59,352 OP Ballyhoo 23,204 BF Shark fins 35 OP Bluefish 38 IP Sheepshead 120 ID Bluerunner 813 BF Hogfish 6,895 ID Bonito (little tunny) 161 OP Snapper, lane 833 GS Bumper, Atlantic 94 BF Snapper, mangrove 3,335 GS Cobia 1,350 OP Snapper, mutton 29,555 GS Croaker 434 ID Snapper, red 337 GS Dolphin 32,704 OP Snapper, silk 68 GS Eels 20 ID Snapper, vermilion 3,358 GS Goggle eye or scad 1,510 BF Snapper, yellowtail 23,443 GS Grouper, black 33,255 GS Snapper, mixed 11,438 GS Grouper, gag 6,155 GS Snapper, other 1,102 GS Grouper, Nassau 40 GS Spot 40 ID Grouper, red 10,612 GS Swordfish 811,896 OP Grouper, scamp 194 GS Tilefish, golden 320 OD Grouper, snowy 939 GS Tilefish, gray 800 OD Grouper, Warsaw 764 GS Triggerfish 2,289 ID Grouper, yellowedge 74 GS Tuna, bigeye 66,437 OP Grouper, yellowfin 8 GS Tuna, blackfin 457 OP Jewfish 232 GS Tuna, bluefin 2,530 OP Grouper, mixed 527 GS Tuna, skipjack 15 OP Grouper, other 1,496 GS Tuna, yellowfin 58,626 OP Grunts 2,700 ID Tuna, mixed 499 OP Jack, crevalle 2,304 IP Wahoo 759 OP Jack, mixed 1,075 IP Whiting 37 ID Jack, other 392 IP Misc. food fish 57,240 UM Mackerel, king 25,848 OP Misc. industrial fish 730 UM Mackerel, Spanish 723 IP Total finfish 1,299,945 Menhaden (pogies) 218 IP Crabs, blue (hard) 7,564 BC Mojarra 697 ID Crabs, stone, large 265 SD Mullet, black 9 IP Lobster, Spanish 225 LB Mullet, silver 1,451 IP Lobster, spiny 37,068 LB Permit 1 IP Octopus 163 ID Pinfish 2 BF Shrimp, pink 5,280 SH Pompano 378 IP Shrimp, bait 453 SH Porgies 2,092 ID Misc. invertebrates Total invertebrates Grand total 4,458 55,475 1,355,421 OD February and June were least similar. About half (28 out of 66) of all possible unique pair-wise compari- sons revealed significantly different assemblages (Table 3). These differences were usually due to changes in the proportion of landings contributed by swordfish and to the prevalence of lobster and baitfish. The composition of the assemblage landed in a particular month varied considerably. The assem- blage landed during any given month was signifi- cantly different from those of as few as two to as many as nine of the other eleven months. Assemblages of adjacent months were not significantly different, with the exception of July-August (owing to a sharp drop in swordfish landings and to a large increase in lob- ster landings at the beginning of the season) and September-October (owing to a sharp increase in swordfish landings and a general reordering of the dominance of other species) (Table 3). October was different from all months, except November and De- cember, and July was different from all months, ex- cept June and September. January was different only Fishery Bulletin 95 ( 1 ), 1997 1 18 from July and October, whereas December was dif- ferent only from February and July. Generally, there were significant differences between winter (large Table 2 Monthly diversity of commercial fisheries landings in Broward County, Florida, during 1989. H' = Shannon- Wiener diversity; V' = evenness; R = species richness. Month H' V' R Jan 1.73 0.40 48 Feb 2.04 0.47 50 Mar 1.49 0.34 52 Apr 1.49 0.34 54 May 1.36 0.31 51 Jun 1.41 0.32 36 Jul 1.90 0.44 44 Aug 2.66 0.62 48 Sep 2.69 0.62 52 Oct 2.35 0.54 54 Nov 1.90 0.44 55 Dec 1.56 0.36 53 Mean 1.88 0.43 50 proportion of swordfish and other offshore pelagics) and late summer-fall (relatively small proportions of offshore pelagics with a mix of species from other groups) assemblages. Classification Cluster analysis classified trips on the basis of the similarity of the species assemblages landed. I used an artificial, but operational, system of general habi- tat associations and species complexes to classify species into eleven groups (Table 1). Groupers and snappers inhabit a wide variety of habitats (Smith, 1976; Robins et al., 1986) and were frequently landed together. They were assigned to their own group rather than limited to a single habitat. Similarly, bait fish often formed a unique cluster and thus a “bait fish” group was used. “Miscellaneous food/industrial fish (UM)” was a “catch all” group used by fishermen to report species that were not explicitly given an identification code by the MFIS. It usually included such species as angelfishes (Pomacanthidae), parrotfishes (Scaridae), butterfish ( Peprilus spp.), JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Month £§ Offshore Pelagics 0 Grouper-snappers ^ Bait Fishes ® Lobsters ■ Unknown Misc. □ Other Figure 2 Total monthly landings in Broward County, Florida, during 1989 and the contribution of each spe- cies group. See Table 1 for species within each group. Each segment of each vertical bar represents the portion of total landings attributable to one of the five major groups (offshore pelagics, grouper- snappers, lobsters, bait fishes, and unknown miscellaneous fishes) or other species. McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida I 19 spadefish ( Chaetodipterus faber), and tripletail ( Lobotes surinamensis). The persistence of each group in the harvest var- ied throughout the year. Offshore pelagics (OP, e.g. swordfish, Xiphias gladius) and tuna (Thunnus spp.), grouper-snapper (GS), bait fish (BF, e.g. ballyhoo, Hemiramphus brasiliensis ), and unknown miscella- neous (UM) fishes occurred in each month. Lobsters (LB ) occurred during each month of the open season (August-March) but declined steadily from the open- ing of the season. Blue crabs (BC, Callinectes sapidus) occurred in summer and fall (May-September and December). Inshore pelagics (IP, e.g. mullet, Mugil spp.) occurred in January, March, April, and August. Stone crabs (SC, Menippe mercenaria) and inshore demersals (ID, e.g. sheepshead, Archosargus probatocephalus) were landed in November and De- cember. Shrimps ( Penaeus spp.) occurred in Febru- ary and April. Offshore demersals (OD, e.g. tileflsh [Malacanthidae]) occurred only in July. Offshore pelagics (OP) accounted for the largest proportion of landings in all months (Fig. 2). They also accounted for the majority of landings on most of the fishing trips from May through July, again in October and November. Groupers and snappers ac- counted for much of the remaining landings and dominated trips in January and December. Together the offshore pelagics (OP) and the grouper-snappers (GS) accounted for over 80% of landings in all months, except August, September, and October. The addi- tion of lobster (LB) landings raises the totals for August and October to more than 80%. Inclusion of bait fish landings helps to account for more than 80% of September landings. Unknown miscellaneous (UM) fishes account for most of the remaining land- ings in each month. Species associations Despite the classification of landings (trip assem- blages) into distinct groups of species assemblages, associations between individual species were weak. Less than 4% of the unique pair-wise comparisons of species occurrence in any given month were signifi- cant. One fourth to half of these accounted for more than 50% of the variation in their ranked abun- dances. Four associations accounted for more than 70% and only one association accounted for more than 80% of the variation in correlated species abun- dances. Roughly half of the significant associations were positive. However, most of these were between two uncommon (landed on less than ten trips per month) species. Only the swordfish-tuna association was consistently strong (r'>50%) and positive. Mut- ton snapper ( Lutjanus analis) was positively associ- ated with black grouper ( Mycteroperca bonaci), mojarras ( Gerreidae), and a number of other species, but these associations were not evident in every month and were usually weak (r'<50%). Only significant as- sociations are considered in the following discussion. Table 3 Dissimilarity and probabilities in comparing all pair-wise combinations of species assemblages landed in Broward County, Florida, during each month of 1989. The upper half matrix contains the dissimilarity values based on the Heterogeneity Ratio (HR), which is a measure of beta diversity. The lower half matrix contains the associated probabilities, generated by faunal similarity analysis (FAUNSIM), that the number of species observed to be common to each pair was less than that expected. Values in boldface are significant at the 0.05 level or greater. Month HR 1 2 3 4 5 6 7 8 9 10 11 12 1 — 1.057 1.073 1.107 1.084 1.179 1.186 1.143 1.104 1.181 1.112 1.112 2 0.07 — 1.071 1.141 1.157 1.262 1.201 1.118 1.137 1.176 1.109 1.128 3 0.53 0.40 — 1.069 1.101 1.233 1.149 1.087 1.050 1.150 1.089 1.089 4 0.80 0.99 0.43 — 1.098 1.205 1.182 1.101 1.064 1.163 1.158 1.105 5 0.38 0.99 0.67 0.75 — 1.160 1.161 1.137 1.098 1.124 1.081 1.082 6 0.71 1.00 0.98 0.87 0.47 — 1.161 1.159 1.201 1.245 1.234 1.212 7 1.00 0.99 0.95 1.00 0.99 0.62 — 1.157 1.137 1.229 1.212 1.151 8 0.93 0.80 0.66 0.73 0.99 0.66 1.00 — 1.063 1.187 1.130 1.089 9 0.80 0.97 0.33 0.51 0.86 0.96 0.83 0.10 — 1.148 1.110 1.090 10 1.00 1.00 1.00 0.99 0.95 0.99 1.00 1.00 0.99 — 1.115 1.095 11 0.76 0.88 0.75 1.00 0.58 0.97 1.00 0.94 0.96 0.86 — 1.070 12 0.77 0.96 0.75 0.84 0.50 0.90 0.95 0.66 0.88 0.67 0.44 — FAUNSIM probability 120 Fishery Bulletin 95( 1 ), 1997 -0 1 2 3 4 5 6 7 8 9 10 11 12 720 a- CD Month Figure 3 Monthly values of alpha diversity for trips landing fish in Broward County during 1989. Species richness, evenness, and the Shannon- Wiener information index are represented by these high-low graphs. Each horizontal tick mark indicates the mean value. Each vertical bar indicates one standard error. Solid rectangles represent the num- ber of fishing trips associated with each high-low bar. A strong, positive association between swordfish and both bigeye tuna ( Thunnus obesus ) and yellowfin tuna ( Thunnus albacares ) existed in spring and fall. In De- cember, the association between each of these tunas and swordfish accounted for over 70% of the variations in their landings. Swordfish showed significant negative as- sociations with shark in the early part of the year and with dolphin ( Coryphaena hippurus ) throughout the year. In 1989, all species of shark landed were reported under the unspecific “mixed shark” code. At least eleven species of shark are landed throughout the state, but blacktip shark ( Carcharhinus limbatus), sandbar shark ( Carcharhinus plumbeus), and shortfin mako ( Isurus oxyrinchus) are most common in the landings from southeast Florida (Brown7). Shark landings were negatively correlated with all significant associates in Broward County. There were strong negative associations between sharks and both groupers and snappers through- out most of the year and between sharks and dolphins in spring and fall. Dolphin landings were negatively corre- lated with all significant associates except for a few rare positive associations with tu- nas in mid-summer. They showed strong negative associations with groupers in the early part of the year and with snappers throughout most of the year. King mackerel ( Scomberomorus cavalla) is a migratory species and a seasonal mem- ber of the offshore pelagics (OP) group (Manooch, 1979; Collette and Russo, 1984). Those caught in the waters off Broward County are considered part of the Atlantic stock from 1 April until 1 November, when they become part of the Gulf-Atlantic stock. The fishery on the Gulf-Atlantic stock is quota-regulated in Florida and usually closes in late December or early January. In 1989, king mackerel landed in Broward County displayed strong, negative associa- tions with dolphin, groupers, and snappers. It also was rarely associated with baitfishes, lobsters, and other offshore pelagics. Spiny lobster ( Panulirus argus) landings in Florida occur only during the open sea- 7 Brown, S.T. 1994. Florida Marine Research Inst., Florida Dep. Environmental Protection, 100 8th Ave SE, St. Petersburg, FL 33701. Personal commun. McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida 121 son (6 August through 31 March). Broward County lobster landings were consistently negatively asso- ciated with yellowtail snapper ( Ocyurus chrysurus) and “mixed snapper.” They were positively associ- ated with grunts (Haemulidae) and “other groupers” in December. A strong positive association with Span- ish lobster ( Scyllarides aequinoctialis ) occurred in October. Black grouper occurred with nine other species and UM. It was negatively associated with lobsters and members of the offshore pelagics group, especially dolphin, king mackerel, and shark. There was a con- sistent positive association only with mutton snapper. Hogfish ( Lachnolaimus maximus) occurred with a dozen other species and UM. This species showed a strong and consistent negative association with sharks and sporadic positive associations with mut- ton snapper in summer and fall. Mutton snapper was significantly associated with the largest number of other species (25) but showed consistent associations with only a few. There was a consistent negative association with sharks and dol- phin throughout most of the year. Landings of mut- ton snapper were positively related to those of grou- pers and mojarra (Gerreidae) in the latter half of the year. This species was frequently associated with hogfish in summer and fall. Yellowtail snapper was also significantly associ- ated with a large number of other species (18), but showed few consistent associations. Positive associa- tions were rare but negative associations with dol- phin, shark, and spiny lobster were common. Species assemblages Seasonal differences suggested by changes in diver- sity and similarity were evident in the species as- semblages landed each month (Table 4). Offshore pelagic species dominated Broward County landings in 1989 (Fig. 2). Swordfish, shark, and dolphin were listed on at least ten trip tickets every month. Sword- fish dominated annual landings as well as those for each month; it accounted for 60% of annual landings (Fig. 4) and more than 50% of landings in all months except August, September, and October. Other off- shore pelagics accounted for 18% of annual landings (Fig. 4). Bigeye and yellowfin tunas occurred com- monly in nine or more months but were uncommon in the summer. King mackerel occurred commonly in all months except January through March, when the fishing season was closed. Spiny lobster ac- counted for less than 3% of the annual landings (Fig. 4) but made a large contribution to the landings in the first part of the open season (August: 15% of the landings) and tapered off throughout the fall. Grou- pers and snappers accounted for more than 6% of the annual landings, but only black grouper, mutton snapper, and yellowtail snapper accounted for more than 1% of annual landings each (Fig. 4). Black grou- per, hogfish, mutton snapper, yellowtail snapper, and mixed snappers occurred commonly in every month. Red grouper ( Epinephalus morio) was common ev- ery month, except January. Gag (Mycteroperca microlepis ) was common only in April, and “other grouper” in winter and September. Unknown mis- Table 4 Species contributions (as a percentage) to monthly landings in Broward County, Florida, during 1989. (See Table 1 for group definitions.) Group Species or complex Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec GS Black grouper 3.0 4.3 2.0 1.8 1.7 2.0 3.7 3.8 4.6 3.4 1.6 1.7 Mutton snapper 1.4 2.3 0.7 0.6 0.9 2.2 3.3 2.5 5.1 5.9 5.1 1.3 Yellowtail snapper 3.2 2.0 1.0 0.7 0.9 2.2 2.9 4.8 5.2 2.3 1.6 0.4 Mixed snapper 0.7 0.5 0.3 0.4 0.4 0.6 2.0 1.5 2.1 2.4 1.3 0.7 LB Lobster 2.0 1.4 1.9 0.0 0.0 0.0 0.0 16.5 9.0 7.8 4.5 4.0 OP Bigeye tuna 5.0 8.3 5.4 1.9 3.2 2.7 1.8 5.8 1.1 6.8 7.6 7.7 Dolphinfish 1.8 1.7 1.6 1.0 1.4 5.4 9.7 8.8 4.7 2.0 0.5 0.7 King mackerel 0.0 0.0 0.0 9.0 1.7 0.4 0.7 2.3 0.7 1.8 0.6 1.0 Shark 4.3 3.4 5.8 5.7 6.6 4.7 2.3 4.1 5.2 4.0 1.9 2.4 Swordfish 62.8 53.5 68.2 67.2 71.4 68.2 54.5 25.4 29.5 38.6 55.2 65.2 Yellowfin tuna 2.3 3.3 1.8 3.6 3.3 5.2 4.4 8.0 5.7 6.8 7.3 4.8 UM Misc. food fish 5.0 5.0 3.1 2.7 3.3 3.7 6.6 5.7 10.6 7.7 3.4 3.0 Other species 12.3 13.1 13.2 11.1 10.4 5.8 8.7 14.0 13.6 8.3 12.0 8.3 122 Fishery Bulletin 95(1 ), 1997 Other 9% Yellowtail snapper 2% Figure 4 Fish species assemblage, based on the total weight of commercial landings in Broward County during 1989. Pie slices shaded with a crosshatch pattern identify species classified as offshore pelagics (OP), those shown as black identify species classified as members of the grouper-snapper (GS) complex, those shown as white identify unknown miscellaneous fishes (UM), those with a dot pattern identify lobsters (LB) landings, and those with a diagonal line pattern identify other species. cellaneous food fish accounted for 4.2% of Broward County landings in 1989. Discussion The diversity of the Broward County harvest in 1989 was relatively low compared with that of a natural marine fish community in Florida. The tropical reef communities of the Florida Keys are some of the rich- est in the world; more than 500 fish species have been reported on Alligator Reef alone (Starck, 1968). Species-rich fish communities, however, are not re- stricted to coral reef habitats. Gilmore (1977) and Gilmore and Hastings ( 1983) reviewed fish collections associated with the Indian River system. They were able to compile a list of 685 species and projected that more than 700 species should be found in that region. The species richness in those studies varied considerably (26-275 species) from habitat to habi- tat. Grass flats, inlets, and offshore reefs had the richest fish faunas (>200 species). From offshore con- tinental-shelf habitats alone, more than 170 species were found. Anderson and Gehringer ( 1965) collected 64 species of fish in 94 hours of trawling over the continental shelf of the Indian River region. Herrema’s (1974) marine fish collections from off parts of Broward and Palm Beach counties included 583 species, although many of these are not commer- cially harvested or are taken in limited number for aquarium collectors. Nevertheless, 76 species offish and shellfish commercially landed from more than 3,000 fishing trips is a poor representation of the fauna known to be present. This low species richness is, of course, a reflection of the selectivity of commercial fishing efforts. Only certain sizes of the vulnerable species are available to the gear and only a fraction of these are captured. However, it is unlikely that a catch will be restricted to the two or three species that were landed per trip, on average (Fig. 3) (Fisher et al., 1943). Fishermen keep only the species and sizes that have a market value, discarding all others. The clear seasonal changes in the assemblages landed reflects a balance between changing availability of different fish spe- cies to the gear and market values of the various species. Presumably, the commonly landed species were the most valuable. Offshore pelagic species, especially swordfish, were clearly targeted in 1989, as were groupers and snappers. The low summer McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida 123 landings of offshore pelagics may reflect a decrease in availability as swordfish moved out of the fishing area (Hoey8) or a decline in market value, or both. An examination of catch records showed no evidence that fishermen who harvest offshore pelagics had switched to another fishery. However, the increase in landings of lobsters by some fishermen who tar- get grouper-snappers and inshore demersals is in- dicative of a shift to the more valuable lobster fish- ery when the season opens. The grouping of trips into distinct clusters that roughly correspond to different habitats indicates a structure among the fishermen, based on what they target. The use of specific gear and fishing sites re- stricts the diversity of the catch. The fisherman’s ability to use different gear (sometimes on the same trip) and visit different sites is a key characteristic of Florida’s fisheries. Twenty-five types of gear were registered by Broward County fishermen in 1989. Such unusual combinations as longlining for sword- fish and pulling traps for lobster commonly occur on the same trip. Six hundred and ninety-eight of the 907 fishermen registered rod-and-reel as one of the gear types they possessed (not necessarily used). Each fisherman may register more than one type of gear. The fishing potential of each gear is also differ- ent. Only 72 fishermen registered surface long lines, but those 72 lines had a total of 29,445 hooks. Florida also requires special licenses for use of cer- tain gear and for landing some species. Two hundred and eighty-two lobster (crawfish) licenses were is- sued to Broward County fishermen in 1989 ( 207 fish- ermen registered a total of 32,433 traps). Other spe- cial licenses included: blue crab (76), stone crab ( 123), shrimp (2), and purse seine (1). Species groups identified by the cluster analysis (Table 1) correspond to those that are vulnerable to different gear types. Species in the offshore pelagic group are caught in offshore surface waters with hook-and-line and surface long lines (Berkely et al., 1981). Most of the common groupers and snappers are caught in shallow, nearshore or shelf waters with hook-and-line. Bait fish are found in all surface wa- ters and are caught with small purse seines and lampara nets. Lobsters and stone crabs are found offshore, whereas blue crabs are harvested from in- shore and estuarine waters. All three are caught in traps, but lobster are also landed with shrimp in trawls. By having more than one gear type, a fisher- man can simply re rig his vessel (and possibly work a Hoey, J. J. 1985. Addendum to the source document for the swordfish fishery management plan. Part I. Prepared by and available from: South Atlantic Fishery Management Council, 1 South Park Circle, Suite 306, Charleston, SC, 132 p. different site) and partake of a completely different component of the fishery. The general negative associations among species is another indication of the selective behavior of com- mercial fishermen. Since the ideal catch for a fisher- man is a monocrop of the most valuable targeted species available, landed assemblages are likely to be as close to that ideal as possible. The catch is sorted and filtered such that the vessel’s hold capac- ity is filled with the greatest amount of the most valu- able species caught by the gear (Gulland, 1983). Thus, one would expect some trips to be monospecific, oth- ers to include a minimum number of other species. Without detailed information on discards and fish- erman behavior, it is difficult to determine if nega- tive associations represent an ecological condition whereby the two species avoid each other (or have different, but overlapping habitat requirements) or if they are an artifact of gear selection and fisher- man behavior. Most likely they are the result of a combination of these factors. The selectivity of the commercial fishing process and the nonrandom sampling of the natural envi- ronment makes it extremely difficult to use commer- cial landings data to gain insight into the natural fish community of a region. The commercial data provide only one component of the mortality affect- ing a fish community. Landings by recreational fish- ermen can be substantial ( Essig et al., 1991 ) but are often unavailable. Moreover, fish discarded at sea often represent the largest component of fishing mortality in a region (FAO, 1973); the market val- ues that drive the selection process are often not available with the landings data. Nonrandom spa- tial and temporal distribution of harvest can yield only biased estimates of fish population sizes, and the extent of that bias cannot be determined. Conclusions There was clear structure to the commercial fishery harvest in Broward County during 1989. The low diversity, classification of trips into habitats fished, and negative species associations were clear indica- tions of the selectivity in the system. A rich variety of species were landed by the fishery as a whole, but fishermen focused individual trips on a restricted subset of these species. The multispecies nature of the fishery and the potential for fishermen to exploit different components of the fishery are important features and should be carefully considered when forming management strategies. These commercial data tell us little about the natu- ral fish community from which the harvest was 124 Fishery Bulletin 95 ( 1 ), 1997 drawn. All that can be stated with certainty about the biological community is the tautology: the fish that were landed were present at the site where the gear was deployed and were available to the gear used at the time. The biases introduced by the selec- tivity of this system obscure events in the natural system and provide little insight into the changes in the fish community. However, these data do show a clear structure of the harvest due to fishing behav- ior and how that structure changes seasonally. The causes of those changes remain unclear. To address this problem, more effort in quantifying discards, recreational fishing mortality, and natural variability is needed, as well as a better understanding of the ac- cessibility and economics driving the social system. Acknowledgments I would like to thank R. G. Muller, F. S. Kennedy, and J. R. O’Hop for their guidance and editorial com- ments. Special thanks go to E. Irby for providing me with valuable insight into the harvest of offshore pelagic species. I am also indebted to J. Quinn for his extensive editorial comments on this document. Literature cited Anderson, W. W., and J. W. Gehringer. 1965. Biological-statistical census of the species entering fisheries in the Cape Canaveral area. U.S. Fish Wildlife Serv., Spec. Sci. Rep. -Fisheries 514, 79 p. Beddington, J. R. 1986. Shifts in resource populations in large marine ecosystems. In K. Sherman and L. M. Alexander (eds.), Variability and management of large marine ecosystems, p. 9-18. Am. Assoc. Adv. Sci. (AAAS) selected symposium 99. Beddington, J. R., and R. M. May. 1982. The harvesting of interacting species in a natural ecosystem. Scientific American 247:62-69. Berkely, S. A., E. W. Irby Jr., and J. W. Jolly Jr. 1981. Florida’s commercial swordfish fishery: longline gear and methods. Mar. Advis. Bull. MAP-14, Florida Coop- erative Extension Service, Florida Sea Grant Program. Bohnsack, J. A., D. E. Harper, and D. B. McClellan. 1994. Fisheries trends from Monroe county, Florida. Bull. Mar. Sci. 54:982-1018. Collette, B. B., and J. L. Russo. 1984. Morphology, systematics, and biology of the Spanish mackerels ( Scomberomorus , Scombridae). Fish. Bull. 82:545-646. Cushing, J. 1961. On the failure of the Plymouth herring fishery. Mar. Biol. Assoc. U.K. 41:899-916. Essig, R., J. F. Witzig, and M. C. Holliday. 1991. Marine recreational fishery statistics survey, Atlan- tic and Gulf coasts, 1987-1989. Current fisheries statis- tics number 1804. U.S. Dep. Commer., NMFS, NOAA, Silver Spring, MD, 363 p. Fable, W. A., Jr. 1990. Summary of king mackerel tagging in the southeast- ern USA: mark-recapture techniques and factors influenc- ing tag returns. Am. Fish. Soc. Symp. 7:161-167. Fisher, R. A., A. S. Corbet, and C.B. Williams. 1943. The relation between the number of species and the number of individuals in a random sample of an animal population. J. Animal Ecology 12:42-58. FAO (U.N. Food and Agriculture Organization). 1973. Report of the expert consultation on selective shrimp trawls; Ijmuiden, The Netherlands, 12-14 June 1973. FAO Fishery Rep. 139, 71 p. Freund, J. E. 1970. Statistics, a first course, 2nd ed. Prentice-Hall, New York, NY, 340 p. Gilmore, R. G., Jr. 1977. Fishes of the Indian River lagoon and adjacent wa- ters, Florida. Bull. Fla. State Mus., Biol. Sci. 22:101-148. Gilmore, R. G., and P. A. Hastings. 1983. Observations on the ecology and distribution of certain tropical peripheral fishes in Florida. Fla. Sci. 46:31-51. Gulland, J. A. 1983. Fish stock assessment: a manual of basic methods. Ch. 1: Introduction, p. 1-20. John Wiley & Sons, New York, NY. Herrema, D. J. 1974. Marine and brackish water fishes of southern Palm Beach and northern Broward Counties, Florida. M.S. thesis, Florida Atlantic Univ., Boca Raton, FL, 179 p. Idyll. C. P. 1973. The anchovy crisis. Scientific American 228:22-29. Kobayashi, S. 1987. Heterogeneity ratio: a measure of beta-diversity and its use in community classification. Ecol. Res. 2:101-111. Manooch. C. S. 1979. Recreational and commercial fisheries for king mack- erel, Scomberomorus cavalla , in the South Atlantic Bight and Gulf of Mexico, U.S. A. In E. L. Nakamura and H. R. Bullis Jr. (eds), Proceedings of the colloquium on the Span- ish and king mackerel resources of the Gulf of Mexico, p. 33-46. Gulf States Mar. Fish. Comm. 4. McKenna, J. E., Jr., and S. B. Saila. 1991. Application of an objective method for detecting changes in fish communities: Samar Sea, Philippines. Asian Fish. Sci. 4:201-210. Moe, M. A. 1991. Lobsters. Florida, Bahamas, the Caribbean. Chapter 6: The commercial fishery, balancing effort and conservation, p. 373-451. Green Turtle Publications, Plantation, FL. Nakamura, E. L. and H. R. Bullis Jr. (eds.) 1979. Proceedings of the colloquium on the Spanish and king mackerel resources of the Gulf of Mexico, no. 4; twenty- eighth annual spring meeting of the Gulf States Marine Fisheries Commission, Brownsville, Texas, March 16, 1978. Newlin, K. (ed.) 1991. Fishing trends and conditions in the Southeast Re- gion 1990. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-SEFC-292, 84 p. Pielou E. C. 1977. Mathematical ecology. Chapter 20: The classifica- tion of communities, p. 312-331. John Wiley & Sons, New York, NY. Raup, D. M., and R. E. Crick. 1979. Measurement of faunal similarity in paleontology. J. Paleontology 53:1213-1227. McKenna: Structure and dynamics of the fishery harvest in Broward County, Florida 125 Robins, C. R., G. C. Ray, and J. Douglass. 1986. A field guide to Atlantic coast fishes of North America. Houghton Mifflin Company, Boston, MA, 354 p. SAS Institute. 1985. SAS user’s guide: statistics, 5th ed. SAS Institute Inc., Box 8000, Cary, NC 27511-8000, 956 p. Sissenwine, M. P. 1986. Perturbation of a predator-controlled continental shelf ecosystem. In Sherman K. and L. M. Alexander (eds.), Variability and management of large marine eco- systems, p. 55-85. AAAS selected symposium 99. Shannon, C. E., and W. Weaver. 1949. The mathematical theory of communication. Univ. Illinois Press, Urbana, IL, 125 p. Smith, G. B. 1976. Ecology and distribution of Eastern Gulf of Mexico reef fishes. Florida Marine Research Publications 19, 78 p. Starck, W. A. II. 1968. A list of fishes of Alligator Reef, Florida, with com- ments on the nature of the Florida reef fish fauna. Under- sea Biology 1:4-40. 126 Abstract .—This study compared the hatching season and the actual spawning season of spring- and au- tumn-spawning herring in the north- ern Gulf of St. Lawrence as determined by otolith characteristics and maturity stages, respectively, to measure the crossover between the two spawning populations. The growth characteristics of two cohorts that showed significant crossover were contrasted with those of a cohort that did not. It was concluded that variable juvenile growth does in- fluence the adoption of the season of first spawning in these populations, and therefore the progeny of a given seasonal-spawning population may re- cruit to a local population that has a different reproductive season. It was also shown that the twinning of year- class strength can be explained by the crossover of a large number of individu- als from one seasonal spawning popu- lation to another. The data presented indicate that the spawning season that is established at the time of first matu- ration is maintained for the remainder of adult life. The present study there- fore does not support the concept of dis- crete sympatric seasonal-spawning populations in Atlantic herring. Manuscript accepted 7 August 1996. Fishery Bulletin 95:126-136 ( 1997). Year-class twinning in sympatric seasonal spawning populations of Atlantic herring, Clupea harengus Ian H. McQuinn Division Poissons et Mammiferes marins, Ministere des Peches et des Oceans Institut Maurice Lamontagne, C.P. 1000, 850 Route de la Mer Mont-Joli (Quebec), Canada G5H 3Z4 E-mail address: l_McQuinn@qc.dfo.ca The current theory on Atlantic her- ring population structure, as it re- lates to sympatric spring- and au- tumn-spawning herring, considers them to be discrete populations with independent life histories (lies and Sinclair, 1982). This concept has largely been based on evidence of significant homing precision to spawning grounds as revealed by tag returns (Hourston, 1982; Wheeler and Winters 1984, a and b; Steven- son et al.1; Hart et al.2; Stobo3) and on studies that have noted signifi- cant differences in meristic and morphometric measurements, such as fin-ray counts and otolith char- acteristics (Messieh, 1972; Parsons, 1973; Postuma, 1974), as well as in life history parameters, such as mean fecundities (Baxter, 1959; Messieh, 1976). However, several observations are difficult to explain within the discrete population concept: 1) typi- cal spring-type otoliths are often found in autumn-spawning herring and vice versa (Messieh, 1972; Aneer, 1985); 2) “twinning” of re- cruitment strength between spring- and autumn-spawning year classes (see below); and 3) the lack of ge- netic divergence between seasonal spawning populations as demon- strated from numerous electro- phoretic and mtDNA studies (Grant, 1984; Kornfield and Bogdanowicz, 1987; Safford and Booke, 1992). In light of the conflicting evidence for stock discreteness, an alternative concept has been proposed whereby seasonal spawning populations are seen as subunits of a larger popula- tion, within which there exists a “dynamic balance” characterized by extensive gene flow (Smith and Jamieson, 1986). Otolith characteristics and matu- rity stages have been used for many decades to determine the spawning affinity of individual herring from sympatric seasonal-spawning popu- lations. Maturity stages are the pre- ferred method for determining the actual spawning season of mature herring because the state of matu- ration can be used reliably to ascer- tain the spawning season through- out the year (McQuinn, 1989). On the other hand, otolith characteris- tics, being related mainly to envi- ronmental conditions at birth, are used to determine the hatching sea- 1 Stevenson, J. C., A. S Hourston, K. J. Jack- son, and D. N. Outram. 1952. Results of the West Coast of Vancouver Island her- ring investigation, 1951-52. Report of the British Columbia Provincial Fisheries Department, Victoria, British Columbia, Canada, p. 57-87. 2 Hart, J. L., A. L. Tester, and J. L. McHugh. 1941. The tagging of herring ( Clupea pallasii) in British Columbia: insertions and recoveries during 1940-41. Report of the British Columbia Provincial Fisheries Department, Victoria, British Columbia, Canada, p. 47-74. 3 Stobo, W. T. 1982. Tagging studies on Scotian shelf herring. Northwest Atlan- tic Fisheries Organization (NAFO) Re- search SCR Document 82/IX/108. Ser. No. N617, 16 p. P.O. Box 638, Dartmouth, Nova Scotia, Canada B2Y 3Y9. McQuinn: Year-class twinning in sympatric spawning populations of Clupea harengus 127 son (Einarsson, 1951; Postuma and Zijlstra, 1958; Messieh, 1972). The comparison of hatching season with spawning season as determined by these two methods, respectively, thus provides us with a rare opportunity to study the reproductive interactions between sympa- tric seasonal-spawning herring populations. As with most herring populations, herring from western Newfoundland (Canada) are characterized by the periodic appearance of very large year classes followed by several years of relatively poor recruit- ment. In addition, the sympatric seasonal-spawning populations in eastern Canadian waters often show year-class twinning (Winters et ah, 1986; de Lafontaine et al., 1991). This phenomenon is most evident when a strong autumn-spawning year class of a given year coincides with a strong spring-spawn- ing year class of the following year. Twinning is rela- tively common with seasonal-spawning populations but does not always occur. It is also true that year- class twinning rarely occurs between successive spring- and autumn-spawning year classes of the same year. Winters et al. (1986) showed a weak, though sig- nificant, relationship between year-class strength (natural log scale) of autumn-spawning herring in eastern Newfoundland and that of spring spawners of the following year. An example where year-class twinning occurred in the western Newfoundland herring populations is with the 1979 autumn-spawn- ing and 1980 spring-spawning year classes, both of which were very large (McQuinn and Lefebvre4). However, the 1982 spring-spawning year class was also very large but had no large autumn-spawning twin in 1981. Again we observed twinning with the 1986 and 1987 autumn- and spring-spawning year classes. What then are the characteristics that dis- tinguish these year classes and that might explain why twinning occurred in 1979-80 and 1986-87, but not in 1981-82? Year-class twinning was first reported in herring by Einarsson (1952), who termed it “year-class strength parallelism.” He speculated that favorable oceanographic and feeding conditions occurring from the fall of one year until the following summer re- sulted in a parallelism in larval survival between the two spawning populations. However, an alternative explanation is that year-class twinning is simply a consequence of straying between sympatric spring- and autumn-spawning populations, i.e. significant numbers of individuals from a large cohort of one 4 McQuinn, I. H., and L. Lefebvre. 1994. An assessment of the west coast of Newfoundland (NAFO division 4R) herring re- source up to 1993. Department of Fisheries and Oceans (DFO) Res. Doc. 94/43, 48 p. Atlantic Stock Assessment Secretariat, RO. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2. seasonal-spawning population subsequently spawn in the other season, creating a strong year class in both populations. Jean5 and Graham (1962) sug- gested that the determination of spawning season of sympatric herring populations may be influenced by juvenile growth rates. Winters et al. (1986) presented data in support of their hypothesis that faster-grow- ing spring-spawned juveniles may become autumn- spawners and conversely that slow-growing autumn- spawned juveniles may spawn in the spring. The objective of the present study is to use the otolith characteristics and maturity stage methods to estab- lish whether indeed juvenile growth rates (as repre- sented by size at age) have an effect on the determi- nation of the onset of first maturation and thus the establishment of spawning season in Atlantic herring. Materials and methods Data for this study were collected from the west coast of Newfoundland herring fishery from 1982 to 1990 (after 1990 otolith characteristics of mature herring were no longer determined by our agers). Samples were frozen and shipped to the Fisheries and Oceans laboratories for detailed analyses. Basic biological data (total length, total weight, gonad weight, and otolith characteristics, as well as the number of win- ter rings from which age was determined) were re- corded for all specimens. The hatching season was ascertained for each fish from otolith characteristics by applying the standard criteria (the size and type — opaque or hyaline — of the nucleus) developed by the Canadian Atlantic Fish- eries Scientific Advisory Committee as described by Cleary et al.6 These criteria were developed from the rationale that rapid growth in the first summer of spring-spawned herring results in a small opaque otolith nucleus. Conversely, the slow first-winter growth of autumn-spawned herring results in a large hyaline otolith center and the first-winter ring is formed only in their second year (Jakobsson et al., 1969; Postuma, 1974). Although the assignment of hatching season is determined subjectively from these criteria, consistency between agers has been shown to be relatively high. A comparative study was 5 Jean, Y. 1956. A study of the spring and fall spawning her- ring Clupea harengus L. at Grande-Riviere, Bay of Chaleur, Quebec. Contribution 49 of the Department of Fisheries, Quebec, Quebec, Canada, 76 p. 6 Cleary, L., J. J. Hunt, J. Moores, and D. Tremblay. 1982. Herring aging workshop; St. John’s, Newfoundland, March 1982. Canadian Atlantic Fisheries Scientific Advisory Com- mittee (CAFSAC) Res. Doc. 82/41, 10 p. CAFSAC, Department of Fisheries and Oceans, P.O. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2. 128 Fishery Bulletin 95 ( 1 ), 1997 conducted involving our ager and several other ex- perienced otolith readers who used otoliths collected from throughout the Gulf of St. Lawrence (Savard and Simoneau7). This study showed a high agree- ment between two agers from our laboratory in Que- bec (87%) and agers from the southern Gulf of St. Lawrence and eastern Newfoundland (75 and 76%, respectively) in the assignment of seasonal-spawn- ing type from otoliths. The actual spawning season of the mature indi- viduals was determined from the stage of sexual maturity of each individual by using a temporal gonadosomatic index model (McQuinn, 1989). This model identifies the spawning season by first deter- mining the maturity stage from the ratio of the go- nad weight to a power function of the total length and by relating this state of maturation to the month of capture. Although spawning can occur from April 7 Savard, L., and M. Simoneau. 1983. Lecture comparative d’otolithes de hareng et utilisation des stades de maturite sexuelle pour l’attribution du groupe reproducteur. Canadian Atlantic Fisheries Scientific Advisory Committee (CAFSAC) Res. Doc. 83/86, 28 p. Department of Fisheries and Oceans, RO. Box 1006, Dartmouth, Nova Scotia, Canada B2Y 4A2. to October, the vast majority of spring herring spawn in May and June, whereas the autumn herring spawn mainly from mid- August to September (Haegele and Schweigert, 1985). The date separating the two spawning seasons was arbitrarily chosen to be 1 July, as relatively little spawning occurs in late June and early July (McQuinn, 1989; Cleary et al.6). Throughout this paper, a distinction is made be- tween the number of rings read from the otoliths and the actual age of the fish because age determination depends upon whether an individual is assigned as a spring spawner or an autumn spawner. Most au- tumn-spawning herring (August-November) do not produce a winter ring on the otolith in their first year of life (Einarsson, 1951; Jakobsson et al., 1969; Rosenberg and Palmen, 1982; Hunt et al.8). The for- mation of the winter ring takes place between Octo- ber and April-May in metamorphosed juveniles 8 Hunt, J. J., L. S. Parsons, J. E. Watson, and G. H. Winters. 1973. Report of the herring ageing workshop; St. Andrews, New Brunswick, 11-13 December 1972. International Com- mission for the Northwest Atlantic Fisheries (ICNAF) Res. Doc. 73/2, Ser. 2901, 2 p. ICNAF, P.O. Box 638, Dartmouth, Nova Scotia, Canada B2Y 3Y9. McQuinn: Year-class twinning in sympatric spawning populations of Clupea harengus 129 (Postuma, 1974; Messieh9), whereas newly hatched autumn spawners are still larvae (Fig. 1). Thus to assign correctly the age of a herring hatched in the autumn, one must add one year to the number of winter rings read from the otolith. Because spring- hatched herring metamorphose before their first win- ter and thus produce a winter ring in their first year, their proper age is equal to the number of winter rings. However, for any spring-hatched herring that subsequently reproduces in the autumn, its spawn- ing season would be considered autumn with the maturity-stage method, and one year would be added to the number of rings read from the otolith. Follow- ing the same logic in reverse, if an autumn-hatched individual subsequently spawned in the spring, a year would not be added to the number of rings read from the otoliths and it would be assigned an age that was one year younger than its actual age. There- fore, the number of rings will be used when compar- 9 Messieh, S. N. 1974. Problems of ageing Atlantic herring (Clupea harengus harengus L.) in the ICNAF area. Inter- national Commision for the Northwest Atlantic Fisheries (ICNAF) Res. Doc. 74/59, Ser. 3274, 6 p. ICNAF, P.O. Box 638, Dartmouth, Nova Scotia, Canada B2Y 3Y9. ing the biological characteristics for a given age be- tween an autumn-spawning year class with the spring-spawning year class of the following year. Results The proportion of spring- and autumn-hatched her- ring was estimated from the mature members of the 1979-80, 1981-82 and 1986-87 autumn- and spring- spawning year classes, respectively (Table 1 ). Accord- ing to their otolith characteristics, the vast majority (>90%) of the 1980 and 1982 spring-spawning her- ring were also spring hatched. This pattern is con- sistent from age 3 through age 5 (age=no. of rings). However, a large percentage of the 1979 (40%) and 1981 (77%) autumn-spawning herring were also spring hatched, as judged from their otolith charac- teristics. Therefore, in both situations, there was a significant crossover from the strong spring-hatched cohort to the autumn-spawning population, in com- parison with the number of autumn-hatched indi- viduals. However, because the resulting 1979 au- tumn-spawning year class was also large, whereas the 1981 autumn-spawning year-class was not, this Table 1 Percentage of autumn- and spring-hatched herring that became mature autumn and spring spawners within the 1979, 1981, and 1986 autumn-spawning and within the 1980, 1982, and 1987 spring-spawning year classes off western Newfoundland (age is expressed as the number of otolith rings — see text). 1979 autumn- ■spawning year class 1980 spring-spawning year class No. of Autumn- Spring- Spring- Autumn- Year rings hatched (%) hatched (%) n hatched (%) hatched (%) n 1983 3 34.5 65.5 712 90.6 9.4 402 1984 4 55.6 44.4 1923 92.3 7.7 1438 1985 5 60.4 39.6 1760 92.4 7.6 2590 1981 autumn-spawning year class 1982 spring-spawning year class No. of Autumn- Spring- Spring- Autumn- Year rings hatched (%) hatched (%) n hatched (%) hatched (%) n 1985 3 54.4 45.6 68 92.2 7.8 192 1986 4 30.7 69.3 140 95.2 4.8 481 1987 5 22.6 77.4 186 97.2 2.8 966 1986 autumn- •spawning year class 1987 spring-spawning year class No. of Autumn- Spring- Spring- Autumn- Year rings hatched (%) hatched (%) n hatched (%) hatched (%) n 1990 3 86.0 14.0 57 68.8 31.2 32 130 Fishery Bulletin 95(1 ). 1997 1979 Autumn-hatched 1980 Spring-hatched Figure 2 Schematic representation of crossover between spring- and autumn-spawning her- ring populations. The relative amount of crossover between the 1979 autumn-hatched and 1980 spring-hatched cohorts is contrasted with that between the 1981 autumn- hatched and the 1982 spring-hatched cohorts. crossover was much less important in absolute terms in the latter (Fig. 2). This pattern is different for the 1986 and 1987 autumn- and spring-hatched cohorts (Table 1). There appears to have been a larger net migration (31%) from the autumn-hatched cohort to- wards the spring-spawners at age 4 (age=no. of rings + 1). Furthermore, the 1979 autumn-spawning year class showed a trend of a decreasing proportion of spring-hatched individuals from age 4 to 6 as more autumn-hatched individuals matured and recruited to the year class (Table 1). Conversely, the 1981 au- tumn-spawning year class showed an increasing per- centage of spring-hatched individuals with age ow- ing to the overwhelming dominance of the large 1982 spring-hatched cohort compared with the small 1981 autumn-hatched cohort (Fig. 2). I have also summarized the mean lengths and stan- dard deviations of the immature 1980 and 1982 spring-hatched cohorts to compare their average growth characteristics prior to their first spawning (Table 2). Although the means are similar at similar ages, the standard deviations are quite different, those for the 1980 cohort being 44% to 79% greater than the 1982 cohort from age 2 to age 3. The differ- ence between the two cohorts is even more obvious when their length-frequency distributions at age 3 are compared (Fig. 3, A and B). The length distribu- tion of the 1980 cohort is wider and bimodal. The McQuinn: Year-class twinning in sympatric spawning populations of Clupea harengus 131 25 15 20 25 30 35 25 15 20 25 30 35 Length (cm) Figure 3 Length-frequency distributions of the (A) 1980 spring-hatched, (B) 1982 spring-hatched, and (C) 1986 autumn-hatched cohorts as immature 3-year-olds in 1983, 1985, and 1989, respectively. Table 2 Mean lengths and standard deviations of immature western Newfoundland herring from the 1980 and 1982 spring-hatched cohorts as 2- and 3-year-olds in the spring (April-June) and the fall (October-December) fisheries. 1980 spring-hatched cohort 1982 spring-hatched cohort Year Age Fishery Mean length (cm) SD n Year Age Fishery Mean length (mm) SD n 1982 2 Spring — — — 1984 2 Spring 187.96 12.87 ii Fall 248.91 27.23 35 Fall 248.47 15.17 75 1983 3 Spring 247.86 20.87 72 1985 3 Spring 252.11 14.50 77 Fall 278.80 17.43 585 Fall 283.27 10.56 310 132 Fishery Bulletin 95 ( I ), 1997 Fall 1984 Spring 1985 Fall 1985 Spring 1986 Length (cm) Figure 4 Length-frequency distributions of the 1982 spring-hatched cohort from the fall of 1984 to the spring of 1986, showing the relative proportions of immature herring that eventually adopted either the spring- or the autumn-spawning season as determined by their maturity stages. same pattern is evident for the 1986 autumn-hatched cohort, which also showed a large bimodal length- frequency distribution at age 3 (Fig. 3C). The significance of these differences in length com- position is shown by following these cohorts as they became mature and began to spawn. We observed that when the length-frequency distribution of the juvenile spring-hatched herring was unimodal and had a relatively small variance (1982 cohort), the majority of them spawned in the spring of 1986 at age 4 (Fig. 4). Conversely, when the juvenile length- frequency distribution was bimodal and had a rela- tively large variance ( 1980 cohort), there was an al- most even split between those that became spring spawners and those that eventually became autumn spawners (Fig. 5). In addition, if one follows the 1980 spring-hatched cohort after maturity, those that be- came spring spawners were significantly smaller la- test: P<0.0001, SAS Institute Inc., 1985) than those that became autumn spawners (Fig. 6A). This length difference was sustained throughout their early adult life, i.e. from age 3 through age 6. A similar pattern was also seen with the 1979 autumn-hatched cohort. Those that became spring-spawners were smaller at age (Fig. 6B), although the differences are not sig- nificant for certain ages owing to small sample sizes. Discussion Einarsson (1952 ) hypothesized that year-class par- allelism (twinning) in sympatric seasonal-spawning herring populations came about through a correla- tion between larval survival conditions in the fall of one year with conditions in the spring of the follow- ing year, assuming the larval stage to be the critical phase that determines year-class strength. However, given the ontogeny of the larvae of sympatric sea- sonal-spawning populations, it is difficult to conceive of a single mechanism by which both cohorts would experience similar survival conditions over a 10- month period, especially since twinning does not normally occur with two successive year classes within the same year. If one follows the development McQuinn: Year-class twinning in sympatric spawning populations of Ctupea harengus 133 of an autumn-hatched cohort and that of the spring- hatched cohort of the following year, from hatching through metamorphosis (Fig. 1), it is apparent that at no time are these two cohorts in the larval stage at the same time, i.e. the autumn spawners have metamorphosed before the spring spawners of the following year have hatched. Autumn-spawned her- ring in the northwest Atlantic hatch from August to November, remain as larvae throughout the winter (lies and Sinclair, 1982), and metamorphose within a “window” between March and May (Sinclair and Tremblay, 1984 ). Larvae hatched in June or July from the spring-spawning event reach the required size for metamorphosis during September or October of their first year. Conditions affecting larval survival would therefore have to be favorable from Septem- ber of one year to June of the next, but unfavorable between July and August for Einarsson’s explana- tion to be credible. Einarsson (1952) speculated that a strong stand- ing stock of copepods in the autumn of one year may be correlated with enhanced copepod egg production the following spring, thus favoring larval herring survival over this extended period, although the data available to him did not show this correlation. In addition, these enhanced survival conditions would not only have to exist over a long, but nonetheless precise period of time (September-June), but would also have to be extremely widespread. Twinning oc- curs in most if not all sympatric herring populations in the northwest Atlantic (de Lafontaine et al., 1991) and sympatric spring- and autumn-spawners do not necessarily use the same breeding locations nor the same larval retention mechanisms, i.e. spring and autumn spawners along the west coast of Newfound- land (McQuinn and Lefebvre4). The present study supports the alternative hypoth- esis that the twinning of year classes can be explained by the crossover or straying of a significant number of individuals from one seasonal-spawning popula- tion to the other. Our results also support the hy- pothesis of Jean5, Graham (1962) and Winters et al. (1986) that variable growth rates in the juvenile phase lead to this crossover. Results from several 134 Fishery Bulletin 95( 1 ), 1997 38 36 34 32 30 28 26 24 1 2 3 4 5 6 7 38 36 34 32 30 28 26 24 1 2 3 4 5 6 7 No. of rings Figure 6 Length at age and 95% Cl of the (A) 1980 spring-hatched and (B) 1979 autumn-hatched cohorts in the late fall (October-December), once the spawning season had been established as determined by their matu- rity stages for each cohort ( age is expressed as the number of otolith rings — see text). t i r S o ■S W) c 1) hJ studies have concluded that either density-dependent (Anthony, 1971; Lett and Kohler, 1976) or density- independent factors (Moores and Winter, 1982), or both (Anthony and Fogarty, 1985; Haist and Stocker, 1985), contribute to the significant inter- and intra- annual variations in the growth rates of juvenile herring. There has developed a general consensus among these authors that differences observed in length at age between year classes of adult herring originated in the juvenile phase, before maturation. Further, Toresen (1990) compared the growth of ju- venile Norwegian herring from the 1950’s with that from the 1970’s and concluded that variable growth rates depended mainly on where the juveniles spent their early years. Large cohorts showed both den- sity-dependent growth when these cohorts were dis- tributed in the fjords, as well as environmentally induced growth variations when components of the cohort were distributed in less productive areas of the Barents Sea. This study demonstrated that dif- ferent components of a single cohort can encounter different growth conditions before maturation and thus can experience different growth rates. Density-dependent and environmentally induced variability in growth and condition in the juvenile phase is believed to affect the onset of first matura- McQuinn: Year-class twinning in sympatric spawning populations of Clupea harengus 135 tion in several teleost species (Lett and Doubleday, 1976; Holdway and Beamish, 1985; Rowe and Thorpe, 1990), including Atlantic herring (Marti, 1959; Raitt, 1961; Anthony and Fogarty, 1985; Haist and Stocker, 1985). Several studies have concluded that length, rather than age, is the “critical” factor determining the onset of first maturation in herring (Burd, 1962; Beverton, 1963; Toreson; 1990). Variations in growth rates within a cohort, whether they are density-de- pendent or not, will therefore influence the age at which different components of a cohort will reach the critical length. We have seen in the present study that variable juvenile growth rates do influence the onset of first maturation in herring (the age of maturity) and thus affect which season is adopted for spawning. When the growth characteristics of a cohort were relatively uniform in early life, as represented by the unimodal length distribution of the immature 1982 cohort as 3- year-olds, most of the cohort subsequently matured in synchrony and spawned in the spring of 1986 as 4- year-olds. However, when the immature 3-year-old length-frequency distribution showed signs of differ- ential growth rates, as with the 1980 spring-hatched and 1986 autumn-hatched cohorts, maturation was asynchronous. Those individuals from the 1980 co- hort with an advanced length at age matured as au- tumn spawners in the fall of 1983 at age 3 years and 4 months. A large proportion of this cohort subse- quently spawned the following spring at age 4, and the remainder took advantage of an additional growth season before spawning in the fall of 1984 as autumn spawners. The autumn-spawning individu- als of this cohort were therefore significantly longer at age than those of the same cohort that remained spring-spawning (Fig. 6A). Winter et al. (1986) also concluded that the faster-growing spring-hatched in- dividuals matured as autumn-spawners. Conversely, the 1979 autumn-hatched individuals that became spring spawners did so the previous spring at age 3 years and 8 months and thus showed a shorter length at age than those that remained autumn spawners and matured at age 4 (Fig. 6B). We also observed that the adopted season was maintained after the initial spawning because this length difference per- sisted until at least age 6. This crossover also explains the observed pattern of twinning — that is to say a strong spring-spawn- ing year class matched with a strong autumn-spawn- ing year class from the previous year. The fact that twinning is seldom seen between spring- and autumn- spawning year classes of the same year is due to the ageing convention for herring (Hunt et al.8), which does not consider the possibility of crossover between these populations. The present study has demon- strated that this crossover can occur in both direc- tions, i.e. spring-hatched herring can contribute to a autumn-spawning year class (1980) and vice versa (1986). It should also be mentioned that although the effects of crossover between sympatric herring populations is more striking when large year classes are involved, resulting in year-class twinning, the significant correlation found between subsequent autumn- and spring-spawning year classes in east- ern Newfoundland (Winters et al., 1986) indicates that crossover undoubtedly occurs to some extent with all year classes. The present study therefore does not support the concept of discrete sympatric seasonal-spawning populations in Atlantic herring. The data presented here suggest that the progeny of a given seasonal population do not necessarily recruit to the parental population but may indeed contribute to a local popu- lation of another reproductive season. Furthermore, the spawning season that is established at the time of first maturation is maintained for the remainder of adult life. Acknowledgments I wish to acknowledge the efforts of Celine Trudeau- Simard and Joanne Hamel for the analysis of the biological samples, including age and hatching-sea- son determinations. I also thank Yvan Lambert and two anonymous reviewers for their helpful comments on the manuscript. Literature cited Aneer, G. 1985. Some speculations about the Baltic herring Clupea harengus membras in connection with the eutrophication of the Baltic Sea. Can. J. Fish. Aquat. Sci. 42 (suppl. 1 ):83— 90. Anthony, V. C. 1971. The density dependence of growth of the Atlantic herring in Maine. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 160:197-205. Anthony, V. C., and M. J. Fogarty. 1985. Environmental effects on recruitment, growth, and vulnerability of Atlantic herring Clupea harengus harengus in the Gulf of Maine region. Can. J. Fish. Aquat. Sci. 42 (suppl. 1 ): 158—173. Baxter, I. G. 1959. Fecundities of winter-spring and summer-autumn herring spawners. J. Cons. Int. Explor. Mer 25( 11:73-80. Beverton, R. J. H. 1963. Maturation, growth and mortality of clupeid and engraulid stocks in relation to fishing. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 154:44-67. 136 Fishery Bulletin 95 ( 1 ), 1997 Burd, A. C. 1962. Growth and recruitment in the herring of the south- ern North Sea. Fish. Invest., ser. 2, 23(5), 42 p. de Lafontaine, Y., S. Demers, and J. Runge. 1991. Pelagic food web interactions and productivity in the Gulf of St. Lawrence: a perspective. In J.-C. Therriault (ed.), The Gulf of St. Lawrence: small ocean or big estu- ary?, p. 99-123. Can. Spec. Publ. Fish. Aquat. Sci. 113. Einarsson, H. 1951. Racial analyses of Icelandic herrings by means of the otoliths. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 128: 55-74. 1952. On parallelism in the year-class strength of seasonal races of Icelandic herring and its significance. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 131:63-70. Graham, T. R. 1962. A relationship between growth, hatching and spawn- ing season in Canadian Atlantic herring Clupea harengus L. J. Fish. Res. Board Can. 19(51:985-987. Grant, W. S. 1984. Biochemical population genetics of Atlantic herring Clupea harengus. Copeia 1984(2):357-364. Haegele, C. W., and J. F. Schweigert. 1985. Distribution and characteristics of herring spawn- ing grounds and description of spawning behavior. Can. J. Fish. Aquat. Sci. 42(suppl. l):39-55. Haist, V., and M. Stocker. 1985. Growth and maturation of Pacific herring Clupea harengus pallasi in the Strait of Georgia. Can. J. Fish. Aquat. Sci. 42(suppl. 1):138-146. Holdway, D. A., and F. W. H. Beamish. 1985. The effect of growth rate, size and season on oocyte development and maturity of Atlantic cod Gadus morhua L. J. Exp. Mar. Biol. Ecol. 85:3-19. Hourston, A. S. 1982. Homing by Canada’s west coast herring to manage- ment units and divisions as indicated by tag recoveries. Can. J. Fish. Aquat. Sci. 39:1414-1422. lies, T. D., and M. Sinclair. 1982. Atlantic herring: stock discreteness and abundance. Science (Wash., D.C.) 215:627-633. Jakobsson, J., H. Vilhjalmsson, and S. A. Schopka. 1969. On the biology of the Icelandic herring stocks. Rit Fiskideildar 4( 6 ): 1— 16. Kornfield, I., and S. M. Bogdanowicz. 1987. Differentiation of mitochondrial DNAin Atlantic her- ring, Clupea harengus. Fish. Bull. 85(31:561-568. Lett, P. F., and W. G. Doubleday. 1976. The influence of fluctuations in recruitment on fisher- ies management strategy, with special reference to southern Gulf of St. Lawrence cod. ICNAF Sel. Pap. 1:171-193. Lett, P. F., and A. C. Kohler. 1976. Recruitment: a problem of multispecies interaction and environmental perturbations, with special reference to Gulf of St. Lawrence Atlantic herring Clupea harengus harengus. J. Fish. Res. Board Can. 33:1353-1371. Marti, Yu.Yu. 1959. The fundamental stages of the life cycle of Atlantic- Scandinavian herring. Fish. Res. Board Can. Trans. Ser. 167:5-68a. McQuinn, I. H. 1989. Identification of spring- and autumn-spawning her- ring ( Clupea harengus harengus) using maturity stages assigned from a gonadosomatic index model. Can. J. Fish. Aquat. Sci. 46(61:969-980. Messieh, S. N. 1972. Use of otoliths in identifying herring stocks in the southern Gulf of St. Lawrence and adjacent waters. J. Fish. Res. Board Can. 29:1113-1118. 1976. Fecundity studies on Atlantic herring from the south- ern Gulf of St. Lawrence and along the Nova Scotia coast. Trans. Amer. Fish. Soc. 105(6): 384-394. Moores, J. A., and G. H. Winters. 1982. Growth patterns in a Newfoundland Atlantic herring Clupea harengus harengus stock. Can. J. Fish. Aquat. Sci. 39:454-461. Parsons, L. S. 1973. Meristic characteristics of Atlantic herring, Clupea harengus harengus L., stocks in Newfoundland and adja- cent waters. ICNAF Res. Bull. 10:37-52. Postuma, K. H. 1974. The nucleus of the herring otolith as a racial char- acter. J. Cons. Int. Explor. Mer 35(2): 121-129. Postuma, K. H., and J. J. Zijlstra. 1958. On the distinction of herring races in the autumn- and winter-spawning herring of the North Sea and En- glish Channel by means of the otoliths and an application of this method in tracing the offspring of the races along the continental coast of the North Sea. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 143(22):130-133. Raitt, D. F. S. 1961. Otolith studies of southern North Sea herring. J. Cons. Int. Explor. Mer 26:312-328. Rosenberg, R., and L.-E. Palmen. 1982. Composition of herring stocks in the Skagerrak- Kattegat and the relations of these stocks with those of the North Sea and adjacent waters. Fish. Res. 1:93-104. Rowe, D. K., and J. E. Thorpe. 1990. Differences in growth between maturing and non-maturing male Atlantic salmon, Salmo salar L. parr. J. Fish Biol. 36: 643-658. Safford, S. E., and H. Booke. 1992. Lack of biochemical genetic and morphometric evi- dence for discrete stocks of northwest Atlantic herring Clupea harengus harengus. Fish. Bull. 90(1):203-210. SAS Institute, Inc. 1985. SAS user’s guide: statistics, version 5 edition. Cary, N.C.: SAS Institute Inc., 956 p. Sinclair, M., and M. J. Tremblay. 1984. Timing of spawning of Atlantic herring Clupea harengus harengus populations and the match-mismatch theory. Can. J. Fish. Aquat. Sci. 41:1055-1065. Smith, P. J., and A. Jamieson. 1986. Stock discreteness in herrings: a conceptual revolu- tion. Fish. Res. 4:223-234. Toresen, R. 1990. Long-term changes in growth of Norwegian spring- spawning herring. J. Cons. Int. Explor. Mer 47:48-56. Wheeler, J. P., and G. H. Winters. 1984a. Homing of Atlantic herring (Clupea harengus harengus ) in Newfoundland waters as indicated by tag- ging data. Can. J. Fish. Aquat. Sci. 41:108-117. 1984b. Migrations and stock relationships of east and south- east Newfoundland herring Clupea harengus , as shown by tagging studies. J. Northwest Atl. Fish. Sci. 5(2):121-129. Winters, G. H., J. P. Wheeler, and E. L. Dailey. 1986. Survival of a herring stock subjected to a catastrophic event and fluctuating environmental conditions. J. Cons. Int. Explor. Mer 43: 26-42. 137 Abstract .■Densities of juveniles of the Hawaiian deepwater snapper Pristipomoid.es filamentosus were sur- veyed for 3 years in relation to their demersal environment at an east Oahu study site. Juveniles settled annually to spatially stable aggregations, occu- pying expanses of uniform sedimentary habitat. Habitat data were collected and used in a logistic regression model to predict correctly 68% of the juveniles’ spatial variability. Premium habitat was identified as a sediment bottom, free of relief, and close to focused sources of drainage (reef platforms, embayments, and anthropogenic sources) in adjacent shallows. Surveys for juve- niles elsewhere on insular slopes of the Hawaiian Archipelago indicated low ju- venile abundance except at infrequent locations close to point sources of coastal drainage. Estimates of recruit production, based on densities of juve- niles from other than premium habi- tat, were a small fraction of the recruits needed (calculated from catch) to ac- count for the fishery’s current landings of adult snappers. The 68-fold higher juvenile abundance at premium habi- tat can reconcile this difference, indi- cating that such infrequent high-qual- ity habitat is an important (perhaps critical) fishery resource. Manuscript accepted 17 July 1996. Fishery Bulletin 95:137-148 ( 1997). Nursery habitat in relation to production of juvenile pink snapper, Pristipomoides filamentosus ; in the Hawaiian Archipelago Frank A. Parrish Edward E. DeMartini Denise M. Ellis Honolulu Laboratory, Southwest Fisheries Science Center National Marine Fisheries Service, NOAA Honolulu, Hawaii 96822-2396 E-mail address: Frank.Parrish@noaa.gov Understanding favorable nursery habitat and its contribution to the standing stock of adults provides an important perspective for managing demersal fisheries. In the tropics, such nursery habitat has been stud- ied effectively for many species in- habiting shallower depths (Bardach, 1959; Parrish, 1989; Birkeland1). Species using deeper, more remote nursery grounds have received less attention, and as a result, habitat often is not considered adequately in fishery modelling or management planning. In places with limited demersal nursery habitat, such as the minimal shelf area of oceanic islands, this habitat may represent a resource of critical importance to the fishery. Accelerated coastal de- velopment on many islands could degrade unrecognized favorable nursery habitat and impact fishery resources. This paper examines the nursery habitat of the deepwater Hawaiian pink snapper, Pristi- pomoides filamentosus, in relation to the spatial variability of its juve- niles. A study site at a productive nursery ground was intensively sur- veyed, and the results compared with surveys made over much of the archipelago. The implications of the variable habitat quality for the stock of adult snappers inhabiting the ar- chipelago were then considered. The pink snapper accounts for more than 40% of the State of Hawaii’s $3 million annual commer- cial bottomfish catch 2 and is well represented in the extensive recre- ational catch. However, study and management of the adult stock has been historically difficult because of its patchy distribution and poorly recorded recreational landings ( Ralston and Polovina, 1982 ). A pro- ductive research approach may be to study the juveniles of the species, which are free of fishing pressure and of the factors that affect recruit- ment to the adult population. Re- cent discovery (F. A. Parrish, 1989) of a dense, stable aggregation of ju- veniles in a nursery area has made this approach feasible. Juveniles (7- 25 cm fork length | FL]) occupy mod- erate depths (60-90 m) in patchy aggregations on the insular shelf for less than a year before moving deeper ( 150-190 m) as they mature 1 Birkeland, C. 1985. Ecological interac- tions between mangroves, seagrass beds, and coral reefs. United Nations Environ- mental Program Regional Series Report and Studies 73, 126 p. 2 WPRFMC (Western Pacific Regional Fish- ery Management Council). 1993. Bot- tomfish and seamount groundfish fisher- ies of the Western Pacific region. NOAA NA17FC0062-02, Honolulu. HI, 57 p. WPRFMC, 1164 Bishop Street, Suite 1405, Honolulu, HI 96813. 138 Fishery Bulletin 95( 1 ), 1997 (Moffitt and Parrish, 1996). Sonic tracks of these ju- veniles indicate a discrete and limited individual home range of 140 m average diameter, suggesting that the locations of these juvenile aggregations could be very stable. Why juvenile aggregations appear spatially stable and how common they are in the rest of the archipelago are the primary focus of this work. Methods Survey of the east Oahu study site The east Oahu study site contains three submarine canyons. Two of the canyons are located just outside Kaneohe Bay, north of Mokapu Point, and the third is located south of Mokapu Point just outside Kailua Bay (Fig. 1, II). Throughout this paper, the three areas will be referred to as the “north Kaneohe,” “south Kaneohe,” and “Kailua” canyons. Positional data from a Global Positioning System (GPS) were entered and manipulated in a raster-based Geographic Information System (GISXIDRISI 4.0 version) (Eastman, 1992). Video index of snapper abundance A baited video camera (Fig. 2) was selected as the primary gear because it provided information on the abundance of snappers and their associated habitat type. In each video drop, the baited camera was placed on the bottom for 10 minutes, where it attracted juvenile snappers in front of the camera lens; the maximum number of snappers seen in a single image was used as the index of abundance. Consecutively deployed video drops were separated by 1,200 m to avoid attraction of fish from previous drops. A description of equipment, method, and vali- dation of the technique for creating a video index of snapper abundance is provided by Ellis and DeMar- tini (1995). Selected video stations at the study site were rep- licated to determine the suitability of unreplicated spatial data for subsequent use in evaluating the persistence of snapper patches over time. Nineteen stations, resampled after 10 days, were used to rep- resent all 3 canyons during February-March 1994. These stations were termed “multicanyon stations.” Figure 1 (I) - Map of the Hawaiian Archipelago with video survey sites denoted as 1 = Kailua-Oahu, 2 = Kaneohe-Oahu, 3 = south Molokai, 4 = Hanalei-Kauai, 5 = Kahului-Maui, 6 = north Molokai, and 7 = French Frigate Shoals in the Northwestern Hawaiian Islands. (II) Loca- tions of the three east Oahu areas studied, including north Kaneohe canyon (A), south Kaneohe canyon (B), and Kailua canyon (C). Also represented are the sources of coastal drainage of north Kaneohe channel (a), south Kaneohe channel (b), and the Kailua waste- water outfall (c). Parrish et al.: Nursery habitat in relation to production of Pristipomoides filamentosus 139 Interannual fidelity of snapper recruitment to the study site was assessed by using 20 video stations in the north Kaneohe canyon during 4 surveys in May 1992, May 1993, September 1993, and June 1994. These were designated as “multiyear stations” and were compared by using date of survey as a covariate. Habitat characteristics Slope, substrate type, sediment particle size, and proximity to closest known point sources of focused coastal drainage (channels through reefs and waste- water outfalls) were determined for all areas. The effect of slope on snapper abundance was assessed by using a GIS slope algorithm with collected bathymetry data. At the depths frequented by juve- nile snappers, the habitat is typically dominated by a featureless expanse of sediment. To test the effect of alternative substrates on snapper abundance, types of substrate (as identified in video and chro- moscope images) were coded as categories: e.g. soft sediments, escarpment-type relief (exposed edges of shelf, about 3 m high), and hard, even bottom. Video drops that recorded alternate substrate, or were within a snapper home range (Moffitt and Parrish, 1996) of such observations, were compared with video drops on soft sediment, presumably away from the influence of the other substrate. Substrate of adja- cent shallower (30-60 m) and deeper (90-120 m) habitats, where juveniles have been historically ab- sent (F.A. Parrish, 1989; Moffitt and Parrish, 1996), were surveyed with a longshore transect of 14 video drops in each of the 2 depth ranges. Ten bottom grab transects perpendicular to the bottom contours, each sampling 3 depths (45, 76, 106 m), were used to assess a possible relationship be- tween snapper abundance and particle sizes in the sedimentary habitat. Replicate grabs were taken in line with and between the axes of the canyons in each area. Samples were wet-sieved into five size catego- ries (>2.0, 0.35-2.0, 0.149-0.35, 0.0625-0.149, and <0.0625 mm). The effect of some notable sources of natural and anthropogenic drainage present in each of the three canyons was considered. In Kaneohe, bay water drains through narrow channels (one in the north and one in the south, each with maximum depth of -15 m at the seaward end) in the reef during ebb tide3 (Fig. 1, II). In Kailua, increased suspended materials are introduced from an island wastewater and sewage outfall.4 * The video index of snapper abun- 3 Bathen, K. H. 1968. A descriptive study of the physical ocean- ography of Kaneohe Bay. Oahu, Hawaii. HIMB Tech. Rep. 14, 353 p. Univ. Hawaii, 2550 The Mall, Honolulu, HI 96822. 4 City and County of Honolulu. 1993-1994. Discharge moni- toring reports. Environmental Protection Agency form 3320- 1. Wastewater Division, 650 South King St. Honolulu, HI 96813. Table 1 Depth, mean daily volume, and suspended load of the east Oahu drainage sources. The Kaneohe channels provide tidal drainage; the Kailua discharge is anthropogenic (24 hours). Source of discharge Discharge volume (m3/day) Discharge depth (m) Suspended solids (kg/day) Source North Kaneohe channel 18.5 x 106 0-15 Bathen' South Kaneohe channel 12.9 x 106 0-15 — Bathen' Kailua wastewater outfall 41,000 30 1,000 City and County of Honolulu2 'See Footnote 3 in the main body of the text. 2See Footnote 4 in the main body of the text. 140 Fishery Bulletin 95( 1 ), 1997 dance was compared with the volume of each source’s discharge (Table 1) divided by the distance separat- ing the video samples from the nearest source of dis- charge. No attempt was made to sample the nutri- ents or suspended materials of these discharges. El- evated organics associated with these water masses are documented in the literature (Bromwell, 1992; City and County of Honolulu4; Laws and Allen5). East Oahu statistical analysis The distribution of the data and the categorical na- ture of the habitat variables required the use of non- parametric analysis (Siegel and Castellan, 1988). The type-I error for statistical significance was set at 0.05 (2-tailed test). Kruskal-Wallis ANOVA (K-W) was used to assess station effects in both “multicanyon and multiyear” analyses and to assess the effect of substrate type. Differences in substrate by depth were tested with chi-square analysis. Replicate bottom grabs were compared by using Wilcoxon matched pairs sign ranks (MPSR), and Spearman’s correlation was used for association of snappers with slope, sediment frac- tions, and influence of drainage. Spatial variation of ranked snapper abundance was related to all habitat variables together by us- ing logistic regression. Snapper abundance was grouped into two categories, aggregation pi'esent (n>5) and aggregation absent (n< 5), and assessed relative to the habitat variables that significantly influenced snapper abundance in the previously de- scribed univariate analyses (Norusis, 1992). Models of the variables and their plausible interaction ef- fects were explored with the simple logistic regres- sion model (Kleinbaum, 1992): where n is the probability of detecting snappers with the linear combination of the habitat variables Xt in a given location. The coefficients estimated with the nonlinear regression by using maximum likelihood are represented by Br The base of the natural loga- rithm is e. The P-value for retention of independent variables in the model was set at 0.01. 5 Laws, E. A., and C. B. Allen. 1993. Impact of land runoff on water quality in Kaneohe Bay, a subtropical Hawaiian estuary. Proceedings of the first biennial symposium for main Hawaiian islands marine resources investigation, November 17- 18. Hawaii Department of Land and Natural Resource Tech- nical Report 95-01, p. 232—248. Hawaii Dep. Land Natl. Re- sources, 1151 Punchbowl, Honolulu, HI 96813. Survey of the archipelago Conventional fishing gear (e.g. trawls, longlines, traps, handlines) was used to survey a total of 332 km of longshore habitat dispersed over seven islands of the archipelago (1989-94). The effectiveness of each gear at catching juvenile snappers was tested at the east Oahu study site. Sites surveyed included areas outside of embayments, places with large shelf areas at snapper depths, and sites of previous re- search fishing where juveniles had been documented incidentally (Struhsaker, 1973). Sites where snap- pers were found were then reassessed with longshore baited video surveys (range 5.5-42.6 km) to permit comparison with snapper abundance at the east Oahu study site. Numbers of juvenile snappers ob- served at each site were standardized by effort. The distance of each video drop from the coastal reef edge (15-m isobath) and the type of substrate seen in the video image were tabulated for each site; these vari- ables were then compared with the respective video index of juvenile snapper abundance. Catch-per-unit- of-effort (CPUE) data from sets of conventional fish- ing gear at these coastlines were included to provide an independent index of snapper abundance. In comparing video data from other coastlines with those of east Oahu, data for the two Kaneohe areas were pooled. Coastlines with point sources of drain- age were identified, and the distance between sources of discharge and the video drops (weighted for maxi- mum depth of discharge) were calculated. Importance of proximity to drainage sources to snapper abun- dance was then evaluated for these archipelago sites. Snapper production estimates To assess the importance of the contribution of juve- niles from a site with premium habitat (e.g. Kaneohe) to the adult fishery, the adequacy of recruit produc- tion from other habitat areas was estimated. The density of snappers at habitat without snapper ag- gregations was compared with the density of snap- pers needed to explain the catch from the main Ha- waiian Islands (MHI) commercial snapper fishery. Derived from mandatory reporting from the commer- cial fishery, the estimate is based on the catch of -3- year-old snappers (termed “immature”) just enter- ing the MHI adult snapper fishery (Ralston, 1981; DeMartini et al., 1994). Based on the years 1989- 92, the estimated mean annual catch, C (i.e. the com- mercial catch [WPRFMC2]) was -22,000 immature (1.3 kg) snapper/year. Recreational fishing produces a significant additional catch in Hawaii, but it is poorly documented and was not considered in this estimate. Parrish et al.: Nursery habitat in relation to production of Pristipomoides filamentosus 141 By using the estimated growth coefficient, k, of 0.25/yr derived for juvenile snappers (DeMartini et al., 1994) in the mortality relationship of M/k~ 2 (Ralston, 1987a), the instantaneous natural mortal- ity coefficient, M, was estimated as 0.50/yr, and a range for the instantaneous fishing mortality coeffi- cient, F, was calculated. The low end of the range assumes M = F, on the basis of the fishery operating at maximum sustainable yield (Ralston and Polovina, 1982), providing an instantaneous F of 0.50/yr. The high end of the range assumes that fishing mortal- ity is twice natural mortality, F = 2 M (Ralston, 1987b), resulting in an F of 1.0/yr. The two estimates of F were used independently to represent the ex- tremes of the probable range. The mean standing stock of immature snappers, N3, can be calculated by use of the conventional formula for the annual rate of exploitation (Everhart and Youngs, 1981; Gulland, 1983): Number of snappers Figure 3 Distribution of occurrence of snappers in video drops at east Oahu sites. n3 = c F ( ]_ _ e~\.F+M~\ } (2) F+M resulting in estimates of 42,500-69,600 fish. Because these immature snappers have been exposed to natu- ral mortality for 2 years since the time t} that they inhabited nursery depths, a back calculation provides Nv an initial estimate of juveniles supported on the MHI grounds. The formula (Gulland, 1983) iV1 = e N3 -M»3-q) (3) yielded values of Nx between 115,600 and 189,200 fish. With this estimate of Nv divided by the amount of bottom area in the MHI between the 60 and 90 m isobaths (2,600 km2, NOS bathymetric charts), an estimate of the overall density of juvenile snappers required to support the current fishery was derived. Results The east Oahu study site Two-hundred and eleven video camera drops with standard bait were dispersed throughout the insu- lar slope (60-90 m depth) of the Oahu study site. Abundance data from the video drops were nonnor- mally distributed (33% zero observations) (Fig. 3). Snappers were found at each of the 3 east Oahu can- yons. Snapper abundance differed significantly among the multicanyon stations (K-W, /2=35.6, P<0.01), confirming that relative spatial differences in snapper distribution remain stable (Fig. 4). In the multiyear stations at north Kaneohe canyon, essen- tially similar spatial differences persisted (K-W, %2=37.3, P<0.01); this finding suggests that succes- sive years of juvenile snappers settle spatially ac- cording to habitat quality. Because the effect of sta- tion was significant for both the multicanyon and multiyear comparisons, the abundances of snappers at unreplicated video drops were considered repre- sentative of the habitat quality at those locations. Bottom slope was unrelated to the video indices of snapper abundance (Spearman’s rs=0.Q13, P-0.12). Substrate at 95% of the video drops (60-90 m) was composed of uniform, smooth sediment. High, escarp- ment-type relief was detected in only 3% of the drops. A significantly lower abundance of snappers occurred in the area surrounding escarpment-type relief than in the even sediment bottom (%2=11.48, P<0.001). The 95% confidence intervals of snapper densities at sites with relief (0-1 snappers) versus sites with sediment bottom (3-4 snappers) did not overlap. A similarly low abundance of snappers was associated with ar- eas near exposed hard substrate (%2=10.50, P<0.01; 95% CI=0-2 snappers). Snapper grounds (60-90 m) and the adjacent deeper (90-120 m) area did not dif- fer in the occurrence of soft sediment substrate (X,2=0.44, P=0.43). However, the adjacent shallow grounds (30-60 m) had significantly more hard bot- tom and relief (%2=11.36, P<0.001); soft sediment oc- curred in fewer (71%) of the shallow video images. The duplicate sediment grabs did not differ, sug- gesting that the sediment sampling effectively rep- resented the soft bottom habitat (Wilcoxon MPSR, P=0.93). Of the 5 sediment fractions, snapper abun- 142 Fishery Bulletin 95(1), 1997 CL 2.000 -0.0426 0.54 0.350-2.000 -0.0932 0.178 0.149-0.350 -0.0922 0.184 0.063-0.149 0.0809 0.244 <0.063 (clay-silt) 0.3555 <0.001 was also assessed. All variables except clay-silt were retained by the model (P<0.01; Table 3). Reasons for the model’s exclusion of clay-silt will be discussed later. The model correctly predicted overall presence (>5) or absence (<5) of snapper aggregations for 68% of the video drops. The model predictions of pres- ence (79% [>5]) were roughly balanced by those for absence (60% [<5]) (Table 4). Ranked snapper abun- dance was interpolated by using all video drops to Parrish et al.: Nursery habitat in relation to production of Pristipomoides filamentosus 143 Table 3 Statistical specifics associated with the regression for presence (>5) or absence (<5), of snapper aggregations at east Oahu. Model chi-square (x2)=54.11, P<0.0001, df=3. Estimated Name of variable coefficient Standard error Probability value P Cross product of clay-silt with proximity of drainage source 1.45 x 10-6 3.47 x 10-7 <0.0001 Distance to drainage -7.5 x 1(E7 1.58 x 10“7 <0.0001 Escarpment relief -1.586 0.435 0.0003 Table 4 Two by two table of presence (>5) or absence (<5) of snapper aggrega- tions predicted by the model versus presence or absence observed from baited video drops. Includes all 211 drops at east Oahu. Predicted by model Aggregations Aggregations Percent Observed absent (<5) present (>5) correct Aggregation absent 73 48 60 Aggregation present 19 71 79 68 overall provide an image of snapper distribution at east Oahu (Fig. 5). Abundance of juveniles in the archipelago Fishing surveys at insular slopes other than east Oahu (total 332 km) detected few juveniles (Table 5). Five of these sites (with snappers) were surveyed with video camera to compare with the east Oahu aggregations. Significant numbers of snappers were found only at a site off the southwest end of the island of Molokai (Sept 1993). A repeat video sur- vey indicated that the significant between-station differences in snapper abundance initially reported at South Molokai, persisted 7 months later (K-W, X2=50.8, P<0.05) (Fig. 6). The video index and CPUE of the conventional fish- ing gear were roughly consistent for all sites (Table 6). Snapper abundance was found unrelated to sub- strate type (rs=0.59, P=0.40, n-1) or distance from the 15-m isobath (rs=-0.84, P=0.15, n-1). However, distance/depth of discharge at the four sites with known sources of coastal drainage (Kaneohe, Kailua, S. Molokai, and Hanalei) were associated with snap- per abundance (rs=-1.0, P<0.001, n- 4). Video- and catch-based production estimates Video abundance data from MHI sites at N. Molokai, Hanalei, and Kahului yielded a mean estimated den- iable 5 Fishing effort, length of slope fished, and total snappers caught on surveys of Hawaiian insular slopes for juvenile snappers. Island Bottom trawls 1990 (no.) Bottom longlines 1992 (no. hooks) Fish traps 1989-94 (no.) Handlining 1993-94 (line-hr) Length of slope fished (km) Total snapper caught (no.) Oahu7 16 (512 — (150) 27 (53) 60 (50) 164 (14) 16(828) Molokai 26 — 101 87 60 256 Maui 6 150 — — 16 4 Lanai 6 — — — 16 0 Kauai — 150 — — 18 3 Necker — — 25 — 16 0 FFS3 — — 63 — 42 5 Total 54 300 216 147 332 284 1 The east Oahu study site values are not included in any of the figured totals. 2 Numbers in parentheses represent additional values from gear validation test done at the east Oahu site. 3 FFS = French Frigate Shoals. 144 Fishery Bulletin 95(1 ), 1997 Figure 5 Interpolation of snapper abundance from all video deployments at the east Oahu study site (north Kaneohe, south Kaneohe, and Kailua). Increasing snapper abundance is signified with darker shading. Both north and south channels of Kaneohe Bay are contoured on the map, and a line is used to indicate the eastward extension of the Kailua outfall from the Mokapu pennisula. Isobaths are in 15-m intervals. West Station numbers of spatial replicates East Figure 6 Mean (filled circle) and range (hollow circles) of number of snappers seen per video drop on south Molokai coastline survey. Each station received two video deployments. The drainage from the Kahanui swamp enters the ocean ~1 km to the east of extreme right of the graph. sity of 6.6 snappers/km2, which was taken as representative of routine “nonpre- mium” habitat. Assuming this density and a uniform distribution of snappers at a large scale, we estimated that the 2,600 km2 of available habitat at 60-90 m depth in the MHI is equivalent to 17,200 juve- nile snappers. This video-based estimate is no more than 15% of the 115,600- 189,200 juvenile snappers (44—72 snap- pers/km2) backcalculated from catch in the commercial fishery. A pilot study of recreational fishing6 suggests that if rec- reational catch was included in the back calculation, the difference between video and catch estimates could be as high as one order of magnitude. Discussion Premium nursery habitat Persistence of specific snapper aggrega- tions on east Oahu was supported by both the multicanyon and multiyear analyses. However, because no multi- year surveys extended beyond the north Kaneohe site, we can only assume that year-to-year variability in the other east Oahu sites was similar. A strong year class of snappers might be expected to force some individuals to occupy mar- ginal habitat, making the distinction between snapper aggregations less clear. Results of the multiyear survey indicated that 1993 and 1994 were rela- tively poor years for recruitment of young snappers, suggesting that the observed snappers in the multicanyon stations occupied favorable habitat. Slope showed no significant effect on the distribution of snappers, but relief did. The deep sediment deposits on the terraces preclude any undetected small- scale relief features to which juveniles might orient. The few areas where es- carpment features protruded from the sediment layer were associated with ab- 6 Hamm, D. C., and H. K. Lum. 1992. Prelimi- nary results of the Hawaii small-boat fisheries survey. Honolulu Laboratory, Southwest Fish. Sci. Cent., Natl. Mar. Fish. Serv., NOAA, Hono- lulu, HI 96822-2396. Southwest Fish. Sci. Cent. Admin. Rep. H-92-08, 35 p. Parrish et al.: Nursery habitat in relation to production of Pristipomoides filamentosus 145 sence of snapper aggregations in the logistic model. This finding supports the hypothesis that structural relief or its associated community repre- sents conditions less favorable or more hazardous to the snappers (greater interspecific competition, risk of predation, etc.) (Johannes, 1978; F. A. Parrish, 1989). Expanses of uniform sediment bottom are obvi- ously an important substrate feature. The relative scarcity of this habitat observed at depths <60 m at least partly explains the absence of snappers on the shallower (30-60 m) grounds. Proximity to point sources of drain- age and its relationship with the presence of clay-silt sediment can explain much of the snappers’ long- shore distribution. Work with first- year juveniles of species of Pagrus has demonstrated that substrate and associated water flow are important to habitat selection (Francis, 1995). Improved availability of food has been proposed as a reason for fish demonstrating habitat preferences (Sudo et al., 1983). Distributions of sediment particle sizes such as clay- silt have been shown to enhance the localized distribution of certain benthic invertebrate infauna (Fegley, 1988). A favorable localized sediment composition might contribute to an enhanced forage base for juvenile fish (Tito de Morais and Bodiou, 1984). However, these longshore variations in clay-silt abundance are simply in- dicative of the longshore differences in coastal water flow that disperse the flocculent clay-silt. The highest fraction of clay-silt is found at the seaward end of the north Kaneohe channel, where snapper abundance is high and bay drainage is most con- centrated. The density of fish in Kailua is greatest near the wastewa- ter outfall, where the clay-silt frac- tion is lowest. The outfall introduces and increases the frequency of drift- ing materials to the area, similar to the flow of natural drainage sources, but without creating a clay-silt dis- persion field. For this reason, the lo- gistic model excluded the variable 146 Fishery Bulletin 95 ( I ), 1997 clay-silt. This finding suggests that the distribution of juveniles within the preferred uniform sediment habitat is related more closely to water flow than to sediment pai tide size. Similar enhanced abundances of fish associated with anthropogenic sources have been proposed elsewhere (Mearns, 1974; Monaco et al., 1992) and in Hawaii (Henderson, 1992; Grigg, 1994). In video deployments at many study sites, snap- pers were observed routinely picking at items in the lower water column and mouthing the substrate. DeMartini et al. (1996) determined that juvenile snappers at the north Kaneohe canyon eat a mix- ture of gelatinous drift, demersal crustaceans (am- phipods, etc.), and benthos (micromollusks, annelids, etc.). The majority of prey were <1 cm, of low motil- ity, and bottom associated. Habitats receiving drainage from shallower envi- ronments might have their food supply enhanced in at least two ways. First, fish may encounter and feed more frequently on suspended organisms and other materials flushed from shallower reef and estuarine environments (Gerber and Marshall, 1974). Second, the flow from shallow sources may elevate the or- ganics in sediments, thereby enhancing production of the benthos that snappers eat. Changes in benthic fauna at comparable depths (50-200 m) have been documented in relation to the flux of organics in the water column— both in natural (Buchanan and Moore, 1986) and anthropogenic situations (Nichols, 1985). Benthos may also become enriched during large episodic movements of nutrient-rich bay sedi- ment to localized areas in the snapper grounds. The significant interaction, identified by the logistic model, of clay-silt with proximity to drainage sources supports the notion of enhanced organic input to the benthic community provided by such drainage. Distribution of juveniles in the archipelago Conventional fishing on the insular slopes of the ar- chipelago (332 km) identified few sites with juvenile snappers; the mode and median of the catch of juve- niles from all the gear was zero. Except for aggrega- tion sites at Oahu and Molokai, catches of juveniles occurred only in token numbers. In a 1967-68 dem- ersal trawl survey (n=6 2), Struhsaker sampled ~90 km of relevant depths in the main Hawaiian Islands and similarly found the occurrence of juveniles to be infrequent and patchy. His catches of juvenile snap- pers had a mode of zero and median of one (Struh- saker, 1973). The 5 sites other than east Oahu that were sur- veyed by video (Table 6) each had substrate and depths consistent with those at east Oahu; 2 had sources of drainage; but only 1, south Molokai, sup- ported a snapper aggregation. South Molokai’s Kahanui swamp, located within the island’s exten- sive fringing reef complex, has a drainage channel similar in width and depth (15 m) to north Kaneohe Bay (U.S. Army Corps of Engineers, 1984). Its asso- ciated snapper aggregation is well situated to exploit the tidal drainage of the reef platform and swamp dispersed by westbound currents of the area7 (Fig. 6). The Hanalei estuary, on the island of Kauai, prob- ably fails to influence snapper depths because it dis- charges at a zone of high-energy mixing (~1 m depth) too far inshore from juvenile snapper grounds.8 The site at Kahului, Maui, would have to have a very large coastal drainage feature to aggregate snappers; the distance between the snapper grounds and such a source would be twice that of the other sites sur- veyed. Presumably, any source of increased sus- pended materials (embayments, reef platforms, or atoll lagoons) could enhance snapper aggregations if depth, distance, and circulation characteristics fo- cused water and increased the frequency of sus- pended materials close to juvenile grounds (Cyrus and Blaber, 1983; Birkeland, 1984). Struhsaker, during his 1967-68 trawl survey, iden- tified one location (north coast of Oahu) with catches as high as 180 individuals in one haul. The substrate at the site was composed of uniform sediment and received discharge from two north Oahu rivers. How- ever, according to the surveys from the present work, the snapper depths at this site seem almost too far offshore (mean=4.5 km) to support an aggregation. Numerous attempts in 1990 (Table 5) to relocate this north Oahu aggregation with the same gear that was used in 1967-68 did not yield any snappers. Many changes that could have modified the suitability of this habitat for juveniles (e.g. heavy exploitation of the snapper stock [WPRFMC2]; collapse of the coast’s large-scale irrigation-based agriculture and its drain- age; effects of increasing relief on juvenile grounds from the accumulation of incidental ocean dumping) have occurred in the 22 years between the surveys. Implications for the fish stock Regardless of what factors create premium habitat, the implications for the snapper stock of the archi- 7 Wyrtki, K., V. Graefe, and W. M. Patzert. 1969. Current ob- servations in the Hawaiian Archipelago. Hawaii Institute of Geophysics HIG-69-15, 27 p. Hawaii Inst. Geophysics, 2525 Correa Rd., Honolulu, HI 96822. 8 U.S. Geological Survey. 1993. Water resources data Hawaii and other Pacific areas, water year 1993. Water-data Report HI-93-l:78-79. U.S. Geological Survey, 677 Ala Moana Suite, Honolulu, HI 96813. Parrish et al.: Nursery habitat in relation to production of Pristipomoides filamentosus 147 pelago are intriguing and potentially important. It is not clear how widespread such habitat (and asso- ciated high densities of juvenile snappers) may be in Hawaii; present surveys and those of 1967-68 sug- gest that it represents a minor fraction of all habitat at appropriate depths. Use of the observed mean density of snappers on other habitats (6.6 snappers/ km2) produced an estimate of juvenile standing stock much lower than that derived from catch records. A possible explanation for the discrepancy is that an abundance of snappers use unidentified habitats sig- nificantly shallower or deeper than 60-90 m. How- ever, extensive diving in shallower waters, observa- tions from submersibles (Moffitt et al., 1989; Haight et al., 1993), and systematic trawl surveys of deeper waters (Struhsaker, 1973) have not disclosed juve- niles in other depth ranges. Conceivably, areas at depths with less than prime habitat for juvenile snap- pers may support loose, mobile aggregations with large home ranges that are difficult to relocate. As of yet, no such aggregations have been documented. According to the Kaneohe GIS data (Fig. 5), juve- nile snappers occurred within an area of 8 km2 and showed a median video-based density index of 7; therefore, Kaneohe is likely to support 450 snappers/ km2 (68-fold above mean estimated density) or a to- tal of 3,600 snappers. This finding suggests that re- cruits from premium habitats like Kaneohe can pro- duce a significant percentage of the MHI juveniles. If Kaneohe snapper abundance values are applied to reconcile the difference between the estimates generated by video densities in nonpremium habi- tats and those obtained by fishery catches, between 9% and 15% of the MHI habitat would have to be of the premium type to account for the current com- mercial snapper catch. If recreational catch is con- sidered, a larger fraction of total habitat must be of a premium type. Exploring the actual extent of this habitat and the adult stock’s dependence on it should be a management priority and a major focus for fu- ture work. Acknowledgments We are grateful to Deborah Goebert, Matt Mc- Granaghan, Ross Sutherland, and Everett Wingert for providing expertise helpful in preparing this re- port. Thoughtful reviews were provided by Wayne Haight, Don Kobayashi, Robert Moffitt, James Parrish, Jeffrey Polovina, and Jim West. The City and County of Honolulu, Department of Wastewa- ter, assisted by providing wastewater discharge data. Literature cited Bardach, J. E. 1959. The summer standing crop of fish on a shallow Ber- muda reef. Limnol. Oceanogr. 4:77-85 Birkeland, C. 1984. Influence of topography of nearby land massess in combination with local water movement patterns on the nature of nearshore marine communities. UNESCO Re- ports Mar. Sci. 27:16-31. Bromwell, K. B. 1992. Assessing the primary biological productivity of Kailua Bay, its influencing streams and 2° treated efflu- ent, through algal biostimulation analysis. Master’s the- sis, Univ. Hawaii, Honolulu, HI, 33 p. Buchanan, J. B., and J. J. Moore. 1986. 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U.S. Army Corps of Engi- neers, Honolulu, HI. 149 Abstract.-Many tuned assessment models, such as sequential population analysis and nonequilibrium produc- tion models, are cast in the form of least-squares minimization routines. It is well known that outliers can substan- tially alter the results of least-squares methods. Indeed, in the process of con- ducting stock assessments, much time and effort are often spent in discussing the merits of individual data points and in evaluating the impact that includ- ing or excluding them has on the per- ceived stock status. Unfortunately, straight-forward statistical tests for detecting outliers have been developed only for univariate statistics or for the simplest of linear models and are gen- erally useful to test for a single outlier only. In this paper, we apply a high- breakdown robust regression tech- nique, least trimmed squares, to two assessment models using North Atlan- tic swordfish and West Atlantic bluefin tuna as examples. We illustrate how robust regression can be used as an ini- tial step in statistically detecting out- liers before the more efficient least- squares minimization can be used. Manuscript accepted 30 July 1996. Fishery Bulletin 95:149-160 (1997). Application of high-breakdown robust regression to tuned stock assessment models Victor R. Restrepo Rosenstie! School of Marine and Atmospheric Science, Cooperative Unit for Fisheries Education and Research University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33 1 49 E-mail address: vrestrepo@rsmas.miami.edu Joseph E. Powers National Marine Fisheries Service, Southeast Fisheries Science Center 75 Virginia Beach Drive, Miami, Florida 33149 Tuned stock assessment models are statistical methods that analyze time series of fishery catch data in conjunction with auxiliary informa- tion (indices of relative abundance, fishing effort, etc. ) to yield estimates of stock abundance and exploitation rates over time. Such methods are widely used today by stock assess- ment working groups throughout the world because they provide an objective and statistically defensible way to assess the status of stocks and to derive management advice. The two primary methods are se- quential population analysis (SPA: Fournier and Archibald, 1982; Deriso et al., 1985; Pope and Shep- herd, 1985; Kimura, 1989; Methot, 1990; Powers and Restrepo, 1992; Gavaris1) and nonequilibrium pro- duction models (Pella and Tom- linson, 1969; Hilborn, 1990; Hilborn and Walters, 1992; Prager, 1994). SPA’s are typically age structured and production models are not, al- though there are exceptions to this generalization in the references just cited. Both types of methods, how- ever, share the commonality of of- ten being cast as nonlinear least- squares minimization problems. Despite efforts to standardize all steps involved in a stock assessment (from data collection, preparation of model inputs, to running the mod- els), stock assessments are rarely automated and, more often than not, generate controversy. In our experience with different fora, a common cause for controversy is as follows: various data sets are pre- sented to a working group and then the group collectively decides on the sets of data and model assumptions to be used. The consensus selection is typically termed the “base case.” Individual data points are then scrutinized for exclusion from fur- ther analyses to determine the ro- bustness of the overall assessment to the sensitivity changes. This par- tial “sensitivity analysis” can, in practice, be undesirable because perceptions of what results ought to be like may influence which data or data points are scrutinized and thus generate controversy; not every working group participant has the same perception. The lack of an a priori objective selection process could lead working groups astray (Restrepo and Powers, 1995). A so- 1 Gavaris, S. 1988. An adaptive frame- work for the estimation of population size. Can. Atl. Fish. Sci. Adv. Comm. (CAFSAC) Res. Doc. 88/29, 12 p. Biological Station, Department of Fisheries and Oceans, St. Andrews, New Brunswick, Canada EOG 2X0. 150 Fishery Bulletin 95( 1 ), 1997 lution to this problem lies in a method that would objectively identify — and deal with — “outliers.” Statistical tests have been developed for identify- ing outliers (see Barnett and Lewis, 1994), but most of the straight-forward approaches can only deal with a few outliers in univariate analyses or in linear re- gression. High-breakdown robust regression meth- ods (Rousseeuw, 1984; Rousseeuw and Leroy, 1987) hold promise for addressing the issue, as suggested by several recent papers in fisheries literature (e.g. Chen et ah, 1994; Chen and Paloheimo, 1994). The goal of high-breakdown robust regression is to pro- vide model estimates that are insensitive to contami- nation (up to 50%) by outliers and thus will serve to identify outlying observations. However, most robust regression applications in fisheries science (Chen et al., 1994; Chen and Paloheimo, 1994) and in statis- tics literature have been developed for linear prob- lems (but, see Stromberg, 1993). In this study we seek to illustrate the application and usefulness of this tool by using two nonlinear examples: a nonequilibrium production model for North Atlantic swordfish, Xiphias gladius, and a sequential popu- lation analysis for West Atlantic bluefin tuna, Thunnus thynnus. Both stocks are assessed by the Standing Committee on Research and Statistics (SCRS) of the International Commission for the Con- servation of Atlantic Tunas (ICCAT). The analyses presented here are illustrative and are not intended to replace those of the SCRS. Methods Assessment models Assuming a normal (Gaussian) error structure, the typical tuned assessment method minimizes the squared deviations (residuals, r) between observed and predicted indices of abundance: m nl m minXi(/y ~4)2 =minSS(ry2)j (1) i= 1 7=1 i=l 7 = 1 for m indices, each with ni observations. The predic- tion of each index, / ■ comes from a population model, such as a surplus production model or a sequential population analysis. Alternatively, the minimization can be made in terms of observed and predicted catches or in terms of observed and predicted fish- ing effort. Note that some maximum-likelihood ap- proaches do not make the normal error assumption (e.g. Fournier and Archibald, 1982); we focus on those approaches that are in a least-squares framework or that can be transformed to one, which include itera- tively reweighted least squares and some forms of maximum likelihood. In this paper, we give robust regression examples using two population models. A detailed explanation of these methods is beyond the scope of this paper and readers are referred to the citations given be- low. The surplus production model corresponds to a Schaeffer (logistic) form, fitted as nonequilibrium time series by using the continuous time method pre- sented by Prager (1994). This method estimates pa- rameters describing the carrying capacity, rate of intrinsic population growth, initial biomass, and catchability coefficients that best explain observed time series of relative abundance according to the criterion in Equation 1. The sequential population analysis corresponds to a tuned virtual population analysis method known as ADAPT, an age-structured assessment framework popular in the east coast of North America. Details on ADAPT can be found in Powers and Restrepo (1992, 1993), Punt (1994), and Gavaris.1 ADAPT estimates age-specific fishing mor- tality rates in the last year of data and catchability coefficients that satisfy Equation 1, while forcing cohorts to conform to exponential survival through time: M , , = N e~Za'y •4¥a + l,j'+l a,y'-' ’ where N denotes stock size in numbers, Z denotes instantaneous total mortality, and a and y are sub- scripts for age and year. Data sets The data set used with the nonequilibrium produc- tion model is for North Atlantic swordfish as em- ployed by ICCAT in its 1994 assessment (ICCAT, 1995). This data set consists of total landings (in weight) for the period 1950-93 and of a single stan- dardized longline series of catch per unit of effort (CPUE, used as a measure of relative abundance), spanning the period 1963-93 (Table 1). After a se- ries of sensitivity tests, ICCAT assumed in its “base case” analysis that the initial biomass in 1950 was a known quantity, equal to 0.875 times the stock’s car- rying capacity. Thus, 3 parameters were estimated: carrying capacity, intrinsic rate of growth, and a con- stant of proportionality ( q ) relating the series of rela- tive abundance (X) to absolute biomass units ( B ). The minimization of Equation 1 was done in log scale, i.e. Ijj = ln(Xy) and 7y = In (qBj). The data for the SPA is for West Atlantic bluefin tuna, also as employed by ICCAT in its 1994 base case assessment (ICCAT, 1995). It consisted of catch Restrepo and Powers: Application of robust regression to tuned stock assessment models 151 Table 1 North Atlantic swordfish, Xiphias gladius, data used for the nonequilibrium production model (from ICCAT, 1995). Relative abundance is in Kg/1,000 standard hooks, standardized from Canadian, Japanese, Spanish, and U.S. longliners. t = metric tons. Year Landings (t) Relative abundance Year Landings (t) Relative abundance 1950 3,646 1973 6,001 — 1951 2,581 — 1974 6,301 — 1952 2,993 — 1975 8,776 421.69 1953 3,303 — 1976 6,587 353.66 1954 3,034 • — 1977 6,352 393.92 1955 3,502 — 1978 11,797 649.61 1956 3,358 — 1979 11,859 338.57 1957 4,578 — 1980 13,527 430.69 1958 4,904 — 1981 11,138 310.18 1959 6,232 — 1982 13,155 356.96 1960 3,828 — 1983 14,464 287.88 1961 4,381 — 1984 12,753 286.12 1962 5,342 — 1985 14,348 265.94 1963 10,189 1,258.10 1986 18,447 255.54 1964 11,258 467.29 1987 20,234 217.30 1965 8,652 294.86 1988 19,614 207.62 1966 9,338 273.50 1989 17,299 196.90 1967 9,084 320.22 1990 15,865 199.20 1968 9,137 269.55 1991 15,224 194.02 1969 9,138 233.95 1992 15,593 182.55 1970 9,425 274.25 1993 16,977 172.27 1971 5,198 — 1972 4,727 — at age from 1970 to 1993 for ages 1 to 10+ (Table 2), and of 7 indices of relative abundance assumed to track different segments of the population (Table 3; see Fig. 4 ). A number of assumptions were made and these can be found in Appendix BFTW-2 of ICCAT (1995). The parameters estimated were 7 constants of proportionality relating each index of relative abundance to absolute biomass or numbers and 4 fishing mortalities in 1993 (for ages 2, 4, 6, and 8). We reiterate that we chose the same data sets and model structures as those in ICCAT (1995) for illus- trative purposes. It may be worthwhile to investi- gate the results of robust regression techniques ap- plied to alternative data (e.g. indices obtained with a different standardization procedure) or to formu- lations (e.g. different assumptions about known quantities and other constraints). Robust regression Several robust minimization criteria discussed in Rousseeuw and Leroy (1987) have been applied to fisheries data (see Chen et al., 1994). In contrast with the method of least squares, the goal of these tech- niques is to moderate the influence of outliers in the parameter fitting process (Eq. 1). Of particular in- terest to us are the so-called “high-breakdown” meth- ods that are insensitive to up to 50% contamination by outliers, because they can effectively be used as an objective method to identify outliers. Two high-breakdown robust regression methods are least median squares (LMS) and least trimmed squares (LTS). LMS minimizes the median of the squared residuals and LTS minimizes the sum of the lowest xn squared residuals, where x is a fraction (less than 1.0 to 0.5) defined by the user. The results of an LMS regression and an LTS regression with a 50% trim are essentially very similar, although the LTS one is statistically more efficient (Rousseeuw and Leroy, 1987). In our initial experimentation with fisheries assessment models, we found that the LTS minimum was somewhat easier to find (the LMS could sometimes not converge, indicating that a large number of restarts may be required). Therefore, we limited our investigation to the LTS minimization criteria discussed below. This can be either m nj 2+1 LTS1 = y min (r2 ) (2) LTS2 = min^T ^V2)y:; i=l 1=1 152 Fishery Bulletin 95 ( 1 ), 1997 Table 2 West Atlantic bluefin tuna, Thunnus thynnus, catch at age data (in numbers) used for the sequential population analysis (from ICC AT, 1995). Age Year 1 2 3 4 5 6 7 8 9 10+ 1970 64,886 105,064 127,518 21,455 3,677 914 176 172 535 3,726 1971 62,998 153,364 38,360 46,074 672 1,673 2,109 1,350 1,133 5,957 1972 45,402 98,578 33,762 3,730 3,857 118 569 576 261 5,519 1973 5,105 74,311 30,482 7,161 2,132 1,451 953 1,544 555 4,444 1974 55,958 20,056 21,094 6,506 3,170 683 916 913 1,081 12,508 1975 43,556 148,027 8,328 11,963 821 547 317 671 1,651 9,472 1976 5,412 19,781 72,393 2,910 2,899 344 206 1,168 558 14,033 1977 1,274 22,419 9,717 32,139 4,946 3,633 957 513 1,109 13,532 1978 5,133 10,863 20,015 6,315 10,530 4,061 655 472 341 11,982 1979 2,745 10,552 16,288 14,916 3,448 3,494 2,612 599 557 12,283 1980 3,160 16,183 11,068 8,881 2,866 2,982 5,533 3,454 1,061 12,213 1981 6,087 9,616 16,541 5,244 6,023 3,721 2,884 3,211 2,764 10,621 1982 3,528 3,729 1,654 498 342 751 477 519 896 3,077 1983 4,173 2,438 3,268 894 866 911 1,402 1,353 1,039 5,628 1984 868 7,504 1,848 2,072 2,077 1,671 594 759 1,091 4,574 1985 568 5,523 12,310 2,814 4,329 4,019 1,024 612 698 5,603 1986 563 5,939 7,135 3,442 1,128 1,726 931 520 345 5,335 1987 1,513 13,340 9,137 5,491 4,385 2,318 1,566 1,251 1,014 3,856 1988 4,850 9,149 11,745 3,933 4,144 4,220 2,258 1,631 1,600 4,555 1989 787 12,877 1,679 3,815 1,713 2,082 2,677 1,864 1,461 5,356 1990 2,368 4,238 17,958 1,947 2,747 1,825 1,629 2,388 1,522 4,253 1991 3,327 14,533 10,761 2,924 1,650 2,166 2,347 1,946 1,915 4,485 1992 420 5,985 1,997 711 1,425 737 1,916 1,870 1,323 4,383 1993 329 1,130 5,215 3,689 2,089 1,883 1,598 2,456 1,479 2,922 where the notation j:ni indicates that the squared residuals are sorted in ascending order from j= 1 to n-; note that nJ2 +1 is actually an integer value equal to nj 2 when ni is even and equal to nJ2 +1 when ni is odd. Equations 2 and 3 are two different minimiza- tion objectives that differ in the way they treat mul- tiple series of relative abundance data. In Equation 2, the trimmed sums of squared residuals are com- puted separately for each index and then added to the objective function being minimized. Thus, the individual indices are de facto given equal weight- ing. In Equation 3, the trimming is done over all avail- able data points, regardless of which relative abundance series they belong to. Thus, the LTSj formulation forces each available series to contribute to the objective func- tion, whereas the LTS2 formulation could plausibly eliminate indices that fit very poorly in comparison with the others. An analogous distinction can be made for the LS fit by giving either equal weight to all available data series (as in Eq. 1) or by assigning weights to each series in proportion to their mean squared errors. The latter has often been accomplished by means of itera- tive reweighting (Powers and Restrepo, 1992) or maxi- mum likelihood (Punt, 1994). Algorithms for high-breakdown robust regression are notoriously computation-intensive, even in the simplest univariate linear regression case (Rousseeuw, 1984; Rousseeuw and Leroy, 1987; Steele and Steiger, 1986) . A typical algorithm for a linear robust regres- sion with p parameters goes like this: For a large number of times, s, select p data points, do a least- squares regression (LS) and compute the correspond- ing robust objective function (e.g. sum of trimmed squares) for the complete data set. The LTS solution is given by the parameter estimates and results in the lowest robust objective function value. In the lin- ear case, the value ofs is chosen such that, for a given fraction of data contamination and a given p, at least one of the s subsamples is not contaminated (Rousseeuw and Leroy, 1987). The choice of s in the nonlinear case is not clearcut. However, in the lin- ear case the values of s grow very rapidly with p and percent contamination; therefore many available al- gorithms set s = 3,000 for p > 9 (Rousseeuw and Leroy, 1987) . Similar values were used here for the nonlin- ear case. Algorithms for nonlinear robust regression are rare, owing partly to the increased computational Restrepo and Powers: Application of robust regression to tuned stock assessment models 153 Table 3 West Atlantic bluefin tuna, Thunnus thynnus, relative abundance indices (from ICCAT, 1995). The larval index is in relative biomass units, while all others are in relative numbers. The numbers below each index label are the ages or range of ages that each index is assumed to represent. TL = tended line, LL = longline, RR = rod and reel, GOM = Gulf of Mexico, NWA = Northwest Atlantic. Year Canada TL 10+ Japan LLGOM 10+ Japan LLNWA 1-9 Larval GOM 8+ US LLGOM 8+ US RR 8+ US RR 1-5 1974 1.4670 1975 — 1.0200 — — — — — 1976 — 0.8960 0.8134 — — — — 1977 — 0.6700 1.7822 1.7704 — — — 1978 — 0.9350 1.4621 4.2341 — — — 1979 — 0.9380 0.5476 — — — — 1980 — 1.5130 1.0327 — — — 1.2109 1981 2.3489 0.5610 1.4812 0.9575 — — 0.1274 1982 2.1095 — 0.7121 1.1008 — — 1.3417 1983 1.5621 — 0.5022 0.8977 — 2.4703 0.7816 1984 1.0718 — 0.8527 0.4750 — 1.0949 — 1985 0.5131 — 0.9967 — — 1.0483 0.5366 1986 0.6157 — 0.5725 0.1897 — 0.7324 0.9995 1987 0.3991 — 1.1490 0.3236 1.7544 0.6933 1.2138 1988 0.6271 — 0.8773 1.4146 0.6842 1.3195 1.6059 1989 0.4561 — 0.7417 0.5803 1.0526 0.6808 1.3339 1990 0.2965 — 0.7754 0.3446 1.1404 0.6204 0.7331 1991 — — 0.7523 0.2652 1.5614 0.7694 1.3277 1992 — — 1.8813 0.4464 0.5263 0.8727 0.7968 1993 — — 1.0675 — 0.2807 0.6981 0.9912 requirements. Although the LS solutions for the lin- ear case (as described in the previous paragraph) can be accomplished with simple matrix manipulations, nonlinear LS solutions require iterative computa- tions. Stromberg (1993) presented a multistage al- gorithm for nonlinear regression that is similar to the one outlined above, succeeded by a direct mini- mization of the robust objective function by using the simplex search of Nelder and Mead (1965). Building upon Stromberg’s ideas, we reviewed algorithms for an LTS1 solution to the bluefin tuna SPA. On the basis of these results and the work of Stromberg (1993), we adopted the algorithm below but acknowl- edge that there are many other possible fruitful op- tions to be explored, such as “simulated annealing” (Corana et al., 1987). Our algorithm uses the fact that the simplex search of Nelder and Mead (1965) requires p+ 1 starting guesses, denoted by v vertices, for each of the p parameters being estimated. 1 Find the LS estimate for the entire data set. The estimates (p)LS are used as starting guesses for step 2. 2 Repeat s times: a) Set initial parameter guesses at random from within 10 times the (p)Lg estimates from step 1. b) Find the LTS estimates for the complete data set by using the starting values from step 2a. c) Restart step 2b until the objective function (ei- ther Eq. 2 or Eq. 3) does not change appreciably. d) Save the parameter estimates corresponding to the (p+l)LTS parameter sets with the lowest objective function value. 3 Initialize the v vertices for the simplex search with the best (p+1) parameter sets from the s solutions from step 2 and find the LTS estimate for the en- tire data set. As in step 2, carry out restarts as needed. This algorithm is a direct robust minimization search that is initialized s times from a Monte Carlo grid centered around the LS solution. It is compu- tationally intensive, but this seems necessary given the multi-modal nature often encountered in the LTS or LMS objective function. For this study we used s = 500. For both the swordfish nonequilibrium pro- duction model and the bluefin tuna SPA analyses, step 2 involved 5 restarts on average and thus made the total number of minimizations greater than 2,500. It should be noted that this search algorithm does not guarantee that a global minimum LTS so- lution is going to be found. Therefore, we favor mul- 154 Fishery Bulletin 95(1 ), 1997 tiple replicates and restarts so that there is some confidence that the solution is globally minimal. At this point we have no firm guidance about the tradeoffs between the number of replicates (s) and the number of restarts other than to say that repli- cates are probably more important than restarts. For example 500 replicates with 5 restarts seems prefer- able to 25 replicates with 100 restarts. Dealing with outliers Aside from biological or fishery considerations, sta- tistical outliers are data points whose residuals, scaled by the dispersion of errors, rJL a are far from the mean scaled residual. For the simple LS minimization (Eq. 1), the overall dispersion of the residuals is the mean squared error (MSE), o = *'= 1 7=1 For the LTS regression, the dispersion is similarly computed as a robust measure of average dispersion ( cr for index i in Eq. 2 or a for all data points in Eq. 3): a, =3.7444 n< >. I>%, for LTSj, Eq. 2, or a = 3.7444 m / 2+ 1 for LTS2, Eq. 3. The constant 3.7444 is a correction factor used to achieve consistency with normal error distributions (Rousseeuw and Leroy, 1987). As a rule of thumb, Rousseeuw and Leroy (1987) suggest that absolute values of scaled residuals larger than 2.5 can be treated as statistical outliers. Owing to the small number of observations in some of our data series, we use a thresh- old based on the t- distribution with a = 0.01 and n- 1 degrees of freedom. After obtaining the LTS estimates, we carried out a new least-squares minimization ex- cluding from the analyses any absolute scaled residu- als greater than the corresponding critical value. We refer to this final result as the “trimmed LS” solution. Results Swordfish nonequilibrium production model The swordfish data represent a simple example with a single index. Nevertheless, there are several very large deviations between the observed and predicted index values, when the traditional least-squares (LS) solution to the nonequilibrium production model fit is computed (Fig. 1). Indeed, these deviations have generated considerable debate (ICCAT, 1995). There- fore, we applied the robust regression techniques of the LTS algorithm and the trimmed LS method of outlier detection to this example. The LTS solution (with a 50% trim) was computed 500 times with 5 restarts each. There did not appear to be problems of multiple minima with this example because vir- £ 0.50 10.0 -10.0 ±_ 65 75 80 Year 90 Figure T Biomass index values for North Atlantic swordfish using least squares (LS), least trimmed squares (LTS), and trimmed least squares (Trimmed LS). The top panel shows observed and predicted index values. The bottom panel shows the scaled residuals from the LTS fit compared with the critical value for the outlier detection criterion (dashed lines); solid circles are those data points considered to be outliers and not included in the final trimmed LS solution. Restrepo and Powers: Application of robust regression to tuned stock assessment models 155 tually all of the 500 solutions converged to the same value. The outlier detection criteria identified five of the original 27 data points (19%), all of which oc- curred before 1981 (Fig. 1). Predicted index values with both the LTS and trimmed LS solutions were higher than the LS predictions prior to the late 1980’s and lower than LS predictions in recent years (Fig. 1). Predicted relative biomass values with either the LTS or trimmed LS solutions are higher than the initial LS solution (ICCAT, 1995), particularly in the 1990’s (Fig. 2) and suggest less of a decline in the population. Abso- lute biomass predictions with the LTS method were generally higher than those from the initial LS solu- tion, whereas trimmed LS solutions were lower (Fig. 2). The trimmed LS solution results in biomass levels Q Q - Ll J I II I i — LJ — LJ — l_l — I — I — L .] 1 I I L_l L_J I L_J LJ L_i_l L_J I 70 75 80 85 90 95 00 05 0 Ll 1 i i i i i i i i i i i 70 75 80 85 90 95 00 05 Year Figure 2 Predicted biomass relative to biomass at maximum sus- tainable yield (B/BMSY, top panel) and absolute biomass (bottom panel) resulting from LS, LTS, and trimmed LS solutions. The left side of the graphs show the production model estimates. The right sides of the graphs are projec- tions made with the fishing mortality rate at maximum sustainable yield (FMSY) and with the fishing mortality rate in 1993 (F 93). Ascending limbs were projected by using Fmsy. descending limbs by using F 93. that are lower than those in the other two methods; however, the decline over the time series is less. Bio- mass projections were made under two strategies: 1) a recovery strategy in which future fishing mortality rate was fixed at the value that would produce maximum sustainable yield and 2) a status quo strategy in which the fishing mortality would be fixed at the 1993 level. The LTS and trimmed LS projections indicate that both recovery and decline is not as rapid as that predicted from the initial LS solution (Fig. 2). The robust regression techniques applied here tend to provide a better fit to the index data points in re- cent years at the expense of the data points in the earlier years of the series. Indeed, several of the points identified through the outlier detection pro- cess were those data points for which there was much debate regarding variability and bias (ICCAT, 1995). However, some of the data points identified here were not identified by ICCAT (1995); therefore, we reem- phasize the point that the selection of outliers should be based on objective criteria. Bluefin tuna SPA As mentioned before, a high-breakdown robust re- gression objective function can possess multiple minima. Figure 3 illustrates this point with the LTSj objective function plotted around ± 50% of the final estimate for one of the parameters, while all other parameter values were fixed at their solution. The fig- ure highlights the need for an exhaustive search ow- ing to the multimodal nature of the response surface. Figure 4 shows the observed indices of relative abundance in the first column, the scaled residuals o 0 05 -I 1 1 0.25 0.5 0 75 Parameter estimate (1CT4) Figure 3 Trimmed squares objective function (Eq. 2) plotted around the solution for one of the parameters estimated in the bluefin tuna sequential population assessment (catch- ability for the U.S. rod and reel large fish index, Table 3). The plot shows that multiple local minima can occur in robust regression problems. Observed indices Scaled residuals (LTS^ Scaled residuals (LTS2) 156 Fishery Bulletin 95( 1 ), 1997 in r- cn I 1 — I 1 1 — I 1 — I ocotoir('io(\i'r(o o cn ♦ h o H 1 1 1 1 TT (N O N 't l£) O r- O) I 1 h O CO ID — I h O • tively. Crossed symbols identify statistical outliers at the 1% significance level. Restrepo and Powers: Application of robust regression to tuned stock assessment models 157 158 Fishery Bulletin 95 ( 1 ), 1997 from the LTSj fit (second column), and the LTS9 fit (last column). The open symbols indicate which data points were identified as outliers according to the t- test criterion mentioned previously. The LTS: regres- sion, which gives equal consideration to all index series, identified 9 outliers (11% of the total index data points). The LTS2 approach, which gives more weight to the better-fitting series, identified the same 9 observations as outliers, and an additional 8 (21% of the total number of data). The 1978 estimate from the larval index stands out as a particularly large outlier (Fig. 4). But perhaps more importantly in terms of the effect on the SPA results, the 1992 data point for the Japanese Northwest Atlantic longline index, is also identified as a large outlier. That is, because of the convergence properties of the ADAPT approach, the more recent data tend to have a larger impact on the estimates of current stock status. Figure 5 shows the estimated stock size trajecto- ries for 3 age groupings that ICCAT assessments fo- cus on: small fish (ages 2 to 5), medium fish (ages 6 and 7), and spawners (ages 8 and older). The solid line without symbols represents the ini- tial LS solution (Eq. 1), as in the 1994 ICCAT assessment. The 2 dashed lines with symbols (virtually indistinguishable from each other) represent the final trimmed LS solutions, i.e. after removal of the outliers identified in Figure 4. Note that all the stock size estimates are iden- tical in the first half of the time series, owing to the convergence properties of the SPA. Differences in 1990’s stock size es- timates before and after trimming are most notable for small and medium blue- fin tuna (Fig. 5). For this example, the final trimmed LS solutions estimate lower current stock sizes (Fig. 5) and cor- respondingly higher current exploitation rates (not shown). The impact that these differences in the estimates have on management recom- mendations can be appreciated in Figure 6, which shows a 10-year projection of the stock’s spawning biomass at two levels of constant landings considered by ICCAT. These projections were made by using the same assumptions as those in the assessment (Appendix BFTW-2 in ICCAT, 1995): essentially, that recruit- ment is constant after a certain parental biomass level and that the 3 most recent recruitment values from the SPA are poorly estimated and are replaced by the geometric mean recruitment from past years. The top panel in Figure 6 is a pro- jection made by assuming 2,000 metric tons (t) landings after 1993: the lower panel assumes 2,660 t landings after 1993. The solid lines represent the LS solution as in the ICCAT assessment, and the dashed lines represent the LS solu- tions after trimming (squares for results from the LTSj solution and circles for re- sults from the LTS2 solution). The pro- jections made without removing outliers Year Figure 5 Bluefin tuna stock size estimates for 3 groups of ages. The solid line represents the estimates from the least-squares solution with all the available data, as in the ICCAT assessment. The dashed lines show the least squares estimates after removal of the data points identified as outliers in Figure 4. Squares = after minimization with Equation 2; circles = after minimization with Equation 3. Squares and circles overlap. Restrepo and Powers: Application of robust regression to tuned stock assessment models 159 are optimistic and suggest a contin- ued increase in parental biomass even at the higher level of landings. The projections made after trimming, on the other hand, are less optimis- tic. These suggest a more modest in- crease in spawning biomass at the 2,000 t level of landings, or a decline in spawning biomass after 7 years of 2,660 t landings (Fig. 6). Discussion The robust regression methods as applied to tuned population assess- ment models may be helpful in sev- eral ways. The methods can be used as an alternative minimization cri- terion to obtain estimates of the population parameters. They can also be used to identify outliers for elimination from subsequent fitting. In either case, much of the subjectiv- ity that can enter discussions about individual data points during work- ing group meetings would be elimi- nated. The latter aspect (identifica- tion and elimination of outliers) is especially useful because, after elimi- nation of the outliers, one can then go on and conduct the normal boot- strap (Punt, 1994) or Monte Carlo (Restrepo et al., 1992) analyses used to evaluate uncertainty in the esti- mates. The robust regression methods could be used to screen the outliers, and then the other methods could be used to estimate variability and to project the population status under different management scenarios. Presently, computation time would pre- clude incorporating bootstrap or Monte Carlo tech- niques directly into the LTS search. Removing outli- ers should, also, have a moderating effect on the so- called retrospective patterns (Sinclair et al., 1990), some of which are caused by outliers in the indices (ICES, 1995). It is important to keep in mind a point of caution when removing statistical outliers from an assess- ment. Observations that appear to be outliers are so in the overall context of data-model. That is, it is possible that a data point is considered as either an outlier or not, depending on the model formulation, constraints, etc. For example, if the bluefln tuna in- dices of abundance had been considered to be log- normally distributed instead of normally-distributed, the LTS regression may have identified more or fewer observations as outliers. A related point is that we do not advocate rushing to eliminate outliers auto- matically from stock assessments. Instead, a first step should be to look into reasons why such obser- vations may seem like outliers, e.g. undetected tran- scription errors or environmental influences that were not accounted for in the analysis. Additionally, the outlier detection would identify candidates for sen- sitivity analysis in an objective manner. Instead of de- termining data points that are influential on the re- sults and trying to determine if those points could be considered outliers, we are advocating the converse. The outlier detection procedures outlined here in- herently assume symmetry in the response surface. Thus, it is expected that the trimmed LS technique will provide results similar to those coming from bias correction procedures used in bootstrapping meth- ods (e.g. Prager, 1994). Both methods assume that the underlying distributions are symmetrical and 160 Fishery Bulletin 95 ( 1 ), 1997 adjust the results in order to maintain that symme- try. However, if model constraints or other features of the model or data force the response surface to have an underlying (but unknown) skewed distribu- tion, then the outlier selection process outlined here might falsely identify some data points as outliers. Conversely, the least trimmed squares (LTS) solu- tions make no assumptions about the shape of the response surface. Therefore, we expect that the LTS method could be robust to those situations where the distribution is skewed. Nevertheless, with judicious application, robust regression is expected to be a useful tool for evaluating and selecting data appro- priate for tuning stock assessment models. Acknowledgments We are grateful to two anonymous reviewers for their critical review of this manuscript. Support for this study was provided through the Cooperative Unit for Fisheries Education and Research (CUFER) by Na- tional Oceanic and Atmospheric Administration Co- operative Agreement NA90-RAH-0075. Literature cited Barnett, V., and T. Lewis. 1994. Outliers in statistical data, 3rd ed. John Wiley and Sons, New York, NY, 584 p. Chen, Y., D. A. Jackson, and J. E. Paloheimo. 1994. Robust regression approach to analyzing fisheries data. Can. J. Fish. Aquat. Sci. 51:1420-1429. Chen, Y., and J. E. Paloheimo. 1994. Robust regression approach to estimating fish mor- tality rates with a cohort-based model. Trans. Am. Fish. Soc. 123:508-518. Corana, A., M. Marchesi, C. Martini, and S. Ridella. 1987. Minimizing multimodal functions of continuous vari- ables with the “simulated annealing” algorithm. ACM Trans. Math. Software 13:262-280. Deriso, R. B., T. J. Quinn II, and P. R. Neal. 1985. Catch-age analysis with auxiliary information. Can. J. Fish. Aquat. Sci. 42:815-824. Fournier, D., and C. P. Archibald. 1982. A general theory for analyzing catch at age data. Can. J. Fish. Aquat. Sci. 39:1195-1207. Hilborn, R. 1990. Estimating the parameters of full age-structured models from catch and abundance data. Bull. Int. N. Pac. Fish. Comm. 50:207-213. Hilborn, R., and C. J. Walters. 1992. Quantitative fisheries stock assessment: choice, dy- namics and uncertainty. Chapman and Hall, New York, NY, 570 p. ICCAT (International Commission for the Conservation of Atlantic Tunas). 1995. Report for the biennial period, 1994-1995, Part I (1994), vol. 2. Int. Comm. Cons. Atl. Tunas, Madrid, Spain, 283 p. ICES (International Council for the Exploration of the Sea). 1995. Report of the working group on methods of fish stock assessments. Int. Coun. Explor. Sea Coop. Res. Rep. 199, 147 p. Kimura, D. K. 1989. Variability, tuning, and simulation for the Doubleday- Deriso catch-at-age model. Can. J. Fish. Aquat. Sci. 46:941-949. Methot, R. D. 1990. Synthesis model: an adaptable framework for analy- sis of diverse stock assessment data. Int. N. Pac. Fish. Comm. Bull. 50:259-277. Nelder, J. E., and R. Mead. 1965. A simplex method for function minimization. The Computer J. 7:308-313. Pella, J. J., and P. K. Tomlinson. 1969. A generalized stock production model. Inter-Am. Trop. Tuna Comm. (IATTC) Bull. 13:420-496. Pope, J. G., and J. G. Shepherd. 1985. A comparison of the performance of various methods for tuning VPAs using effort data. J. Cons. Int. Explor. Mer 42:129-151. Powers, J. E., and V. R. Restrepo. 1992. Additional options for age-sequenced analy- sis. ICCAT Coll. Vol. Sci. Pap. 39: 540-553. 1993. Evaluation of stock assessment research for Gulf of Mexico king mackerel: benefits and costs to manage- ment. N. Am. J. Fish. Manage. 13:15-26. Prager, M. H. 1994. A suite of extensions to a nonequilibrium surplus- production model. Fish. Bull. 92: 374-389. Punt, A. E. 1994. Assessments of the stocks of Cape hakes Merluccius spp. off South Africa. S. Afr. J. Mar. Sci. 14:159-186. Restrepo, V. R., and J. E. Powers. 1995. Useful methods for dealing with outliers in stock assessments. Int. Comm. Cons. Atl. Tunas, Coll. Vol. Sci. Pap. Vol. XLIV:151-154. Restrepo, V. R., J. M. Hoenig, J. E. Powers, J.W. Baird, and S. C. Turner, 1992. A simple simulation approach to risk and cost analy- sis, with applications to swordfish and cod fisheries. Fish. Bull. 90:736-748. Rousseeuw, P. J. 1984. Least median of squares regression. J. Am. Stat. Assoc. 79:871-880. Rousseeuw, P. J., and A. M. Leroy 1987. Robust regression and outlier detection. John Wiley and Sons, New York, NY, 329 p. Sinclair, A., D. Gascon, R. O’Boyle, D. Rivard, and S. Gavaris 1990. Consistency of some Northwest Atlantic groundfish stock assessments. North Atlantic Fisheries Organization (NAFO) Sci. Council Study 16:59-77. Steele, J. M., and W. L. Steiger 1986. Algorithms and complexity for least median of squares regression. Discr. Appl. Math. 14:93-100. Stromberg, A. J. 1993. Computation of high breakdown nonlinear regression parameters. J. Am. Stat. Assoc. 88:237-244. 161 Abstract . — Growth and mortality rates of 0+ English sole were estimated from field data collected from estuarine and nearshore nursery areas off Wash- ington during 1985-88. Growth of 0+ English sole was approximately linear over time and was estimated with the length modal progression method. Point estimates of growth rates during May through September were in the range of 0.33 to 0.49 mm/day. Statisti- cal analysis with a general linear model showed significant year and settlement time effects on growth of 0+ English sole but failed to detect any density or tem- perature effect. Instantaneous mortal- ity rate varied significantly with sea- son, declining from 0.0175 per day in July and August to 0.0075 per day in September. Changes in population den- sity appeared to play a minor role in causing this decline. Manuscript accepted 30 July 1996. Fishery Bulletin 95:161-173 (1997). Growth and survival of 0+ English sole, Pleuronectes vetulus, in estuaries and adjacent nearshore waters off Washington Yunbing Shi* Donald R. Gunderson School of Fisheries, 357980, University of Washington Seattle, Washington 98195 E-mail address: yshi@HARZA.com Patrick J. Sullivan International Pacific Halibut Commission RO Box 95009, Seattle, Washington 98145 Fish growth depends on numerous factors, e.g. supply of suitable prey items, ambient temperatures, and oxygen concentration. Laboratory studies have shown that growth of juvenile plaice ( Pleuronectes pla- tessa), sole (Solea solea), and En- glish sole ( Pleuronectes vetulus) de- pends strongly on ambient tempera- ture (Williams and Caldwell, 1978; Fonds, 1979; Yoklavich, 1981). Field observations also show that growth of flatfishes is regulated by ambient temperature. Applying a model based on Fonds’s ( 1979) labo- ratory experiment and observed temperature data for predicting monthly growth of North Sea pla- ice, van der Veer et al. (1990) showed a close overall agreement between predicted growth incre- ments and those observed in the field. Simulated growth rates, how- ever, were consistently lower than field-observed growth rates in June, and this tendency was reversed in August (Fig. 7 in van der Veer et al., 1990). This finding suggests that in addition to temperature there are other factors that also af- fect the growth of plaice. Laboratory studies of juvenile English sole (Williams and Cald- well, 1978; Yoklavich, 1981) have shown that food limitation can sig- nificantly reduce growth. Edwards and Steele (1968) suggested that food limitation was the controlling factor for the growth of North Sea plaice in Loch Ewe. Bergman et al. ( 1988) reported that growth reduction of 0-group plaice occurred in specific areas of the Wadden Sea where there was low food abundance, although this phenomenon was restricted to only a small part of the population. Isolating the effects of fish den- sity, food supply, and ambient tem- perature on growth is difficult with field data. Within a certain range of population density or food abun- dance, growth may be regulated primarily by temperature and, within a certain range of tempera- ture, population density may have a dominant influence. Survival is the key element in determining success of recruitment. Early research was largely focused on the “critical period” theory (Hjort, 1913), i.e. survival of small first-feeding larvae is critical to sub- sequent year-class strength. More recent studies have shown that low * Present address: HARZA Consulting En- gineers and Scientists, 2353 130th Avenue N.E., Suite 200, Bellevue, WA 98005. 162 Fishery Bulletin 95(1), 1997 124°30'W 124°00'W Figure 1 The study area along the southern Washington coast. Shown are subsystem boundaries, nearshore transect lines, trawl stations (filled circles), and stratum numbers (open circles). Dashed lines indicate survey stratum boundaries. larval abundance may indicate poor year- class strength, but high larval abundance will not guarantee a strong year class (Bailey and Spring, 1992; Bradford, 1992). Survival dur- ing the juvenile stage is critical to year-class success. On the basis of simulation, Bradford (1992) concluded that correlation between recruitment and abundance at early life stages increases monotonically with age, es- pecially during the first 100 days of life, be- cause variation in survival weakens the re- lationship between recruitment and abun- dance of early life stages. Although monthly or annual instantaneous mortality is usually higher during the larval stage than that at the juvenile stage, the cumulative mortality might be higher, and more variable, during the juvenile stage because it usually lasts much longer. Therefore any variation in mor- tality at this stage would induce much greater variation in recruitment. English sole spawn in offshore areas. Tim- ing of spawning is variable and duration of the spawning period is protracted (August to May, Shi, 1994). The egg and larval stages last from two to two-and-a-half months, and survival and transport of eggs and larvae are dependent on oceanographic conditions (Boehlert and Mundy, 1987, 1988; Shi, 1994). Once metamorphosis and benthic settlement have occurred, English sole actively seek out estuarine nursery areas, and oceanographic influence becomes less im- portant. Analyses of tagging data, distribution of adults, available spawning habitat, and egg distribution (Shi, 1994) suggest that the Grays Harbor and Willapa Bay estuaries serve as nursery areas for English sole that spawn as far south as central Oregon. This study summarizes results from a series of trawl surveys of Grays Harbor, Willapa Bay, and the adjacent nearshore, 1985-88. Previous work (Gun- derson et al., 1990; Shi et al., 1995) has shown that these estuaries provide critical nursery habitat for ju- venile English sole during their first year of life. The abundance of 0+ English sole in our study area (Fig. 1) was relatively stable during September, show- ing only a threefold difference during 1985-88, de- spite great variation in settlement in May (Shi et ah, 1995). If survival is density dependent, then den- sity could function to stabilize recruitment. In this paper, growth and survival rates will be estimated by using data from field surveys, and we will inves- tigate statistically the effects of population size and ambient temperature on the growth and survival of juvenile English sole. Methods Study area and field methods Grays Harbor (8,545 ha) and Willapa Bay (11,200 ha) are two major Washington coastal estuaries char- acterized by numerous channels, sandflats, and eel- grass beds that provide excellent habitat for 0+ En- glish sole. The nearshore portion of the study area is bounded to the north at 47°15'N, and to the south at about 46°30’N, and extends from the shoreline sea- ward to 60 m. It encompasses an area of nearly 146,600 ha. A stratified random trawl survey was performed to estimate population sizes for English sole in both estuaries (Fig. 1) with the area-swept method: P - Ad , where P = population size, A = area of survey stra- tum (ha), and d = mean density (no. of fish caught/ Shi et al.: Growth and survival of Pteuronectes vetulus 163 ha) (Shi et al., 1995). Within each stratum, stations were randomly selected from sampling units super- imposed on nautical charts, with the constraint that no two stations were immediately adjacent to one another. The effort (number of stations) allocated to each stratum was proportional to the abundance of English sole in that stratum (Shi et al., 1995). The nearshore area was sampled along fixed transects oriented east-west and trawl stations were located at discrete depths (Fig. 1). Five transects were established, and sampling stations were located at depths of 5, 9, 18, 27, 36, 46, and 55 m. The 55-m station was not sampled on the northernmost transect because of frequent gear damage at this lo- cation. Additional effort was allocated to the inter- mediate stratum; two trawl samples were taken at all 27- and 36-m depths. Sampling stations were stratified according to depth to obtain population estimates. The outer boundary for the nearshore study area was the 59-m (32.5-fm) isobath, and the mean low low water (MLLW) mark was the inner boundary. The boundary separating the inner and middle strata followed the 14-m (7.5-fm) isobath, whereas the boundary between the middle and outer strata was located at 41 m (22.5 fm). The northern and southern limits of the survey area were posi- tioned 5 km beyond the northernmost and southern- most transects. Each of the three areas was visited once a month. Sampling in estuaries was planned during low spring tides of the month (April or May through Septem- ber) so that we could navigate among unmarked channels, which otherwise are difficult to see. Sta- tions in close proximity to intertidal areas were sampled preferentially at low tide to minimize bias associated with fish movement onto the tideflats at higher stages of tide. More exposed sites were typi- cally sampled at high water. Trawling operations ceased when tidal currents were judged sufficiently strong that the trawl gear would not tend the ocean bottom properly. Nearshore sampling trips were usu- ally made between the two estuary trips in that month. Survey samples throughout the study area were collected with a 3-m beam trawl specifically devel- oped for this study (Gunderson and Ellis, 1986). Ef- fective width of the net was 2.3 m, whereas the esti- mated vertical opening was 0.6 m. The body of the net was composed of 7-9 mm (lumen) knotless ny- lon, and the codend was lined with 4-mm stretch mesh. A double tickler chain array was attached to a 9.5-kg wingtip weight at each corner of the net. The tickler chain array, together with the turbulent zone it creates, dislodges small animals from the sub- strate, thus promoting capture by the net. Nearshore sampling was conducted from the 17- m stern trawler F/V Karelia. Tows in the nearshore were taken parallel to isobaths. Scope was routinely 5:1, except at the 5 and 9 m stations where it was 8:1 and 9:1, respectively. Time on the ocean bottom was estimated by using a trigonometric relationship be- tween water depth and wire out, whereas the linear distance towed (mean: 750 m) was determined from LORAN-C readings. Tow duration was routinely 20 minutes at a mean towing speed of 2.6 km/hr (1.4 knots), except at the 5- and 9-m stations, which of- ten yielded excessive quantities of sand dollars ( Dendraster excentricus) and gravel; tows in these areas were limited to 5 or 10 minutes. A 6.4-m Boston whaler with a 150-hp outboard engine was used for estuarine trawling. Buoys were deployed at the points where the net first contacted the bottom and subsequently left bottom upon re- trieval. The distance towed (mean: 260 m) was esti- mated with an optical rangefinder. Mean towing speed was 2.8 km/hr (1.5 knots), comparable to that used in the nearshore area. Data analysis Length Growth rate estimates were obtained by regressing the mean length of a recruitment influx (indicated by a mode in the length-frequency distri- bution [Shi et al., 1995]) against the time when samples were taken. There was a linear relationship between modal length and time, as was the case in previous growth studies on juvenile English sole (Ketchen, 1956; Kendall, 1966; Rosenberg, 1982). Because size-dependent migration between near- shore and estuarine systems occurs, with smallest juveniles migrating into estuaries and larger fish moving offshore (Gunderson et al., 1990; Shi et al., 1995), separate estimates of growth rates for nearshore and estuarine fish would be inappropri- ate. To minimize the effect of interregional migra- tions, the mean lengths at each mode (defined on the basis of visual inspection of monthly length frequency plots [Shi et al., 1995]) were calculated from the es- timated size composition of the overall population. The length statistic used was the mean modal length (MML), which is defined as the mean length within a mode, weighted by the estimated population size for each size group: MML = (1) 164 Fishery Bulletin 95(1), 1997 where, Pt = estimated population (millions) of fish in length group l; lu and llow = length (mm) at upper and lower limits of^ the mode, which are so defined that lu and l/ow are the length groups at which abun- dance fms declined to half that at the modal size (Fig. 2). If length is normally distributed, l~N( l , s2), the population within the upper and lower limits of the mode so defined would account for about 75% of the total population of that cohort (Shi, 1994). Date The dates used in growth and mortality esti- mation were also population-weighted means. The dates when the samples were taken cannot be used directly in growth and mortality estimation without being standardized. Estuarine samples had to be taken during low low tide (LLT) periods, and we were often forced to take estuarine samples at unequal time intervals. We made every effort to carry out the monthly nearshore surveys during the intervals between the Grays Harbor and Willapa Bay surveys, but they sometimes had to be done either before or after the estuarine trips owing to logistic difficulties. This made the time between the first and last surveys for a given month more than a half-month apart. The population-weighted mean date (PWMD) was cho- sen to standardize the “date” of monthly surveys and is the best estimate of the average sampling date for the total population in our study area. The PWMD was computed from 3 X P'Jm d(lte‘J PWMD jm = -^h-g , (2) y p ;=i where, PWMD/m = population-weighted mean date in month j (May, June, July, August, and Septem- ber) for mode m (1 or 2); P = population of mode m in system i (GH, WB, NS), month j; dateij = mean date of a survey carried out in month j and system i, i.e. number of days from 1 May. Temperature A population-weighted mean bottom temperature (PWMBT) was developed in this study because of extensive seasonal ontogenetic migrations «/->0»o©‘r>0*no*nou-)©v>©»''}Ow">Qio©‘n©«o©*n©»/->© Length (mm) Figure 2 A diagram illustrating how the upper (/ ) and lower (llow) limits of a mode were determined. Pm is the estimated modal population and Plow, Pup are the estimated population of the lower and upper limits of the mode. Shi et al.: Growth and survival of Pleuronectes vetulus 165 between estuarine and nearshore areas and because of differences in mean bottom temperatures between estuarine and nearshore systems (2-8°C, Fig. 3). 3 % P T isjm isj PWMBTjm = > (3) p isjm i= 1 s= 1 where, PWMBT/m = population-weighted mean bot- tom temperature in month j for mode m\Pisjm - popu- lation of mode m in system i, stratum s, month j; Tiy = mean bottom temperature in system i, stratum s, and month j; and s- = number of strata in system i. PWMBT]m is the best estimate of the average tempera- ture experienced by the population in our study area. Growth rates A linear model was developed to de- termine competing factors that had significant ef- fects on the growth of 0+ English sole. Iji = a + axc + a2jy j + /ft, + /^c^, ( 4 ) +A> jJ + (34Tltl + e Jt , where, F = mean modal length (MML) at time F and year j; y; and c are dummy variables for year and settlement time; ti = population weighted mean date (PWMD); = density (no/ha), monthly mean den- sity from May through September; T = population weighted mean bottom temperature ( PWMBT ); and Eji = residual. The dt and T terms were used to ex- amine whether or not there were density or tempera- ture effects (or both) on the growth of 0+ English sole in the study areas. The dummy variable y . (year) was defined as follows: [ 1 1986 f 1 1987 Vi = 1 ’ y 2 ={ ’ [0 otherwise [0 otherwise _ ( 1 1988 ' 3 jo otherwise Since, settlement time obviously differed between cohorts (Fig. 4), the dummy variable (c) was used to denote early (1) and the late (0) settlement groups: j 1 early settlements ( 1985, 1986 - 1, 1988) j 0 late settlements ( 1986 - 2, 1987 ). 166 Fishery Bulletin 95 ( 1 ), 1997 Early and late settlement was defined by visual inspection of length frequency (Shi et al., 1995). A recruitment influx with mean modal length less than 40 mm during the May survey was defined as late settlement, and that with mean modal length greater than 40 mm in May was defined as early settlement. A multiple-partial F-test was used here to test the significance of settlement time (/3;c), year (/1 .), den- sity (P3dt), and temperature (p4Tp effects on growth. The computer program MGLH (SYSTAT [Wilkinson, 1989]) was used to carry out all calculations. Mortality rates The significance of density and season effects on mortality was examined by using the restated Beverton-Holt equation (Beverton and lies, 1992): dP — = -^1+fi2]nP)t (5) Pdt 1) By integrating Equation 5 with fi2 = 0, the den- sity-independent mortality model is P,,=Poie~’“‘‘- <6> 2) Integrating Equation 5 over the time period from t = 0 to t = ti without any constraint on /li1 or /u2, the full model is Pt = eu "-I) ; (7) 3) Integrating Equation 5 with = 0, and allowing to vary, the model becomes Pole M t, =0 to 31 (July) < P0;e-#i'(32)e"/J"(ti“32) tt =32 to 62 (August) P e-^e-^62-32)e-rure2) t > 62 (September) l “ 1 (8) where P is the population of juvenile English sole in our study area; /u1 is the density independent coeffi- cient, as defined in Beverton and lies (1992), and |r9 is the density-dependent coefficient. Population estimates from the surveys were fitted to the following three competing models, by using nonlinear least-squares regression (Wilkinson, 1989): where ti = time, number of days elapsed since 1 July for surveys conducted in year i ( 1985-88); Pt - the ob- served total population size (0+ group ) for all areas com- bined at time ti in year i; and Poi = initial total popula- tion size on 1 July in year i. Pm was estimated as a parameter along with the coefficients q, and q2. n', ji", and fi'" are the density-independent mortality Shi et a I.: Growth and survival of Pleuronectes vetulus 167 Table 1 Monthly population-weighted mean date (PWMD), mean modal length (MML), population-weighted mean bottom temperature (PWMBT) and overall mean densities of 0+ English sole, 1985-88. Year Settlement time Month PWMD (days) MML (mm) PWMBT (°C) Density' (No. /ha) 1985 Early May 21.66 60.56 13.07 55.03 Jun 55.34 68.08 14.66 130.95 Jul 83.75 79.95 12.94 200.60 Aug 112.39 84.55 14.34 132.69 Sep 135.72 99.96 12.24 96.42 1986 Early May 17.05 67.54 13.61 118.25 Jun 44.71 72.97 16.24 77.06 Jul 77.54 88.58 14.66 96.75 Aug 106.18 99.17 12.07 72.55 Sep 141.32 112.98 12.49 65.80 1986 Late May 22.51 25.99 12.33 118.25 Jun 48.11 36.09 15.31 77.06 Jul 75.63 55.01 15.43 96.75 Aug 102.64 66.10 14.63 72.55 Sep 144.51 84.67 14.81 65.80 1987 Late May 21.47 25.17 10.48 188.77 Jun 45.58 36.64 13.70 219.35 Jul 76.00 48.83 15.28 346.38 Aug 106.47 63.19 12.48 186.98 Sep 130.45 71.49 13.65 200.48 1988 Early May 12.48 45.01 13.21 269.44 Jun 57.61 61.24 15.20 193.18 Jul 86.10 79.77 14.91 182.15 Aug 113.75 91.58 14.78 116.13 Sep 149.10 103.72 12.26 90.31 1 The estimated densities are combined densities of early and late recruits. coefficients during July, August, and September, respec- tively. Model selection was based on the Bayesian In- formation Criterion (BIC) proposed by Schwarz ( 1976). Results The mean modal length (MML), population- weigh ted mean date (PWMD), population-weighted mean bot- tom temperature (PWMBT), and overall mean den- sities of 0+ English sole from May through Septem- ber are shown in Table 1. Growth Growth of 0+ English sole was linear over time (Fig. 4). A general linear model pooled all data together and considered the effects of year, time of settlement, density, and temperature on growth. The final, best- fitted (R2 = 0.99, P < 0.001) model was lfi - 8.44 + 43.48c + 6.76jq +7.79y2 - 13.06y3 + 0.43/,- 0.11c/, +0.063/^ + 0.12y3/, + . The results of partial / -tests (used in all compari- sons unless specified otherwise), indicated there was a significant settlement time effect on growth (PcO.Ol), late-settling cohorts growing the fastest. The year effect was also significant (multiple partial E-test P<0.Q5, Table 2). Multiple-partial P-tests in- dicated that there were no density (P=0.80) or tem- perature (P- 0.37) effects on the growth of 0+ English sole. The date of settlement was estimated by fitting separate regression equations to each cohort in Fig- ure 4, then by backcalculating to a length of 20 mm TL (Table 3), or by inverse prediction (Neter et al., 1985). We estimated that settlement of the 1985 and 1986 group-1 cohorts peaked in January, with 95% prediction intervals ranging from 27 November to 22 March. Settlement of the 1986 group-2 and 1987 cohorts peaked in May (with 95% prediction inter- vals ranging from 21 April to 29 May), and that of the 1988 cohort peaked in March (ranging from 17 February to 16 April). Mortality The following equations were obtained from nonlinear least-square regression: 168 Fishery Bulletin 95 ( 1 ), 1997 Table 2 A summary of effects of year and settlement time on the growth of 0+ English sole. Variable Parameter Coefficient Partial-f P (2-tail) Intercept Constant a 8.44 3.19 < 0.01 Settlement Time (c) «i 43.48 17.03 < 0.01 Year (yp y2, y3) — — — <0.01; yi a21 6.76 2.61 < 0.05 y2 a22 7.79 3.58 < 0.01 y3 a23 -13.06 -4.19 < 0.01 Slope (Growth) PWMD (t,) P 0.43 18.45 < 0.01 Settlement time (c) Pi -0.11 -4.04 < 0.01 Year (yp y2, y3) — — < 0.052 yi P2I 0.06 2.13 < 0.05 y3 P23 0.12 3.66 < 0.01 1 Based on the result of an F-test, F3 I5 = 4.35. 2 Based on the result of an F-test, F3 I5 = 13.25. Model 1 : q2 = 0, Pt = P0le = \ 42.82 1985 21.02 1986 66.63 1987 40.40 1988 -0.0066f -0 00561, _o 0056(, Model 2 :Pt = e 00056 ]Peoi \Pol = 43.80 18.62 73.33 40.46 1985 1986 1987 1988 Model 3: P, n -0.0175 f, Po,e P,,e -0. 0 175x62^, -0.0075U, -62). t, = 0 to 62 (July and August) t; > 62 (September) (49.60 P.. = 23.23 74.33 47.28 1985 1986 1987 1988 The estimated instantaneous mortality rates of 0+ English sole in July and August were equal, there- fore model 3 was reduced from a three-step to a two- step model. The data fitted model 3 best (Fig. 5) with instantaneous mortality rates of 0.0175 per day in July and August and 0.0075 per day in September. The value of the Bayesian information criterion was 3.68 for model 1, 4.04 for model 2, and 2.65 for model 3. As a result, we concluded that model 3 was the best for estimating mortality. Table 3 Back-calculated settlement dates with their ranges, assum- ing average length at settling, lsettling = 20 mm TL. Settlement Date of 95% Year cohort settlement prediction interval 1985 1 24 Jan 27 Nov-22 Mar 1986 1 16 Jan 17 Dec -16 Feb 2 10 May 21 Apr -29 May 1987 2 8 May 30 Apr -16 May 1988 1 18 Mar 17 Feb -16 Apr Discussion Gear efficiency Our estimates of growth and mortality might be bi- ased if gear selectivity varied with size. Edwards and Steele (1968) suggested that beam trawl efficiency depends on a number of factors, such as towing speed, bottom type, and fish size. At a speed of 35 m/min, the efficiency of their 2-meter beam trawl was 25- 35%, depending on fish size. They point out that their results apply only to their particular gear and the special conditions in Loch Ewe. Kuipers (1975) found that the efficiency of a 2-m beam trawl in the Dutch Wadden Sea declined from 100% at lengths below 70 mm to 15-30% for plaice larger than 150 mm. Our gear was a 3-m beam trawl with effective fishing width of 2.3 m, wider than the gear used by either Kuipers or Edwards and Steele, and was towed faster (41-47 m/min vs. 30-35 m/min). Also the ratio of fish- Shi et al.: Growth and survival of Pieuronectes vetulus 169 Figure 5 Population sizes (millions) plotted against time (number of days from 1 July) for O' English sole off Washington during July through September, 1985-88. All lines represent the predicted tra- jectories with the month-specific, density independent mortality model (model 3); solid symbols represent the observed data. ing line out to bottom depth used in our survey (10- 15 in shallow waters) was greater than that in Kuipers’ experiment (4-8), resulting in better bot- tom contact and reduced vessel avoidance. During a series of 15 pairs of day-night compara- tive tows, for which gear and operating procedures were the same as those described in this paper, Gunderson and Ellis (1986) failed to detect any sig- nificant net avoidance by either butter sole ( Pieuro- nectes isolepis) over the length range from 40 to 280 mm, or Pacific tomcod ( Microgadus proximus) over the length range from 60 to 220 mm. In the present study, the data for English sole did not show any decline in estimated growth with size (Fig. 4). We conclude that the efficiency of the gear used in this study does not decrease with fish size over the length range from 20 to 150 mm and has little influence on estimates of growth or mortality. Growth It has been shown that English sole juvenile migra- tion in and out of estuaries is size dependent (Gunderson et al., 1990; Shi et al., 1995), and our approach to the length modal progression method (LMP), namely pooling length data from coastal and estuarine areas to estimate growth, accounts for the effect of such migrations. Our nearshore survey area covered the outer limit of 0+ English sole bathymet- ric distribution; less than 1% of the total population was found in the deepest nearshore stratum (Shi et al., 1995). To minimize the effects of inter-area mi- gration, we pooled data from all three areas surveyed (Fig. 1), which cover a major portion of waters avail- able to English sole juveniles along the Washington coast. The resulting estimates fall between faster growth rates estimated from previous LMP analy- ses (Westrheim, 1955; Smith and Nitsos, 1969; Krygier and Pearcy, 1986) and slower growth rates estimated from fortnightly ring counts by Rosenberg (1982) (Table 4). Gunderson et al. (1990) and Shi et al. (1995) suggested that the population of English sole juveniles in this study may not be closed, how- ever, and that some migrations, especially during May and June each year, involve areas outside the study area shown in Figure 1. Continuous recruit- ment of young juveniles from outside the study area would result in an underestimation of growth rates, as could emigration of larger juveniles. Our data do not show any decline in growth at either the begin- ning or end of the survey season (Fig. 4); thus the influence of continuous recruitment of small fish or emigration of larger fish on growth estimation ap- peared to be minimal. 170 Fishery Bulletin 95 ( 1 ), 1997 Table 4 A summary of daily growth rate estimates from field studies. Location Size at age 1-yr (mm TL) Daily growth rate (mm/day) Data source Willapa Bay, Grays Harbor, and adjacent neashore, WA <150 0.33-0.49 (May-Sep) This study, 1985-88 Yaquina Bay, OR 130-160 0.49 (May-Oct) Westrheim, 1955 Monterey Bay, CA 130-150 0.55 (May-Oct) Smith and Nitsos, 1969 Yaquina Bay, OR 100-140 0.33 Rosenberg, 1982i2 Moolach Beach, OR 100 0.34 Yaquina Bay, OR <150 0.46-0.49 (Mar-Oct) Krygier and Pearcy, 19862 Moolach Beach, OR <150 0.26-0.32 (Dec-Apr) 0.28-0.42 (Apr-Oct) 1 The original daily growth rates were estimated from fortnightly ring counts. 2 Length at age 1 and daily growth rates were converted from standard length (SL) to total length (TL) by using the relationship: SL = -0.205 + 0.848 TL (senior author, unpubl. data). Several previous studies with the LMP technique have attempted to estimate growth rates for English sole juveniles from estuaries and open coast but failed to consider the effect of interarea migration on the growth estimates. As a consequence, their results often show significant differences between coastal and estuarine populations (Westrheim, 1955; Smith and Nitsos, 1969; Krygier and Pearcy, 1986). In con- trast, growth estimated from fortnightly ring counts showed no differences between coastal and estuarine populations (Rosenberg, 1982). Previous laboratory studies where ration was held constant (Williams and Caldwell, 1978) showed that ambient temperature had no statistically significant effect on English sole growth rate between 9.5 and 15. CPC but significantly reduced growth between 15.0 and 18.0°C. The artificial food pellets used in that study may have been nutritionally inadequate, how- ever, making it difficult to extrapolate the results to field conditions. Laboratory studies by Yoklavich (1981), where live polychaetes were used as food, showed a significant decline in the mean growth rate of0+ English sole (from 1.87% to 1.17% of body weight per day) between 13.0 and 17.5°C. Our results do not indicate any statistically significant interannual effect of temperature on the growth of English sole juveniles over the range of population-weighted mean temperatures (10.5-16.2°C) observed under field con- ditions. Higher temperatures presumably result in in- creased benthic productivity (Johnson and Brinkhurst, 1971) and in more food available to juveniles. On the other hand, metabolic requirements increase at high temperatures. Whether the juveniles grow faster or slower under field conditions probably depends on the bioenergetic balance at higher temperatures. Peterman and Bradford ( 1987) found that density had a significantly negative effect on the growth of 1+ English sole off the Oregon and Washington coasts (P=0.024, one-tailed C-test); therefore it would be rea- sonable to expect that growth of 0+ English sole is also density dependent. Nevertheless growth rate and mean population size varied over relatively nar- row ranges in this study, and we were unable to de- tect any statistically significant density effect. The spawning season for English sole can extend from September to April (Kruse and Tyler, 1983), and recruitment processes are also protracted. Multiple peak recruitments are common (Kendall, 1966; Boehlert and Mundy, 1987; Gunderson et al., 1990; Shi et al., 1995), and peak recruitment occurs at dif- ferent times each year, depending on ocean tempera- ture and transport mechanisms (Ketchen, 1956; Boehlert and Mundy, 1987). Kendall (1966) reported that for Puget Sound English sole juveniles that were recruited earlier, growth was slower than that of later recruits during the same period. Our results led to the same conclusion, that is, timing of settlement influences the growth trajectory (Fig. 4). Different size groups of English sole have differ- ent prey requirements and suffer from different de- grees of food limitation (Gunderson et al., 1990). Off the Oregon coast, English sole 17-35 mm standard length (SL) fed primarily on polychaete palps, juve- nile bivalves, and harpacticoid copepods, whereas 35- 82 mm fish fed on larger prey such as amphipods and cumaceans (Hogue and Carey, 1982). In the Humboldt Bay estuary, English sole smaller than 50 mm TL fed almost exclusively on harpacticoid cope- pods whereas the diet of 66-102 mm fish was domi- nated by polychaetes (Toole, 1980). Shi et al.: Growth and survival of Pleuronectes vetulus 171 Winberg (1956) found that individual metabolic requirements and food consumption increase as a function of W° 8 (where W=body weight), and Fonds (1979) showed a similar relation for young sole, Solea solea. The larger sizes attained by the early-settle- ment cohorts of English sole would probably also entail increased food requirements for those fish during May-September. The lower growth rates ob- served for early-settlement cohorts in comparison with those that settled later probably resulted from a combination of higher metabolic demands and re- duced availability of suitable prey. Growth of 0+ En- glish sole does not appear to be strictly linear if a sufficiently long period of time is examined. Mortality Estimates of mortality rates were subject to some of the same sources of error and bias that the growth estimates were. Previous work (Gunderson et al., 1990; Shi et al., 1995) has shown that migrations of 0+ English sole are size dependent. Typically, larger fish emigrate from estuaries and perhaps out of our nearshore survey area, whereas smaller fish immi- grate into our survey area. Immigration of small fish would cause underestimates of mortality. Previous analysis indicated that most immigration probably occurred near the settling period, i.e., May and June or earlier (Shi et al., 1995). Therefore, it is unlikely that immigration had much effect on the mortality estimates because only population estimates for July through September were used in this analysis. Emigration of large 0+ fish would cause overesti- mation of mortality rates. Although we cannot com- pletely ignore the possibility of emigration, previous analysis of length increment patterns in estuarine and nearshore areas has indicated that net emigra- tion of large fish from the study area is minor during July-September (Shi et al., 1995). In addition, had substantial emigration occurred during July through September, estimated mortality rates would be con- sistently higher during September than dui’ing July and August, rather than the opposite (0.0075 per day vs. 0.0175 per day). Seasonally differentiated daily mortality could be related to differences in temperature, individual size, or population density. Water temperatures, however, remained relatively stable in the study area during July through September (Fig. 3). Size-dependent mortality may have occurred, because 0+ English sole grow rapidly during the summer, with increases in individual size of 0+ English sole ranging from 20 to 30 mm TL from July to September. Kramer (1991) estimated the mortality rates for each 5-mm size group of California halibut, Paralichthys californicus , on the basis of daily production by size group, and found that mortality was size specific for fish less than 70 days old (< 30 mm SL), smaller fish suffer- ing higher mortality than larger ones. For older 0+ California halibut (70-115 days of age or 31-70 mm SL), mortality varied little (0.011- 0.014 per day, with mean=0.0124 per day and SD=0.001 per day) and no trend was observed. Beverton and lies (1992) found a significant density-dependent mortality (/t2) effect for North Sea plaice ranging from <15 mm to 35 mm, although this effect was not significant for fish larger than 35 mm. Both Kramer (1991) and Beverton and lies (1992) found that mortality of juvenile flatfish is highest during and immediately after settlement, and our results for English sole suggest the same. The effects of density and individual size were clearly confounded in our study (Fig. 6). An empiri- cal relation between population size in the survey area (Pf) and mean length (lt) was fitted as In Pt = A + Blt , where, Pt = the estimated total population size at time t; lt = the mean length of all 0+ English sole at time t (July through September); and the correla- tion between the two confounding factors was highly significant (r2=0.56, PcO.Ol). It should always be borne in mind that it is very difficult, if not impos- sible, to isolate these two confounding factors in field observations; controlled enclosure experiments would probably be required to disentangle them. There is both a theoretical (Peterson and Wroblewski, 1984) and an empirical (McGurk, 1986; Kramer, 1991) ba- sis for assuming that mortality decreases with indi- vidual size, and if this is the case, n2 (the density- dependent mortality coefficient) would have been overestimated. If mortality of Q+ English sole is den- sity dependent, it appears that this dependence is weak because adding a density-dependent term to the model (model 2) did not improve the fit to the data. Our surveys showed only a threefold difference in abundance of 0+ English sole during 1985-88, but stock synthesis analysis of commercial fisheries data indicates this was a period of relatively stable re- cruitment (Sampson1). The estimated recruitment of age-2+ females to the commercial fishery varied by no more than a factor of 1.8 for the 1985-88 year classes, whereas it varied by a factor of 6.5 for the 1 Sampson, D. B. 1993. An assessment of the English sole stock off Oregon and Washington in 1992. Appendix H in Status of the Pacific coast groundfish fishery through 1993 and recom- mended allowable biological catch for 1994. Pacific Fish. Man- agement Council, 2000 SW 1st Ave., Suite 420, Portland, OR, 43 p. 172 Fishery Bulletin 95( 1 ), 1997 1975-90 year classes. Although our results appar- ently did not encompass periods of poor recruitment, they show that surveys of the nursery areas of 0+ English sole have the potential to provide estimates of year-class strength several years in advance of the commercial fishery, as well as provide insight into the processes that generate recruitment variability. Acknowledgments The senior author is grateful to his graduate com- mittee members and fellow graduate students for their support and assistance during this study. This research was supported by grants from Washington Sea grant (NA 86AA-D-SG004) and the National Marine Fisheries Service (NOAA). Literature cited Bailey, K. M., and S. M. Spring. 1992. Comparison of larval, age-0 juvenile and age-2 re- cruit abundance indices of walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska. ICES J. Mar. Sci. 49:297-304. Bergman, M. J. N., H. W. van der Veer, and J. J. Zijlstra. 1988. Plaice nurseries: effects on recruitment. J. Fish. Biol. 33 (suppl. A):210-218. Beverton, R. J. H., and T. C. lies. 1992. Mortality rates of 0-group plaice ( Pleuronectes platessa L.), dab ( Limanda limanda L.) and turbot ( Scophthalmus maximus L.) in European waters. Ill: Den- sity-dependence of mortality rates in 0-group plaice, and some demographic implications. Neth. J. Sea Res. 29:61-79. Boehlert, G. W., and B. C. Mundy. 1987. Recruitment dynamics of metamorphosing English sole, Parophrys vetulus, to Yaquina Bay, Oregon. Estu- arine Coastal Shelf Sci. 25:261-281. 1988. Roles of behavioral and physical factors in larval and juvenile fish recruitment to estuarine nursery areas. Am. Fish. Soc. Symp. 3:51-67. Bradford, M. J. 1992. Precision of recruitment predictions from early life stages of marine fishes. Fish. Bull. 90:439-453. Edwards, R., and J. H. Steele. 1968. The ecology of 0-group plaice and common dabs at Loch Ewe. I. Population and food. J. Exp. Mar. Biol. Ecol. 2:215-238. Fonds, M. 1979. A seasonal fluctuation in growth rate of young plaice (. Pleuronectes platessa ) and sole ( Solea solea) in the labo- ratory at constant temperatures and a natural daylight cycle. In E. Naylor and R. G. Hartnell (eds.), Proceed- ings of the 13th European marine biology symposium, p.151-156. Pergamon Press, Oxford, 477 p. Gunderson, D. R., and I. E. Ellis. 1986. Development of a plumb staff beam trawl for sam- pling demersal fauna. Fish. Res. 4:35-41. Gunderson, D. R., D. A. Armstrong, Y. B. Shi, and R. A. McConnaughey. 1990. Patterns of estuarine use by juvenile English sole ( Parophrys vetulus) and Dungeness crab (Cancer magi- ster). Estuaries 13:59-71. Hjort, J. 1913. Fluctuations in the great fisheries of northern Eu- rope viewed in the light of biological research. Rapp. R- V. Reun. Cons. Int. Explor. Mer 19:1-228. Shi et a L Growth and survival of Pleuronectes vetulus 173 Hogue, E. W., and A. G. Carey Jr. 1982. Feeding ecology of 0-age flatfishes at a nursery ground on the Oregon Coast. Fish. Bull. 80:555-565. Johnson, M. G., and R. O. Brinkhurst. 1971. Production of benthic macroinvertebrates of Bay of Quinte and Lake Ontario. J. Fish. Res. Board Can. 28:1699-1714. Kendall, A. W., Jr. 1966. Sampling juvenile fishes on some sandy beaches of Puget Sound, Washington. M.S. thesis, Univ. Washing- ton, Seattle, WA, 77 p. Ketchen, K. S. 1956. Factors influencing the survival of the lemon sole (Parophrys vetulus) in Hecate Strait, British Columbia. J. Fish. Res. Board Can. 13(5):647-694. Kramer, S. H. 1991. Growth, mortality, and movements of juvenile Cali- fornia halibut, Paralichthys californicus , in shallow coastal and bay habitats of San Diego County, California. Fish. Bull. 89:195-207. Kruse, G. H., and A. V. Tyler. 1983. Simulation of temperature and upwelling effects on the English sole (Parophrys vetulus) spawning sea- son. Can. J. Fish. Aquat. Sci. 40:230-237. Krygier, E. E., and W. G. Pearcy. 1986. The role of estuarine and offshore nursery areas for young English sole, Parophrys vetulus Girard, off Ore- gon. Fish. Bull. 84:119-132. Kuipers, B. R. 1975. On the efficiency of a two-metre beam trawl for juve- nile plaice (Pleuronectes platessa). Neth. J. Sea Res. 9: 69-85. McGurk, M. D. 1986. Natural mortality of marine pelagic fish eggs and larvae: role of spatial patchiness. Mar. Ecol. Prog. Ser. 34:227-242. Neter, J., W. Wasserman, and M. H. Kutner. 1985. Applied linear statistical models. Regression, analy- sis of variance and experimental designs, 2nd ed. Irwin, Homewood, IL, 1127 p. Peterman, R. M., and M. J. Bradford. 1987. Density-dependent growth of age 1 English sole (Parophrys vetulus) in Oregon and Washington coastal waters. Can. J. Fish. Aquat. Sci. 44:48-53. Peterson, I., and J. S. Wroblewski. 1984. Mortality rate of fishes in the pelagic ecosystem. Can. J. Fish. Aquat. Sci. 41:1117-1120. Rosenberg, A. A. 1982. Growth of juvenile English sole, Parophrys vetulus, in estuarine and open coastal nursey grounds. Fish. Bull. 80:245-252. Schwarz, G. 1976. Estimating the dimension of a model. Ann. Statist. 6:461-464. Shi, Y. 1994. Recruitment of juvenile English sole, Pleuronectes vetulus, in estuaries and nearshore areas of Wash- ington. Ph.D. diss., Univ. Washington, Seattle, WA, 148 p. Shi, Y., D. R. Gunderson, and D. A. Armstrong. 1 995. Population dynamics of 0+ English sole, Pleuronectes vetulus, in estuaries and nearshore areas of Wash- ington. In Proceedings of the international symposium on North Pacific flatfish, p. 343-365. Alaska Sea Grant College Program Report 95-04, Univ. Alaska, Fairbanks. Smith, J. G., and R. J. Nitsos. 1969. Age and growth studies of English sole, Parophrys vetulus, in Monterey Bay, California. Pac. Mar. Fish Comm. Bull. 7:74-79. Toole, C. L. 1980. Intertidal recruitment and feeding in relation to op- timal utilization of nursery areas by juvenile English sole (Parophrys vetulus: Pleuronectidae). Env. Biol. Fish. 5(41:383-390. van der Veer, H. W., L. Pihl, and M. J. N. Bergman. 1990. Recruitment mechanism in North Sea plaice, Pleuronectes platessa. Mar. Ecol. Prog. Ser. 64:1-12. Westrheim, S. J. 1955. Size composition, growth, and seasonal abundance of juvenile English sole (Parophrys vetulus) in Yaquina Bay. Res. Briefs. Fish Comm. Oregon 6(2):4-9. Wilkinson, L. 1989. SYSTAT: the system for statistics. SYSTAT, Inc., Evanston, IL, 822 p. Williams, S. F., and R. S. Caldwell. 1978. Growth, food conversion and survival of 0-group En- glish sole (Parophrys vetulus Girard) at five temperatures and five rations. Aquaculture 15:129-139. Winberg, G. G. 1956. Rate of metabolism and food requirements of fishes. Nauchye Trudy Belorusskogo Gosudarstvennogo Universiteta Imeni. V. I. Lenina, Minsk, 253 p. (Fish. Res. Board Can. Trans. Ser. 194, 202 p.). Yoklavich, M. 1981. Growth, food consumption, and conversion efficiency of juvenile English sole (Parophrys vetulus). Proceeding of the 3rd Pacific workshop on fish food habits studies, p. 97-105. Washington Sea Grant Rep WSG-WO-82-2. 174 Mitochondrial DMA diversity in and population structure of red grouper, Epinephelus morio, from the Gulf of Mexico* Linda R. Richardson John R. GoJd Department of Wildlife and Fisheries Sciences Texas A&M University College Station, Texas 77843-2258 E-mail address: lindafish@tamu.edu Red grouper, Epinephelus morio, is a protogynous hermaphrodite found exclusively in the Atlantic Ocean from the coast of Massachusetts southward to Brazil (Smith, 1961). It is most abundant along the west- ern Florida shelf and off the north coast of the Yucatan Peninsula, Mexico (Brule and Canche, 1993). Studies on the biology of red grou- per are few. Adults are known to be associated with rocky reef bottoms and caverns, ledges, and crevices formed by limestone outcroppings (Moe, 1969). Other available data include food habits and some as- pects of early life history and pat- terns of migration (Moe, 1969; Brule and Canche, 1993). Red grouper are important to both commercial and recreational fisheries in the United States (U.S.) and Mexico (Moe, 1969). In recent years, declines in recreational and commercial landings have led to regulation of both fisheries in U.S. waters. The Mexican red grouper fishery, reportedly working above maximum sustainable yield (Solis Ramirez, 1970; Arreguin-Sanchez, 1987), however, remains essentially unregulated. Important in formu- lating management plans for ma- rine fish species such as red grou- per is information on population structure or stocks and on levels of genetic variation. This information is critical for both stock assessment and adjustment of fishery regula- tions within regions. In a previous study (Richardson and Gold, 1993), we examined mi- tochondrial (mt)DNA variation among a sample of red grouper from the west coast of Florida. Es- timated within-population mtDNA diversity in this sample was among the lowest reported for a marine fish species. In this note, we report mtDNA variation within a sample of red grouper from the Campeche Banks, Mexico (Fig. 1). The main objectives were 1) to determine whether red grouper from Florida and Mexico represent different ge- netic stocks and 2) to compare lev- els of mtDNA diversity in red grou- per from Campeche Banks, Mexico, with those from west Florida. Materials and methods Specimens were obtained from commercial fishermen in Celestun and San Felipe, Mexico, during November, 1991 (Fig. 1). Heart and muscle tissue were removed from each individual, stored at -20°C in a fish house in Merida, Mexico, and transported on wet ice to Houston, Texas, where they were frozen in liq- uid nitrogen. Upon arrival at Texas A&M, tissues were stored at — 80°C. Details of DNA isolation, storage, restriction enzyme digestion, aga- rose electrophoresis of DNA frag- ments, and Southern blot hybrid- ization with a mtDNA probe may be found in Gold and Richardson (1991). The mtDNA probe used (MCm-mt2) was an entire red grou- per mtDNA genome cloned into lambda bacteriophage (Richardson and Gold, 1993). The ten restriction endonucleases used in this study were those previously identified to be polymorphic in red grouper (Richardson and Gold, 1993) and included Apa I, Kpnl, Ncol, Nde I, Nhe I, Nsil, Pvull, Sspl, Xbal, and Xmnl. Within-sample mtDNA diversity was assessed by nucleon diversity (probability that any two individu- als drawn at random will differ in mtDNA haplotype) and by intra- populational nucleotide sequence diversity (average nucleotide differ- ence between any two individuals drawn at random). Both estimates of mtDNA diversity were generated by using equations in Nei and Tajima (1981). Geographic partitioning of mtDNA variation was assessed by homoge- neity testing of mtDNA haplotype frequencies and by searching for phylogeographic cohesion or struc- ture of mtDNA haplotypes with a parsimony approach. Homogeneity tests included 1) a log-likelihood (G) test and 2) a Monte Carlo random- ization procedure developed by Roff and Bentzen ( 1989). Analyses were carried out with the BIOM-PC (a package of statistical programs, Applied Biostatistics Inc.; Rohlf, 1987) and REAP (restriction en- * This paper represents number XV in the series “Genetic studies in marine fishes” and contribution number 48 of the Cen- ter for Biosystematics and Biodiversity at Texas A&M University, College Station, Texas 77843-2258. Manuscript accepted 25 July 1996. Fishery Bulletin 95:174—179 (1997). NOTE Richardson and Gold Mitochondrial DNA diversity in and population structure of Epmephelus mono 175 Figure 1 Sampling localities of red grouper ( Epinephelus morio). zyme analysis package; McElroy et al., 1992) com- puter software packages. A minimum-length parsi- mony network of mtDNA haplotypes was constructed by connecting haplotypes in increments of (inferred) single site gains and losses. Results Digestion patterns of the ten restriction enzymes revealed 16 mtDNA haplotypes among all red grou- per assayed to date (exclusive of the five individuals from the Dry Tortugas sampled by Richardson and Gold [1993, Table 1]). Of the 16 mtDNA haplotypes, one (haplotype 1) accounted for 77% of all individu- als sampled. Four haplotypes were present in both geographic regions. The remaining 12 haplotypes were unique to a geographic region (Table 1). Per- centage nucleotide sequence divergence between in- dividual haplotypes ranged from 0.09 to 0.59 (mean ±SE = 0.27 ±0.01). MtDNA nucleon diversity among individuals from the Campeche Banks was 0.365. This value is lower than that found among individuals from the west coast of Florida (0.457). Percentage intrapopulational nucleotide sequence diversity among individuals from the Campeche Banks was 0.042 ± 0.001 (mean ± SE), as compared with 0.078 ± 0.003 among indi- viduals from the west coast of Florida. Nucleon and intrapopulational nucleotide sequence diversities among all red grouper assayed to date are 0.389 and 0.059 ±0.001, respectively. Values obtained are based on the 28 restriction enzymes surveyed by Rich- ardson and Gold ( 1993) and on the assumption that the restriction enzymes previously found to be monomorphic among red grouper from the west coast of Florida are monomorphic among red grou- per from Mexico as well. Results of tests for homogeneity of mtDNA hap- lotype frequencies between the two localities were nonsignificant (G=20.02, P~ 0.21 and %2=14.71, P= 0.55). The parsimony network (Fig. 2) included a single “assumed” haplotype (i.e. one not detected in the survey ). All the haplotypes in the network, including the “assumed” haplotype, could be de- rived from adjacent haplotypes by one or two re- striction site changes. The most common haplo- type (haplotype 1) was considered to be central, and nine of the remaining 14 haplotypes were de- rived from the central haplotype by a single re- striction site change. Haplotypes 5 and 8 are most divergent and are separated from the common haplotype by 4 restriction site changes. Except for haplotypes 5 and 8, and 6 and 7 (all from Campeche Banks, Mexico) which are grouped by a single re- striction site change, no geographic partitioning was evident. Table 1 Distribution of 16 mitochondrial DNA composite genotypes (haplotypes). Composite Haplotype MtDNA number genotype7 Locality Campeche Banks West Florida Shelf 1 AAAAAAAAAA 43 34 2 AAABAAAAAA 1 1 3 AAAAAAAAAB 3 2 4 AAAABAAAAA 1 1 5 ABAAAABAAC — 1 6 AAAAAABAAA — 1 7 AABAAABAAA — 1 8 ABAAAABCAB — 1 9 CAAAAAAAAA — 1 10 AAAAABAAAA — l 11 AAAAAAABBA — 1 12 BAAAAAAAAB — 1 13 AAAACAAAAA 2 — 14 ACAAAAAAAA 2 — 15 AAAAAAADAB 1 — 16 AAAAAAABAA 1 — 1 Letters (from left to right) are digestion patterns for Apal, Kpnl, Ncol, Nde I, Nhel , Nsil, Pvull, SspI, Xbal, and Xmnl. Restric- tion fragment sizes may be found in Richardson and Gold ( 1993). Fragment sizes for three restriction fragment patterns not pre- viously identified are as follows: (in base pairs) KpnKC). 16800; NheUC): 6800, 3200, 2950, 2450, 1300, 50; and SspI(D): 6000, 5400, 2900, 1700, 800. 176 Fishery Bulletin 95( 1 ), 1997 Figure 2 Minimum-length parsimony network of red grouper mitochon- drial DNA haplotypes. Numbers refer to mtDNA haplotypes listed in Table 1. Hatch marks represent the number of restriction site changes among individual haplotypes. The haplotype designated by “a” refers to a haplotype assumed to exist, but not detected in the survey. Shaded and solid circles refer to haplotypes unique to a locality (West Florida Shelf and Campeche Banks respectively). Open circles are haplotypes found in both localities. Discussion Homogeneity in mtDNA haplotype frequency and absence of phylogeograhic structure among haplo- types are consistent with the hypothesis that red grouper from the west Florida shelf and the Campeche Banks represent a single unit stock. There are caveats to this hypothesis. First, genetic homo- geneity does not unequivocally establish occurrence of a unit stock, in part because proof of a null hy- pothesis is impossible. Genetic homogeneity in this case is simply consistent with the hypothesis that samples are drawn from a single population with the same parametric haplotype frequencies. In addition, small amounts of gene flow are sufficient to homog- enize populations genetically (Allendorf and Phelps, 1981), even though geographic samples may be dis- continuous demographically. Another caveat is that observed homogeneity may reflect historical rather than current events. Present-day populations could be isolated spatially but have had enough contact in the recent past such that haplotype frequencies are overshadowed by historical gene flow. Examination of a more rapidly evolving nuclear marker (e.g. microsatellite loci) may provide data that suggest such a scenario. Within-population mtDNA diversity among red grouper from the Campeche Banks was lower than that reported previously for red grouper from the west Florida shelf, and overall, red grouper have among the lowest levels of mtDNA diver- sity reported for marine fish species (Table 2). Levels of intrapopulational mtDNA diversity are thought to reflect evolutionary-effective population sizes of females (Avise et al., 1988), although there is some evidence (Gold et al., 1994) that intrapopulational mtDNA diversities may also reflect contemporary (female) popu- lation sizes as well. The latter is of interest given that some of the species with low intra- populational mtDNA diversities (e.g. weakfish, orange roughy) have experienced significant reductions in population sizes over the past sev- eral years (Graves et al., 1992b; Smolenski et al., 1993). The mtDNA diversity observed in red grouper may thus indicate that red grouper warrant immediate attention in terms of man- agement regulation. Alternatively, red grouper and black sea basses, the species with the low- est reported mtDNA diversities (Table 2), are protogynous hermaphrodites (Manooch, 1988), and it is possible that this mode of reproduc- tion may affect estimates of mtDNA diversity. Estimates for black sea bass (Bowen and Avise, 1990), however, may be somewhat compromised by the low sample sizes, given that a significant pro- portion of the sampling variance for estimates of nucleotide diversity stems from population sampling (Lynch and Crease, 1990). Further study of mtDNA diversity in other hermaphroditic fishes and in sea bass is clearly warranted. Present-day gene flow The observed genetic homogeneity in E. morio from west Florida and Mexico was surprising because, a priori, we expected gene flow between the two areas to be minimal and the two populations to be diver- gent in mtDNA haplotypes. This expectation was based on available information about the life history of red grouper and on reported discontinuity in red grouper distribution. Observations from divers and aquaria personnel have shown that juvenile red grou- per are fairly sedentary, preferring to hide in crev- ices or shells of shallow nearshore habitat (Moe, 1969). Adult red grouper also are important mem- bers of the benthic community, frequently occupying crevices, ledges, and caverns formed by rugged lime- stone reefs. There is, however, evidence that red grou- per do migrate at least to some extent, and tagging data suggest “developmental” migration from shal- low coastal waters to depths greater than 36 m at approximately 5 years of age (Moe, 1966, 1967; NOTE Richardson and Gold: Mitochondrial DNA diversity in and population structure of Epmephelus mono 177 Tabie 2 Comparison of estimates of intrapopulational mtDNA di- versity in various species of marine fishes. Species Number of individuals surveyed Number of mtDNA haplotypes Nucleotide sequence diversity (%) Bluefish7 372 40 1.23 Atlantic herring2 69 26 1.09 Gulf Menhaden3 16 16 0.99 Red drum4 693 99 0.88 Spanish sardine5 73 24 0.52 Red snapper4 421 68 0.50 Black drum4 300 37 0.48 Greater amberjack6 59 23 0.34 Spotted seatrout7 384 73 0.31 Orange roughy3 244 22 0.13 Weakfish9 370 11 0.13 Red grouper 105/0 16 0.05i; Atlantic black sea bass2 19 3 0.03 Gulf black sea bass2 9 2 0.03 I Graves et al., 1992a. 2Kornfield and Bogdanowicz, 1987. 3 Bowen and Avise, 1990. 4 Gold et al., 1994. 5Tringali and Wilson, 1993. 6Richardson and Gold, 1993. 7 Gold, J. R. 1995. Dep. of Wildlife and Fisheries Sciences, Texas A&M Univ., College Station, TX 77843-2258. Unpubl. data. 8Smolenski et al., 1993. 9Graves et al., 1992b. ;oNumber of individuals includes 5 additional specimens from the Dry Tortugas surveyed by Richardson and Gold (1993). II Value obtained by using 28 restriction enzymes surveyed in Richardson and Gold (1993). Value obtained from the ten poly- morphic restriction enzymes surveyed here is 0.15. Beaumariage1). Presumably, this migration corre- sponds to the onset of sexual maturity. Other tag- ging data (Moe, 1966) suggest that adult red grou- per may move as much as 18 to 50 miles over a pe- riod of time from several months to a year. Finally, on the basis of data from other species of Epinephelus (Mito et al., 1967), the pelagic larval stage in E. morio is presumed to last 30-40 days, during which larvae are dispersed by ocean currents along a great por- tion of the Florida shelf (Moe, 1969). However, de- spite the evidence suggesting that individual red grouper could move considerable distances, consis- tent patterns of migration in red grouper are not re- 1 Beaumariage, D. S. 1969. Returns from the 1965 Schlitz tag- ging program including a cumulative analysis of previous re- sults. Fla. Dept. Nat. Resources, Mar. Res. Lab., Tech. Ser. No. 59:1-38. Div. of Mar. Resources, Dep. of Environmental Protection, Florida Mar. Res. Inst., 100 Eighth Ave. SE, St. Pe- tersburg, FL 33701. ported, and it is generally presumed that adult red grouper do not undergo large-scale movements offshore. Gene flow between the west Florida shelf and the Campeche Banks via migration of adults would have to occur either 1 ) along the north-central and west- ern Gulf or 2) across the Florida Straights. Rivas ( 1970) noted circumstantial evidence suggesting that there may be seasonal migration of red grouper be- tween the northern and southern Gulf, most prob- ably via a western route. Red grouper, however, are rarely taken in the Gulf west of the Mobile Basin (Springer and Bullis, 1956), and virtually no land- ings of red grouper occur along most of the Texas coast (Osburn2; Campbell3). The apparent paucity of red grouper in the northwestern Gulf may reflect either the absence of suitable habitat along the Texas-Louisiana shelf or be a result of some other extrinsic barrier (McEachran4). These observations suggest that movement of adult red grouper through the western Gulf is unlikely, if it occurs at all. With respect to movement across the Florida Straights, Rivas ( 1970) considered it unlikely that red grouper, a bottom dwelling fish, would cross great depths. The Florida Straights are characterized by 100 to 2,000 fathom depths that separate the Campeche Banks from the west Florida shelf (Rezak et al., 1985). This range also suggests limited movement, if any, of red grouper from west Florida to the Campeche Banks. Alternatively, present-day gene flow among red grouper could occur through dispersal of larvae by ocean currents. Shulman and Bermingham (1995) recently examined variation in mtDNA data among eight species of reef-associated fishes and searched for correlations between gene flow and egg type (pe- lagic and nonpelagic) and length of planktonic (usu- ally larval) life, two life history traits which could potentialy affect dispersal capability. Although sur- face currents that might explain observed genetic homogeneity in five of the species were identified, neither egg type nor length of larval stage appeared to be an adequate predictor of geographic structure in reef associated fishes (Shulman and Bermingham, 1995). Therefore, even though red grouper may have 2 Osburn, H. R. 1988. Trends in finfish landings by sport-boat fishermen in Texas marine waters, May 1974-May 1987. Texas Parks Wildl. Dep., Manag. Data Ser., no. 150, Austin, TX. Fish- eries and Wildlife Div., Coastal Fisheries Branch, Texas Parks and Wildlife Dep., 4200 Smith School Road, Austin, TX 78744. 3 Campbell, R. P. 1993. Trends in Texas commercial fishery landings, 1972-1992. Texas Parks Wildl. Dep., Manag. Data Ser., no. 106, Austin, TX. Fisheries and Wildlife Div., Coastal Fisheries Branch, Texas Parks and Wildlife Dep., 4200 Smith School Road, Austin, TX 78744. 4 McEachran, J. D. 1995. Dep. of Wildlife and Fisheries Sci- ences, Texas A&M Univ., College Station, TX 77843-2258. Per- sonal commun. 178 Fishery Bulletin 95( I ), 1997 a lengthy pelagic larval stage, there is no reason to assume a priori that gene flow occurs via dispersal of red grouper larvae. Nonetheless, it cannot be ruled out as a contributing factor. Historical bottleneck In a general sense, genetic homogeneity and absence of phylogenetic structure are compatible with lim- ited gene flow under models where isolated popula- tions (or subpopulations) have recently diverged from a panmictic population that possessed low levels of genetic variation and where each subpopulation has a small effective population size. An example of iso- lated subpopulations that are genetically homoge- neous and also genetically depauperate are African cheetahs, where subspecies in east and south Africa are essentially monomorphic for the same alleles at numerous genetic loci (O’Brien et al., 1987). To ac- count for both genetic homogeneity between, and low genetic variability within, subpopulations, O’Brien et al. (1987) hypothesized the past occurrence of at least two genetic bottlenecks. Their hypothesis was based on the premise that genetic homogeneity of isolated subpopulations was consistent with a histori- cal event; whereas low genetic variability in extant populations was consistent with a more recent event. Red grouper fit the cheetah model in that the sub- populations surveyed are genetically homogeneous and each possesses limited genetic variation. We suggest the possibility that red grouper from west Florida and Mexico are isolated genetically, but that recurring genetic bottlenecks continue to generate high frequencies of the most common genotype. In addition, we suggest that red grouper from these two regions were not isolated historically and that the historical population underwent a severe bottleneck that reduced much of the extant genetic variation. These suggestions account for the observed genetic data and how isolated populations can be genetically homogeneous. A historical bottleneck could have oc- curred during late Pleistocene times when environ- mental fluctuations impacted the biota of the region (Rezak et al., 1985; Graham and Mead, 1987). Our suggestions could be tested, in part, by asking whether rare haplotypes found in both locals are identical by descent (i.e. independently derived). In red grouper, two of the three haplotypes shared be- tween west Florida and Campeche Banks are the result of a site loss from the common haplotype and could be the result of a nucleotide substitution at any one of six nucleotide positions. Examination of a more rapidly evolving nuclear marker in individu- als from each locality that share these rare haplotypes would address this issue. Acknowledgments We thank C. Furman and K. Burns for help in pro- curing specimens of red grouper from Mexico. Work was supported by the Marfin Program of the U.S. Department of Commerce Award NA90AA-H-MF755, administered by the National Marine Fisheries Ser- vice and by the Texas Agricultural Experiment Sta- tion under Project H-6703. Part of the work was car- ried out in the Center for Biosystematics and Biodiversity, a facility funded, in part, by the Na- tional Science Foundation under grant DIR-8907006. Literature cited Allendorf, F. W., and S. R. Phelps. 1981. Use of allelic frequencies to describe population structure. Can. J. Fish. Aquat. Sci. 38:1507-1514. Arreguin-Sanchez, F. 1987. Present status of red grouper fishery of the Campeche Bank. Proc. Gulf Caribb. Fish. Inst. 37:498-509. Avise, J. C., R. M. Ball, and J. Arnold. 1988. Current versus historical population sizes in verte- brate species with high gene flow: a comparison based on mitochondrial DNA lineages and inbreeding theory for neutral mutations. Mol. Biol. Evol. 5: 331-344. Bowen, B. W., and J. C. Avise. 1990. Genetic structure of Atlantic and Gulf of Mexico popu- lations of sea bass, menhaden, and sturgeon: influence of zoogeographic factors and life-history patterns. Mar. Biol. 107:371-381. Brule, T., and L. G. Rodriguez Canche. 1993. Food habits of juvenile red groupers, Epinephelus morio (Valenciennes, 1828), from Campeche Bank, Yucatan, Mexico. Bull. Mar. Sci. 52:772-779. 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 F. Sun. 1994. Mitochondrial DNA diversity and population struc- ture in marine fish species from the Gulf of Mexico. Can. J. Fish. Aquat. Sci. 51:205-214. Graham, R. W., and J. I. Mead. 1987. Environmental fluctuations and evolution of mam- malian faunas during the last deglaciation in North America. In W. F. Ruddiman and H. E. Wright (eds.), North America and adjacent oceans during the last degla- ciation, p. 371-402. Geological Society of America, Boul- der, CO. Graves, J. E., J. R. McDowel, A. M. Beardsley, and D. R. Scoles. 1992a. Stock structure of the bluefish Pomatomus saltatrix along the mid-Atlantic coast. Fish. Bull. 90:703-710. Graves, J. E., J. R. McDowell, and M. L Jones. 1992b. A genetic analysis of weakfish, Cynoscion regalis, stock structure along the Mid-Atlantic coast. Fish. Bull. 90:469-475. Kornfield, I., and S. M. Bogdanowicz. 1987. Differentiation of mitochondrial DNA in Atlantic her- ring, Clupea harengus. Fish. Bull. 85:561-568. NOTE Richardson and Gold: Mitochondrial DNA diversity in and population structure of Epinephelus mono 179 Lynch, M., and T. J. Crease. 1990. The analysis of population survey data on sequence variation. Mol. Biol. Evol. 7:377-394. Manooch, C. S., III. 1988. Fisherman’s guide to the fishes of the southeastern United States. NC State Mus. Nat. Hist., Raleigh, NC. McElroy, D., P. Moran, E. Bermingham, and I. Kornfield. 1992. REAP-The Restriction Enzyme Analysis Package. J. Hered. 83:157-158. Mito, S., M. Ukawa, and M. Higuchi. 1967. On the larval and young ages of a serranid fish, Epinephelus akaara (Temminck et Schegel). Bull. Naikai Region. Fish. Res. Lab. 25:337-347. Moe, M. A. 1966. Tagging fish in Florida offshore waters. Fla. Bd. Conserv., Prof. Papers Ser., no. 4, 117 p. 1967. Prolonged survival and migration of three tagged reef fishes in the Gulf of Mexcio. Trans. Am. Fish. Soc. 96: 228-229. 1969. Biology of the red grouper Epinephelus morio (Valenciennes) from the eastern Gulf of Mexico. Prof. Pap. Ser. Fla. Dep. Nat. Resour. Mar. Sci. Lab. 34:1-331. Nei, M., and F. Tajima. 1981. DNA polymorphism detectable by restriction endonucleases. Genetics 97:145-163. O’Brien, S. J., D. E. Wildt, B. Mitchell, T. M. Caro, C. FitzGibbon, I. Aggundey, and R. E. Leakey. 1987. East African cheetahs: Evidence for two population bottlenecks? Proc. Natl. Acad. Sci. 84:508-511. Rezak, R., T. J. Bright, and D. W. McGrail. 1985. Reefs and banks of the northwestern Gulf of Mexico. John Wiley and Sons Inc., NY, 259 p. 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. Rivas, L. R. 1970. The red grouper of the Gulf of Mexico. Commer. Fish. Rev. 32:24-30. Roff, D. A., and P. Bentzen. 1989. The statistical analysis of mitochondrial polymor- phisms: chi-square and the problem of small samples. Mol. Biol. Evol. 6:539-545. Rohlf, F. J. 1987. BIOM: A package of statistical programs to accom- pany the text Biometry. Applied Biostatistics, Inc. Setauket, NY, 70 p. Sh u I man, M. J., and E. Bermingham. 1995. Early life histories, ocean currents, and the popula- tion genetics of Caribbean reef fishes. Evol. 49:897-910. Smith, C. L. 1961. Synopsis of biological data on grouper ( Epinephelus and allied genera) of the western North Atlantic. FAO Fish. Bio. Synop. 23:1-61. Smolensk!, A. J., J. R. Ovenden, and R. W. G. White. 1993. Evidence of stock separation in southern hemisphere orange roughy ( Hoplostethus atlanticus , Trachichthyidae) from restriction-enzyme analysis of mitochondrial DNA. Mar. Biol. 116:219-230. Solis Ramirez, M. 1970. The red grouper fishery of Yucatan Peninsula, Mexico. Proc. Gulf Caribb. Fish. Inst. 22:122-129. Springer, S., and H. R. Bullis. 1956. Collections by the Oregon in the Gulf of Mexico. Sci. Rep. U.S. Fish. Wildl. Serv., no. 196:1-134. Tringali, M. D., and R. R. Wilson Jr. 1993. Differences in haplotype frequencies of mtDNA of the Spanish sardine, Sardinella aurita, between specimens from the eastern Gulf of Mexico and southern Brazil. Fish. Bull. 91:362-370. 180 Entanglement of California sea lions, Zalophus californianus californianus, in fishing gear in the central-northern part of the Gulf of California, Mexico Alfredo Zavala-Gonzalez Eric MeSSiok Departamento de Ecologia, Centro de Investigacion Cientifica y de Educacion Superior de Ensenada Apdo. Postal 2732, Ensenada, Baja California, Mexico U S. mailing address: CICESE; PO. Box 434844, San Diego, CA 92 1 43 E-mail address: alzago@cicese.mx The range of the California sea lion, Zalophus californianus califor- nianus, extends from British Co- lumbia south to Mazatlan, Mexico, and includes the Gulf of California. The population of sea lions in Mexico has been estimated at 74,467 individuals along the Pacific coast (Lowry et al.1), 28,220 in the Gulf of California (Zavala, 1990). Little is known about the Pacific coast population, but there are probably 8 breeding colonies (Lowry et al., 1992). In the Gulf there are 40 rookeries: 13 breeding colonies and 27 haulouts (Zavala et al., in press). Strong tidal forces cause a constant upwelling condition in the central-northern part of the Gulf of California that sustains high nutri- ent and phytoplankton concentra- tions, especially around the Midriff Islands in the central Gulf (Al- varez-Borrego, 1983; Alvarez-Bor- rego and Lara-Lara, 1991). This upwelling condition allows the exist- ence of large populations of fish, marine mammals, and marine birds. Between the 1960’s and the 1980’s, the population at some breeding colonies of California sea lions in the Gulf of California in- creased 30% (Le Boeuf et al., 1983). During the 1980’s and early 1990’s, the yearly increase in those popu- lations was between 2% (Morales, 1990; Zavala et al., in press) and 4.7% (Aurioles and Arizpe2). After 1991 some populations experienced a slight reduction in size and later a partial recovery (Heath et al., 1994; Zavala et al.3). Since 1985, we have censused annually 10 of the 11 reproductive sea lion colonies that account for 94.9% of all sea lions in the Gulf of California (Fig. 1 in Aurioles and Zavala, 1994). In 1991 we com- menced seeing more sea lions with pieces of fishing gear entangled around their head and neck than we had remembered seeing during previous years (Zavala and Garcia4) and began documenting the inci- dence of entanglement. We report the numbers of entangled sea lions observed between 1991 and 1995 in the central-northern part of the Gulf of California and comment on the effect this may have on the con- servation of the species. Methods Ten of 11 breeding colonies in the central-northern Gulf of California were studied (Fig. 1). Only Roca Consag (31°12'N, 114°29'W) was excluded. Between 1991 and 1995, we made eight cruises to the 10 breeding colonies: 16 Jun-19 Jul 1991, 8 Jul-4 Aug 1992, 16-25 Jun 1993, 10-29 Jul 1993, 16-25 Jun 1994, 11-20 Jul 1994, 1-4 Aug 1994, and 15-28 Jun 1995. San Jorge and El Coloradito were vis- ited only once each year whereas Los Cantiles and San Pedro Martir were not surveyed in 1991 and 1992, respectively. All other islands were surveyed on every trip. All cruises were made on patrol ships of the Mexican Navy, leaving from Guaymas, Sonora. Surveys around the islands were made aboard small (7 m in length) fiber- glass boats with 35-55 hp outboard motors. We cruised at about 2 knots, 30-50 m from the coast, to census the animals. They were clas- sified as adult males, subadult males, females, juveniles, or pups (sensu LeBoeuf et al., 1983; Auri- oles and Zavala, 1994). Entanglement frequencies were calculated by dividing the total number of entangled animals (those animals with pieces of fishing gear around head and neck) by the total number of adult, subadult, and 1 Lowry, M. S., P. Boveng, R. J. DeLong, Ch. W. Oliver, B. S. Stewart, H. DeAnda, and J. Barlow. 1992. Status of California sea lion (Zalophus californianus cali- fornianus) population in 1992. Admin. Rep. LJ-92-32, 35 p. Southwest Fisher- ies Science Center, NMFS, NOAA, P.O. Box 271, La Jolla, CA 92038. 2 Aurioles, D., and O. Arizpe. 1989. Unpubl. data. Departamento de Pes- querias y Biologia Marina. Centro Inter- disciplinario de Ciencias Marinas. Apdo. Postal 592, La Paz, B.C.S., Mexico. 3 Zavala, A., H. de la Cueva, and E. Mellink. 1991. Unpubl. data. Departa- mento de Ecologia, Centro de Investi- gacion Cientifica y Educacion Superior de Ensenada, Apdo. Postal 2732, Ensenada, B.C., Mexico. 4 Zavala, A., and M. C. Garcia. 1991. De- partamento de Ecologia, Centro de Inves- tigacion Cientifica y Educacion Superior de Ensenada, Apdo. Postal 2732, Ensen- ada, Baha California, Mexico. Personal obs. Manuscript accepted 4 September 1996. Fishery Bulletin 95:180-184 (1997). NOTE Zavala-Gonzalez and Mellink: Entanglement of California sea lions in fishing gear 181 Figure 1 Central-northern Gulf of California, showing the study localities. young animals. We excluded dead animals because time since death could not be assessed. Pups spend most of their time on land and have virtually no in- teraction with fishing gear during the survey peri- ods. When we made more than one survey per year, we considered the highest rate of entanglement as the best estimator. We used the “differences-between-proportions” test (Zar, 1974) to compare the proportions of age and sex classes between the entangled animals and the population. Differences between sea lion colonies and years were compared by using a two-factor Analysis of Variance (ANOVA) and Newman-Keuls (N-K) tests (Zar, 1974) on the basis of the number of entangled animals (adjusted by a square-root transformation) and entanglement rates (adjusted by an arcsine transformation). Variation in the number of en- tangled animals and in the average transformed en- tanglement rates over time, as well as the relation- ship between number of entangled animals and colony size, were analyzed with simple linear regres- sions (Zar, 1974). In the two cases where we lacked data, we used the eight colonies, for which we had complete infor- mation, to calculate the relationship between the year with missing data (1991 or 1992) and the average of the remaining four years, and then used the average of the problem colony (Los Cantiles or San Pedro Martir) to estimate the missing value. This was done in order to perform the statistical tests on a stan- dard basis. Types of fishing gear involved were recorded from dead entangled animals. This information was com- pleted with records about the size and characteris- tics of the fishing gear used by fishermen working in the islands. Similarly, fishing gear debris found on sea lion rookeries and showing evident signs of hav- ing been associated with a sea lion (bites, sea lion fat,) was noted. We also interviewed local fishermen about their problems with sea lions. Results and discussion Sea lion entanglement During the study, we counted 237 entangled animals (Table 1), 207 of which could be assigned to a par- ticular sex and age class: 46.4% were young animals (1-3 yr), 41.5% females, 7.2% subadult males, and 4.8% adult males. The percentage of young animals among the entangled animals was statistically higher than their proportion in the censused population (25.8%, Z=6.70, P=7.13xl0-11), whereas that of fe- males was lower (60.5%, Z--5.53, P=0. 913x10 8). This is probably a result of young animals being more curious, less experienced, and weaker, in addition to foraging closer to the surface (senior author, personal observation [1994-95]). In other species (northern fur seals, Callorhinus ursinus ) high rates of juvenile mortality might have caused a decrease in the popu- lation (Trites, 1992). Our data are not sufficient to establish or discard any such links. Percentages of entanglement of subadult and adult males were not different from percentages of proportion of subadult and adult males in the population (6.3%, Z=0.78, P=0.294 and 7.4%, Z=1.5, P=0.13, respectively). Between 1991 and 1995, the number of recorded entangled sea lions for all 10 rookeries combined varied from 34 (±2.27, 95% Cl) (adjusted to 36) to 72, with a low of 24 (±4.02, 95% Cl) (adjusted to 27) in 1992 (Table 1). Regression analysis between num- ber of entangled animals and year was significant (n= 5, r2=0.79, F=11.05, P=0.045). Significant differ- ences were found only between 1992 and 1995: the other years were not different from one other (F=3.09, P=0.027). In 1992, there were not only fewer entangled ani- mals, but also there were fewer sites with entangled animals. This year saw the worst recent fishing sea- son in the central Gulf according to information at the Bahia de Los Angeles fishing office, and this find- ing is likely a reflection of the prevailing El Nino Southern Oscillation conditions, which had strong 182 Fishery Bulletin 95 ( 1 ), 1997 Table 1 Number of entangled and total (in parentheses, including entangled) California sea lions, except pups, in 10 breeding colonies in the central-northern Gulf of California, 1991-95. Rookeries Locations Year Lat. Long. 1991 1992 1993 1994 1995 San Jorge 31°01'N 113°15'W 8 (4,536) 14 (2,915) 10 (2,183) 4 (2,208) 24 (3,200) Coloradito 30°03'N 114°29'W 4 (2,100) 2 (1,610) 7 (1,662) 3 (1,749) 13 (1,688) Granito 29°34'N 113°33'W 2 (609) 1 (603) 6 (390) 2 (923) 4 (631) Los Cantiles 29°32'N 113°29'W — 1 (712) 3 (620) 1 (602) 6 (916) Los Machos 29°18'N 113°31'W 0 (718) 0 (601) 1 (718) 4 (659) 1 (512) El Partido 28°53'N 113°02’W 5 (463) 2 (524) 4 (798) 3 (311) 1 (402) El Rasito 28°49'N 113°00'W 1 (353) 0 (326) 1 (101) 5 (223) 0 (198) San Esteban 28°43'N 112°35'W 10 (4,758) 4 (3,135) 10 (2,610) 20 (3,859) 14 (3,396) S.P. Martir 28°23'N 112°20'W 1 (1,379) — 11 (676) 8 (770) 7 (937) S.P. Nolasco 27°58'N 111°23'W 3 (1,009) 0 (338) 1 (517) 3 (340) 2 (358) Total 34 (15,925) 24 (10,764) 54 (10,275) 53 (11,644) 72 (12,238) effects in the Gulf (Hamman et al., 1995). Entangle- ment rate did not exhibit any tendency through time (n=5, r2= 0.44, F=2.35, P= 0.223), and the ANOVA de- tected only 1992 as statistically inferior to 1993 and 1994, whereas 1991 and 1995 were not different from any other year (F=4.20, P=0.007). San Jorge and San Esteban exhibited the largest overall numbers of sea lions (>2,000 California sea lions) (Table 1); El Coloradito had intermediate val- ues (>1500, <2200), and the other colonies <1400 sea lions. The ANOVA indicated that the first two sites had statistically more entangled sea lions than did El Partido, Granito, Cantiles, San Pedro Nolasco, El Rasito, and Los Machos, whereas San Pedro Martir and El Coloradito were not different from any other site (F= 7.67, P<0.001). This pattern corresponds to differences in the size of the colonies; a regression linking total number of entangled animals at each colony and size of the different colonies was highly significant (n=10, r2=0.92, F=89.71, P<0.001). We have no detailed records of differences in the fishing effort throughout the study area, although it seems to be larger in the Midriff region than in the northern Gulf (E. Mellink, personal obs. [1995]). The ANOVA did not show differences in the entanglement rates between colonies (F=0.64, P=0.76), and the number of incidents seemed to be more a function of the size of the colony than of local and yearly varia- tions in fishing effort, although, as suggested by the 1992 data, these effects cannot be neglected. Entanglement rates in our region varied between 0% and 2.24%. These values are substantially lower than the 3. 9-7. 9% detected in Los Islotes, Bahia de La Paz, in the southern Gulf of California (24°35'N, 110°23'W; Harcourt et ah, 1994). In the latter local- ity, high values may have been due to the proximity of Los Islotes to a moderate-size city (La Paz, approx. 250,000 inhabitants) and to abundant sport and com- mercial fishing. However, our entanglement values are higher than those at California islands (0.08%, Stewart and Yochem, 1987) and, again, this could be due to differences in the intensity and type of fish- ing practiced. The main fishing gear involved in sea lion entangle- ment in the study area were nets and, to a lesser degree, lines and ropes. The nets included monofila- ment, purse seine, and gill nets (with stretched mesh sizes 3”, 3.2”, 4,5”, 5”, and 8”), cotton gill nets (1.5” and 5.1” mesh size), and trammel nets (either cotton or nylon monofilament with 14” to 16” mesh size). These nets originally measure 120-180 fathoms long and 7 fathoms high. The lines involved were all ny- lon of different thickness and, in most cases, were found tied around the animal. In only one case did we see a hook in a sea lion’s mouth, or a line coming out of it. Most entanglement occurs during fishing, either when the net and the catch are hauled out or when a net is deployed during 24-48 hr periods. In other regions of the North Pacific, in addition to entangle- ment during events involving active fishing, entangle- ment occurs because marine mammals encounter drift- ing debris, especially when they are foraging or mi- grating (Fowler, 1987). In the central-northern Gulf of California it is rare to encounter fishing gear de- bris drifting in the water. Artesanal fishermen, who carry out most of the fishing in the area, cannot af- ford to lose nets; therefore nets are usually fixed, not drifted. When part of a net or a complete net is no longer usable, it is usually discarded in a local gar- NOTE Zavala-Gonzalez and Mellink: Entanglement of California sea lions in fishing gear 183 bage dump. When a fisherman finds a lost drifting net at sea, he takes it for his own use. Unlike other areas of the world (Croxall et al., 1990) and the south- ern Gulf of California (Harcourt et al., 1994), the cen- tral-northern portion of the Gulf of California did not provide evidence of sea lions entangled in nonfishing plastic debris. Sea lion conservation In addition to accidental entanglements reported here, there is a deliberate (although illegal) killing of sea lions in the region for baiting shark longlines. At Isla San Pedro Martir, Thomson and Mesnick5 found 14 sea lions entangled in a gill net in a cave about 15-20 m from a breeding site, in July 1993. They concluded that the net had been set to inten- tionally capture sea lions. In December 1993, about 20 sea lions were captured in a gill net in San Pedro Nolasco (El Imparcial, Hermosillo, Sonora, 25 De- cember 1993). The fishermen involved argued that the capture had been accidental, resulting from their lack of expertise. In addition to their intentional cap- ture, sea lions are sometimes shot with firearms be- cause fishermen believe that they interfere with fish- ing gear (Belgado-Estrella et al., 1994). Our data were limited to a single season in each year of a 5-yr span and did not include animals that died without us having seen them on the islands. However, according to our assessment, the current entanglements rate of 0.49% does not seem to pose a threat to the conservation of California sea lions in the central-northern Gulf of California. We believe, however, that entanglement should be routinely monitored and studied in further detail. Acknowledgments Maria del Carmen Garcia, Elisa Peresbarbosa, Leonardo Inclan, Maria Concepcion Garcia, Jaime Luevano, Martin Escoto, Luis Bourillon, Maria de la Soledad Tordecillas, Sarah Mesnick, Carlos Godinez, Dulce Maria Broussett, and several students from the Laboratorio de Mamiferos Marinos, Facultad de Ciencias, Universidad Nacional Autonoma de Mexico, assisted with field work. Oscar Sosa, Gregory Hamman, Fred Julian, Mark Lowry, and Tim Gerrodette provided editorial assistance. All cruises and surveys were carried out with the support of the 5 Thomson, D. A., and S. L. Mesnick. 1993. Biodiversity of the marine fauna around islands in the Gulf of California. Unpubl. report. Department of Ecology and Evolutionary Biology, Univ. Arizona, Tucson, AR. Armada de Mexico (6ta Zona Naval), to which we are greatly in debt. Consejo Nacional de Ciencia y Tecnologia (Mexico) supported A. Zavala with an aca- demic scholarship and partially supported this project through a grant to E. Mellink. The Oficina Federal de Pesca in Baja California kindly provided fishing records for Bahia de los Angeles. All work was carried out under permits from the Direccion General de Aprovechamiento Ecologico de los Recursos Naturales, Institute Nacional de Ecologia. Literature cited AIvarez-Borrego, S. 1983. The Gulf of California. In B. H. Ketchum ( ed. ), Es- tuaries and enclosed seas of the world, p. 427-449. Else- vier Sci. Publ., Amsterdam. AIvarez-Borrego, S., and R. Lara-Lara. 1991. The physical environment and primary productivity of the Gulf of California. In J. P. Dauphin and V. R. T. Simoneit (eds.), The Gulf and Peninsular province of the Californias: memoir 47, p. 555-567. Am. Assoc. Petroleum Geologists. Aurioles, G. D., and A. Zavala G. 1994. Ecological factors that determine distribution and abundance of the California sea lion Zalophus califor- nianus, in the Gulf of California. Ciencias Marinas 20:535-553. Croxall, J. P., S. Rodwell, and I. L. Boyd. 1990. Entanglement in man-made debris of antartic fur seals at Bird Island, South Georgia. Mar. Mamm. Sci. 6:221-233. Delgado-Estrella, A., J. G. Ortega-Ortiz, and A. Sanchez-Rios. 1994. Varamientos de mamiferos marinos durante prima- vera y otono y su relacion con la actividad humana en el norte del Golfo de California. Anales del Institute de Bio- logta, Universidad Nacional Autonoma de Mexico, serie Zoologia 65:287-295. Fowler, C. W. 1987. Marine debris and northern fur seals: a case study. Mar. Pollut. Bull. 18:326-335. Hamman, M. G., J. S. Palleiro-Nayar, and O. Sosa-Nishizaki. 1995. The effects of the 1992 El Nino on the fisheries of Baja California, Mexico. Calif. Coop. Oceanic Fish. In- vest. (CALCOFI) Report 36:127-133. Harcourt, R. D. Aurioles, and J. Sanchez. 1994. Entanglement of California sea lions at Los Islotes, Baja California Sur, Mexico. Mar. Mamm. Sci. 101:122- 125. Heath, C. B., D. Aurioles, M. C. Garcia, and A. Zavala. 1994. Factores que indican la disminucion de la dis- ponibilidad de presas de la poblacion de lobos marinos en la Isla Angel de la Guarda, Mexico. Reunion Internacional para el estudio de los Mamiferos Marinos (Resumenes) 19, 7 P- LeBouf, B. J., D. Aurioles, R. Condit, C. Fox, R. Gisiner, R. Romero, and F. Sinsel. 1983. Size and distribution of the California sea lion popu- lation in Mexico. Proceedings of the California Academy of Sciences 43:77-85. 184 Fishery Bulletin 95(1), 1997 Morales V., B. 1990. Parametros reproductive^ del lobo marino en la Isla Angel de la Guarda, Golfo de California, Mexico. M.Sc. thesis, Universidad Nacional Autonoma de Mexico. Mexico, D.F., 110 p. Stewart, B. S., and P. K. Yochem. 1978. Entanglement of pinnipeds in synthetic debris and fishing net and line fragments at San Nicolas and San Miguel Islands, California. Mar. Pollut. Bull. 18:336—339. Trites, A. W. 1992. Northern fur seals: why have they declined? Aquat. Mamm. 18:3-18. Zar, J. H. 1974. Biostatistical analysis. Prentice Hall. Englewood Cliffs, NJ, 620 p. Zavala, A. G. 1990. La poblacion de lobo marino comun, Zalophus californianus (Lesson 1828) en las islas del Golfo de Cali- fornia, Mexico. B.Sc. thesis. Universidad Nacional Autonoma de Mexico, Mexico, D.F., 253 p. Zavala G. A., A. Aguayo L., D. Aurioles G., and M. C. Garcia R. In press. Distribucion y abundancia del lobo marino Zalophus californianus californianus (Lesson 1828), en el Golfo de California, Mexico. Biotica. 185 Erratum Quinn, T. P., J. L. Nielsen, C. Gan, M. J. Unwin, R. Wilmot, C. Guthrie, and F. M. Utter. 1996. Origin and genetic structure of chinook salmon, Oncorhyncus tshawytscha, transplanted from Cali- fornia to New Zealand: allozyme and mtDNA evi- dence. Fish. Bull. 94:506-521. Corrections: In Table 1 (p. 509) and Table 3 (p. 514): Locus PEP-B2*, Allele *108 should be Locus PEP-B1*, Allele *103 In Table 4 (p. 516) Locus PEP-B2* should be Locus PEP-B1* 186 Fishery Bulletin 95( 1 ), 1997 Superintendent of Documents Order Form *5178 □yes, please send me the following publications: subscriptions to Fishery Bulletin ( FB ) for $32.00 per year ($40.00 foreign). The total cost of my order is $ . Price includes regular shipping and handling and is subject to change. International customers please add 25%. Charge Company or personal name (Please type or print) your order. It’s Additional address/attention line easy! Street address City, State, Zip code Daytime phone including area code P3S Purchase order number (optional) Check method of payment: □ Check payable to Superintendent of Documents -□ □ GPO Deposit Account □ VISA □ MasterCard (expiration date) To fax your orders (202) 512-2250 To phone your orders (202) 51 2-1800 Thank you for your order! 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Articles 187 Alvarez, Federico, and Francisco Alemany Birthdate analysis and its application to the study of recruitment of the Atlanto-lberian sardine Sardina pilchardus 195 Barber, Willard E., Ronald L. Smith, Mark Vallarino, and Robert M. Meyer Demersal fish assemblages of the northeastern Chukchi Sea, Alaska 209 Broadhurst, Matt K., Steven J. Kennelly, John W. Watson, and Ian K. Workman Evaluations of the Nordmore grid and secondary bycatch- reducing devices (BRD's) in the Hunter River prawn-trawl fishery, Australia 219 Gunderson, Donald R. Spatial patterns in the dynamics of slope rockfish stocks and their implications for management 231 Johnson, Lyndal L., Sean Y. Sol, Daniel R Lomax, Gregory M. Nelson, Catherine A. Sloan, and Edmundo Casillas Fecundity and egg weight in English sole, Pleuronectes vetulus, from Puget Sound, Washington: influence of nutritional status and chemical contaminants 250 Lamkin, John The Loop Current and the abundance of larval Cubiceps pauciradiatus (Pisces: Nomeidae) in the Gulf of Mexico: evidence for physical and biological interaction 267 Leber, Kenneth M., H. Lee Blankenship, Steve M. Arce, and Nathan R Brennan Influence of release season on size-dependent survival of cultured striped mullet, Mugil cephalus, in a Hawaiian estuary Fishery Bulletin 95(2), 1 997 280 Legault, Christopher M., and Nelson M. Ehrhardt Correcting annual catches from seasonal fisheries for use in virtual population anlysis 293 Lenarz, William H., and Franklin R. Shaw Estimates of tag loss from double-tagged sablefish, Anoplopoma fimbria 300 Lutcavage, Molly, Scott Kraus, and Wayne Hoggard Aerial survey of giant bluefin tuna, Thunnus thynnus, in the Great Bahama Bank, Straits of Florida, 1 995 3 1 1 Martini, Frederic, John B. Heiser, and Michael R Lesser A population profile for Atlantic hagfish, Myxine glutinosa (L. ), in the Gulf of Maine. Part I: Morphometries and reproductive state 321 McDermott, Susanne F., and Sandra A. Lowe The reproductive cycle and sexual maturity of Atka mackerel, Pleurogrammus monopterygius, in Alaska waters 334 Robertson, Kelly M., and Susan J. Chivers Prey occurrence in pantropical spotted dolphins, Stenella attenuata, from the eastern tropical Pacific 349 Timmons, Maryellen, and Richard N. Bray Age, growth, and sexual maturity of shovelnose guitarfish, Rhinobatos productus (Ayres) Notes 360 Abitia-Cardenas, Leonardo A., Felipe Galvan-Magana, and Jesus Rodriguez-Romero Food habits and energy values of prey of striped marlin, Tetrapturus audax, off the coast of Mexico 369 Biggs, Douglas C, Robert A. Zimmerman, Rebeca Gasca, Eduardo Suarez-Morales, Ivan Castellanos, and Robert R. Leben Note on plankton and cold-core rings in the Gulf of Mexico 376 Lazzari, Mark A., David K. Stevenson, and Stephen M. Ezzy Physical environment and recruitment variability of Atlantic herring, Clupea harengus, in the Gulf of Maine 386 Sekiguchi, Keiko, and Peter B. Best In vitro digestibility of some prey species of dolphins 394 Tucker, John W., Jr., and Sabine R. Alshuth Development of laboratory-reared sheepshead, Archosargus probatocephalus (Pisces: Sparidae) 402 Subscription form 187 Birthdate analysis and its application to the study of recruitment of the Atlanto- Iberian sardine Sardina pilchardus Federico Alvarez Francisco Alemany Instituto Espanol de Oceanografia Muelle de Poniente s/n. Apdo. 291, 07080 Palma de Mallorca, Spain E-mail address: falvarez@ctv.es Abstract .—The ages of 217 juve- niles from the Atlanto-Iberian sardine (Sardina pilchardus ) stock were deter- mined by means of counts of daily growth rings in otoliths. These juve- niles were caught by the commercial purse-seine fleet off Galicia (NW Spain) between June and November 1992. The back-calculated hatching period was 13 December 1991 to 2 April 1992, with a mean date of 2 February and a stan- dard deviation of ± 17 days. The origi- nal aim of the study was to relate the birthdate distribution of the recruits to environmental, biological, and physical data taken during a series of oceano- graphic cruises. Oceanographic cruises, carried out between March and July 1992, covered the spring-spawning area of the stock (Cantabrian Sea and coasts of Galicia, the supposed area of origin for the recruits) but such a relationship was not documented because the re- sults of the study showed that most surviving juveniles were spawned be- fore the period considered during the oceanographic cruises. However, the observed birthdate distribution of the recruits, together with hydrographic data, does suggest that a larval drift from the northern Portuguese coasts to the Galician coast took place. Thus, at least in 1992, there is evidence to sug- gest that the winter-spawning zone, located along the coast of northern Por- tugal, may have been the area of origin for recruits off Galicia, in contrast to the previous assumption that these fish were spawned in the Cantabrian Sea. Manuscript accepted 25 September 1996. Fishery Bulletin 95:187-194 (1997). The Atlanto-Iberian sardine Sardina pilchardus (also known as Euro- pean pilchard [Robins et al., 1991]) is the dominant coastal pelagic fish species along the Atlantic coasts of the Iberian peninsula, as much for the biomass of the stock as for its importance in the pelagic fisheries of Spain and Portugal. Landings, carried out by purse seiners, reached a maximum value for the period 1975-92 of 214,000 metric tons (t) in 1981, and a minimum of 126,000 t in 1992. In the same period, the estimated biomass of the spawning stock varied between 160,000 t in 1976 and 510,000 t in 1985 (Anony- mous1). An analysis of the abun- dance trend of the stock (Pestana, 1989) suggests that, in the short term, catches are dependent on re- cruitment, characteristic of short- lived pelagic species (Ulltang, 1980). In recent years more intensive work has been done on document- ing the oceanographic characteris- tics for the area of distribution of the Atlanto-Iberian sardine stock. Among these, the most noticeable is the seasonal upwelling that af- fects the west coast of the Iberian peninsula (Fiuza, 1984; Lavin et al., 1991; Cabanas et al., 1992). Numer- ous studies have also been carried out on the biological aspects of the species, such as areas and periods of reproduction (Re et al., 1990; Lopez-Jamar et al., in press; Cunha and Figueiredo2; Garcia et al.3; Sola et al.4), frequency of spawning (Perez et al., 1992a), batch fecun- dity (Perez et al., 1992b), and lar- val growth (Re, 1983; Alemany and Alvarez5), as well as the spatial dis- tribution by age classes, of which the latter suggests a northward dis- placement of early age groups as they grow (Porteiro et al.6). In this area, the sardine has a protracted spawning season that can last prac- 1 Anonymous. 1993. Report of the Work- ing Group on the assessment of mackerel, horse mackerel, sardine and anchovy. ICES Council Meeting 1993/H:19, 274 p. (mimeo). 2 Cunha, E., and I. Figueiredo. 1988. Re- productive cycle of Sardina pilchardus in the central region off the Portuguese coast (1970/1987). ICES Council Meeting 1988/ H:61, 54 p. (mimeo). 3 Garcia, A., C. Franco, A. Sola, and M. Alonso. 1988. Distribution of sardine Sardina pilchardus egg and larval abun- dance off the Spanish North Atlantic coast (Galician and Cantabrian areas) in April 1987. ICES Council Meeting 1988/H:27, 8 p. (mimeo). 4 Sola, A., L. Motos, C. Franco, and A. lago de Lanzos. 1990. Seasonal occurrence of pelagic fish egss and larvae in the Canta- brian sea (VIIIc) and Galicia (IXa), from 1987 to 1989. ICES Council Meeting 1990/H:25, 15 p. (mimeo). 5 Alemany, F., and F. Alvarez. 1992. Re- gional growth differences in sardine Sardina pilchardus larvae from Canta- brian and Galician coasts. ICES Council meeting, 22 p. (mimeo). 6 Porteiro, C., F. Alvarez, and J. A. Per- eiro. 1986. Sardine ( Sardina pilchar- dus Walb.) stock differential distribution by age class in Divisions VIIIc and Ixa. ICES Council Meeting 1986/H:20, 13 p. (mimeo). 188 Fishery Bulletin 95(2), 1997 tically all year. The main spawning periods, however, are in the spring (April-May) along the northern coast of Spain (Cantabrian Sea) (Sola et al.4) and in winter (December^January) off the northern Portu- guese coast (Re et al., 1990). Thus, in a given year, a wide range of likely birthdates exist. Depending on the particular biotic and abiotic conditions that af- fect survival during the prerecruitment period, the abundance and the age composition (birthdate dis- tribution) of recruits will be restricted, however, to a relatively short period. An important part of the present study was initi- ated through Spanish-USA collaboration (Anony- mous, 1990) within the framework of the Sardine Anchovy Recruitment Project (SARP). The work car- ried out in this program continued for the next 3 years under the auspices of a European program that also sponsored research on the sprat Sprattus sprattus in the German Bight and the anchovy Engraulis encrasicolus within Portuguese estuaries. The origi- nal aim of this project was to identify the biological and environmental factors affecting interseasonal larval mortality of short-lived coastal pelagic fish. Birthdate analysis is one of the more relevant tools for the study of recruitment processes (Campana and Jones, 1992). Birthdates of juvenile fishes were first calculated by Methot (1983) who showed that the data can be used to determine periods of high sur- vival. This technique is a key element in the work of SARP, i.e. to test the critical survival-period hypoth- esis (Hjort, 1914 and 1926) and its later variants (Cushing, 1975; Lasker, 1981; Parrish et al., 1981). Only one paper, in which this technique was applied, is available for the area studied (Alvarez and Butler, 1992). It shows that birthdates of surviving fish oc- curred at the beginning of a period of calm weather in May and is consistent with Lasker’s (1981) stable- ocean hypothesis. The aims of the present work were 1) to calculate the birthdate distribution of recruits in the stock of the Atlanto-Iberian sardine in 1992, as inferred from daily otolith growth increment analysis; 2) to relate the birthdate distribution of recruits to environmen- tal conditions during earlier development stages; and 3) to verify the previous hypothesis that sardine re- cruits in the Galician area originate in the Canta- brian Sea. Material and methods Age determination Juvenile sardines were sampled fortnightly, 30 June- 19 November 1992, from landings of the commercial purse-seine fleet at the ports of Vigo and La Coruna (Fig. 1), providing a total of 22 samples, each of 50 specimens. These ports are located in the area of re- cruitment of the Atlanto-Iberian sardine (age-0 fish) (Anonymous1). A subsample of ten individuals was taken at random from each sample for birthdate analysis from counts of the daily growth-ring incre- 8°W 7° 6° Figure 1 Map of the northwestern Iberian peninsula showing sampling ports (dots) and area of distribution of age-0 sardines, Sardina pilchardus (shadowed), during the 3rd and 4th quarters in 1992 (redrawn from Anonymous, 1993). Alvarez and Alemany: Birthdate analysis and its application to the study of recruitment of Sardina pilchardus 189 ments (Methot, 1981). The length ranges by sample of the analyzed specimens (n= 220) are given in Table 1. Fish were measured (standard length and total length by 1-mm size classes), weighed (0.1 g), and their otoliths were removed and mounted on micro- scope slides with Eukitt mounting medium. It is pos- sible to determine the ages of juveniles and their daily growth rates because the daily deposition of growth increments has been validated for sardine (Re, 1984), the time of formation of the first daily growth ring is known (Alemany and Alvarez, 1994), and because it is possible to distinguish false or subdaily increments from true daily growth rings. It should be pointed out that no gaps were observed in the daily growth rings in the sagittal otoliths of the analyzed speci- mens. Each daily ring was counted and its width was measured, along a transect located at ± 5° of the long- est radius from the focus to the posterior margin of the otolith, with a video coordinated digitizer con- nected to a microcomputer (Methot, 1981). To reveal increments, the otoliths were progressively polished between readings by using 30-, 9-, and 0.3-micron lapping film. Magnifications of x60, x640, and xl,000 were used. Data from several replicate transects per otolith, at different magnifications, were combined to estimate age. Occasionally, daily increments were difficult to resolve within short (<60 microns) seg- ments of the otoliths. In these cases, widths of rings, and hence their number, were interpolated by using linear approximation based on the widths of previ- ous and later clearly visible daily rings. Data from an otolith reading were rejected as unreliable if the interpolation process affected more than 5% of the readings. Environmental conditions During the main sardine spawning season off the northern and northwestern coasts of the Iberian pen- insula, 5 cruises were conducted between March and July 1992 (Lopez-Jamar et al., in press). The proto- cols established during a pilot cruise carried out in April 1991 (Lopez-Jamar et al.7) were followed. The 7 Lopez-Jamar, E., S. Coombs, F. Alemany, J. Alonso, F. Alvarez, C. Barrett, J. M. Cabanas, B. Casas, G. Diaz del Rio, M. L. Fernandez de Puelles, C. Franco, A. Garcia, N. C. Halliday, A. Lago de Lanzos, A. Lavin, A. Miranda, D. Robins, L. Valdes, and L. M. Varela. 1991. A SARP pilot study for sardine Sardina pilchardus off north and northwest Spain in April/May 1991. ICES Council Meeting 1991/L:69, 36 p. (mimeo). Table 1 Summary statistics for the Atlanto-Iberian sardine Sardina pilchardus by port and sampling date in 1992. TL=total length (mm), SD=standard deviation, BD=birthdate, Age=days. Port and sampling date Mean TL SD Min. TL Max. TL Mean BD Mean age SD Min. age Max. age La Coruna 7 Jul 90 6 82 103 18 Jan 173 11 156 193 24 Jul 113 7 105 127 13 Jan 194 13 172 215 9 Sep 116 3 111 122 23 Jan 231 8 216 246 17 Sep 130 3 125 133 21 Jan 243 13 225 265 1 Oct 128 9 121 155 20 Jan 256 19 231 294 6 Oct 126 6 118 135 1 Feb 249 19 228 295 13 Oct 118 11 105 139 1 Feb 254 22 215 298 28 Oct 133 6 121 140 30 Jan 273 13 241 291 4 Nov 115 2 112 120 14 Feb 265 22 217 293 13 Nov 109 3 105 113 13 Feb 275 21 256 290 Vigo 30 Jun 83 3 77 87 26 Jan 157 5 151 169 8 Jul 89 5 80 97 25 Jan 165 5 154 173 16 Jul 97 3 94 105 22 Jan 177 13 162 207 8 Sep 104 3 92 102 9 Feb 212 11 193 224 22 Sep 112 6 107 127 3 Feb 235 13 218 254 1 Oct 104 5 93 111 11 Feb 234 10 220 247 9 Oct 107 6 99 122 10 Feb 243 7 238 260 14 Oct 113 5 103 120 30 Jan 259 11 240 279 22 Oct 131 6 121 141 8 Feb 259 9 249 273 27 Oct 138 13 101 145 1 Feb 270 14 254 292 11 Nov 122 4 117 132 1 Mar 258 17 231 280 19 Nov 121 8 112 137 23 Feb 271 11 247 287 190 Fishery Bulletin 95(2), 1997 seasonal production and distribution of sardine lar- vae and their nutritional condition were determined during these cruises as were the spatial and tempo- ral distributional patterns of larvae in relation to hydrographic and biological parameters. During the first 3 cruises, additional transects were located in French waters to estimate the extension of spawn- ing in the northeastern area of the Cantabrian Sea. In the western area, spawning during these months is very low from Cape Finisterre southwards (Garcia et al., 1992); therefore sampling was curtailed at the Portuguese border. The sampling design for the present study did not cover, either spatially or tem- porally, spawning along the entire Iberian peninsula, because sampling on the Portuguese shelf during winter was not possible owing to logistical reasons. Sampling of this area was not considered important because other studies (Robles et al., 1992; Cabanas et al.8; Roy et al.9) have suggested that recruitment 8 Cabanas, J. M., C. Porteiro, and M. Varela. 1989. A possible relation between sardine fisheries and oceanographic conditions in NW Spanish coastal waters. ICES Council Meeting 1989/ H:18, 12 p. (mimeo). 9 Roy, P., C. Porteiro, and J. M. Cabanas. 1993. The optimal environmental window hypothesis in the ICES area: the ex- ample of the Iberian sardine. ICES Council Meeting 1993/L:76, 13 p. (mimeo). of the Atlanto-Iberian sardine stock depends mainly on spring spawning in the Cantabrian Sea, as well as on upwelling features along the west coast of the Iberian peninsula. Results The length distribution of a subsample of 220 juve- niles used for the birthdate analysis was not signifi- cantly different from that of the entire sample of 1,100 juveniles (Kolmogorov-Smirnov test, P>0.2). Of these 220 juveniles, 3 were rejected because daily rings were not visible in more than 5% of the transect readings. The age of the remaining 217 fishes ranged from 151 to 298 days, with birthdates from 13 De- cember 1991 to 2 April 1992 (Fig. 2). Data by sample date are shown in Table 1. The average birthdate of juveniles from La Coruna was 28 January 1992 (n- 98, SD=19 d, birthdate range: 13 December 1991 to 2 April 1992), and the average birthdate of juve- niles from Vigo was 6 February 1992 (n=119, SD=16 d, birthdate range: 23 December 1991 to 26 March 1992). The observed differences between the two birthdate distributions (Fig. 2) were significant (Kolmogorov-Smirnov test, P<0.001). Specimens younger than 5 months were not caught by the fish- Alvarez and Alemany: Birthdate analysis and its application to the study of recruitment of Sardina pilchardus 191 ery (see Table 1; Robles et al., 1992). Thus, no cor- rections for cumulative mortality were made because the corrected birthdate distribution would be quite similar to the uncorrected distribution (Methot, 1983). The most significant aspect of these results is that they indicate a period of birthdates outside the main period of larval production in the Cantabrian Sea. The relationship between the estimated age and length of the sampled recruits from Vigo and La Coruna are shown in Figure 3. In both cases, the relationship was linear, and significant (ANOVA, P<0.000). The slopes (£=1.26,P>0.10, df=213) and the intercepts (£=0.96, P>0.20, df=214) were not signifi- cantly different. Thus, a regression from pooled data was fitted (ANOVA, P<0.000). The precision of ageing within each sample and within each 1-cm length range was assessed with the calculated coefficient of variation, CV (standard de- viation divided by the the mean estimated age). The precision was good (CV<20%), stabilizing at a level of about 5-10% as fish grew in length (Fig. 4). The within-sample CV did not show any trend and re- mained at values less than 10% across all ages (Fig. 5). These results suggest that the intrinsic variabil- ity that may exist between otoliths of different fish of the same length range is low and that the preci- sion of the age estimates is not affected by age. To assess seasonal changes in the estimated birthdate distribution, we performed a test to com- pare birthdate distribution from early samples (June-September, n= 89) with a distribution calcu- lated from late samples (October-November, n = 128). There was no significant difference (Kolmogorov- Smirnov test, P>0.05), which indicated that the samples came from the same cohort and that mor- tality during the period was not age selective. Discussion The juvenile birthdate distribution, which shows that the 1992 recruits were winter spawned, does not support the hypothesis that sardine recruits in the Galician area are mainly spawned during spring in the Cantabrian Sea, as was suggested by a previous study on the birthdate distribution of juveniles in this area (Alvarez and Butler, 1992). In fact, the sur- viving juveniles observed in our study were spawned earlier than the time when the sampling cruises were carried out in 1992. Thus, it was not possible to draw any detailed conclusions on the relationship between larval survival and environmental factors from otolith data. This was a significant obstacle for the achievement of the objectives of SARP because the “within year” exercise relies on a comparison of the birthdate distribution of juveniles with environmen- tal conditions that occur during their larval develop- ment (Bakun et al.10). 10 Bakun, A., J. Alheit, and G. Kullenberg. 1991. The sardine- anchovy recruitment project (SARP): rationale, design and development. ICES Council Meeting 1991/L:43, 17 p. (mimeo). 192 Fishery Bulletin 95(2), 1997 However, if the spatial-temporal features of spawn- ing of this species in this area are taken into account, the results of Alvarez and Butler ( 1992), cited above, cannot be considered to be based on an unbiased sam- pling of the 1988 recruitment because they were de- rived from two samples only. 2 - 8 9 10 11 12 13 14 Total length (cm) Figure 4 Ageing precision for juveniles of the Atlanto-Iberian sardine Sardina pilchardus within 1-cm length ranges. Precision is defined as the standard deviation divided by the mean (CV). Alvarez and Alemany: Birthdate analysis and its application to the study of recruitment of Sardina pilchardus 193 An alternative area of origin for the 1992 recruits is the coastal waters off northern Portugal, where eggs are presumably spawned during the winter. Relatively large larvae were found in March-April in southern and western Galicia. These larvae may not have been locally spawned, because spawning is low in this area (Garcia et al., 1992; Lopez-Jamar et al., in press), but rather spawned during the winter off the coasts of north Portugal where high winter spawning has been observed (Re et al., 1990). Sup- porting evidence for such an area of spawning was deduced by Lopez-Jamar et al. (in press) from the results of a 1992 sampling, where the progressive northwards and westwards displacement (around Galicia) of a group of larger larvae (mean length >13 mm) was documented. Using a larval growth-rate estimate of 0.59 mm/day in March off Galicia (Alemany and Alvarez5), we found that the length range of this group of larvae is in accordance with a February birthdate. Moreover, such a spawning area is consistent with the poleward flow of winter circu- lation in the coastal ocean off southwest Europe (Frouin et al., 1990). The significant differences between the mean birthdates of recruits sampled at Vigo and those sampled at La Coruna are also consistent with the hypothesis of a larval drift from the south. The lar- vae that were produced at the beginning of the pe- riod, when the displacement northwards took place, would reach areas farthest from the spawning zone. Thus, the mean birthdate of specimens at La Coruna would be earlier than that of specimens at Vigo, as was observed. It is suggested from the evidence of the overall distribution pattern of sardine larvae in the Cantabrian Sea that, during the 1992 spawning season, most larvae drifted westwards and were dis- persed offshore in northern Galicia (Lopez-Jamar et al., in press). This pattern could be explained by the hydrographically dynamic area observed off north- western Galicia in spring (Lopez-Jamar et al., in press; Chesney and Alonso-Noval11). On the other hand, several studies have also suggested that re- cruitment of the Atlanto-Iberian sardine stock de- pends mainly on spring spawning in the Cantabrian Sea and on upwelling features along the west coast of the Iberian peninsula. These studies have been based on empirical relationships (Dickson et al., 1988; Cabanas et al.8) and a qualitative approach (Robles et al., 1992; Roy et al.9). Possible mechanisms asso- ciated with physical factors that could influence early 11 Chesney, E. J., and M. Alonso-Noval. 1989. Coastal up- welling and the early life history of sardine Sardina pilchardus along the Galician coast of Spain. ICES Council Meeting 1988/ H:61, 54 p. (mimeo). life-stage larvae from spring spawning in the Cantabrian Sea are suggested in these works. How- ever, the present study is a process-oriented ap- proach, which accounts for intraseasonal effects of the biotic and physical environment on the survival of a fish cohort. There is a possibility that spring-spawned recruits could exist off Galicia, but owing to their later incor- poration into the juvenile fishery, they may not have been present before the sampling of recruits was fin- ished in November 1992, when the March-June lar- vae may not have yet recruited to the fishery. How- ever, on the basis of the age range given in Table 1, these spring-spawned recruits would be caught by the fishery from August and should be distinguish- able in birthdate distribution. Moreover, the routine sampling of sardine length-frequency distributions carried out for stock assessments at the same area from January 1993 onwards has not revealed the presence of smaller sizes,12 which would be an indi- cation of recruitment from spring-spawned larvae in the Cantabrian Sea. In summary, our results reinforce the suggestion of an alternative origin for the Atlanto-Iberian sar- dine recruits of the Galician area, at least in some years. The particular hydrological conditions along the northern coast of Portugal would favor either spring- or winter-spawned recruits, as outlined by Lopez-Jamar et al., (in press). If upwelling in spring is weak, larvae spawned at this time in the Cantabrian Sea could be transported to the Galician area. On the other hand, if upwelling is intense, they may drift offshore at Cape Ortegal, as was postu- lated by Lopez-Jamar et al. (in press) for the 1992 spawning in the Cantabrian Sea. In this latter case, the recruits in the Galician area would come from winter-spawned larvae in northern Portugal, which could reach the Galician area by northward-flowing surface currents during the winter. The influence of these mechanisms on year-class abundance remains to be investigated. Acknowledgments We express our gratitude to P. Cubero for the pa- tient work of sampling juveniles, and to the person- nel of the laboratories participating in this Project (Centro Oceanografico de La Coruna and Centro Oceanografico de Malaga (IEO), Plymouth Marine Laboratory, Alfred Wegener Institut fur Polar und Meeresforschung, and Institut fur Hydrobiologie und 12 Porteiro, C. 1994. Institute Espanol de Oceanografia, P.O. Box 1552, 36280 Vigo, Spain. Personal commun. 194 Fishery Bulletin 95(2), 1 997 Fischereiwissenschaft) and involved in the larval sampling cruises, especially to E. Lopez-Jamar, S. H. Coombs, A. Garcia, R. Knust, and W. Nellen. We are also grateful for the contribution and the support of J. M. Cabanas and C. Porteiro, and we would like to thank E. Moksness and two anonymous reviewers who offered useful suggestions for improvement of the manuscript. This work was carried out with partial funding from the Consejo Interministerial de Ciencia y Tecnologia (CICYT) of Spain, contract 91.0089, and from the Fish- eries Aquaculture Research (FAR) programme of the European Union, contract MA196. Literature cited Alemany, F., and F. Alvarez. 1994. Formation of initial daily increments in sagittal otoliths of reared and wild Sardina pilchardus yolk-sac larvae. Mar. Biol. (Berl.) 121(l):35-39. Alvarez, F., and J. Butler. 1992. First attempt to determine birthdates and environ- mental relationship of juvenile sardine, Sardina pilchar- dus, in the region of Vigo (NW Spain) during 1988. Bol. Inst. Esp. Oceanogr. 8:115-121. Anonymous. 1990. IOC workshop report of the expert consultation on the Sardine/Anchovy Recruitment Programme (SARP). Intergovernmental Oceanographic Commission, Workshop Report 66, UNESCO, Paris, 51 p. Cabanas, J. M., G. Diaz del Rio, A. Lavin, and M. T. Nunes. 1992. Hydrographic conditions off the Galician coast, NW of Spain, during an upwelling event. Bol. Inst. Esp. Oceanogr. 8(l):7-26. Campana, S., and C. Jones. 1992. Analysis of otolith microstructure data. In D. K. Stevenson and S. E. Campana (eds.), Otolith microstruc- ture examination and analysis, p. 73-100. Can. Spec. Publ. Fish. Aquat. Sci. 117. Cushing, D. H. 1975. The natural mortality of the plaice. J. Cons. Int. Explor. Mer 36:150-157. Dickson, R. R., P. M. Kelly, J. M. Colebrook, W. S. Wooster, and D. H. Cushing. 1988. North winds and production in the eastern North Atlantic. J. Plankton. Res. 10:151-169. Fiuza, A. F. G. 1984. Hidrologia e dinamica das aguas costeiras de Portugal. Ph.D. diss., Univ. Lisbon, Portugal, 294 p. Frouin, R., A. F. G. Fiuza, I. Ambar, and T. J. Boyd. 1990. Observations of a Poleward Surface Current off the Coasts of Portugal and Spain during winter. J. Geophys. Res. 95:679-691. Garcia, A., C. Franco, and A. Sola. 1992. Sardine Sardina pilchardus egg and larval distribu- tion off North Atlantic coast (Galician and Cantabrian ar- eas) in April 1987. Bol. Inst. Esp. Oceanogr. 8:87-96. Hjort, J. 1914. Fluctuations in the great fisheries of northern Eu- rope. Viewed in light of biological research. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 20:1-228. 1926. Fluctuation in the year classes of important food fishes. J. Cons. Int. Explor. Mer 1:1-38. Lasker, R. 1981. The role of a stable ocean in larval fish survival and subsequent recruitment. In R. Lasker (ed.). Marine fish larvae: morphology, ecology and relation to fisheries, p. 80- 87. Univ. Washington Press, Seattle, WA. Lavin, A., G. Diaz del Rio, J. M. Cabanas, and G. Casas. 1991. Afloramiento en el noroeste de la peninsula Iberica. Indices de afloramiento para el punto 43°N 11°W. Periodo 1966-1989. Inf. Tec. Inst. Esp. Oceanogr. 91:1-33. Lopez-Jamar, E., S. Coombs, A. Garcia, N. C. Halliday, R. Knust, and W. Nellen. In press. The distribution and survival of larvae of sar- dine, Sardina pilchardus (Walbaum) off the North and Northwestern Atlantic coast of Spain in relation to envi- ronmental conditions. Bol. Inst. Esp. Oceanogr. 11. Methot, R. D. 1981. Growth rates and age distributions of larval and ju- venile northern anchovy, Engraulis mordax, with infer- ences on larval survival. Ph. D. diss., Univ. California at San Diego, San Diego, CA, 209 p. 1983. Seasonal variation in survival of larval northern an- chovy, Engraulis mordax estimated from the age distribu- tion of juveniles. Fish. Bull. 81(4):741-750. Parrish, R. H., C. S. Nelson, and A. Bakun. 1981. Transport mechanisms and reproductive success of fishes in the California Current. Biol. Oceanogr. 1: 175-203. Perez, N., I. Figuereido, and N. C. H. Lo. 1992a. Batch fecundity of Sardina pilchardus off the At- lantic Iberian coast. Bol. Inst. esp. Oceanogr. 8:155-162. Perez, N., Y. Figuereido, and B. J. Macewicz. 1992b. The spawning frequency of sardine, Sardina pilchardus off the Atlantic Iberian coast. Bol. Inst. esp. Oceanogr. 8:175-189. Pestana, G. 1989. Manancial ibero-atlantico de sardinha Sardina pilchardus, sua avaliagao e medidas de gestao. Ph. D. diss., Univ. Lisbon, Portugal, 188 p. Re, P. 1983. Daily growth increments in the sagitta of pilchard larvae Sardina pilchardus Walb. (Pisces: Clupeidae). Cy- bium 7(3):9-15. 1984. Evidence of daily and hourly growth in pilchard Sardina pilchardus larvae based on otolith growth increment. Cybium 8:33-38. Re, P., R. Cabral e Silva, E. Cunha, A. Farinha, I. Meneses, and T. Moita. 1990. Sardine spawning off Portugal. Bol. Inst. Nac. Invest. Pescas 15:31—44. Robins, C. Richard, R. M. Bailey, C. E. Bond, J. R. Brooker, E. A. Lachner, R. N. Lea, and W. B. Scott. 1991. World fishes important to North Americans. Am. Fish. Soc. Spec. Publ. 21, Bethesda, MD, 243 p.. Robles, R., C. Porteiro, and J. M. Cabanas. 1992. The stock of Atlanto-Iberian sardine: possible causes of variability. ICES Mar. Sci. Symp. 195:418-423. Ulltang, O. 1980. Factors affecting the reactions of pelagic fish stocks to exploitation and requiring a new approach to assess- ment and management. Rapp. P.-V. Reun. Cons. Int. Explor. Mer 177:489-504. 195 Demersal fish assemblages of the northeastern Chukchi Sea, Alaska Robert M. Meyer USGS Biological Resources Division, Eastern Regional Office 1 700 Leetown Road Kearneysville, West Virginia 25430 Abstract .—We documented the dis- tribution and abundance of demersal fishes in the northeastern Chukchi Sea, Alaska, in 1990 and 1991, and de- scribed 1990 demersal fish assemblages and their relationship to general oceanographic features in the area. We collected samples using an otter trawl at 48 stations in 1990 and 16 in 1991, and we identified a total of 66 species in 14 families. Gadids made up 83% and 69% of the abundance in 1990 and 1991, respectively. Cottids, pleuronectids, and zoarcids together made up 15% of the species in 1990, 28% in 1991. The number of species, species diversity (H), and evenness (V') generally were greater inshore than offshore and greater in the south than in the north. There were significant differences in ranks of species, species diversity, and evenness at 3 of 8 stations sampled both years. From data collected in 1990, 3 nearshore and 3 offshore station group- ings were defined. The northern off- shore assemblages had the fewest spe- cies, lowest diversity and evenness, and least abundance, whereas two southern assemblages had the most species, high- est diversity and evenness, and great- est abundance. We determined that bottom salinity and percent gravel were probably the primary factors influenc- ing assemblage arrangement. Manuscript accepted 9 September 1996 Fishery Bulletin 95:195-209 ( 1997). Willard E. Barber School of Fisheries and Ocean Sciences University of Alaska Fairbanks, Fairbanks, Alaska 99775-7220 E-mail address: wbarber@ims.alaska.edu Ronald L. Smith Mark Vallarino Institute of Marine Sciences University of Alaska Fairbanks Fairbanks, Alaska 99775-722 0 The distribution and abundance of commercially important demersal fishes inhabiting temperate and tropical seas are relatively well studied (e.g. Pearcy, 1978; Mahon and Smith, 1989; Weinberg, 1994). Results from such studies have been used to examine relationships be- tween environmental factors and fish assemblage distributions. Im- portant environmental variables that have been identified include sediment type, water depth, bottom temperature, and bottom salinity. Overholtz and Tyler ( 1985) found that six species assemblages on Georges Bank, northwestern Atlan- tic, remained consistent over depth for a number of years. Fargo and Tyler (1991), sampling at depths of 18-240 m in Hecate Strait off Brit- ish Columbia, found four species assemblages separated by depth. Pearcy ( 1978) described shallow and deep demersal fish assemblages in the northeast Pacific Ocean off the coast of Oregon at depths ranging from 70 to 102 m. Although there was an interaction between depth and sediment type, he concluded that depth was a primary factor and that sediment type was of second- ary importance. Mahon and Smith (1989) looked for interactions be- tween sediment characteristics, water depth, bottom temperature, and bottom salinity but concluded that assemblages were related more to depth than to other attributes. Scott ( 1982 ) reported that although fish distributions were related to sediment types, the latter was re- lated to depth. Studies of other fishes indicated that temperature and salinity are important; Jahn and Backus (1976), using salinity and temperature to characterize slope waters, the Gulf Stream, and northern and southern Sargasso Sea waters in the Atlantic Ocean, concluded that mesopelagic fishes associated with slope and Gulf Stream waters were distinct and different from fish assemblages as- sociated with the other two water masses. Bianchi (1992, a and b) de- termined that water depth, bottom temperature, bottom salinity, and 196 Fishery Bulletin 95(2), 1997 dissolved oxygen content determined benthic fish as- semblages observed off the west coast of central Africa. Relatively few fisheries resource surveys have been conducted in Arctic waters off Alaska; only three have been conducted in the northeastern Chukchi Sea (A1 verson and Wilimovsky, 1966; Frost and Lowry, 1983; Fechhelm et al.1). These were limited in geo- graphic coverage and not designed to address ques- tions on environmental factors influencing fish dis- tribution. The studies were, however, important first steps in determining factors influencing the distri- bution and abundance of fishes in Arctic waters. The goal of our study was to determine the distri- bution and abundance of demersal fishes, the pres- ence of species assemblages, and the relationship of such assemblages to oceanographic features in the northeastern Chukchi Sea, Alaska. Results from in- vestigations of the distribution and relative abun- dance of infaunal and epifaunal mollusks in the east- ern Chukchi Sea suggest that invertebrate assem- blages may be associated with differences in hydro- graphic conditions and sediment types (Feder et al., 1994, Feder et al.2). On the basis of these findings, we hypothesized that there would be onshore-off- shore and north-south differences in demersal fish abundance, biomass, and assemblages, and that these differences would be related to hydrographic conditions and sediment type. Materials and methods Our study area was north of 68°N (Point Hope, Alaska), east of 168°58'W, and limited in northward extent by weather and sea ice (Fig. 1). The shelf of the northeastern Chukchi Sea is rela- tively shallow, gently sloping offshore to depths of 30-50 m in the study area. Bottom sediments in the region are poorly sorted, trending to relatively coarse sediments on the inner shelf between Point Hope and Point Barrow, and shifting offshore to muds contain- ing various proportions of gravel and sand (Sharma, 1979; Naidu, 1988). Sediments in the more north- erly offshore region contain a higher percentage of water and a lower percentage of gravel than sedi- ments found in the more southernly offshore area (Feder et al.2). 1 Fechelm, P, C. Craig, J. S. Baker, and B. J. Gallaway. 1985. Fish distribution and use of near shore waters in the northeastern Chukchi Sea. U.S. Dep. Commer., NOAA, OCSEAP Final Rep. 32, p. 121-297. 2 Feder, H. M., A. S. Naidu, M. J. Hameedi, S. C. Jewett, and W. R. Johnson. 1990. The Chukchi Sea continental shelf: benthos-environmental interactions. U.S. Dep. Commer., NOAA, OCSEAP Final Rep. 68:25-311. The Chukchi Sea consists of several water masses ( Weingartner, in press): Alaska Coastal Water (ACW) and the Resident Chukchi Water (RCW) commonly dominate the study area. The ACW is relatively warm, low-salinity water lying nearshore. It is a mixture of Bering Shelf water and freshwater that comes from western Alaskan rivers, primarily the Yukon. The RCW is relatively cold, high-salinity water that lies seaward of the ACW. The RCW is ei- ther advected onshore from the upper layers of the Arctic Ocean or is remnant ACW from the previous winter. The ACW and RCW masses are separated by a hydrographic front that tends to be located between the 25-m and 40-m isobaths and that intersects the coast between Icy Cape and Point Franklin (Johnson, 1989; Weingartner, in press; Feder et al.2). Sampling occurred during August and September in 1990 and 1991. In 1990, 48 stations were occu- pied along 11 transects perpendicular to shore; 16 stations were occupied in 1991, including 8 that were sampled in 1990 (Fig. 1; station locations, water depths, bottom temperatures, and bottom salinities are given in Smith et al., in press, b). In 1990, nearshore stations were established closer to one another than were stations farther offshore in order to increase the probability of having two stations in each transect inshore of the historical position of the “bottom (hydrographic) front.” Weather and ice con- ditions dictated the sequence of stations sampled. Sta- tions were numbered to reflect the sampling sequence. Two samples for each category (fishes and inver- tebrates) were collected at each station by towing a standard 83-112 survey otter trawl3 for 30 minutes. However, because of weather condition and torn nets, only one haul was made at station 31 in 1990 and at stations 16, 91-33, 91-34, and 91-35 in 1991. The trawl had a 25.2-m head rope, 34.1-m footrope, tick- ler chain, and codend of 8.9-cm stretched mesh with a 3.2-cm stretched mesh liner. The area swept by the trawl was calculated by multiplying the length of each trawl haul (beginning and ending location of each tow was determined with “Global Positions Sys- tem”) by the width of the trawl during fishing (the trawl width at the wings and height of the headrope above the footrope were determined with a Scanmar™ electronic mensuration unit). Upon retrieval of the trawl, the entire catch was either weighed in the net with an electronic load cell or in baskets on a mechanical platform. Fish were sorted to the lowest taxa possible, counted, and weighed with a mechanical platform scale. Fish abun- dance (fish/km2) and biomass (g/km2) were deter- 3 Sample, T. E. 1994. Alaska Fisheries Science Center, Natl. Mar. Fish. Serv., NOAA, Seattle, WA. Personal commun. Barber et al.: Demersal fish assemblages of the northeastern Chukchi Sea 197 Figure 1 General location of stations sampled for demersal fishes in the northeastern Chukchi Sea, Alaska, during August and September 1990 and 1991. Specific locations for stations are given in Smith et al. (1996b). mined by the area-swept method (Wakabayashi et al., 1985). Following the last trawl at each station, bottom temperature and bottom salinity were measured with a Seabird™ SBE 19 internally recording conductiv- ity-temperature-depth recorder. Owing to a malfunc- tion, however, salinity and temperature could not be recovered for 7 of 48 stations sampled in 1990. The total number of unique species captured at each station was determined by pooling results from 198 Hshery Bulletin 95(2), 1997 the 2 trawl hauls. Mean abundance and biomass of each species at each station were determined by aver- aging the results from the 2 trawl hauls, except at the few stations where only 1 sample could be collected. To investigate diversity, we used the number of species for richness (S) and calculated Shannon’s Index ( H ) (Pielou, 1977). Abundance and total unique species of both samples were combined for each sta- tion. Shannon’s index was calculated as k n log n - ^ log ft H = ^ n where n = total number of fish; ft - number of individuals in species i ; and k = the number of species (Zar, 1984). By using Shannon’s Index (H), “evenness” was esti- mated with the equation , H v = > Ins where v' = measure of evenness; and s = the number of species present. Fish assemblages were identified and their rela- tionship to physical oceanographic conditions deter- mined in a two-stage process. The first stage used cluster analysis of species abundance by station, fol- lowed by discriminant function and principal coordi- nate analyses of environmental data. Cluster analy- sis based on species abundance at each station was used to determine fish assemblages. Following the recommendation of Clifford and Stevenson (1975), the most commonly occurring species (21 species, each of which made up >0.1% of abundance) were chosen on the basis of a preliminary examination of abundance data. These species made up 99.6% of the total abundance, 98% of the biomass. Prior to calcu- lating similarity indices, abundance (X) was trans- formed (In [X+l]) to normalize the data (Clifford and Stevenson, 1975). Similarity indices were calculated as 1 - D, where D is the Bray-Curtis dissimilarity index (Clifford and Stevenson, 1975) adapted from Lance and Williams (1967). The algorithm for D is ZK-M D = , n ^Xlj+X2j) i= 1 where n = number of individuals in species i; and j - number of stations. Similarity index values range from 0 to 1; a value of 1 indicates identical species composition between 2 stations and a value of 0 indicates no common spe- cies between stations. Following Clifford and Stevenson (1975), a range of similarity indices was used to determine major groupings. From prelimi- nary inspection of the data, it appeared that group- ings could be distinguished with indices of 0.5-0. 6. These indices were used as our reference for exam- ining the resulting dendrograms. Relationships between environmental conditions and fish assemblages were evaluated by using the following data subsets: 1) environmental (water depth, bottom temperature, and bottom salinity); 2) sediment type (arcsine-transformed percent of mud, sand, and gravel); and 3) abundance of infaunal and epifaunal mollusks. Sediment type and mollusk data values for those stations nearest ours were taken from Feder et al. (Footnote 1, sediment type) and Feder et al. (1994, mollusks). Multiple discriminant function analysis (DFA) and principal coordinate analysis (PCA) were used to evaluate the relationship between fish assemblages and environmental parameters. Mud, bottom tem- perature, epifaunal biomass, and invertebrate infau- nal biomass were not included in the analyses be- cause they were highly correlated with gravel, bot- tom salinity, epifaunal abundance, and infaunal in- vertebrate abundance, respectively. PCA was used to validate the results of the DFA and to determine whether other variables were influencing assem- blages. To control for multicollinearity, we discarded one of any pair of variables with -0.8 < r > 0.8. Abundance, commonality in species occurrence, ranks, and diversity were used to determine whether there was congruity between years at stations sampled in 1990 and 1991. Species ranks were com- pared by using the Wilcoxon signed-ranks test (Siegel and Castellan, 1988). Results Abundance and biomass A combined total of 66 species of 14 families were collected in 1990 and 1991 (Table 1). In 1990, two species of gadids, Boreogadus saida and Eleginus gracilis, made up 82% of the abundance and 69% of the biomass. Cottids, pleuronectids, and zoarcids made up an additional 15% of total abundance and 24% of total biomass in 1990. On the basis of percent Barber et a!.: Demersal fish assemblages of the northeastern Chukchi Sea 199 Table 1 Estimated mean abundance (no. of fish/km2), biomass (g/km2), and the percent (%) of each demersal fish species collected in the northeastern Chukchi Sea, Alaska, during 1990 and 1991. The 21 most abundant species are labeled in parentheses according to a decreasing scale of abundance from 1 (most abundant) to 21 (less abundant). 1990 1991 Species Abundance (%) Biomass (%) Abundance (%) Biomass (%) Cottidae (sculpins) Icelus spatula1 I. spiniger1 Cottidae sp. Artediellus sp. (21) A. pacificus A. scaber (7) Blepsias bilobus Enophrys diceraus Eurymen gyrinus Gymnocanthus tricuspis (4) Hemilepidotus papilio (20) Megalocottus platycephalus Microcottus sellaris1 Myoxocephalus sp. (3) M. polyacanthocephalus M. quadricornis M. verrucosus (6) Myoxocephalus sp. 2 Myoxocephalus sp. 1 Nautichthys pribilovius1 Triglops forficatus1 T. pingeli Pleuronectidae (flounders) Hippoglossoides robust us (5) Pleuronectes aspera P. proboscideus P. sakhalinensis1 P. quadrituberculatus Platichthys stellatus Reinhardtius hippoglossoides Hippoglossus stenolepis1 Zoarcidae (eelpouts) Lycodes palearis (11) L. polaris (14) L. raridens (15) L. turneri L. rossi Lycodes sp. 1 Lycodes sp. 2 Lycodes sp. Gymnelis hemifasciatus1 G. viridis Agonidae (poachers) Aspidophoroides bartoni1 A. olriki Podothecus acipenserinus (16) Occella dodecaedron1 Pallasina barbata 2 3 12 3 2 3 10 3 0.00 0.00 0.00 0.00 26 (0.10) 280 (0.06) 2 (0.01) 47 (0.01) 141 (0.55) 583 (0.12) 1 3 169 (0.03) 5 (0.02) 188 (0.04) 2 3 31 (0.01) 783 (3.06) 9,070 (1.84) 28 (0.11) 571 (0.12) 15 (0.06) 944 (0.19) 2 3 12 3 1,573 (6.15) 49,167 (9.99) 1 (.01) 167 (0.03) 6 (0.02) 442 (0.09) 238 (0.93) 12,604 (2.56) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 3 12 3 2 3 20 3 137 (0.54) 1,698 (0.35) (11.56) (15.46) 486 (1.90) 17,406 (3.54) 20 (0.08) 746 (0.15) 5 (0.02) 181 (0.04) 2 3 12 3 18 (0.07) 2,467 (0.50) 2 (0.01) 1,365 (0.28) 2 (0.01) 85 (0.02) 2 3 256 (0.05) (2.11) (4.59) 133 (0.52) 4,802 (0.98) 83 (0.33) 7,780 (1.58) 67 (0.26) 8,078 (1.64) 8 (0.03) 580 (0.12) 4 (0.02) 137 (0.03) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2 3 12 3 1 3 30 (0.01) (1.16) (4.37) 1 3 24 3 2 (0.01) 85 (0.02) 57 (0.22) 1,077 (0.22) 2 3 11 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 5 (0.05) 272 (0.20) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 197 (2.28) 704 (0.51) 0.00 0.00 0.00 0.00 130 (1.50) 1,106 (0.81) 1 (0.01) 17 (0.01) 494 (5.71) 5,228 (3.81) 9 (0.11) 414 (0.30) 10 (0.12) 944 (0.72) 0.00 0.00 0.00 0.00 90 (1.05) 1,295 (0.94) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 1,033 (11.95) 35,017 (25.51) 108 (1.25) 4,550 (3.31) 2 (0.02) 318 (0.23) 4 (0.05) 15 (0.01) 0.00 0.00 0.00 0.00 131 (1.52) 1,294 (0.94) (25.61) (37.29) 25 (0.29) 940 (0.68) 101 (1.17) 1,505 (1.10) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 16 (0.19) 2,016 (1.47) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (1.65) (3.25) 24 (0.27) 536 (0.39) 0.00 0.00 0.00 0.00 71 (0.82) 5,241 (3.82) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 8 (0.09) 92 (0.07) 8 (0.09) 92 (0.07) 4 (0.04) 112 (0.08) 0.00 0.00 0.00 0.00 30 (0.35) 72 (0.05) (1.66) (4.48) 0.00 0.00 0 (0.00) 0.00 0.00 0 (0.00) 24 (0.28) 147 (0.11) 0.00 0.00 0 (0.00) 2 (0.02) 9 (0.01) continued on next page 200 Fishery Bulletin 95(2), 1 997 Table 1 (continued) 1990 1991 Species Abundance (%) Biomass (%) Abundance (%) Biomass (%) Stichaeidae (pricklebacks) Chirolophis snyderi Lumpenus fabricii (13) L. medius1 Stichaeus sp. S. punctatus Eumesogrammus praecisus Gadidae (cods) Boreogadus saida (1) Eleginus gracilis (2) Gadus macrocephalus (17) Theragra chalcogramma (8) Cyclopteridae (snailfishes) Eumicrotremus andriashevi1 3 E. orbis Liparis sp. L. tunicatus L. gibbus ( 18) Osmeridae (smelts) Osmerus mordax (19) Mallotus villosus (10)4 Hexagram midae (greenlings) Hexagrammos stelleri Clupeidae (herring) Clupea harengus pallasi (12) Ammodytidae (sand lances) Ammodytes hexapterus Anarhichadidae (wolffish) Anarhichas orientalis1 0.00 0.00 0.00 0.00 90 (0.35) 1122 (0.23) 1 3 38 (0.01) 0.00 0.00 0.00 0.00 2 (0.01) 107 (0.02) 1 (0.01) 61 (0.01) 19,456 (76.06) 301,878 (61.34) 1642 (6.42) 38,769 (7.88) 44 (0.17) 1869 (0.38) 138 (0.54) 1883 (0.38) (83.19) (69.98) 2 3 11 3 4 (0.02) 116 ( 0.02) 1 3 34 (0.01) 10 (0.04) 373 (0.08) 44 (0.17) 442 (0 90) 32 (0.13) 1903 (0.39) CO CO (0.52) 710 (0.14) 4 (0.01) 151 (0.03) 126 (0.49) 17,469 (3.55) LOO 0.00 0.00 0.00 1 3 61 (0.01) 1 (0.01) 57 (0.04) 52 (0.61) 102 (0.07) 0.00 0.00 0.00 0.00 2 (0.02) 48 (0.03) 1 (0.01) 28 (0.02) 3 (0.04) 151 (0.11) 5,728 (66.27) 63,913 (46.56) 255 (2.95) 7150 (5.21) 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (69.22) (51.77) 31 (0.34) 753 (0.55) 2 (0.02) 112 (0.08) 4 (0.05) 373 (0.27) 0.00 0.00 0.00 0.00 17 (0.20) 2408 (1.75) 13 (0.15) 129 (0.09) 1 (0.01) 6 3 00 0.00 0.00 0.00 1 (0.01) 57 (0.04) 5 (0.06) 10 (0.01) 0.00 0.00 0.00 0.00 1 Found at only one station in 1990. 2 Less than 0.49. 3 Less than 0.01%. 4 Found at only one station in 1991. of total abundance, the 45 species captured in 1990 fell into four general categories: category 1 (extremely abundant) consisted only of B. saida and made up 76.1% of total abundance and 61.3% of total biomass; category 2 included five moderately abundant species ( Myoxocephalus verrucosus, Myoxocephalus sp., E. gra- cilis, Gymnocanthus tricuspis, and Hippoglossoides robustus) and made up 18.4% and 25.8% of total abun- dance and biomass, respectively (Table 1); category 3 included 13 occasional species and made up 5.9% and 13.7% of total abundance and biomass, respectively; and category 4 included 26 rare species that accounted for only 0.46% of the abundance and <5.0% of the bio- mass in 1990 (Table 1). The fish in the first two catego- ries accounted for more than 94.4% and 87.1% of the total abundance and biomass, respectively. This pat- tern was generally reflected in the 1991 catches. In 1990, there was a tendency for abundance and biomass of all species combined to be greatest in the southern part of the study area and lowest in the northern part of the study area (Fig. 2). Seven sta- tions south of Ledyard Bay yielded more than 50,000 fish/km2. In contrast, many stations off and north of Icy Cape had fewer than 10,000 fish/km2. Barber et at: Demersal fish assemblages of the northeastern Chukchi Sea 201 170° 155° 170° 155° Figure 2 Relative abundance ([number/km2] x 103) and biomass (g/km2) estimates of demersal fishes at 48 and 16 stations sampled during 1990 and 1991, respectively, in the northeastern Chukchi Sea, Alaska. In 1991, abundance and biomass estimates were low over the entire study area and there was no trend towards greater abundance or biomass in the south- ern area (Fig. 2). At the eight stations sampled in both 1990 and 1991, biomass and abundance esti- mates differed widely between years (Table 2). For example, at station 22, B. saida was 2.4 times as abundant in 1990 as in 1991, and H. robustus was 23 times as abundant in 1991 as in 1990. Species richness and diversity Families contributing the most species were Cottidae (21), Zoarcidae (10), Pleuronectidae (8), Stichaeidae (6), and Agonidae (5) (Table 1). Ten families contrib- uted only 16 additional species. Fifty-five percent of the species were represented by less than 10 indi- viduals and some 45% were represented by a single specimen. 202 Fishery Bulletin 95(2), 1 997 Table 2 Estimated mean abundance (fish/km2) of demersal fishes collected at stations sampled during both 1990 and 1991 in the north- eastern Chukchi Sea. Species sequence is based on the overall abundance of 1990 (Table 1), and the probability value ( P ) is from the Wilcoxon signed-ranks test. Species diversity was calculated from Shannon’s Index (H). Station 6 Station 16 Station 21 Station 22 Station 23 Station 43 Station 36 Station 27 Species 1990 1991 1990 1991 1990 1991 1990 1991 1990 1991 1990 1991 1990 1991 1990 1991 Boreogadus saida 56,373.8 14,183.5 22,386.5 2,273.4 32,184.6 393.0 20,475.3 8,527.7 3180 2,379.4 13,684.7 5,090.2 19,104.8 2,139.4 3,017.3 2,180.3 Gymnocanthus tricuspis 207.3 3,047.0 157.6 0 386.8 0 494.5 124.8 778.3 2,041.0 160.2 244.5 728.4 969.1 0 0 Myoxocephalus verrucosus 324.7 0 0 27.0 630.5 0 0 568.8 1,163.1 1,016.2 170.6 189.8 246.7 702.2 0 0 Enophrys diceraus 59.4 1,932.8 11.6 0 0 0 0 0 0 0 0 0 0 0 0 0 Myoxocephalus sp. 0 0 712.2 0 599.4 0 608.9 0 6 55.9 0 11 0 0 0 0 Pleuronectes aspera 400.8 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Hippoglossoides robustus 0 37.5 1,113.5 0 229.7 0 254.8 10.8 0 0 66.8 88.2 0 33.4 0 0 Lycodes raridens 0 102.4 0 54.1 0 0 1,061.0 0 0 0 550.5 22.0 0 0 0 34.6 Myoxocephalus sp.2 0 1,621.1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Lycodes palearis 0 37.5 492.1 162.4 199.8 0 416.1 0 0 0 0 0 0 0 0 0 Lumpenus fabncii 22.1 651.8 147.3 0 129.8 0 124.0 0 0 0 43 111.0 0 11.1 0 0 Triglops pingeli 0 519.1 0 0 27.4 0 0 0 57.8 111.8 375.4 22.2 49.1 11.1 0 0 Clupea harengus 221.8 0 10.7 0 804.1 0 26 0 0 0 34.1 0 0 0 0 0 Gadus macrocephalus 830.6 0 102.7 0 0 0 62.5 0 6 0 0 0 0 0 0 0 Number of other species 2,853.2 6,807.0 492.1 0 376.5 0 551.6 135.6 28.9 86.3 178.0 99.2 49.0 100.3 0 251.1 P 0.562 0.003 0.001 0.004 0.444 0.42 0.975 0.498 Number of species 19 24 15 4 15 1 17 8 7 9 14 13 8 10 1 5 Total number of species, both years combined 32 17 15 19 10 18 14 5 Percent in common both years 40.6 17.6 6.7 42.1 60 50 28.6 20 Species diversity 0.47 1.83 0.62 0.4 0.52 — 0.74 0.4 1.01 1.25 0.53 0.54 0.38 1.18 — 0.37 There was a trend towards higher species richness in the southern and offshore areas than in the north- ern and inshore areas (Fig. 3). The greatest num- bers of species per station (19) were recorded at sta- tions 6 (Point Hope), 45 (Point Lay), and 48 (Ledyard Bay) in 1990 and at station 6 (23 species) in 1991 (Fig. 3). The fewest species (2 or 3) occurred at four stations in the more northern area (stations 28 through 32). There was a tendency for the stations south of Icy Cape to have 11 or more species and those stations to the north to have 10 or less; the majority of the latter had fewer than 8 species. The number of species at stations sampled during both 1990 and 1991 differed considerably (Table 2). For example, catches at three stations northeast of Cape Lisburne consisted of 15 and 17 species in 1990 but in 1991 comprised 1 to 8 species. In contrast, farther north at station 21, 1 species was collected in 1990 and 5 species in 1991. Those stations with a species diversity of >0.90 oc- curred south of a line extending south-westward from Point Franklin. The greatest species diversity (1.99) occurred at station 45 off Point Lay; species diver- sity at two stations off Cape Lisburne (15 and 14) was nearly as large (1.56 and 1.87, respectively). Nearly all stations with a diversity of >1.0 occurred alongshore from Point Franklin to Point Hope. Low- est species diversity occurred at station 39 (0.02). Evenness followed the same pattern as species di- versity indices (Fig. 3). Assemblages Fishes collected in 1990 formed, at a similarity level of 0.5-0. 6, three nearshore (I, III, and V) and three offshore (II, IV, and VI) associations (Fig. 4). One sta- tion (15) was not classifiable (Fig. 4). Two clusters of stations formed an association (I) off the Lisburne Barber et at: Demersal fish assemblages of the northeastern Chukchi Sea 203 170° 155° Figure 3 Relative richness (number of species), species diversity (Shan- non index), and evenness of demersal fishes at 48 and 16 sta- tions sampled during 1990 and 1991, respectively, in the north- eastern Chukchi Sea, Alaska. SW = Shannon Wiener. Peninsula. A second association ( II ) was formed near a station cluster that bisected the north- ern offshore association (VI) but was more closely related to association I. The northern offshore association (VI) consisted of two rela- tively distant clusters, whereas the northern inshore association (III) consisted of two closely related clusters, one made up of two stations. The central offshore association (IV) was formed by two clusters. Finally, there was the central on- shore association (V) in Ledyard Bay, which con- sisted of four closely related and two distantly related stations. The cluster analysis yielded simi- lar results when B. saida was not included in the analysis. In all associations, B. saida made up over 90% of the abundance (Table 3). The most distinctive assemblage was VI, which had the fewest species, lowest abundance, and least diversity and evenness (Table 3). In comparison, associations I and V had much greater values for all these measures. Associa- tion I had the greatest number of species; the top five species in order of abundance were B. saida , Myoxocephalus sp., H. robustus, G. tricuspis, and Lrycodes palearis. Association II had the second most abundant species; the top five species in order of abundance were B. saida, L. raridens, M. verrucosus, G. tricuspis, and Clupea harengus pallasi. Bottom salinity and percent gravel were iden- tified through discriminant analysis as key fac- tors separating assemblage groups. The first axis accounted for 72%, the second axis for 28% of the variation (Table 4). Bottom salinity showed the strongest association with axis 1, whereas percent gravel was strongest in axis 2. The lines superimposed on Figure 5 enclose stations of similar environmental conditions. There is relatively little overlap of groups III and V; the former is characterized by low bot- tom salinity and high gravel, whereas the lat- ter is intermediate in salinity and gravel (Fig. 5). Stations 14 and 15 were classified together, with lowest salinity and percent gravel. There is overlap at the boundaries of groups I and VI, which suggests that there is a gradation in en- vironmental conditions. Group VI is associated with more saline water but includes a wide range of percent gravel. A principal component analysis, which in- cluded all environmental data, supports the discriminant analysis but suggests that other variables are also important determinants of fish associations (Table 5). This analysis indi- cated that bottom salinity, water depth, and Similarity 204 Fishery Bulletin 95(2), 1997 0.3- stalion number association group 170° 156° Figure 4 Similarity dendrogram (upper) and demersal fish associations (lower) for fishes captured in the northeastern Chukchi Sea, Alaska, during 1990. The criterion for determining asso- ciations was a similarity index of 0.5-0. 6. Arabic numerals are station numbers and Ro- man numerals represent station associations. Barber et a I,: Demersal fish assemblages of the northeastern Chukchi Sea 205 gravel accounted for 37.1% of the variance among stations, that epifaunal and infaunal abundances and gravel accounted for 27.8% of the variation, and that gravel and sand accounted for an additional 15.6% of the variation. Discussion The northeastern Chukchi Sea lies between the Arc- tic Ocean and the Bering Sea and serves as a con- duit for water flowing between these two bodies of water. In terms of oceanographic flow, this is a dy- namic region, with a net water transport from the Bering Sea into the Arctic Ocean. Flow reversals oc- cur in response to regional storm events (primarily during the seasonal ice-forming period). Therefore, oceanographic information used in this study repre- sents but a short-term snapshot of environmental conditions within the region. Information on sedi- ment distribution and associated invertebrate fauna was considered to provide a long-term integration of oceanographic conditions within the region. Even though invertebrate fauna may be influenced by hy- drographic conditions in much the same way as ich- thyofauna are influenced by these conditions, in this study they were used as independent variables. This designation was made in part because invertebrates tend to be less mobile than fishes and because, in eco- logical terms, invertebrates provide habitat and food for many fish species. During this study, 66 species representing 14 fami- lies were collected, 56 in 1990 and an additional 10 in 1991. This number is similar to the number of species (52) collected in the Chukchi Sea by A1 verson and Wilimovsky (1966) and is greater than the 29 species taken in the nearshore Chukchi Sea by Fechelm et al.1 and the 19 species captured west of Point Barrow by Frost and Lowry (1983). As in our study, Boreogadus saida was the dominant species captured during each of these surveys. Other impor- tant species reported by these authors that were im- portant in our study included Mallotus villosus, Liopsetta glacialis, Lycodes polaris, and Icelus bicornus. The number, diversity, and biomass of fish species documented during our study are comparable to those in more southerly areas of the North Pacific Ocean. Day and Pearcy (1968) found 67 species represent- Table 3 Estimated mean abundance (fish/km2), number of species, Shannon Wiener diversity, and evenness found in the six assemblages for the 21 most abundant demersal fish species determined from the cluster analysis with the Bray-Curtis dissimilarity index. Assemblage Species 1 2 3 4 5 6 Boreogadus saida 43,733 16,419 5,280 8,172 16,096 6,100 Eleginus gracilis 684 2 170 19 10,956 0 Myoxocephalus sp. 3,391 49 44 2 4,492 0 Gymnocanthus tricuspis 1,005 87 889 156 2,618 7 Hippoglossoides robustus 1,599 72 0 61 15 3 Myoxocephalus verrucosus 178 0 429 177 773 9 Artediellus scaber 20 0 0 11 1,061 4 Theragra chalcogramma 69 0 0 26 861 0 Triglops pingeli 70 3 120 59 722 0 Mallotus villosus 437 0 0 40 0 0 Lycodes palearis 453 0 0 7 0 0 Clupea harengus pallasi 195 0 0 139 323 0 Lumpenus fabricii 235 18 2 14 141 0 Lycodes polaris 260 64 2 0 6 0 L. raridens 76 7 4 284 13 5 Podothecus acipenserinus 60 0 18 5 280 0 Gadus macrocephalus 21 0 1 6 273 0 Liparis gibbus 129 2 0 15 29 0 Osmerus mordax 0 0 0 0 258 0 Hemilepidotus papilio 89 0 0 13 0 0 Artediellus sp. 80 0 0 0 20 0 Number of species 20 10 11 18 18 6 Shannon Wiener diversity 0.35 0.05 0.37 0.25 0.72 0.02 Evenness 0.27 0.05 0.35 0.20 0.57 0.02 206 Fishery Bulletin 95(2), 1997 Increase in bottom salinity (0 CD C 0 Q_ C 0 C/) CO 0 O c -3-2-10 1 2 3 Discriminant Function 1 Figure 5 Station associations based on results of the discriminant function analysis using the key discrimination factors salinity and percent gravel. • Group I O Group II a Group III v Group IV < Group V > Group VI ■ Group VII ing 21 families offshore of central Oregon at depths of 40-1,829 m. Fargo and Tyler (1991) reported more than 50 species of demersal fish in Hecate Strait, British Columbia. Spe- cies diversity seems to be somewhat lower in our study area than off Or- egon, where diversity indices varied from 0.7 to 2.47 (Pearcy, 1978). As noted, in terms of biomass and abundance, B. saida was the most common species in our study area; however, this species varied exten- sively between stations and years. For example, at station 15 (off Cape Lisburne), B. saida accounted for 0.23% of the number and 0. 18% of the biomass. In contrast, at station 27 (northwest of Point Franklin), 100% of the catch comprised B. saida. Observed trends of fish distribu- tion, abundance, biomass, and as- semblages were qualitatively similar to those of epi- faunal mollusks found by Feder et al. (1994) but not to those of infaunal mollusks. These qualitative simi- larities suggest that common variables are influenc- ing the distribution of fishes and epifaunal mollusks in the study area. Feder et al. (1994) found epifau- nal mollusk abundance and biomass to be highest along the coast, with very high values adjacent to Point Hope and north of Cape Lisburne. Addition- ally, the 5 epifaunal mollusk assemblages described by Feder et al. were configured in the same way as the fish assemblages described in our study7. How- ever, in contrast to results from our study, abundance and distribution of infaunal mollusks were highest north of and adjacent to the hydrographic front as- sociated with the Alaska Coastal Current (ACC) and along the coast north of Icy Cape and adjacent to or north of Cape Lisburne. The multivariate, cluster, Table 4 Discriminant function analysis of environmental factors with Chukchi Sea demersal fish abundance as the class criterion. Significant relationships are underlined. Standardized discriminant function coefficients Independent variable 1st axis 2nd axis Bottom salinity Percent gravel Percent variance Eigen value 0.94189 0.48469 -0.14688 1.04905 71.81 28.19 1.887 0.741 discriminant, and principal component analyses yielded similar results: stations tended to be grouped by bottom salinity and percent gravel. Because of the relatively shallow (30-50 m) depth of the northern Chukchi Sea and its gradual, fea- tureless northward slope (Fig. 1), it seems surpris- ing that the principal component analysis identified depth as a significant variable. Depth may have been significant because it acted in concert with other fac- tors, such as sediment (which tends to be relatively coarse, grading to muds containing various proportions of gravel and sand) on the inner shelf between Point Hope and Point Barrow (Sharma, 1979; Naidu, 1988). Fargo and Tyler (1991) found assemblages related to depth and sediment type, where sediment type Table 5 Results of the principal component (PC) analysis using both environmental factors and infaunal and epifaunal abun- dance. Significant relationships are underlined. Variable PCI PC2 PC3 Percent sand 0.563 -0.451 -0.643 Percent gravel 0.663 -0.421 0.771 Depth -0.796 0.398 -0.238 Bottom salinity —0.882 0.118 0.105 Epifaunal abundance 0.461 0.861 0.060 Infaunal abundance 0.318 0.880 0.040 Cumulative variance 0.371 0.649 0.805 Eigenvalue 2.596 1.951 1.095 Barber et al.: Demersal fish assemblages of the northeastern Chukchi Sea 207 was different for each species assemblage. Their spe- cies assemblages and sediment types, however, did not coincide exactly; two sediment types were found in the same depth range of species assemblages. They suggested that faunal similarities were maintained in regions of sediment transition and that factors other than sediment type governed distribution of assemblages. Similarly, Pearcy (1978) found a clear separation in the effects of depth but not in the ef- fects of sediment for two assemblages, one shallow and one deep. There was, however, an interaction between depth and sediment type where the shal- low assemblages showed a high similarity between stations of different sediment types. In respect to the hydrography of this area, the ACC sweeps through the area in a general northwest flow. However, change in wind conditions may cause peri- odic and persistent reversals in the southerly flow of the ACC (Johnson, 1989; Weingartner4). Flow rever- sals tend to be more common during winter ice cover. A review of long-term ice records suggests that in summer, an oceanographic front (as represented by the southern ice edge) may exist to the south and east of Point Franklin. However, there is much interannual variation in the location of this front, in related flow patterns, and in potential transport of adult and larval fishes into the area from the south. Variations in hydrographic conditions, coupled with differences in catches and changes in year-class strength, strongly suggest that there are interannual changes in abundance and distribution of fishes within the study area. How, or if, the dynamics of oceanographic parameters are translated into dis- tributions and relative abundances of fishes and fish assemblages is unknown. Differences in catches at stations sampled in 1990 and 1991 may have been due to interannual changes in fish distribution and abundance, or even to sampling error. However, dif- ferences in the age-class structure of fishes captured during the two years are striking. In 1990 approxi- mately 42% of G. tricuspis (Smith et al., 1996b) were older than 4 years, but in 1991 only 9% were older than 4 years. Similarly, ages of H. robustus in 1990 ranged from 1 to 11, and age class 5 was most abundant (Smith et al., 1996a), whereas ages reported by Pruter and Alverson ( 1962) for this species were 6 to 13, and ages 7 through 9 accounted for 90% of the fish samples. Interannual change in the distribution and rela- tive abundance of fish species may not lead to differ- ent associations or result in a change in the loca- tions of these fish within the study area. Overholtz 4 Weingartner, T. J. 1994. Institute of Marine Sciences, Univ. Alaska Fairbanks, Fairbanks, AK 99775-7220. Personal commun. and Tyler (1985) concluded that, even though some assemblages changed dramatically in species rich- ness and relative abundance, the spatial integrity of each complex remained constant over time. Similarly, there were seasonal changes in species associations on the Scotian Shelf, but these were relatively con- stant over 9 years within seasons (Mahon and Smith, 1989). Colvocoresses and Musick (1984) examined 9 years of trawl data from the Middle Atlantic Bight, and the distributional patterns that were found were largely structured by temperature on the innershelf and midshelf and by depth on the outer shelf and shelf break. They also found that there was sedimen- tary and topographical uniformity for both the innershelf and midshelf and that there were no strong relationships between species group and sedi- ment. Like Mahon and Smith (1989), Colvocoresses and Musick (1984) found good geographic definition in both autumn and spring groups and overlap be- tween groups. The groups that made up the commu- nities had much in common but differed between seasons. Colvocoresses and Musick ( 1984) also found relationships between groups and depth, and shifts in the groups with changes in temperature. For ex- ample, the geographic extent of assemblages varied between years depending on the southward extent of the cooler 8°C water. The fish apparently behave as a group in response to environmental variation. The fish assemblages in our study were depicted as having clear assemblage boundaries related to sediment type and oceanographic features. Results from a principal coordinate analysis, however, indi- cate that these boundaries are related to other fea- tures as well. Therefore, the assemblages shown in the ordination plots should more appropriately be thought of as transitional species abundances and proportional compositions. This conclusion is simi- lar to that reached by McKelvie ( 1985), namely that assemblages of mesopelagic fishes were best inter- preted as gradations between faunas associated with different water masses. Consequently, our study area may be viewed as a transition zone between fish com- munities of the southern Chukchi Sea and those of the Arctic Ocean. In this view, the presence of differ- ent species assemblages in the northeastern Chukchi Sea represents a mixture of 2 fish communities whose abundance and biomass vary, shifting somewhat off- shore-onshore or northerly-southerly, according to variations in the oceanographic structure of the area. Acknowledgments We would like to thank T. Sample, C. Armistead, and T. Dark, National Marine Fisheries Service, as well 208 Fishery Bulletin 95(2), 1997 as the crew of the Ocean Hope III for their assistance and camaraderie that helped make this study pos- sible and enjoyable under adverse conditions. Data on sediment type, biomass, and abundance of inverte- brates were provided by H. M. Feder, Institute of Ma- rine Science, University of Alaska Fairbanks. We ap- preciate the assistance of R. Baxter and A. E. Peden in identifying some of the fishes. Finally, we thank A. V. Tfyler, S. C. Jewett, H. M. Feder, and unknown review- ers for comments that led to considerable improvements in the manuscript. This study was funded by the Alaska Outer Continental Shelf Region of the Minerals Man- agement Service, U. S. Department of the Interior, Anchorage, Alaska, Contract 14-35-0001-30559. Literature cited Alverson, D. L., and N. J. Wilimovsky. 1966. Fishery investigations of the southeastern Chukchi Sea. In N. J. Wilimovsky and J. N. Wolfe (eds.). Environ- ment of the Cape Thompson region, Alaska, p. 843- 860. U.S. Atomic Energy Commission, Washington, DC. Bianchi, G. 1992a. Demersal assemblages of the continental shelf and upper slope of Angola. Mar. Ecol. Prog. Ser. 81:101-120. 1992b. Study of the demersal assemblages of the continen- tal shelf and upper slope off Congo and Gabon Based on the trawl surveys of the RV ‘Dr. Fridtjof Nansen.’ Mar. Ecol. Prog. Ser. 85:9-23. Clifford, H. T., and W. Stevenson. 1975. An introduction to numerical classification. Aca- demic Press, New York, NY, 229 p. Colvocoresses, J. A., and J. A. Musick. 1984. Species associations and community composition of Middle Atlantic Bight continental shelf demersal fishes. Fish. Bull. 82(21:295-313. Day, D. S., and W. G. Pearcy. 1968. Species associations of benthic fishes on the continen- tal shelf off Oregon. J. Fish. Res. Board Can. 25:2665-2675. Fargo, J., and A. V. Tyler. 1991. Sustainability of flatfish-dominated fish assemblages in Hecate Strait, British Columbia, Canada. Neth. J. Sea Res. 27 (3/41:237-253. Feder, H. M., N. R. Foster, S. C. Jewett, T. J. Weingartner, and R. Baxter. 1994. Molluscs in the northeastern Chukchi Sea. Arctic 47:145-163. Frost, K. J., and L. F. Lowry. 1983. Demersal fishes and invertebrates trawled in the north- eastern Chukchi and western Beaufort seas 1976-1977. U.S. Dep. Commer., NOAATfcch. Rep. NMFS-SSRF-764, 22 p. Jahn, A. E., and R. H. Backus. 1976. On the mesopelagic fish faunas of Slope Water, Gulf Stream and northern Sargasso Sea. Deep-Sea Res. 23:223-234. Johnson, W. R. 1 989. Current response to wind in the Chukchi Sea: a regional coastal upwelling event. J. Geophys. Res. 94:2057-2064. Lance, G. N., and W. T. Williams. 1967. A general theory of classificatory sorting strategies. 1: Hierarchical systems. Comput. J. 9:373380. Mahon, R., and R. W. Smith. 1989. Demersal fish assemblages on the Scotian Shelf, Northwest Atlantic: spatial distribution and persistence. Can. J. Fish. Aquat. Sci. 46 (suppl. 1:134-152. McKelvie, D. S. 1985. Discreteness of pelagic faunal regions. Mar. Biol. 88:125-133. Naidu, A. S. 1988. Marine surficial sediments. Section 1.2 in Bering, Chukchi, and Beaufort seas: coastal and ocean zones stra- tegic assessment data atlas, p. 1.4. U.S. Dep. Commer., NOAA, Strategic Assessment Branch, Ocean Assessment Division, Rockville, MD. Overholtz, W. J., and A. V. Tyler. 1985. Long-term responses of the demersal fish assem- blages of Georges Bank. Fish. Bull. 83(4):507-520. Pearcy, W. G. 1978. Distribution and abundance of small flatfishes and other demersal fishes in a region of diverse sediments and bathymetry off Oregon. Fish. Bull. 76(31:629-640. Pielou, E. C. 1977. Mathematical ecology. John Wiley and Sons, New York, NY, 385 p. Pruter, A. T., and D. L. Alverson. 1962. Abundance, distribution and growth of flounders in the southeastern Chukchi Sea. J. Cons. Cons. Int. Explor. Mer 27:81-99. Scott, J. S. 1982. Selection of bottom type by groundfishes of the Scotian Shelf. Can. J. Fish. Aquat. Sci. 39:943-947. Sharma, G. D. 1979. The Alaskan shelf: hydrographic, sedimentary and geochemical environment. Springer- Verlag, New York, NY, 498 p. Siegel, S., and N. J. Castellan Jr. 1988. Nonparametric statistics for the behavioral sciences. McGraw-Hill Book Co., New York, NY, 399 p. Smith, R. L., W. E. Barber, M. Vallarino, E. Barbour, and E. Fitzpatrick. In press, a. Biology of the Bering flounder, Hippoglossoides robustus , from the northeastern Chukchi Sea. In J. B. Reynolds (ed.l. Fish ecology ofArctic North America. Am. Fish. Soc. Symp. Smith, R. L., W. E. Barber, M. Vallarino, J. A. Gillispie, and A. Ritchie. In press, b. Biology of Gymnocanthus tricuspis, the Arctic staghorn sculpin, from the northeastern Chukchi Sea. In J. B. Reynolds (ed.), Fish ecology of Arctic North America. Am. Fish. Soc. Symp. Wakabayashi, K., R. G. Bakkala, and M. S. Alton. 1985. II: Methods of the U.S. -Japan demersal trawl surveys. In R. G. Bakkala and K. Wakabayashi (eds.), Results of cooperative U.S. -Japan groundfish investiga- tions in the Bering Sea during May-August 1979, p. 7— 29. Int. North Pac. Fish. Comm. Bull. 44. Weinberg, K. L. 1994. Rockfish assemblages of the middle shelf and upper slope off Oregon and Washington. Fish. Bull. 92(3 ):620-632. Weingartner, T. J. In press. The physical oceanography of the northeastern Chukchi Sea: a review. In J. B. Reynolds (ed.), Fish ecol- ogy ofArctic North America. Am. Fish. Soc. Symp. Zar, J. H. 1984. Biostatistical analysis. Prentice Hall, Englewood Cliffs, NJ, 718 p. 209 Evaluations of the Nordmore grid and secondary bycatch-reducing devices (BRD's) in the Hunter River prawn-trawl fishery, Australia Matt K. Broadhurst Steven J. Kennedy NSW Fisheries Research Institute RO. Box 2 1 , Cronulla, New South Wales 2230, Australia E-mail address: broadhum@fisheries.nsw.gov.au John W. Watson Ian K. Workman Mississippi Laboratories, National Marine Fisheries Service Pascagoula Facility, RO. Drawer 1207, Pascagoula, Mississippi 39568-1207 Abstract .—Several bycatch-reduc- ing devices IBRD’s) were compared for their effectiveness in reducing bycatch while maintaining catches of prawns in an estuarine prawn-trawl fishery in New South Wales (NSW), Australia. A solid separator-panel (the Nordmpre grid), a soft separator panel (the com- mercially used blubber chute), and four secondary BRD’s (the fisheye, extended mesh funnel, Allerio Brothers grid, and square-mesh panel) each attached to a Nordmpre grid, were compared against each other in a series of paired compari- sons in the Hunter River prawn-trawl fishery. The results showed that the Nordmpre grid and all secondary BRD’s caught less bycatch and more prawns than the commercially used blubber chute. Most bycatch seemed to escape with use of the Nordmpre grid, and there was no significant advantage in adding a secondary BRD to this design. The efficiency of the Nordmpre grid has led to its voluntary adoption by many commercial prawn-trawl fishermen throughout NSW estuaries. Manuscript accepted 15 November 1996. Fishery Bulletin 95:209-218 (1997). In New South Wales (NSW), Aus- tralia, estuarine prawn-trawling occurs in five localities and is val- ued at approximately A$7 million per annum. Like the majority of the world’s prawn-trawl fisheries, sig- nificant numbers of nontarget or- ganisms, or bycatch, are captured incidentally with targeted prawns (for reviews see Saila, 1983; Andrew and Pepperell, 1992; Alverson et al., 1994; Kennelly, 1995). In recent years, bycatch from these fisheries has become of in- creasing concern to a broad cross section of the fisheries community. As a result, a 3-yr observer-based study was undertaken from 1990 to 1992 to quantify the distributions and abundances of bycatch species (Liggins and Kennelly, 1996; Ken- nelly1). The results from these stud- ies showed that, despite large spa- tial and temporal variabilities in the bycatches of many species, some juveniles of commercially and recreationally important species were caught in large numbers throughout the trawling seasons. The quantities involved raised con- cerns over the potential impacts of prawn-trawling on subsequent stocks of these species. These con- cerns led to the current investiga- tion, which examines various modi- fications to trawling gear and trawl- ing practices that minimize unde- sirable bycatches while maintaining catches of prawns. A number of recent attempts to exclude bycatch from prawn-trawls have concentrated on modifications that incorporate bycatch-reducing devices (BRD’s) (Christian and Harrington, 1987; Averill, 1989; Kendall, 1990; Isaksen et al., 1992; Rulifson et al., 1992; Broadhurst et al., 1996). In previous experiments (Broadhurst and Kennelly, 1994, 1995, 1996; Broadhurst et al., 1996) we showed that the successful ap- plication of various BRD’s is specific to individual fisheries and depends upon several factors, including the type of species to be excluded. Fur- ther, to promote acceptance by in- dustry, BRD’s should be designed so that they do not adversely influence normal commercial operations. 1 Kennelly, S. J. 1993. Study of the by- catch of the NSW east coast trawl fishery. Final rep. to the Fisheries Research and Development Cooperation. Project 88/ 108, ISBN 0 7310 2096 0, 520 p. 210 Fishery Bulletin 95(2), 1997 In estuarine prawn-trawl fisheries in NSW, many of the individual fish in bycatch are larger than the targeted prawns and include organisms such as jel- lyfish or jelly “blubber” — Catostylus spp. For the past 30 years, many of the estuarine prawn-trawlers in NSW have routinely used a BRD designed specifi- cally to exclude these individuals. Commonly called “blubber-chutes,” these BRD’s consist of a funnel of soft mesh inserted into the aft belly of the trawl. Organisms larger than the mesh in the funnel are guided through an opening in the top of the trawl, while prawns and smaller individuals pass through the mesh into the codend (see Broadhurst and Kennelly, 1996). In the Hunter River (HR) prawn- trawl fishery (Fig. 1), the abundance of jellyfish means that commercial fishermen use blubber chutes throughout most of the trawling season. In a series of experiments that examined the per- formance of several types of BRD’s (Broadhurst et al., 1996; Broadhurst and Kennelly, 1996), we showed that a rigid separator-panel (the Nordmpre grid) sig- nificantly reduced the mean weight of bycatch in two estuaries and had no effect on the catches of prawns. Compared with the commercially used blubber chute, the Nordmpre grid also retained significantly less bycatch but caught more prawns. Bycatch-reducing devices, such as the Nordmpre grid and the blubber chute, function by mechanically partitioning the catch according to size (see Broadhurst et al., 1996), and therefore are generally not as effective in excluding unwanted individuals that are of a similar size or that are smaller than the targeted prawns. Previous studies have shown, how- ever, that it may be possible to exclude these smaller individuals by exploiting behavioral differences be- tween some species of fish and prawns (Watson et al., 1986; Broadhurst and Kennelly, 1994, 1995; Broadhurst et al., 1996). For example, studies by Watson et al. (1993) in the Gulf of Mexico showed that small individuals of red snapper ( Lutjanus campechanus), Atlantic croaker ( Micropogon undulatus), Atlantic bumper ( Chloroscombrus chrysurus ) and whiting (Menticirrhus sp.) were pas- sively excluded from trawls by various BRD designs comprising strategically placed panels of netting and escape exits. These designs were located posteriorly to a larger mechanical separating grid (designed to exclude turtles) and effectively functioned as second- ary BRD’s. It is apparent that several options exist for ways of excluding bycatch from prawn trawls. In the present study we wanted to determine which of these various devices (i.e. the Nordmpre grid, blubber chute, or some type of secondary BRD) is most ap- propriate for use in the HR prawn-trawl fishery. Our Figure 1 The location of the Hunter River in New South Wales. specific goals, therefore, were 1) to assess the perfor- mance of four secondary BRD’s located behind the Nordmpre grid (including designs previously tested in the Gulf of Mexico by Watson et al., 1993) in re- ducing smaller unwanted individuals in the HR prawn-trawl fishery; 2) to compare the two most ap- propriate secondary BRD’s from 1) against a stan- dard Nordmpre grid and the commercially used blub- ber chute; and 3) to test a standard Nordmpre grid (with no secondary BRD) against the commercially used blubber chute. Materials and methods Two experiments were performed on commercial prawn-trawl grounds in the Hunter River (32°53'S, 151°45'E, Fig. 1), between November and December 1995 with a chartered commercial prawn-trawler (12.72 m). Three Florida flyers (mesh size=40 mm), each with a headline length of 9.14 m, were rigged in a standard triple gear configuration (see Andrew et al., 1991, for details) and towed at 2 knots across a combination of sandy and muddy bottoms in depths ranging from 2 to 8 m. Each of the identical outside nets were rigged with zippers to facilitate changing the codends (see Broadhurst et al., 1996). Because the middle net was not rigged in an identical man- ner to that used on the outside nets, its catch was excluded from analysis. The codends used in the experiments measured 50 meshes long (2 m) and were constructed from 40- Broadhurst et a I.: Evaluations of Nordmore grid and secondary bycatch-reduction devices 21 I mm netting. They comprised two panels. The ante- rior panel was 100 meshes in circumference, 25 meshes in length, and constructed of 400/36 ply, UV- stabilized, high-density polyethylene twine. The pos- terior panel was 150 meshes in circumference, 25 meshes in length, and constructed of 3-mm diam- eter braided polyethylene twine. Two standard Nordmpre grids (each measuring 600 x 400 mm and weighing 1.9 kg, Fig. 2) were constructed and located in 2-m extension pieces (made from 400/36 ply, UV-sta- bilized, high-density polyethylene twine, mesh size = 40 mm) immediately anterior to each codend (Fig. 3A, see also Broadhurst and Kennedy, 1996, for details). Experiment 1 (comparisons of secondary BRD's) Four designs of secondary BRD’s were constructed and installed into the codends described above, be- hind the Nordmpre grids. The first design (termed the fisheye) consisted of a stainless steel pyramid- shaped frame inserted 12 meshes to the left of the center of the top anterior section of the codend (Fig. 3B, see also Watson and Taylor2; Watson3). The sec- ond design (termed the square-mesh panel) had a panel of 50-mm knotless netting, hung on the bar and inserted into the top anterior section of the codend (Fig. 3C). The third design (termed the ex- 2 Watson, J. W., and C. W. Taylor. 1996. Technical specifica- tions and minimum requirements for the extended funnel, ex- panded mesh and fisheye BRDs. Mississippi Laboratory, NMFS, NOAA, P.O. Drawer 1207, Pascagoula, MS 39567. Watson, J. W. 1996. Summay report on the status of bycatch reduction devices development. Mississippi Laboratory, NMFS, NOAA, P.O. Drawer 1207, Pascagoula, MS 39567. tended mesh funnel or EMF) comprised a guiding funnel surrounded by larger square-shaped mesh (see Watson and Taylor2; Watson3) and was located in the anterior section of the codend (Fig. 3D). The fourth design (termed the Allerio Brothers grid, Watson4) was constructed like the Nordmpre grid but included additional lateral fish escape windows posterior to the aluminium grid (Fig. 4). All four designs were compared against each other, one pair of each design on the outside nets of the triple-rigged gear (i.e. 6 separate paired compari- sons). The position and order of each secondary BRD was randomly determined, and during 6 days in the trawling season in the Hunter River, we completed a total of 12 replicate 30-min tows for each paired comparison. The location of each tow was randomly selected from the available prawn-trawl locations that were possible under the particular conditions. Prior to the trials, we rigged both nets with normal commercial codends to ensure that there were no differences in fishing characteristics. Experiment 2 (comparison of two secondary BRD's, standard Nora more grid and blubber chute) In this experiment, the fisheye and EMF, each at- tached to a Nordmpre grid, were compared against a standard Nordmpre grid (with no secondary BRD) and the commercially used blubber chute. The stan- dard Nordmpre grid and blubber chute were also com- pared against each other (providing a total of five 4 Watson, J. W. 1995. Mississippi Laboratory, NMFS, NOAA, P.O. Drawer, 1207, Pascagoula, MS 39567. Personal commun. 212 Fishery Bulletin 95(2), 1997 A Figure 3 Diagrammatic representation of prawn-trawl and (A) Nordmpre grid, (B) fisheye, (C) square-mesh panel, and (D) extended mesh funnel (EMF) bycatch-reducing devices. T = transversals; B = bars; and N = normals. paired comparisons). The blubber chute comprised a panel of netting (36-ply, UV-stabilized, high-density polyethylene with a mesh size of 90 mm) sewn into a funnel (with an anterior circumference of 100 meshes) located in a 2-m panel of mesh (mesh size of 40 mm) measuring 150 meshes in circumference (see Broadhurst and Kennedy, 1996, for details). The pos- terior point of the blubber chute was attached five meshes from the end of the 2-m panel. A 30-mesh opening (termed the escape exit) was cut immedi- ately anterior to this point of attachment. As was the case for experiment 1, the position and order of each design was randomly determined and used in normal commercial tows of 30-min duration. Over 8 days, we completed a total of 23 replicate tows for each of the five paired comparisons. Data collected After each tow in each paired experiment, the two codends were emptied onto a partitioned tray. All organisms were sorted according to species. The fol- lowing data were collected from each tow: the total weight of prawns; the total weight of bycatch; the weights; numbers and sizes of commercially or recreationally (or both) important finfish (to the near- est 0.5 cm); the numbers of noncommercial or nonrecreational species; and the total numbers of noncommercial and commercial species in the assem- blage. All prawns in a subsample of the total prawn catch from each tow in experiment 2 were measured in the laboratory (to the nearest 1-mm carapace length). Several species were caught in sufficient quantities to provide meaningful analyses. These were the commercially important school prawns ( Meta - penaeus macleayi ) and large tooth flounder (Pseudo- rhombus arsius) and the commercially unimportant fortesque ( Centropogon australis), narrow banded sole ( Synclidopus macleayanus), bridle goby ( Arenigobius bifrenatus), and catfish ( Euristhmus lepturus). Bata from all replicates that had sufficient num- bers of each variable (defined as >2 fish in at least 8 Broadhurst et al.: Evaluations of Nordmore grid and secondary bycatch-reduction devices 213 50 N i Figure 4 Diagrammatic representation of the Allerio Brothers grid, dia = diameter. replicates) in experiment 1 were analyzed by using two-tailed, paired f-tests. Because a previous experi- ment in the Clarence River prawn-trawl fishery showed that the Nordmpre grid caught more prawns than the blubber chute (Broadhurst and Kennelly, 1996), in experiment 2 we tested the hypothesis that each of the three designs incorporating a Nordmpre grid caught more prawns but less bycatch than the commercially used blubber chute. These data were analyzed by using one-tailed paired Gtests. Size fre- quencies of prawns from experiment 2 were graphed and compared by using two-sample Kolmogorov- Smirnov tests (P=0.05). Results Experiment 1 (comparisons of secondary BRD's) Apart from a significant reduction in the number of noncommercial species caught as bycatch by the Allerio Brothers grid, compared with the number caught with the square-mesh panel, there were no other detectable differences between any of the sec- ondary BRD’s tested (Table 1). However, because previous studies in the Gulf of Mexico showed that the EMF and fisheye were most effective in exclud- ing small fish from the codend (Watson and Taylor2; Watson3), these two designs were tested further in experiment 2. Experiment 2 (comparison of two secondary BRD's, standard IMordmore grid and blubber chute| Compared with the commercially used blubber chute, the standard Nordmpre grid, EMF, and fisheye all significantly increased the weight of prawns caught (means increased by 24%, 41%, and 23%, respec- tively) and decreased the weight of total bycatch (means reduced by 58%, 45%, and 55%, respectively) and number of noncommercial species in bycatch (Fig. 5, A, B, and H; Table 2). The fisheye also sig- nificantly reduced the mean number of catfish caught by 79.5% (there were insufficient catfish from the 214 Fishery Bulletin 95(2), 1 997 Table 1 Summaries of two-tailed paired f-tests in a series of comparisons of various secondary BRD’s in experiment 1. ** = significant (P<0.01); * = significant (P<0.05); n = the number of replicates that had sufficient data available for analysis (i.e. >2 fish in 8 replicates). Allerio Bros. vs. EMF Allerio Bros. vs. square- mesh Allerio Bros. vs. fisheye Paired f-value P n Paired f-value P n Paired f-value P n Wt. of prawns -0.602 0.559 12 0.193 0.850 12 0.689 0.505 12 Wt. of total bycatch -0.967 0.354 12 -1.827 0.095 12 0.958 0.358 12 No. of fortesque 0.000 0.999 9 0.886 0.398 10 2.200 0.052 11 No. of noncommercial sp. -0.860 0.407 12 -2.46 0.031* 12 -1.146 0.276 12 No. of commercial sp. -0.232 0.821 12 1.517 0.157 12 -1.698 0.120 12 Square-mesh vs. EMF Fisheye vs. square-mesh Fisheye vs. EMF Paired f-value P n Paired f-value P n Paired f-value P n Wt. of prawns -0.225 0.826 12 -1.36 0.200 12 -1.795 0.100 12 Wt. of total bycatch -0.318 0.756 12 -1.821 0.095 12 -0.513 0.618 12 No. of fortesque 0.808 0.440 10 -0.683 0.544 8 -0.455 0.659 10 No. of noncommercial sp. 1.216 0.249 12 -1.431 0.180 12 -1.383 0.194 12 No. of commercial sp. -0.890 0.392 12 -0.364 0.722 12 -1.190 0.256 12 Table 2 Summaries of one-tailed paired f-tests in a series of comparisons of various BRD’s in experiment 2. Ng = Nordmpre grid, significant (P<0.01); * = significant (P<0.05); n = the number of replicates that had sufficient data available for analysis (i. fish in 8 replicates). ** _ e. >2 Standard Ng vs. blubber chute EMF vs. blubber chute Fisheye vs. blubber chute Paired f-value P n Paired f-value P n Paired t-value P n Wt. of prawns 2.864 0.004** 23 3.764 0.0005** 23 2.020 0.027* 23 Wt. of total bycatch 3.515 0.001** 23 2.930 0.003** 23 3.306 0.002** 23 Wt. of large tooth flounder 0.979 0.173 14 0.729 0.239 14 1.394 0.103 8 No. of large tooth flounder 0.061 0.476 14 -0.879 0.802 14 0.747 0.239 8 No. of fortesque 0.286 0.389 19 -0.261 0.601 20 0.761 0.228 18 No. of narrow banded sole 1.064 0.164 8 — — — 1.440 0.090 10 No. of bridled goby -0.414 0.654 8 — — — -0.078 0.531 11 No. of catfish — — — — — — 3.490 0.003** 10 No. of noncommercial sp. 2.626 0.007** 23 2.040 0.026* 23 1.931 0.033* 22 No. of commercial sp. -1.190 0.876 23 0.000 0.500 23 0.282 0.390 23 Standard Ng vs. fisheye Standard Ng vs. EMF Paired t-value P n Paired f-value P n Wt. of prawns 0.618 0.271 23 -1.418 0.914 23 Wt. of total bycatch 0.721 0.239 23 0.512 0.307 23 Wt. of large tooth flounder 0.410 0.346 9 -0.507 0.689 13 No. of large tooth flounder 1.835 0.052 9 -0.456 0.672 13 No. of fortesque -0.647 0.736 16 -0.128 0.449 19 No. of narrow banded sole -0.147 0.556 9 0.741 0.241 8 No. of bridled goby — — — 3.468 0.004** 8 No. of catfish — — — — — — No. of noncommercial sp. 0.530 0.300 23 2.688 0.007* 23 No. of commercial sp. 1.156 0.130 23 0.755 0.229 23 Broadhurst et al.: Evaluations of Nordmore grid and secondary bycatch-reduction devices 215 Figure 5 Differences in mean catch (+ SE) between the various designs of (A) the weight of prawns (Metapenaeus macleayi ), (B) the weight of to- tal bycatch, (C) the number of large tooth flounder (Pseudorhombus arsius ), (D) the number of fortesque ( Centropogon australis ), (E) the number of narrow banded sole ( Synclidopus macleayanus), (F) the number of bridle goby ( Arenigobius bifrenatus), (G) the number of catfish (Euristhmus lepturus), (H) the number of noncommercial species, and (I) the number of commercial species. * = P< 0.05; ** = P<0.01. Ng = Nordmpre grid; EMF = extended mesh funnel. standard Nordm0re grid and EMF for meaningful analyses) (Fig. 5G; Table 2). There were no signifi- cant differences detected between the standard Nordm0re grid and fisheye, whereas the EMF caught significantly fewer bridled gobies and noncommer- cial species than did the standard Nordm0re grid (Fig. 5, F and H; Table 2). Two sample Kolmogorov-Smirnov tests comparing the size-frequency distributions for school prawns showed that, apart from a significant difference be- tween the standard Nordmpre grid and the EMF (Fig. 6E), there were no other differences in the relative size-compositions between any of the codends tested in experiment 2. 216 Fishery Bulletin 95(2), 1997 1 1 CO CM C F ii ii l. 61 4- 2- i ** ■ 1 il o> Year Figure 4 Estimated biomass (t) of Pacific ocean perch in the U.S. Vancouver-Columbia management area based on Stock Synthesis analysis (Ianelli et al.3) and ex- trapolated CPUE ( 1968 CPUE = q x 1968 Stock Synthesis biomass, where q is the catchability coefficient) at the northern Washington index sites for the years 1968-92. the case of Pacific ocean perch, allozyme differences follow a cline from the Washington coast to the Bering Sea rather than show discrete differences between stocks (Seeb and Gunderson, 1988). Strong year classes of Pacific ocean perch tend to occur synchro- nously throughout the Oregon-British Columbia region (Gunderson, 1977; Hollowed, et al., 1987), in- dicating recruitment strength is determined by oceanographic conditions operating over broad spa- tial scales. 226 Fishery Bulletin 95(2), 1 997 However, migrations of adult Pacific ocean perch appear to be quite limited. For example, Westrheim et al. (1974), documented a virtually unexploited stock of Pacific ocean perch in Moresby Gully, B.C., located immediately north of heavily fished stocks in Mitchell’s Gully, also within Queen Charlotte Sound (Fig. 1). Pacific ocean perch habitat in Moresby Gully is contiguous with that of Mitchell’s Gully at 25 Age (yr) Figure 6 Age composition of Pacific ocean perch in the northern Washington index sites in 1992, contrasted with that of lightly exploited (F=0.02, Moresby Gully, 1982) and intensively exploited (F=0.60, Langara Spit, 1982) stocks (Leaman, 1991). Length (cm) Figure 7 Size composition of Pacific ocean perch at the northern Washington index sites, 1968-92. Gunderson: Spatial patterns in the dynamics of slope rockfish stocks 227 the shallower extremes, and the 200-m contours are separated by only about 30 km. Nevertheless, the size and age composition in these two areas differed sharply (Gunderson et al., 1977). The composition of the parasite fauna on adult Pacific ocean perch in Moresby Gully has also been shown to differ signifi- cantly from that found on adult ocean perch in Goose Island Gully (immediately south of Mitchell’s) (Leaman and Kabata, 1987). It seems clear that, whereas spawner-recruit processes probably operate over broad geographic scales, adult migrations are limited, and changes in abundance and age compo- sition for this species in response to fishing are highly localized. The results of this study indicate that responses to fishing also occur over very small spatial scales off Oregon-southern Vancouver Island as well as in Queen Charlotte Sound, and that this is true for rougheye, splitnose, and darkblotched rockfish as well as for Pacific ocean perch. The index areas off the northern Washington coast were only 28 km south of the Canadian experimental overfishing zone (Fig. 1), yet rockfish stocks in this area appear to have experienced little change in abundance between 1968 and 1992. Shortspine thornyhead abundance within the 219-366 m depth interval actually in- creased between 1968 and 1972, although these fish represent only the shallowest part of the range and the younger age groups in a stock that can extend to depths greater than 1,100 m (Ianelli et al.4). Although the rockfish assemblage in the northern Washington index sites was not depleted to the same extent as its counterpart in Canadian waters during 1970-92, it is far from being at pristine abundance. The effects of the overfishing during 1966-68 are still evident because the abundance of fish older than age 15 is still much lower than that characteristic of lightly exploited stocks (Fig. 6). Pacific ocean perch stocks in the index areas appear to have undergone significant exploitation between 1970 and 1992 but have not declined to the same extent as those in Canadian waters to the north (Tables 1 and 4) or in U.S. waters to the south (Fig. 4). The size and age at maturity for Pacific ocean perch in the northern Washington index areas appear to have declined significantly between 1968 and 1972 (Gunderson, 1977) and 1992. Length at 50% maturity declined from 34.4 cm to 31.6 cm (Table 5), whereas age at 50% maturity declined from 10.1 years to 8.1 years. This shift in size and age at maturation should be viewed with some cau- tion, however, because most adult fish examined in 1992 were still in the”maturing” stage. Only one of the 382 adults examined was in a more advanced 4 Ianelli, J. N., R. Lauth, and L. D. Jacobson. 1994. Status of the thornyhead ( Sebastolobus sp. ) resource in 1994. App. D in Status of the Pacific coast groundfish fishery through 1994, and recommended acceptable biological catches for 1995. Pacific Manage. Council, Portland, OR, 58 p. Table 5 Estimated length and age at maturity for female Pa- cific ocean perch off the northern Washington coast, 1968-72 versus 1992. 1968-1972 1992 a -29.0258 -20.3196 p 0.8439 0.6428 l05(cm) 34.40 31.61 Var tf0.6> 0.0437 0.1498 Z-statistic' 6.33 Wyears^ 10.1 8.1 1 See Gunderson (1977). 228 Fishery Bulletin 95(2), 1997 maturation stage, i.e. “fertilized.” In contrast, the fish used in constructing the length-maturity curve for 1968-72 were collected during February-June, when Pacific ocean perch are closer to the embryo-release period. Nevertheless, it appears that after 20 years in a depleted state, the stocks of Pacific ocean perch off Washington have partially compensated for a loss in reproductive potential by reducing their age at ma- turity from 10 years to 8. Comprehensive studies have shown long-term declines in age at maturity from 10.5 years (1923) to 8 years (1976) in a heavily fished stock of Northeast Arctic cod (Jprgensen, 1990), and from 5-7 years (early 1900’s) to 4-5 years in North Sea plaice (Rijnsdorp, 1989). Although a genetic basis for such changes has been documented in some species (Policansky, 1993), disentangling genetic changes and phenotypic plasticity is often difficult. Given the long generation time of Pacific ocean perch and the relatively short time span involved, these changes probably reflect reaction norms of phenotypic plasticity rather than changes in genotype. In contrast, growth, as reflected in size attained at age 15, showed no substantial changes between 1968-70 and 1992. Although monitoring length-age relationships at fixed bathymetric locations allows the depth effect to be controlled for, it is difficult to maintain the sampling depth within a range of less than about 18 m with trawls on the continental slope. Aggregations of Pacific ocean perch often have dif- ferent growth characteristics and vary interannually in their availability (Gunderson, 1974), and further sources of bias and variability are inherent in using different age-determination techniques. All of these factors make it difficult to detect changes in growth rates unless they are substantial. Nevertheless it should be kept in mind that interannual variations in food availability can often be more influential than changes in population density in determining growth rates (Rijnsdorp et al., 1991; Rijnsdorp and van Leeuwen, 1992). Most adult rockfish in the Oregon- Vancouver Is- land region probably migrate to a very limited ex- tent, and stocks within these regions represent a mosaic of small, highly localized stocks. Neverthe- less, practical considerations in terms of data collec- tion, data assessment, and management enforcement often force the geographic scale of fishery manage- ment to be relatively broad. For example, although the Pacific Fishery Management Council has at- Gunderson: Spatial patterns in the dynamics of slope rockfish stocks 229 tempted to eliminate directed fishing on Pacific ocean perch in the U.S. Vancouver-Columbia management area, fishermen have been observed fishing for this species in the northern Washington index areas, where Pacific ocean perch and other rockfish are the most abundant fish in catches (Fig. 2). Only distance and time act as disincentives for fishermen, who have yet to achieve their “incidental” allotment of Pacific ocean perch, from moving to areas such as these to “top off’ their catch. It is not surprising then that stocks in the index area have failed to rebuild as the Council had hoped. While often ignored in manage- ment considerations owing to a lack of information, other species in the slope rockfish assemblage (nota- bly rougheye and splitnose rockfish) have probably experienced the same pattern of overfishing as Pa- cific ocean perch and should be considered when con- templating future rebuilding plans. One possible solution to many of the problems that currently exist in managing slope rockfish stocks is to delineate areas such as the index sites, where rock- fish dominate the exploitable fish biomass (Fig. 2), and to eliminate all fishing within them. A variety of questions remain as to the optimal size and spatial dispersion of such closed areas (or “refugia”), as well as the enforcement problems associated with main- taining them, but it seems clear that if managers cannot rebuild rockfish stocks in areas of prime habi- tat, it is unlikely that they will be able to rebuild them over broader scales. Acknowledgments This research was supported in part by grants from the U.S. National Marine Fisheries Service and Washington Sea Grant. Assistance by the scientists and staff at the NMFS Alaska Fisheries Science Cen- ter in constructing the trawl gear, implementing the 1992 survey, and ageing the otoliths collected is grate- fully acknowledged. I also thank Dan Ito and Bruce Leaman for their comments on an earlier draft of this paper. Literature cited Chilton, D. E., and R. J. Beamish. 1982. Age determination methods for fishes studied by the groundfish program at the Pacific Biological Station. Can. Spec. Publ. Fish. Aquat. Sci. 60, 102 p. Cochran, W. G. 1977. Sampling techniques, 3rd ed. John Wiley, New York, NY, 428 p. Gunderson, D. R. 1969. Pacific ocean perch cruises off the northern Wash- ington coast during 1967 and 1968. Wash. Dep. Fish. Groundfish Data Rep. Ser., No. 3, 35 p. 1974. Availability, size composition, age composition, and growth characteristics of Pacific ocean perch ( Sebastes alutus ) off the northern Washington coast during 1967- 72. J. Fish. Res. Board Can. 31:21-34. 1977. Population biology of Pacific ocean perch, Sebastes alutus , stocks in the Washington-Queen Charlotte Sound re- gion, and their response to fishing. Fish. Bull. 75:369-403. Gunderson, D. R., S. J. Westrheim, R. L. Demory, and M. E. Fraidenburg. 1977. The status of Pacific ocean perch ( Sebastes alutus ) stocks off British Columbia, Washington, and Oregon in 1974. Can. Fish. Mar. Serv. Tech. Rep. 690, 63 p. Hollowed, A. B., K. M. Bailey, and W. S. Wooster. 1987. Patterns in recruitment of marine fishes in the north- east Pacific Ocean. Biol. Oceanogr. 5:99-131. Jprgensen, T. 1990. Long-term changes in age at sexual maturity of Northeast Arctic cod ( Gadus Morhua L.). J. Cons. Cons. Int. Explor. Mer 46: 235-248. Leaman, B. M. 1991. Reproductive styles and life history variables rela- tive to exploitation and management of Sebastes stocks. Environ. Biol. Fishes 30:253-271. Leaman, B. M., and Z. Kabata. 1987. N eobrachiella robusta (Wilson, 1912) (Copepoda: Lernaeopodidae) as a tag for identification of stocks of its host, Sebastes alutus (Gilbert, 1890) (Pisces: Teleostei). Can. J. Zool. 65:2579-2582. Leaman, B. M., and R. D. Stanley. 1993. Experimental management programs for two rock- fish stocks off British Columbia, Canada. In J. Smith, J. J. Hunt, and D. Rivard (eds.), Risk evaluation and biologi- cal reference points for fisheries manaagement, p. 403- 418. Can. Spec. Publ. Fish. Aquat. Sci. 120. McDermott, S. F. 1994. Reproductive biology of rougheye and shortraker rockfish, Sebastes aleutianus and Sebastes borealis. M.S. thesis, Univ. Washington, Seattle, WA, 76 p. Policansky, D. 1993. Evolution and management of exploited fish popula- tions. In Proceedings of the international symposium on management strategies for exploited fish populations, p. 651- 664. Alaska Sea Grant Program, Univ. of Alaska Fairbanks. Ralston, S., and D. F. Howard. 1995. On the development of year-class strength and co- hort variability in two northern California rockfishes. Fish. Bull. 93:710-720. Richards, L. J. 1994. Slope rockfish. 7n Groundfish stock assessments for the West Coast of Canada in 1993 and recommended yield options for 1994, p. 230-287. Can. Tech. Rep. Fish. Aquat. Sci. 1975. Rijnsdorp, A. D . 1989. Maturation of male and female North Sea plaice ( Pleuro - nectes platessa L). J. Cons. Cons. Int. Explor. Mer 46:35-51. Rijnsdorp, A. D ., N. Daan, F. A. van Beek, and H. J. L. Heessen. 1991. Reproductive variability in North Sea plaice, sole, and cod. J. Cons. Cons. Int. Explor. Mer 47:352-375. Rijnsdorp, A. !)., and P. I. van Leeuwen. 1992. Density-dependent and independent changes in so- matic growth of female North Sea plaice Pleuronectes platessa between 1930 and 1985 as revealed by back-cal- culation of otoliths. Mar. Ecol. Prog. Ser. 88:19-32. 230 Fishery Bulletin 95(2), 1997 Seeb, L. W., and D. R. Gunderson. 1988. Genetic variation and population structure of Pacific ocean perch (Sebastes alutus). Can. J. Fish. Aquat. Sci. 45:78-88. Tagart, J. V. 1984. Comparison of final ages assigned to a common set of Pacific ocean perch otoliths. Wash. Dep. Fish. Tech. Rep. 81, 36 p. Weinberg, K. L. 1994. Rockfish assemblages of the middle shelf and upper slope off Oregon and Washington. Fish. Bull. 92:620-632. Westrheim, S. J. 1967. Sampling research trawl catches at sea. J. Fish. Res. Board Can. 24:1187-1202. 1973. Age determination and growth of Pacific ocean perch ( Sebastes alutus) in the northeast Pacific Ocean. J. Fish. Res. Board Can. 30:235-247. Westrheim, S. J., W. R. Harling, D. Davenport, and M. S. Smith. 1974. G. B. Reed groundfish cruise no. 74-4, 4-25 Septem- ber 1974. Can. Fish. Mar. Serv. Tech. Rep. 497, 37 p. Wilkinson. L. 1989. SYSTAT: the system for statistics. SYSTAT, Inc. Evanston, IL, 822 p. 231 Fecundity and egg weight in English sole, Pleuronectes vetulus, from Puget Sound, Washington: influence of nutritional status and chemical contaminants Lyndal L. Johnson Sean Y. Sol Daniel R Lomax Gregory M. Nelson Catherine A. Sloan Edmundo Casillas Environmental Conservation Division, Northwest Fisheries Science Center National Marine Fisheries Service, NOAA 2725 Montlake Blvd. East, Seattle, Washington 98112 E-mail address: Lyndal.L.Johnson@noaa.gov Abstract .—Differences in fecundity and egg weight were evaluated in En- glish sole, Pleuronectes vetulus, from four sites in Puget Sound (the Duwamish Waterway, Eagle Harbor, Sinclair Inlet, and Port Susan) with differing concen- trations and types of sediment contami- nation. Duwamish Waterway sediment has high concentrations of both poly- chlorinated biphenyls (PCB’s) and poly- cyclic aromatic hydrocarbons (PAH’s), Eagle Harbor sediment has high con- centrations of PAH’s, and Sinclair In- let sediment has low concentrations of PAH’s and moderate concentrations of PCB’s, whereas sediments at Port Su- san, the reference site, are minimally contaminated. Fish from the Duwa- mish Waterway and Eagle Harbor had significantly higher levels of fluorescent aromatic compounds (FAC’s) in bile than sole from Port Susan and Sinclair Inlet, and fish from the Duwamish Waterway had significantly higher con- centrations of PCB’s in ovary and liver tissue than fish from the other sam- pling sites. Fecundity and egg weight were compared in fish of equivalent size, age, and reproductive maturity from the four sites; fish from the Duwamish Waterway showed signifi- cantly higher relative fecundity and lower egg weight than fish collected from the three other sites. Production of more and smaller eggs in fish from the Duwamish Waterway site was as- sociated with elevated hepatosomatic indices, elevated plasma triglyceride levels, and elevated levels of PCB’s in liver and ovarian tissue, and reduced plasma vitellogenin levels (as esti- mated from alkali-labile protein (ALP) concentrations). Fish from the Duwa- mish Waterway and Sinclair Inlet also had higher age-specific fecundity than animals from other sites because of their larger size at age. On an indi- vidual fish basis, elevated tissue PCB concentrations were significantly cor- related with low plasma ALP, reduced egg weight, and increased egg number, whereas elevated biliary FAC’s were associated with increased ovarian atre- sia, increased egg weight, and reduced egg number. The results of this study suggest that English sole exposed to chemical contaminants may experience alterations in egg development; how- ever, nutritional or other environmen- tal factors may also contribute to the observed intersite differences in egg weight and fecundity. Manuscript accepted 4 November 1996. Fishery Bulletin 95:231-249 (1997). Reproductive impairment is poten- tially one of the most damaging ef- fects of aquatic pollution on marine fish and shellfish because of its im- pact on population growth and con- sequently on the abundance of ma- rine resources (Hose and Guillette, 1995; Grosse et ah, in press). Envi- ronmental contaminants exert their effects on reproductive function through a variety of mechanisms; they may have direct toxic effects on germ-cell tissue or may disrupt the endocrine mechanisms that regulate reproduction and early de- velopment, causing inhibited or ab- normal gonadal development or re- duced fertility (Donaldson, 1990; Colburn et ah, 1993; Kime, 1995). Sediments from several areas of Puget Sound, Washington, are pol- luted with xenobiotic compounds such as polychlorinated biphenyls (PCB’s), and polycyclic aromatic hydrocarbons (PAH’s) (Malins et al., 1984, 1985; PSWQA1). These com- pounds are known or suspected dis- rupters of endocrine function (Col- burn et ah, 1993) and, as such, pose a potential threat to the reproduc- tive health of marine fish that re- side in these areas. In previous studies, we examined the effects of these contaminants on several as- pects of reproductive function in English sole, Pleuronectes vetulus , a commercially important bottom- fish species that is widely distrib- uted in Puget Sound (Johnson et ah, 1988, 1993; Casillas et al., 1991; Collier et ah, 1992). These investi- gations revealed that sole from two heavily polluted sites, Eagle Harbor and the Duwamish Waterway, ex- hibited various types of reproduc- tive dysfunction, including inhib- ited gonadal development (Johnson et ah, 1988), depressed plasma es- tradiol levels and reduced ovarian estradiol production in vitro (John- son et ah, 1988, 1993), and reduced spawning success (Casillas et ah, 1991). In contrast, fish from Port Susan and Sinclair Inlet, two sites 1 PSWQA (Puget Sound Water Quality Au- thority). 1994. Puget Sound update: fifth annual report of the Puget Sound ambient monitoring program. Puget Sound Water Quality Authority, Seattle, WA, 122 p. 232 Fishery Bulletin 95(2), 1997 with low to moderate levels of sediment contamina- tion (Malins et ah, 1984), showed little evidence of reproductive dysfunction. Although the causative agents were not definitively identified, aromatic and chlorinated hydrocarbons present in sediments at the Duwamish Waterway and Eagle Harbor sites were shown to be significant risk factors for the develop- ment of these reproductive abnormalities (Johnson et ah, 1988; Casillas et al., 1991). The present study extends our previous work by examining egg weight and fecundity in English sole from the same four sites in Puget Sound. Fecundity and egg size are important determinants of reproductive output in fish (Bagenel, 1973). Fe- cundity provides a measure of the potential number of offspring a female can produce, whereas egg size is an indicator of the nutritional reserves available to developing embryos and may strongly influence the growth and survival of larval fish (Blaxter and Hempel, 1963; Miller et ah, 1988). Both egg size and fecundity vary considerably among stocks, species, and individuals. However, in most marine teleosts, fecundity within a stock or species is highly corre- lated with fish size or weight. Egg size is generally more constant but may vary with factors such as fish age or spawning time or with genetic, nutritional, or environmental factors (Hempel and Blaxter, 1967; Bagenal, 1971; Gall, 1974; Zastrow et ah, 1989; Zamaro, 1992). Teleost fish may have either determinate fecun- dity, in which egg production is set before the spawn- ing season, or indeterminate fecundity, in which egg production can be increased, by recruiting additional oocytes into vitellogenesis during gonadal develop- ment, or reduced through atresia (Hunter and Macewicz, 1985a). In Puget Sound English sole popu- lations, potential fecundity appears to be determined several months before the spawning season because these fish recruit a single clutch of oocytes in late summer or early fall and no additional oocytes enter vitellogenesis prior to spawning in February or March (Johnson et ah, 1991). However, the extent to which fecundity declines as a result of atresia of de- veloping oocytes is unclear. In most fish species, both egg size and fecundity can be influenced by environmental conditions such as water temperature, salinity, and food supply. In herring ( Clupea sp. ), for example, water temperature 60 to 90 days before spawning may be critical in de- termining the balance between egg size and num- ber. Unusually warm temperature leads to high fe- cundity and smaller eggs (Tanasichuk and Ware, 1987). Other stressors, such as handling or crowd- ing, may also be associated with alterations in egg size and number; typically, stressed animals produce more and smaller eggs than do controls (Contreras- Sanchez et ah, 1995; Short et ah, 1995). Field and laboratory studies have demonstrated that fish exposed to certain chemical contaminants exhibit alterations in both egg size and fecundity. Contaminant-associated declines in egg size, fecun- dity, or in both, have been noted in several marine fish species collected from urban embayments, in- cluding white croaker and kelp bass from the Los Angeles area (Hose et ah, 1989), striped bass from San Francisco Bay (Setzler-Hamilton et ah, 1988), and winter flounder from Boston Harbor and Long Island Sound (Nelson et ah, 1991; Johnson et ah, 1994). Similarly, white sucker exposed to pulp mill effluent in a contaminated lake of Ontario, Canada, showed a decrease in egg size and fecundity (McMaster et ah, 1991). Reductions in egg size and fecundity have also been observed in a number of other fish species in conjunction with controlled ex- posure to chlorinated and aromatic hydrocarbons and to other organic pollutants (reviewed in Kime, 1995). These compounds may have direct toxic effects on oocytes and supporting cells (Armstrong, 1986), or alternatively, may disrupt normal hormonal regula- tion of gonadal growth (Donaldson, 1990; Thomas, 1990). Egg production is strongly influenced by the nu- tritional status of fish; if this status is extremely poor, animals may not reproduce at all (e.g. Burton and Idler, 1987), or fecundity may be reduced (Penzak, 1985; Springate et ah, 1985; Chappaz et ah 1987; Rozas and Odum, 1988). Studies also suggest that egg size and number may change seasonally or as environmental conditions vary, thus maximizing the larvae’s probability of survival as food availability changes (Buckley et ah, 1991). Interactive effects of toxicant exposure and nutrition may also occur. For example, toxicants may influence reproductive out- put indirectly, by reducing food quality, food abun- dance, or the ability of animals to digest food or to forage effectively. In some studies where fish were exposed to oil or other organic compounds, declines in fecundity were associated with reduced food in- take and weight loss; thus the contaminants were likely affecting reproductive success by reducing the animal’s condition (e.g. Ghatak and Konar, 1991). In order to account, more precisely, for possible effects of nutritional status on egg weight and fecundity in English sole from contaminated Puget Sound sites, we measured several indicators of nutritional status (i.e. condition, plasma glucose and triglyceride lev- els, and hepatosomatic index [HSI] ) in sampled fish, as well as parameters associated with reproductive development and contaminant exposure. Our objec- tive in this paper is to describe age and size-specific Johnson et al. : Fecundity and egg weight in Pleuronectes vetulus 233 LEGEND Sediment PCB’s (ng/g) Sediment AH’s (ng/g) jail 500 ' 80,000 •10000 Sediment PCB’s ro cn O (V.'V' TTTJ Sediment AH’s o o o LO Eagle Harbor Viv%>^vere“ 500 •10000 on U « 250 c 5000 3 B t3 pm Port Susan § // Bremerton A/ | \ - ^Jj} 500 250 fe’i 10000 5000 I > Sinclair Inlet ST (?) i r/Jir 1 y}i 5 j- Tacoma ~^FT7 Olympia X r 500 250 ■ A mm® 'mm 10000 5000 Duwamish Waterway Figure 1 Chart of Puget Sound, showing locations of sampling sites and concentrations of aromatic hydrocarbons (AH’s) (ng/g dry weight) and polychlorinated biphenyls (PCB’s) (ng/g dry weight) in sediments collected from these areas. Data from Malins et al. ( 1984, 1985); contaminant levels in the same range have been observed in more recent samplings (see Footnote 1 in the main text). patterns of egg production in Puget Sound English sole and to examine how these patterns vary in rela- tionship to collection site, chemical contaminant ex- posure, and nutritional status. Materials and methods Collection of samples Vitellogenic female English sole (greater than 250 mm total length [TL]) were collected by otter trawl from Eagle Harbor, Sinclair Inlet, Port Susan, and the Duwamish Waterway in Puget Sound, Washing- ton (Fig. 1 ). Sampling was conducted during the win- ters of 1986-87 and 1989-90 in mid-December and from mid to late January, to coincide with the period in which vitellogenesis normally occurs in this spe- cies, before substantial migration to spawning areas has taken place (Johnson et al., 1991). Aside from a relatively brief spawning migration, sole are relatively territorial and reside at these sites throughout the year (Day, 1976). It should be noted that because these ani- mals were not actively spawning, fecundity determi- 234 Fishery Bulletin 95(2), 1997 nations estimated potential rather than actual fecun- dity. However, we chose to collect animals at this ear- lier stage of development to ensure that specimens came from resident subpopulations at sites with known sedi- ment contaminant concentrations. This selection would not have been possible if animals had been collected on their spawning grounds. Fecundity determinations were carried out on 5 to 10 animals from each site at each sampling time. Fish sampled in 1986-87 were collected as part of a study on gonadal development in English sole (Johnson et al., 1988); ovary samples were preserved for fecundity determination and archived, but not analyzed, at that time. Fish collected in 1989-90 were sampled specifically for fecundity determination. Fish were caught by otter trawl in 5-min tows and held in aerated saltwater in holding tanks on the deck of the research vessel until they could be pro- cessed. Within an hour of capture, fish were weighed and measured. From each animal, a 1-mL blood sample was collected with a heparinized syringe from the caudal vessel. Blood samples for measurement of plasma estradiol concentrations were centrifuged at 3,000 xg, and the plasma was stored at -20°C. Fish were sacrificed by severing the spinal cord. Ovaries from vitellogenic females were removed and weighed; one ovary was slit longitudinally and pre- served in modified Gilson’s fluid (Simpson, 1951) for later determination of fecundity and egg weight. Ovarian tissues for histological examination were preserved in Davidsons’ fixative (Mahoney, 1973). Additionally, tissue samples for determination of PCB concentrations were collected from the liver and ovary and stored at -20°C. Bile for measurement of fluorescent aromatic compounds (FAC’s) was col- lected and stored at -0°C. Analysis of samples Ovaries collected for histology were embedded in paraffin, stained with hematoxylin and eosin (Luna, 1968), and examined microscopically to confirm their developmental stage and to record ovarian atresia and related lesions by using criteria outlined in Hunter and Macewicz (1985b) and Johnson et al. ( 1991). Ovarian lesion severity was ranked on a sub- jective scale of 1 to 7, with 1 being minimal and 7 being severe. Fecundity was determined by using the gravimet- ric method described by Bagenal and Braum (1971). Ovaries were preserved in Gilson’s fluid for at least 3 months to allow eggs to harden and ovarian con- nective tissue to disintegrate. Preserved eggs were washed with water, filtered to separate them from residual ovarian connective tissue fragments, and dried at 60°C for 24 hours. All eggs were weighed, and then 3 subsamples of 200 eggs each were weighed. Fecundity, relative fecundity, and reproduc- tive rate were subsequently determined by using the formulas below: >• (total weight of eggs) (#of eggs in subsample)] Fecundity = - (mean weight of eggs in subsample) Relative fecundity = (Fecundity/gutted body weight (g)]; and Reproductive rate (g of eggs/year) = [Fecundity x egg weight in (g)]. Additionally, gonadosomatic index (GSI), hepato- somatic index (HSI), and condition factor were cal- culated as follows: GSI = ovary weight (g) gutted body weight (g) x 100 HSI = liver weight (g) gutted body weight (g) x 100 Condition factor = gutted body weight (g)/length3 (cm). Levels of fluorescent aromatic compounds (FAC’s) in bile were measured according to the method of Krahn et al. (1987), which provides a semiquanti- tative determination of the concentrations of metabo- lites of PAH’s (Krahn et al., 1993). Bile sampled from fish was injected directly into a Spectra-Physics Model 8800 high performance liquid chromatograph (HPLC ) equipped with a Phenomenex reversed-phase C18 analytical column. The polar analytes (prima- rily metabolites of AH’s) in bile were eluted with a linear gradient from 100% water containing 5 mL of acetic acid/L to 100% methanol and monitored by two fluorescence detectors connected in series. Fluores- cence of metabolites was measured at two wave- lengths: 290/335 nm, where metabolites of naphtha- lene (NPH ) and related two-ring aromatic compounds from petroleum fuels fluoresce; and 380/480 nm, where metabolites of benzo[a]pyrene (BaP) and re- lated multi-ring AH’s from combustion sources fluo- resce (Krahn et al., 1987). Levels of biliary FAC’s were reported as equivalents of known concentrations of BaP or NPH standards on the basis of biliary protein be- cause recent studies (Collier and Varanasi, 1991) have shown that such normalization can account for varia- tion in FAC levels associated with the feeding status of Johnson et al.: Fecundity and egg weight in Pleuronectes vetulus 235 sampled fish. Concentrations of biliary protein were determined by the method of Lowry et al. (1951) with bovine serum albumin (BSA) as the standard. Liver and ovary tissue were analyzed for PCB’s by following the method described by MacLeod et al. ( 1985 ) and modifications later described in Stein et al. ( 1987). Tissue samples (approximately 2 g) were ground with 10 g of silica and then added to a column (270 x 23 mm) containing 3 g of activated silica gel (Amicon Corp., Danvers, MA) held in place by a glass wool plug. PCB’s were eluted with pentane: methylene chloride (90:10, V/V). The first 50 mL of eluant were collected, concen- trated, and exchanged with 1 mL of hexane prior to analysis by gas chromatography with electron capture detection (GC/ECD) (MacLeod et al., 1985). Selected samples of ovary tissue required the removal of lipids by size-exclusion HPLC (Krahn et al., 1988) prior to analysis with GC/ECD. Plasma estradiol- 17/? concentrations were deter- mined by radioimmunoassay as described by Sower and Schreck (1982). Plasma glucose and triglycer- ides were determined as described by Casillas et al. (1983) and Casillas and Ames (1986), respectively. Fish age was estimated from length by using site- specific age-length curves calculated from length and age data collected from female English sole sampled during previous studies in Puget Sound. Fish ages were determined from otolith analysis (Chilton and Beamish, 1982). Site-specific growth relationships were fitted by using the von Bertalanffy growth curve (Ricker, 1987), and age was then estimated with the formula age = t - ((ln( 1 - length / ))) / K , where t = time at which length = 0; Lm= asymptotic length; and K = Brody’s growth coefficient. Substituting site-specific values for t , Lm, and K into the general formula, age-length equations for female sole from the specific sites were as follows: age Port Susan = -3-41 - (dn(l-length/487)))/ 0.096; a£eSmciair inlet = -1-82 - ((ln( 1-length/ 445)))/ 0.200; ^Duwaimsh Waterway = ~2-87 ~ ((ln(l-fe/lgffc/586)))/ 0.085; a^Eagie Harbor = -2.4 1 - (( ln( l-length/394 )))/ 0.209. Statistical analyses Data were initially analyzed to identify the major biological factors affecting fecundity and egg weight so that potential confounding factors could be ad- justed before evaluating the impacts of sampling site and contaminant exposure on these endpoints. Analy- sis of variance ( ANOVA), and Fisher’s protected least- significant difference multiple-comparison test (Fisher’s PLSD) were used to examine the effects of site, year, and month of capture on fecundity and egg weight. Intersite differences in ovarian atresia sever- ity, an ordinal variable, were compared by using the nonparametric Kruskall-Wallis test. Linear regression analysis was used to examine the relationships of fe- cundity and egg weight with biological variables (i.e. fish size, condition, and gonadosomatic index (GSI). Stepwise multiple-regression analysis was subse- quently used to assess the relationships of fecundity and egg weight with indicators of contaminant expo- sure (e.g. tissue PCB levels, biliary FAC’s, and site of capture) after adjusting for relevant biological factors identified in initial regression analyses. Data were log- transformed as necessary prior to statistical analyses to normalize data and reduce heteroscedasticity. These standard statistical analyses are described in detail in Sokal and Rohlf ( 1981 ) and Dowdy and Wearden ( 1991 ). For all statistical tests, a was set at 0.05. Results Biological factors affecting egg production The number of eggs produced by sole from our sam- pling sites ranged from approximately 120,000 in a 28-cm TL fish to approximately 1.2 million in a 43- cm TL fish. In multiple regression analysis (Table 1), fish length was the strongest predictor of fecun- dity, but GSI (an indicator of the level of gonadal de- velopment) also showed significant associations with fecundity. Fish length explained the highest propor- tion (48%) of variation in fecundity; GSI accounted for 8% of variation in fecundity. Fish age was also positively correlated with fecundity (r2=0.41, P=0.0001, /?=47), but the association was not as strong as the association between fecundity and length because of the high variability in size, and consequently, in egg production, among fish of the same age class. After the influences of fish length and GSI had been accounted for, fish age and sam- pling time had weak but significant negative rela- tionships with fecundity, a finding that suggests a tendency for fecundity to decline in older animals and at the end of the sampling season. Sampling time and age accounted for approximately 3%’ and 4% of variation in fecundity, respectively. In contrast to fecundity, egg weight was not highly correlated with either fish size or age but was re- 236 Fishery Bulletin 95(2), 1 997 Table 1 Results of multiple regression analysis examining effects of biological factors (length, age, GSI, sampling year, and sampling time in Julian day) on egg weight and fecundity. Factors that did not contribute significantly to the model are not included in the table. Regression results Model variables df F-test r2 t-value p -value Fecundity Overall model 98 42.25 0.63 — <0.0001 length 0.48 8.31 <0.0001 +GSI 0.56 5.98 <0.0001 +sampling time 0.59 -3.41 0.0010 +age 0.63 -3.13 0.0023 Egg weight Overall model 98 154.23 0.76 — <0.0001 GSI 0.73 8.674 <0.0001 +sampling time 0.76 3.164 0.0004 lated primarily to the degree of gonadal development (i.e. GSI), and increased as vitellogenesis progressed. In vitellogenic sole sampled in December, mean egg weight was 4.6 ± 0.4 pg (n=66), whereas in January, it was 13.0 ± 0.9 pg (n=34) (P<0.0001, 1-way ANOVA). In multiple regression analysis, the best predictors of egg weight were GSI (73% of variation) and sam- pling time (3% of variation), both of which were posi- tively correlated with egg weight (Table 1). Because of their strong influence on fecundity and egg size and because of their high degree of individual variability, the basic parameters included in Table 1 were adjusted for in subsequent analyses to assess the effects of contaminant exposure, nutritional fac- tors, and site of capture on fecundity and egg size. Site-specific patterns of egg production Mean fecundity, relative fecundity, egg weight, and reproductive output for English sole from Port Su- san, Sinclair Inlet, the Duwamish Waterway, and Eagle Harbor are shown in Table 2, along with re- sults of 2- way ANOVA examining the effects of site and sampling time on these parameters. Overall, fecundity did not change significantly with sampling time, and no significant site-month interactions were seen for fish from Port Susan, the Duwamish Water- way, or Eagle Harbor. Fish from Sinclair Inlet, how- ever, had significantly higher fecundity in Decem- ber than in January (£=2.613, P=0.0105). Site of cap- ture had a significant effect on fecundity; fish from the Duwamish Waterway exhibited higher fecundity than fish from the Port Susan reference site (£=2.016, P=Q.0467). Like fecundity, relative fecundity did not show any consistent overall change with sampling time, but significant site-month interactions were seen for fish from Sinclair Inlet, Port Susan, and Eagle Harbor. Relative fecundity of fish from Sinclair Inlet was significantly higher in the December than in the January sampling (£=2.829, P=0.0058), whereas in fish from Eagle Harbor (£=-2.747, P=0.0072) and Port Susan (£=-4.232, P=0.0001), rela- tive fecundity was significantly lower in December than in January. There was also a significant effect of site on relative fecundity, which was lower in Sinclair Inlet than in Port Susan sole (£ =-3.46, P=0.0006). Egg weight increased significantly, by 2- to 3-fold, between December and January at all sampling sites. No significant site-month interactions were seen. Site of capture did not have a significant effect on egg weight, although Duwamish Waterway fish tended to exhibit lower egg weights than fish from Port Su- san (£ =-1.878, P=0.0635). Like egg weight, repro- ductive rate was significantly higher in January than in December. However, no significant intersite dif- ferences or month-site interactions were seen for re- productive rate. The effects of site of capture on egg weight and fecundity were also evaluated by using multiple re- gression, after adjusting for the influence of fish length, age, GSI, and sampling time (see Table 1). Results showed that even after fish length, sampling time, and GSI had been accounted for, Duwamish Waterway fish had significantly higher fecundity (£=4.52, P=0.0001) than comparable animals from the Port Susan reference site (see Fig. 2A). Age and fe- cundity relationships were also analyzed by using multiple regression (excluding fish length from the model); the results showed that Duwamish Water- Johnson et a I.: Fecundity and egg weight in Pleuronectes vetulus 237 Table 2 Mean values (± SE) of fecundity, egg weight, relative fecundity, and reproductive rate in English sole from four sites in Puget Sound, and results of 2-way analysis of variance (ANOVA) assessing effects of site and month of collection on these variables. All variables were normalized by log transformation prior to statistical analysis. EH = Eagle Harbor, DW = Duwamish Waterway, SI = Sinclair Inlet, and PS = Port Susan, ns = not significant. Site Fecundity (egg no. x 105) Egg weight (gg) Relative fecundity (eggs/g gutted wt) Reproductive rate (g eggs/yr) Dec Jan Dec Jan Dec Jan Dec Jan Port Susan 2.34 ±0.25 2.65 ± 0.28 5.3 ± 0.2 16.7 ± 1.8 770 ± 50 1,250 ± 126 1.4 ± 0.40 4.2 + 0.70 (n=19) («= 9) (71 = 19) (77=10) (77=19) (77=9) (77 = 19) (77=9) Sinclair Inlet 5.44 + 0.70 2.78 ± 0.27 4.1 + 0.5 13.9 ± 2.2 1,080 + 82 670 ± 62 2.4 ± 0.45 3.7 + 0.27 (n= 15) (4) (n=15) (77=4) (77 = 15) (77=4) (77 = 15) (77=4) Duwamish 4.74 ± 0.38 3.92 + 0.43 4.4 ± 0.7 10.7+ 1.4 1,190 ± 57 1,220± 106 2.2 ± 0.41 4.0 ± 0.64 (re=17) (77=10) (71 = 17) (77=10) (77=17) (77= 10 ) (77=17) (77 = 10) Eagle Harbor 3.22 ± 0.32 3.64 ± 0.46 4.6 ± 0.9 11.4+ 1.2 760 ± 64 1,020 ± 73 1.7 + 0.44 4.3 ± 0.74 (n= 15) (n=10) (77=15) (77=10) (77 = 15) (77=10) (77= 17 ) (77=10) 2-way ANOVA results Month ns ns F= 94.42 P = 0.0001 ns ns F= 36.57 P = 0.0001 Dec < Jan Dec c Jan Site F=8.83 P=0.0001 ns ns F= 6.22 P=0.0004 ns ns DW > PS, DW Jan, t = 2.61, P = 0.011 SI: Dec > Jan, t = 2.83, P = 0.0058 EH: Dec < Jan, t = -2.78, P = 0.0078 PS: Dec < Jan, t = -4.23, P = 0.0001 way (£=6.23, P=0.0001) and Sinclair Inlet sole (£=4.33, P=0.0001 ) had significantly higher age-specific fecun- dity than Port Susan reference fish (see Fig. 2B). Mean age-specific fecundity in sole from the four sam- pling sites is shown in Table 3. Similarly, results of multiple regression analysis indicated that intersite differences in egg weight were only partially ex- plained by variation in the degree of gonadal devel- opment (i.e. GSI) or sampling time. Even after ad- justing for these factors, Duwamish Waterway fish exhibited significantly lower egg weight (£=-4.070; P=0.0001) than comparable fish from the other sites (Fig. 2C). Intersite differences in biological and chemical factors Indicators of contaminant exposure Mean levels (± SE) of biliary FAC’s in English sole from Port Su- san, Sinclair Inlet, the Duwamish Waterway, and Eagle Harbor are shown in Fig. 3A; mean concentra- tions of PCB’s (± SE) in liver and ovary tissue are shown in Fig. 3B. English sole from Eagle Harbor had significantly higher biliary FAC-BaP levels than fish from any of the other sampling sites, including the Duwamish Waterway, and significantly higher FAC-NPH levels than fish from either Port Susan or Sinclair Inlet. Duwamish Waterway fish had signifi- cantly higher biliary FAC-BaP and FAC-NPH levels than Port Susan or Sinclair Inlet fish. No significant differences were found between biliary FAC concen- trations in Port Susan and Sinclair Inlet fish. Duwamish Waterway fish had significantly higher liver PCB concentrations than fish from any of the other sampling sites and significantly higher ova- rian PCB concentrations than fish from either Port Susan or Eagle Harbor. Concentrations of PCB’s in liver and ovary tissues of fish from Sinclair Inlet, although not as high as those observed in Duwamish fish, were significantly elevated in comparison with those found in Port Susan and Eagle Harbor fish. Tissue PCB concentrations in Port Susan and Eagle Harbor fish were not statistically different. Size and nutritional status Mean values (± SE) of length, age, condition factor, HSI, plasma triglyceride levels, and plasma glucose levels for sole collected from the four sampling sites are shown in Table 4. Port Susan fish were significantly smaller (in length) and Sinclair Inlet fish significantly larger than ani- 238 Fishery Bulletin 95(2), 1997 log length (mm) Figure 2 (A) Linear regression of log fecundity vs. log length in gravid English sole from Port Susan, Sinclair Inlet, Eagle Harbor, and the Duwamish Waterway. Multiple regression analysis indicated that fecundity was significantly higher in fish from the Duwamish Waterway than in fish of comparable size from the other sites (<=3.601, P=0.0005). (B) Linear regression of log fecundity vs. log age (in years) in gravid English sole from Port Susan, Sinclair Inlet, Eagle Harbor, and the Duwamish Waterway. Multiple regression analysis indicated that fecundity was significantly higher for a given age in fish from the Duwamish Waterway (<=6.23, P=0.0001) and Sinclair Inlet (t=4.33, P=0.0001) than in animals from other sites. (C) Linear regression of egg weight vs. GSI in gravid English sole from Port Susan, Sinclair Inlet, Eagle Harbor, and the Duwamish Waterway. Multiple regression analysis indicated that egg weight was significantly lower for given GSI in fish from the Duwamish Waterway than in animals from other sites (<=-3.218, P=0.0018). mals collected from the other sites; fish age (as estimated from length) was significantly lower in fish from the Duwamish Waterway and Port Susan than in fish from Sinclair Inlet and Eagle Harbor. No significant intersite differences were found in either condition factor or length-weight relationship. In contrast, other indicators of nutritional status showed significant intersite differences. Duwamish Waterway fish had sig- nificantly higher HSI than fish from other sites, as well as significantly higher triglyceride lev- els in plasma. Plasma glucose levels, on the other hand, were significantly lower in Eagle Harbor fish than in those from the other sam- pling sites. Moreover, condition factor and the other proposed indicators of nutritional status were not consistently correlated. A significant positive correlation was found between condi- tion factor and plasma triglyceride concentra- tions (r=0.312, P=0.014, n-6 2), but no signifi- cant relationship was found between condition factor and either HSI (r=0.093, P= 0.356, T— i II - (22=17) Eagle Harbor 350 ± 22c 8.8 ± Q.66 0.0091 ± 0.0010 2.22 ± 0.10'1 73 ± 21" 25 ± 41h (72=25 ) (22= 25) (22 = 25) (22 = 15) (22=15) (22= 15) P=0.0001 P=0.G076 P=0. 1016 P=0.0001 P=0.0001 P=0.0005 statistic, P<0.05). At Eagle Harbor, 43% of sampled fish exhibited atresia, in comparison with 28%, 21%, and 17% of fish at Sinclair Inlet, Port Susan, and the Duwamish Waterway, respectively. Atresia sever- ity also tended to be greatest at Eagle Harbor (aver- age rankings were 1.33 at Eagle Harbor, 1.00 at Sinclair Inlet, 0.759 at Port Susan, and 0.542 at the Duwamish Waterway), but intersite differences in severity were not statistically significant (P=0.2659 in the nonparametric Kruskall-Wallis test). Chemical and nutritional parameters as predictors of egg production patterns Results of multiple regression analysis examining associations of egg weight, fecundity, and relative fecundity with biomarkers of contaminant exposure and nutritional factors are shown in Table 6 (repro- ductive rate was not included in this analysis because no significant intersite differences were seen in this variable). As noted above, potentially confounding biological factors ( i.e. fish length, age, sampling time, and GSI in the case of fecundity, and GSI and sam- pling time in the case of egg weight; see Table 1 ) were incorporated into the regression analyses along with bioindicators of exposure in order to adjust for their contribution to the observed variation in fecundity and egg weight. Both fecundity and relative fecundity were found to be significantly and positively associated with PCB concentrations in liver. Egg weight showed a signifi- cant positive association with biliary FAC’s-BaP; the relationship with FAC’s-NPH was also positive, but not statistically significant. Additionally, a near-sig- 240 Fishery Bulletin 95(2), 1 997 A JV "ab a U < U- I S 1,000,000 100,000 10,000 1,000 100 □ FAC-BaP !g FAC-NPH b' Port Susan Sinclair Inlet Eagle Harbor Duwamish 100,000 '5 10,000 - CQ 1 ,000 u Cu Port Susan Sinclair Inlet Eagle Harbor Duwamish Figure 3 (A) Mean levels (± SE) of aromatic compounds (ng/mL) fluorescing at benzo[a]pyrene (FAC-BaP) and napthalene (FAC-NPH) wave lengths in bile of gravid female English sole collected from Eagle Harbor, Sinclair Inlet, Port Susan, and the Duwamish Waterway. (B) Mean (± SE) con- centrations of PCB’s (ng/g wet weight) in liver and ovary of gravid fe- male English sole collected from Eagle Harbor, Sinclair Inlet, Port Su- san, and the Duwamish Waterway. Numbers of samples analyzed per treatment are indicated in parentheses. Significant differences in site means (as determined by ANOVA and Fisher’s PLSD, P < 0.05) are indicated by letter superscripts. Series a-c is used for FAC-BaP and liver PCB concentrations; series a'-c' is used for FAC-NPH and ovarian PCB concentrations. nificant negative association (P=0.0617) was found between liver PCB concentrations and egg weight. A significant negative association was also observed between fecundity and atresia severity of yolked oo- cytes, and a positive association between atresia and egg weight. Vitellogenin concentration showed sig- nificant or near-significant negative associations with fecundity (P=0.0579) and relative fecundity (P=0.Q164) and was significantly positively correlated with egg weight (P=0.0164). Plasma estradiol concentration showed a similar relationship with fecundity, rela- tive fecundity, and egg weight, but the associations were not statistically significant at a - 0.05 (0.06 < P < 0.18). In addition to bioindicators of contaminant expo- sure and reproductive condition, fecundity and rela- Johnson et al.: Fecundity and egg weight in Pleuronectes vetulus 241 Table 5 Mean values (± SE) of gonadosomatic index (GSI), plama estradiol concentration and plasma vitellogenin concentrations (as estimated from plasma alkali-labile phosphate (ALP)) in English sole from four sites in Puget Sound, and results of 2-way analy- sis of variance (ANOVA) assessing effects of site and month of collection on these variables. All variables were normalized by log- transformation prior to statistical analysis. No significant month-site interactions were observed for either GSI or plasma estra- diol concentration, so the interaction term was suppressed in the final model. EH=Eagle Harbor, DW=Duwamish Waterway, SI=Sinclair Inlet, and PS=Port Susan. nd=not determined. Site GSI Plasma estradiol 17-p (pg/mL) Plasma ALP (vitellogenin) (mg/mL) Dec Jan Dec Jan Dec Jan Port Susan 5.5 ± 0.7 15.0 ± 1.4 5000 ± 600 12000 ± 1700 35 ± 3 nd (72=19) (72 = 9) (72=19) (72 =10) (72=19) Sinclair Inlet 5.8 ± 0.6 9.4 ± 0.1.3 5300 ± 400 11000 ± 2100 24 ± 8 nd (n=15) (72=4) (72=15) (72=4) (72=19) Duwamish Waterway 6.4 ± 0.7 11.4 ± 1.3 3300 ± 400 9000 ± 900 29 ± 2 nd (72=17) (72=10) (72 =17) ( 72 =10) (72 = 17) Eagle Harbor 4.5 ± 0.6 9.6 ± 1.1 4900 ± 700 8500 ± 800 49+3 nd (72=15) (72 = 10) (72 =13) ( 72 =10) (72=15) 2-way ANOVA Results Month F=65.9 P=0.0001 P=49.4 P=0.0001 nd nd Dec < Jan Dec < Jan Site F=3.05 P=0.032 F= 3.57 P=0.016 P=11.25 P=0.0001 EH< PS, t=- -2.57, P=0.012 DW < PS, t =-2.46, P=0.016 EH > PS, t=2.86, P=0.006 SI < PS, *=-3.00, P=0.004 tive fecundity were significantly related to several indicators of nutritional status. Fecundity was posi- tively associated with condition factor (P=0.0006) and showed a near-significant tendency (P=0.0603) to increase with increasing plasma triglyceride levels. Relative fecundity was significantly positively asso- ciated with plasma triglyceride levels. These rela- tionships were also observed when only sole from the least contaminated sites (Port Susan and Sinclair Inlet) were examined (e.g. for fecundity vs. condition factor, £=3.177, P=0.0028, n- 47; for fecundity vs. plasma triglyceride concentration, £=1.923, P=0.0647, n= 32). In addition, significant positive associations were observed between both fecundity and relative fecundity and HSI (P=0.0011) when fish from all sites were included in the analysis. These associations were not apparent when only reference fish were examined (e.g. for fecundity vs. HSI, £=0.776, P=0.4423, n=47). None of the nutritional factors ex- amined were significantly associated with egg weight. Significant correlations were found between indi- cators of contaminant exposure and several of the factors related to fish nutritional status (Table 7). Hepatosomatic index showed strong positive corre- lations with both biliary FAC and tissue PCB con- centrations (0.0001 < P < 0.0002), and plasma trig- lyceride and glucose levels were significantly posi- tively correlated with PCB concentrations in the liver. Plasma triglyceride and glucose levels also tended to increase as ovarian PCB concentrations increased and to decrease as biliary FAC levels increased (0.05 < P < 0.07), but the correlation was not statistically significant at a - 0.05. Condition factor was not sig- nificantly correlated with any biomarker of contami- nant exposure. No significant correlations were seen between either GSI or plasma estradiol concentra- tions and either tissue PCB levels or biliary FAC’s. However, significant negative correlations were found between plasma vitellogenin (ALP) concentra- tions and both hepatic and ovarian PCB concentrations. Discussion Significant intersite differences in both egg weight and fecundity were detectable in English sole sampled in this study, even after variation in fish size and sampling time had been taken into account. One notable finding was the tendency for Duwamish Waterway and Sinclair Inlet fish to exhibit higher age-specific fecundity in comparison with fish from Eagle Harbor and Port Susan. This difference ap- peared to be due, at least in part, to a larger size at age in the Duwamish and Sinclair Inlet fish. Al- though additional data, particularly on older fish, 242 Fishery Bulletin 95(2), 1997 Table 6 Associations between bioindicators of contaminant exposure, atresia severity, and nutritional factors in English sole and egg weight, fecundity, and relative fecundity as determined through multiple regression, while adjusting for effects of fish size, GSI, and sampling time. For each independent variable, t-value, P-value, and sample number (n) are shown. The sign of the f-value indicates the direction of the association (positive or negative). Statistically significant associations (P<0.05) are indicated in bold. Independent variable Fecundity7 Egg weight2 Relative fecundity2 -1.379 2.314 -1.083 Biliary FAC-BaP P=0.1733 P=0.0242 P=0.2835 (n= 62) (71=62) (77=62) 0.842 1.593 -0.029 Biliary FAC-NPH P=0.4032 P=0.1165 P= 0.9767 (71=61) (77=62) (77=61) 2.402 -1.897 2.350 Liver PCB’s P=0.0187 P=0.0617 P=0.0214 (n=79) (77 = 72) (77=79) 0.933 -0.105 1.641 Ovary PCB’s P=0.3544 P=0.9166 P=0.1054 CM II (77 = 72) (77 = 72) -2.162 2.901 -1.852 Atresia severity3 P=0.0334 P=0.0047 P=.0674 (yolked oocytes) (71=91) (71=92) (77=91) -1.907 1.340 -1.775 Plasma estradiol P=0.0598 P=0.1834 P=0.0792 (n=96) (77=97) (77=96) -1.934 2.462 -2.468 Vitellogenin P=0.0579 P=0.0164 P=0.0164 (77=65) (77=65) (77=65) 3.552 -1.440 0.330 Condition factor P=0.0006 P=0.1531 P=0.7419 (ti=99) (77 = 100) (77=99) 3.381 -1.440 4.350 HSI P=0.0011 P=0.1531 P=0.0001 (71=99) (77 = 100) (77=99) 1.722 0.896 1.355 Glucose P=0.0902 P=0.3735 P=0.1803 (77=65) (77=65) (77=65) 1.917 1.680 2.629 Triglycerides P=0.0603 P=0.0983 P=0.0109 (77=62) (77=62) (77=62) 1 Adjusted for fish length, age, GSI and sampling time. 2 Adjusted for GSI and sampling time. 3 Ranked on a scale of 1-7, where 1 is miminal and 7 is severe. are needed to confirm that Duwamish and Sinclair Inlet fish do in fact have a higher growth rate or longer period of growth than fish from the other sam- pling areas, the present results suggest that intersite differences in growth rate may have a significant effect on age-specific egg production in Puget Sound English sole. This problem deserves further investi- gation because of its potential impact on sole popu- lation dynamics. Another notable finding was the tendency of En- glish sole from the Duwamish Waterway to produce more and smaller eggs than fish of comparable size Johnson et at: Fecundity and egg weight in Pleuronectes vetulus 243 Table 7 Associations between nutritional and reproductive factors and biomarkers of contaminant exposure in English sole as deter- mined through multiple regression, while adjusting for effects of sampling time. P-value for regression factor, P-value, and sample number are shown. The sign of the t-value indicates the direction of the association (positive or negative). Statistically significant associations are indicated in bold. Nutritional and reproductive factors Exposure biomarkers Condition HSI Glucose Triglycerides Estradiol Vitellogenin GSI 1.701 6.297 -1.902 -1.880 -0.739 0.493 -0.237 Biliary P=0.0942 P<0.0001 P=0.0654 P=0.0699 P=0.4628 P=0.6248 P=0.8133 FAC-BaP (n=6 2) (n=62) (n= 37) (77=33) (77=59) ( 72=37 ) (77=62) 1.287 6.660 -1.692 -0.851 -0.636 -0.280 -0.262 Biliary P=0.2032 P<0.0001 P=0.0994 P=0.4041 P=0.5274 P=0.7809 P=0.7939 FAC-NPH (t? = 62) (71=62) (n= 37) (72=33) (77=59) ( 77 =37 ) (77=62) 0.143 3.905 2.357 3.786 -1.209 -2.717 0.096 Liver P=0.8870 P=0.0002 P=0.0228 P=0.0005 P=0.2305 P=0.0093 P=0.9328 PCB’s o GO II (77=80) (77=47) (77=43) (77 = 77) (77=47) 3 II 00 o 0.132 5.279 1.997 1.920 0.066 -2.603 0.676 Ovary P= 0.8956 P=0.0001 P=0.0534 P=0.0630 P=0.9476 P=0.0133 0.5010 PCB’s (n= 72) (77=72) (77=38) (77=34) (77=69) (77=38) (77=72) and maturity from other sites. Although this intersite difference in egg production pattern could represent a genetic adaptation of the Duwamish sole stock to its particular habitat, this is not likely on the basis of current knowledge of the population structure of English sole in Puget Sound. Sole populations resid- ing at our sampling sites do not appear to constitute discrete breeding populations but migrate to com- mon breeding areas, such as University Point or Duwamish Head in central Puget Sound, for spawn- ing (Collier et al., 1992). Moreover, their eggs and larvae are pelagic and therefore are transported from the breeding area to nearshore nursery ground set- tling sites in accordance with current patterns (Lassuy, 1989). Site-specific genetic adaptation would be unlikely in animals with such a breeding system, although some genetic divergence between subpopu- lations in northern and southern Puget Sound with distinct spawning areas is a possibility. Overall, marine fish show relatively little geographic diver- sity (Gyllensten, 1985), and their genetic structure is thought to be determined largely by the dispersal potential of the pelagic stages, rather than by adap- tation to local environmental conditions (Waples, 1987). Studies of marine flatfish, such as the com- mon sole ( Solea vulgaris), turbot ( Scopthalmus maxi- mus), and flounder ( Platichthys flesus) in the north- eastern Atlantic and Mediterranean (Galleguillos and Ward, 1982; Blanquer et al., 1992; Kotoulas et al., 1995), indicate that these species exhibit some geo- graphic isolation or differentiation due to tempera- ture gradients that inhibit larval transport and sur- vival but that they show fairly substantial gene flow on a regional level. In the common sole, for example, a species with a life history strategy similar to that of English sole, the geographic unit of population structure appears to lie within a radius of approxi- mately 100 km (Kotoulas et al., 1995). Changes in egg weight and number could also be associated with alterations in habitat characteristics, such as water temperature, food supply, and food qual- ity, all of which have been shown to influence egg de- velopment in English sole or other fish species. Win- ters et al. ( 1993), for example, demonstrated that win- ter temperature 2 to 3 months before spawning can affect fecundity and egg size in herring from the north- west Atlantic. Temperature can also affect the rate of gonadal development in English sole, and consequently egg size at a particular sampling time ( Kruse and Tyler, 1983). In general, however, bottom temperature in Puget Sound is not highly variable over the geographic range encompassed by this study (Collias et al., 1974; Malins et al., 1980, 1982), and water temperatures in the Duwamish Waterway are comparable to those from sites in the main basin (Collier, 1988). Consequently, it is unlikely that temperature is a major contributing factor to the intersite differences in patterns of egg de- velopment that we observed in this study. The present findings suggest, on the other hand, that contaminant exposure and nutritional variables, 244 Fishery Bulletin 95(2), 1997 or their interaction, could be important contributing factors to the observed changes in fecundity and egg weight. Although indicators of chemical contaminant exposure were not among the strongest predictors of fecundity and egg weight in the sole examined in this study, some significant associations were observed between tissue PCB and biliary FAC concentrations in individual fish and patterns of egg production. Elevated concentrations of PCB’s in liver or ovarian tissue, which were characteristic of fish from the Duwamish Waterway, were associated with reduced plasma ALP (vitellogenin) concentrations, as well as with production of more but smaller eggs. These data suggest that exposure to PCB’s might affect egg de- velopment, perhaps by inhibiting either the produc- tion or uptake of vitellogenin. However, reports of the effects of PCB’s on vitellogenin production in fish are somewhat inconsistent. In the larger set of fish sampled in our earlier study (Johnson et al., 1988), a correlation between elevated tissue PCB concen- trations and reduced plasma ALP in vitellogenic fish was also observed (n- 60, Spearman’s p=-0.30, P=0.023), as well as a tendency for plasma ALP con- centrations to be lower in fish from the Duwamish Waterway and Sinclair Inlet, although intersite differ- ences were less pronounced than in the smaller set of fish for which fecundity and egg weight determinations were performed. In other studies such compounds have proved to be estrogenic and have enhanced vitellogenin production in fish and reptiles (von der Decken et al., 1992; Guillette et al., 1994) or have exerted little effect on plasma vitellogenin concentrations (Monosson et al., 1994). The impact of PCB exposure on egg development might be better clarified through congener-specific analysis of PCB’s because the various coplanar and noncoplanar PCB congers present in complex PCB mixtures are known to differ in toxicity (Safe, 1990), as well as in their ability to enhance or inhibit vitellogenin synthesis (Anderson et al., 1996). Exposure to PAH’s also appeared to have some influence on egg develop- ment because we found that elevated biliary FAC-BaP levels were correlated with both increased atresia of yolked oocytes and a trend toward increased egg weight and lowered fecundity. Interestingly, atresia tended to be most prevalent and of greatest severity at Eagle Harbor, where biliary FAC concentrations in fish were particularly high. In earlier studies of reproductive function in English sole (Johnson et al., 1988), we observed reduced plasma estradiol concentrations in female fish from both Eagle Harbor and the Duwamish Waterway. These differences were partially associated with inhibited ovarian devel- opment in significant proportions of adult fish from these sites, but differences persisted even when only vitellogenic fish were examined. A similar trend was observed in the fish examined in this study, all of which were vitellogenic, although the intersite difference was statistically significant only for fish from the Duwamish Waterway. Depressed plasma estradiol concentrations tended to be associated with increased fecundity but were not strongly correlated with changes in egg production patterns. Nutritional status appeared to have a significant effect on fecundity in English sole because a strong correlation was found between condition factor and fecundity in fish from minimally to moderately con- taminated sites. Similar relationships between fe- cundity and food supply, condition factor, and other indicators of nutritional status have been observed in other fish species, including winter flounder (Tyler and Dunn, 1976), temperate and tropical clupeids (Hay and Brett, 1988; Milton et al., 1994), plaice (Horwood et al., 1986, 1989), and rainbow trout (Bromage et al., 1992). When sole from the contami- nated sites (Eagle Harbor and the Duwamish Wa- terway) were included in the analyses, additional nutrition-related factors showed correlations with fecundity in English sole. Of these factors, HSI showed a particularly strong relationship with fe- cundity and was significantly higher in Duwamish Waterway fish than in animals from the other sam- pling sites. Animals from the Duwamish Waterway also exhibited elevated plasma triglyceride levels, which, like increased HSI, appeared to be associated with production of more and smaller eggs. Milton et al. (1994) also observed production of more, but smaller, eggs in tropical clupeids with increased HSI, and interpreted the alteration in egg size and num- ber as a response to the good nutritional status of the female and a possible adaptation to environmen- tal conditions in which food was abundant. It is pos- sible that elevated HSI and plasma triglyceride lev- els in Duwamish Waterway fish could be related to favorable feeding conditions. Previous studies have, in fact, shown that benthic invertebrates such as mollusks and polychaetes, which form a significant proportion of the diet of English sole (Varanasi et al., 1989), are relatively abundant in the Duwamish Waterway (Malins et al., 1980, 1982). However, the Duwamish Waterway fish did not have a significantly higher mean condition factor than that of animals from the other sampling sites, and although plasma triglyceride concentrations showed some correlation with condition factor, HSI did not. Both HSI and plasma triglyceride concentrations, however, showed strong correlations with bioindicators of contaminant exposure. Increased HSI in association with expo- sure to toxicants, particularly agents that induce cell proliferation, is well documented in a number of fish species (Heath, 1987), and toxicant-related increases Johnson et a I : Fecundity and egg weight in Pleuronectes vetulus 245 in serum triglycerides have also been observed in previous studies. For example, English sole showed increased serum triglyceride levels in response to laboratory exposure to model toxicants bromo- benzene and o-bromophenol (Casillas and Myers, 1989). Consequently, elevated levels of these param- eters in Duwamish Waterway fish could be a reflec- tion of toxicant exposure rather than good nutritional status. Moreover, the contaminants may affect fecun- dity or egg size indirectly through their impact on liver function, lipid disposition, or lipid metabolism. Results of this study suggest the possibility of such interactive effects of contaminants and nutritional factors on egg development. In addition to PCB’s and PAH’s, other contami- nants, such as heavy metals and organotins, which are present in the Duwamish Waterway and to a lesser extent at Sinclair Inlet (Krone et al., 1989; PSWQA, 1994; Dutch et al.2), could affect egg weight or other aspects of gonadal development in English sole. Previous studies have shown that a number of toxic trace elements, including copper, lead, mercury, and cadmium (Kaviraj, 1983; Munkittrick and Dixon, 1988; Dethlefson, 1989), as well as tributyl tin (TBT) (Walker et al., 1990), can affect egg size or gonadal de- velopment in fish. Previously, Krone et al. (1989) showed increased tissue concentrations of TBT in En- glish sole from the Duwamish Waterway. However, many of these trace elements in their organic form do not bioaccumulate in English sole (Meador et al., 1994) and indicate low bioavailability. Increased egg production or production of more but smaller eggs is not the most commonly observed re- sponse in fish exposed to environmental contami- nants, but such trends have been observed in some previous studies. Slooff and DeZwart ( 1983) reported increased fecundity in bream exposed to a mixture of chlorinated and aromatic compounds in the Rhine River, and Walker et al. (1990) reported a similar finding for medaka exposed to TBT. Reduced egg size, although not necessarily in conjunction with in- creased egg production, has been reported in a num- ber of studies in which fish were exposed to PAH’s, PCB’s, orboth(Kime, 1995). Interestingly, since 1900, North sea plaice have also exhibited a trend toward production of more but smaller eggs (Rijnsdorp, 1991). The causes of these changes are unknown, but it is suspected that they are most likely related to changes in environmental conditions, which could 2 Dutch, M., H. Dietrich, and P. L. Striplin. 1993. Puget Sound Ambient Monitoring Program 1992: marine sediment monitor- ing task — annual report 1992. Environmental Investigations and Laboratory Services Program, Washington State Dep. Ecol- ogy, Olympia, WA. Publ. 93-87. include environmental degradation associated with anthropogenic activities. In summary, the results of this study suggest that egg weight and number in English sole are influenced by a variety of factors, including exposure to organic chemical contaminants such as PCB’s and PAH’s, nutritional status, and growth rate. Although chemi- cal contaminant exposure did not appear to have a major impact on egg development in English sole, high concentrations of contaminants in tissues or body fluids showed significant associations with cer- tain potentially detrimental changes: elevated PCB concentrations in liver were correlated with reduced plasma vitellogenin levels and reduced egg weight, and high levels of biliary FAC’s were associated with increased ovarian atresia and reduced fecundity. The impact of these alterations in egg weight and num- ber on the reproductive fitness of affected fish is not clear. It is likely that smaller eggs will tend to pro- duce smaller larvae, and reduced larval size has been associated with lower growth and survival rates in other flatfish species (Buckley et al., 1991). However, the detrimental effects of reduced egg weight could be offset by increased egg production, or, at least in the case of the Duwamish Waterway fish, by a rela- tively fast growth rate and high age-specific fecun- dity. In order to gain a better understanding of the relationships between chemical contaminants, nu- tritional factors, and alterations in gonadal develop- ment and egg production, we are currently investi- gating the effects of PCB’s, AH’s, and food supply on egg weight and fecundity in further laboratory stud- ies with English sole. Acknowledgments We thank Sue Pierce and other members of the En- vironmental Chemistry Branch, Environmental Con- servation Division, for assistance in measurement and calculation of PCB concentrations in tissue samples; William Gronlund, Paul Plesha, and Herbert Sanborn for assistance in fish collection; Mark Myers and Paul Olson for assistance in histo- logical examination of liver and ovary tissue; Tom Horn and Sylvester Spencer for measurement of bil- iary FAC’s; Ethel Blood for measurement of plasma triglyceride and glucose concentrations; and Herbert Sanborn, Casimir Rice, Tracy Collier, and two anony- mous referees for reviewing the manuscript. Literature cited Anderson, M. J., M. R. Miller, and D. E. Hinton. 1996. In vitro modulation of 17-/) estradiol-induced 246 Fishery Bulletin 95(2), 1997 vitellogenin synthesis: effects of cytochrome P4501A1 in- ducing compounds on rainbow trout (Oncorhynchus mykiss) liver cells. Aquat. Toxicol. (Amst.) 14:327-350. Armstrong, D. T. 1986. 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Determination of fecundity in Amerian plaice ( Hippoglossoides plastessoides ) and its variation from 1987 to 1989 on the tail of the Grand Bank. Neth. J. Sea Res. 29:205-209. Zastrow, C. E., E. D. Houde, and E. S. Saunders. 1989. Quality of striped bass (Morone saxtilis) eggs in rela- tion to river source and female weight. Rapp. R-V. Reun. Ciem. 191:34-42. 250 Abstract.-By analyzing annual ichthyoplankton survey data from 1983 to 1988, I found a significant positive correlation in distribution and abun- dance between larval Cubiceps pauci- radiatus and the Loop Current in the Gulf of Mexico. The data indicate that C. pauciradiatus is a species whose adult spawning grounds and larval habitat are tied to sharp temperature gradients. These gradients occur along the edge of the Loop Current in the eastern Gulf of Mexico and along the anticyclonic-cyclonic rings in the west- ern Gulf of Mexico. Transects made across the Loop Current, in 1987 and 1988, show that larval C. pauciradiatus is found close to the frontal interface and that peak abundance occurs before peak SST (sea surface temperature). Variation in the extent of the frontal systems in the Gulf of Mexico would be expected to affect annual recruitment of a species that is tied to a frontal habi- tat. Annual abundance of C. pauci- radiatus varied considerably but was similar to that of other pelagic species. This finding suggests that the physical processes in the Gulf of Mexico may affect a wide range of species. Manuscript accepted 10 October 1996. Fishery Bulletin 95:250-266 (1997). The Loop Current and the abundance of larval Cubiceps pauciradiatus (Pisces: Nomeidae) in the Gulf of Mexico: evidence for physical and biological interaction John Lamkin Southeast Fisheries Science Center National Marine Fisheries Service, NOAA 75 Virginia Beach Dr., Miami , Florida 33149 E-mail address: John.Lamkin@noaa.gov Bigeye cigarfish, Cubiceps pauci- radiatus, is a member of the family Nomeidae (suborder Stromateoidei) characterized by Haedrich ( 1967 ) as “oceanic fishes of tropical and sub- tropical waters.” Fishes of this fam- ily are widely distributed across the Gulf of Mexico, in Caribbean wa- ters, and in the tropical Atlantic, Pacific, and Indian Oceans (Butler, 1979). The family Nomeidae com- prises three genera: Cubiceps, Psenes, and Nomeus. Nomeus is monotypic, Cubiceps has seven spe- cies, and Psenes six (Haedrich, 1967, 1972; Butler, 1979). Cubiceps pauciradiatus Gunther is a world- wide tropical species and an impor- tant forage fish for porpoises (Perrin et al., 1973) and tuna (Alverson, 1963). Because of their oceanic habi- tat, cigarfishes are poorly known, and only limited information is available on their distribution. Ahlstrom et al. (1976) described the larval stages of five species of this suborder, including C. pauciradi- atus. All identifications in this study are based upon their work. In the central and South Atlan- tic, Oven et al. (1984) found that Cubiceps pauciradiatus was always present in the upper sound-scatter- ing layer. It was also the dominant species in the Gulf of Guinea, ac- counting for 46-85% of the catch (Salekhov, 1989). Like many fishes, C. pauciradiatus migrate to the sur- face waters at night, concentrating in the upper 70 m. Salekhov (1989) reported that they were abundant and at times the dominant species in night-collected samples where surface-water temperatures were 26.2-28.30°C. Juveniles do not mi- grate but remain in the 30-90 m stratum. Cubiceps pauciradiatus is an intermittent spawner and has a life span of 1 to 2 years. Peak spawning occurs from December to April in tropical waters. The distribution of this fish in the North Atlantic and Gulf of Mexico is poorly known. In a 1979 study by Houde et al.,1 C. pauciradiatus were the most abundant nomeid in the eastern Gulf. Richards (1984) found this species widely distributed in the eastern Caribbean Sea. In the central and South Atlantic, Salekhov (1989) showed that largest catches of C. pauciradiatus occurred in tropical waters along the periphery of cyclonic gyres, the Equatorial Counter Current, and the upwelling region of the Sierra-Leone Ridge. The Gulf of Mexico contains simi- lar frontal areas, such as the Loop 1 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. Contract Report to the Bureau of Land Manage- ment, rep. AA550-ct7-28, 546 p. Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 251 Current in the eastern Gulf and the large warm-core anticyclonic and smaller cold-core cyclonic eddies in the western Gulf. These features form frontal zones across a wide area of the Gulf of Mexico and may be areas in which adult C. pauciradiatus are abundant. If C. pauciradiatus are abundant around the edges of gyres and upwelling areas, the Gulf of Mexico could be expected to support an extensive population. This relationship between larval fish and frontal zones has been an area of intense research since lies and Sinclair (1982) first proposed the existence of larval retention zones caused by oceanographic fea- tures. Thermal fronts are defined as a boundary be- tween two water masses that usually have a sharp temperature gradient over short (<10 km) distances (Brandt and Wadley, 1981; Owen, 1989). The biologi- cal implications of these features have been recog- nized by several authors (Brandt and Wadley, 1981; Le Feure, 1986; Richardson et al., 1986, 1989). Ther- mal fronts are often associated with abrupt changes in salinity, color, turbidity, primary productivity, and phytoplankton species composition and abundance. Fronts may also be considered ecotones and may pose a zoogeographic barrier to both adult and larval fish (Brandt and Wadley, 1981; Richards et al., 1993). Changes in the distribution and abundance of phy- toplankton species across frontal zones have been reported by Seliger et al. (1981), Holligan et al. (1984), Richardson et al. (1985), and Richardson et al. (1986). These authors have reported increased abundance across these features, but the duration and long-term effect of increased phytoplankton abundance on trophic levels have yet to be deter- mined. In a series of papers examining larval her- ring patches in the Buchan area of Scotland, Richardson et al. (1986) found phytoplankton bio- mass was highest at a transition zone created by warming waters and tidal mixing. Increased zoo- plankton abundance across fronts has also been re- ported (Tranter et al., 1983; Kiprboe and Johansen, 1986; Richards et al., 1989). Trantor et al. (1983) and Kiprboe and Johansen ( 1986) both reported increased zooplankton biomass concurrent with increased phy- toplankton abundance. In a series of transects across the Loop Current, Richards et al. (1989) found in- creased zooplankton volumes in thermally mixed water close to the outer perimeter of highest surface- current velocity. This occurrence coincided with in- creases in surface chlorophyll measurements. The purpose of this paper is to describe the large- scale (Gulf-wide) distribution and abundance of lar- val C. pa uciradiatus and their interaction with meso- scale oceanographic features in the Gulf of Mexico. I will show that C. pauciradiatus are retained on the cool side of thermal fronts in areas of high produc- tivity. I hypothesize that the temporal persistence of northern excursions of the Loop Current directly af- fects the abundance and probably the survival of this species. Because this study focuses on larval, rather than adult, C. pauciradiatus , the results of this study will help define the role that these oceanographic features play in larval distribution and may help to determine the size of future year classes. Physical oceanography of the Gulf of Mexico The Gulf of Mexico is a semi-enclosed body of water, the circulation of which is dominated by the Loop Current. Water enters through the Yucatan Chan- nel and exits through the Straits of Florida. The Loop Current is very dynamic and unstable, pushing as far as 29 degrees north latitude into the Gulf of Mexico and at other times flowing almost directly out through the Straits (Vukovich et al., 1979). These characteristics have caused considerable confusion over the years, and only recently have we begun to understand the dynamics of this system (Leipper, 1970; Behringer et al., 1977; Maul, 1977; Vukovich et al., 1979; Vukovich, 1988; Maul and Vukovich, 1993). Among the more significant features of the Loop Current are the large (200-300 km at formation) anticyclonic rings generated when the northward intrusion separates from the rest of the Current. These rings are pinched off from the Loop Current and move into the western Gulf shelf where they eventually spin down and break up (Merrell and Vazquez, 1983; Lewis and Kirwan, 1987; Lewis, 1992). The exact mechanism of ring genesis is un- clear, but it seems to involve the formation of a nar- row intrusion of cold water between the ring and the remainder of the Loop Current (Cochrane, 1972; Vukovich and Maul, 1985; Vukovich, 1986). Hurlburt and Thompson (1980, 1982) used numeric models that showed that inherent instabilities exist within the flow field and eventually result in ring separa- tion. Ring separation occurs every 6-17 months (on average every 11 months [Maul and Vukovich, 1993]). As these warm-core anticyclonic rings move west- ward, adjacent mesoscale (20-80 km) cyclonic circu- lations may develop (Elliot, 1979; Merrell and Morrison, 1981; Merrell and Vazques, 1983; Lewis and Kirwan, 1985). These cyclonic rings may be im- portant biologically; Biggs (1992) found elevated ni- trate concentrations just below the mixed layer. Cy- clones such as these exist for 6 months or more, dur- ing which time they may move tens to hundreds of km (Hamilton, 1992). However, their cold surface 252 Fishery Bulletin 95(2), 1997 expression is limited, and generally these rings can be recognized better by direct sampling from ships or aircraft than from satellites (Hamilton, 1992). Materials and methods Collections in this study were gathered from the NOAA Ship Oregon II. Annual surveys were con- ducted that covered most of the U.S. Exclusive Eco- nomic Zone (Richards, 1984). These surveys followed a grid pattern with stations at every 30 minutes of latitude and longitude. Each station consisted of con- ductivity-temperature-depth (CTD) casts to 200 meters or consisted of an expendable bathythermo- graph (XBT) drop. Biological samples were collected with 60-cm paired bongo nets of 0.333-mm mesh towed to 200 m or to within 5 m of the bottom at stations <200 m. The nets were towed at a speed of approximately 1.5 kn with a wire angle of 45° and retrieved at a rate of 20 m per minute. A neuston- net tow of 10-min duration (vessel speed approxi- mately 2.5 kn) was also conducted at each station. In 1983 and 1984 only one survey of the northern Gulf of Mexico was completed (Table 1). In the fol- lowing years, two surveys were completed (1986, 1987, and 1988), each about two weeks apart, al- though with fewer stations and reduced geographic coverage. There was no survey in 1985. In addition to the normal survey, six transects across the Loop Current were made in 1987. The transect locations were selected on the basis of real-time satellite im- agery and frontal analysis and the frontal positions were radioed to the vessel (Richards et al., 1989). Transects 1-6 consisted of stations 2 km apart, and transects 7-8 were 3.6 km apart. In 1988, a line of stations was sampled along 86°W running from 29.5°N to 27.6°N. All stations except Table 1 NOAA ship Oregon II cruise dates covering the period 1983-88. Cruise Leg Date Year No. of bongo stations QT-134 25 April-16 May 1983 99 OT-143 23 April-7 May 1984 98 OT-159 1 22 April-6 May 1986 36 2 9 May-22 May 1986 37 OT-166 1 15 April -2 May 1987 35 2 7 May-20 May 1987 35 OT-173 1 15 April -2 May 1988 34 2 12 May-26 May 1988 35 the first two were 8.3 km apart. Biological samples were taken until the 22°C isotherm rose to 100 m. The XBT drops were continued in order to provide a more detailed definition of the water mass. A 1-m Tucker trawl, with three nets and two opening and closing bongo nets, was deployed in addition to the stan- dard gear. Samples were fixed in buffered formalin and transferred to 70% ethanol within 48 hours. Bulk zooplankton biomass was estimated from wet displacement volume (dv) (Ahlstrom and Thrailkill, 1960). Samples from the annual surveys were pro- cessed at the Plankton Sorting and Identification Center, Szczucin, Poland. The samples from the transects in 1987 and 1988 were processed at the Southeast Fisheries Science Center, Miami, Florida. Fish were identified from the descriptions of Ahlstrom et al. (1976). Catches of larvae were stan- dardized to number under 10 m2. Bulk plankton standing stocks were standardized to mL of wet dis- placement volume per 1,000 m3. To test the relationship between larval C. pauci- radiatus and the close proximity (<5 km) of a frontal feature, a chi-square (^2) test for one-dimensional count data was performed. Because the sampling grid was held constant, the total number of stations <5 km from a zone of surface-temperature gradient var- ied in relation to the spatial location and size of the frontal features present in that year. To account for the fact that in some years most of the stations were within 5 km of a frontal zone and in other years few were, the analysis was performed to test whether the percentage of stations with C. pauciradiatus <5 km from a frontal feature was greater than the percent- age of stations with C. pauciradiatus >5 km from a frontal feature. This procedure has the added ben- efit of accounting for random distribution; i.e. if 90% of the stations are within 5 km of a feature, then expectations are such that 90% of the stations with C. pauciradiatus would be within 5 km. Pearson cor- relation coefficients (r) were also obtained from the relationship between larval C. pauciradiatus and plankton volume and between total larvae (number under 10 m2) and plankton volume from the transects. The frontal edge of the Loop Current and anticy- clonic rings was defined as 22°C at 100 m depth fol- lowing Leipper, ( 1970) and Maul and Herman (1985). Results Physical oceanography The circulation patterns preceding and during the 1983-88 April-May ichthyoplankton cruises varied Lamkin. The Loop Current and abundance of larval Cubiceps pauciradiatus 253 considerably from year to year and within years (cruises) in both the eastern and western Gulf of Mexico. A brief synopsis of the circulation patterns present throughout this study are presented below. Figure 1 shows the position of the 22° isotherms at 100 meters (Loop Current frontal edge) in the east- ern Gulf of Mexico for each of the 5 years covered in this study. In 1983, 1984, and 1988, the Loop Cur- rent extended north to 27°N. However, in 1986, the Loop barely penetrated into the sample area, and in 1987, a broad front stretched from 88°W to 84°W, al- though not as far north as in other years. The posi- tions of the Loop Current and cyclonic-anticyclonic rings are detailed in Figures 2—11. Although Figure 1 indicates the position of the Loop current at the time of the survey, it does not reflect the dynamics of the system nor the formation of warm-core eddies. The northward penetration and stability of the Loop Current front is directly im- pacted by formation and separation of warm-core eddies. In 1983, a ring began to form in January but did not separate until March and left the Loop Cur- rent extended to the northeast with the northern boundary at 27°N (Fig. 2). It remained in that posi- tion throughout most of the spring. In contrast, the Loop Current underwent ring-shedding events in January 1984 and 1986, and the front remained far- ther south and with a much narrower frontal area (Figs. 3-5). In 1987, and 1988, ring formation had taken place in September-November of the previ- ous year. However, the Loop Current did not push north until the spring of the following year. The presence or absence of the anticyclonic (warm- core) rings and their companion cyclones (cold-core) also strongly influences the circulation in the west- ern Gulf of Mexico. Anticyclonic rings were present in the western Gulf of Mexico in all years, although their influence on the area sampled varied consider- ably. In 1983, 1984, and 1986, warm-core rings were present in the western Gulf. In 1987, and 1988, their influence was restricted to the southern part of the survey area, and in 1988, the temperature signature was evident only at 200 m. Cyclones were also present, although the number and position varied considerably from year to year. However, in some years it was not possible to resolve the circulation patterns because of difficulty in obtaining sufficient sample density. Distribution and abundance In each year of this study, larval C. pauciradiatus were most abundant in temperature gradients: in the Loop Current front between the the 22° and 20° iso- therms and in the gradients of 16-20°C associated with cold cyclonic rings. Abundance varied consider- ably from year to year and between the eastern and western Gulf. Overall, abundance was greatest in 1983 and lowest in 1987. In 1983, C. pauciradiatus were present at 41 of 99 stations, with peak catches in the southeast of 162 and 188 individuals under 10 nr (Fig. 2). Thirty six of the 41 sta- tions were in the eastern Gulf. In succeeding years, abundance was greatly reduced. In 1984, C. pauci- radiatus were present at only 9 stations in the eastern Gulf (Fig. 3). This pattern continued through 1988, when C. pauciradiatus was found at no more than 9 stations and at as few as 4 (leg 2, 1988). Although C. pauciradiatus were present at a few stations inshore, most were found concentrated around the Loop Current, but not in its interior. Peak catches often occurred when a station coincided with a cyclonic meander at the Loop Current or in areas associated with cyclonic rings and cold water intrusions. -90 -89 -88 -87 -86 -85 -84 -83 -82 -81 Figure 1 Position of Loop Current (22°C at 100 m) for each year and leg. 254 Fishery Bulletin 95(2), 1997 Distribution and abundance were considerably dif- ferent in the western Gulf than in the eastern. Lar- val C. pauciradiatus were found at few stations (ex- cept in 1984), and generally in smaller numbers. They tended to be found around the edge of rings (warm and cold) when present. The number of stations occupied in the western Gulf varied considerably from year to year and created difficulties for evaluating the available data and for constructing a meaningful interpreta- tion of the physical oceanography. Both interannual and east-west differences, however, were evident. In 1983, C. pauciradiatus was abundant in large numbers at 36 stations in the eastern Gulf of Mexico but were present in only five stations west of 90°W. The follow- ing year they were taken at only 9 sta- tions in the eastern Gulf but were found at 17 stations throughout the western Gulf, although in fewer numbers. They were found infrequently in the follow- ing years. Total larvae (number under 10 m2) and plankton displacement volumes showed similar trends in abundance. Larval abundance and plankton dis- placement volumes were generally high- est inshore and along the 100-m curve. However, at stations along the edge of the Loop Current and around the cold core rings, numbers of larvae and plank- ton were often equal to and sometimes exceeded values at the inshore stations. Transects Measurements at each station and sat- ellite sea-surface temperatures indicate that in 1987, transects I, II, III, V, VI, and VII crossed surface fronts (Figs. 12 and 13), as did the only transect in 1988 (Fig. 14). Transect I crossed a warm fila- ment extending north from the eastern edge of the Loop. Transect II was south of the origin of the warm filament. Transect III was made in a north-south direction, and transect V was the only transect to cross a cold-core cyclonic ring. Transect VI was on the cool side of the front and is discussed in detail by Richards et al. (1989). Transect VII be- gan on the cool side of the Loop Current along longitude 87°W and crossed south into the northern edge of the Loop. Sea- surface temperatures increased from Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 255 -96 © - 5 larvae © - 175 larvae -94 Figure 4 NOAA ship Oregon II cruise 159, leg 1, 22 April-6 May 1986. Symbols as in Figure 2. Number of larvae under 10 m2 ranged from 6 to 100 individu- als per station. Figure 5 NOAA ship Oregon II cruise 159, leg 2, 9 May-22 May 1986. Symbols as in Figure 2. Number of larvae under 10 m2 ranged from 6 to 24 individuals per station. 25.1°C to a high of 29°C in the middle of the transect. The transect in 1988 (Fig. 14) bisected a northward intrusion of the Loop Current between a cyclonic ring to the west and cooler shelf water to the east. Figures 15 and 16 show the distribu- tion of C. pauciradiatus in relation to SST fronts. Few were found in areas of peak temperatures, and the larvae were generally more abundant on the cool side, although SST was not as impor- tant as spatial orientation to the front. Peak abundances occurred at a variety of temperatures (21.5°C-28.7°C) but were always greatest in regions of great- est horizontal temperature gradients. This pattern can best be seen at transect III in 1987 and at the single transect completed in 1988. In transect III, C. pauciradiatus were found on both sides of the temperature front but were con- centrated around the steepest slope. In 1988, the transect was much longer, and stations were 5 nautical miles apart (Fig. 16). Cubiceps pauciradiatus were absent until sea-surface temperatures began to increase significantly and peaked prior to maximum SST tempera- tures. Few C. pauciradiatus were found in the warm Loop Current waters or in the cooler continental shelf waters. Plankton volumes, total larvae (un- der 10 m2), and larvae in the neuston net showed similar patterns (Fig. 17). Perhaps the most important of these is plankton volume because this may show an abundance of the potential prey of larval C. pauciradiatus . There was no significant correlation between abun- dance of larval C. pauciradiatus and plankton volume across all transects. However, few larvae were found in the second series of transects (V, VI, VII). When each transect was examined separately, II and III showed high cor- relations between C. pauciradiatus and plankton volume (0.829, P=0.0410, and 0.896, P=0.0002, respectively). In both these transects, larval C. pauciradiatus were present in consecutive stations, and the frontal features were well de- fined. Total larvae (number under 10 m2) were cor- related with plankton volume when examined across all transects (0.236, P=0.0349) but not when com- pared transect by transect. Within each transect, correlations were highest in III and VII (0.822, P=0.0019, 0.697, P=0.017). The results of the test be- 256 Fishery Bulletin 95(2), I 997 ® - 5 larvae © - 175 larvae Figure 6 NOAAship Oregon II cruise 166, leg 1, 25 April-16 May 1987. Symbols as in Figure 2. Number of larvae under 10 m2 ranged from 7 to 50 individuals per station. tween larval C. pauciradiatus and the close proxim- ity of a temperature front across all cruises yielded a value of 34.128 with a P-value of <0.01, indicating a very strong correlation between the presence of C. pauciradiatus and fron- tal zones. Of stations with C. pauci- radiatus, 84.7% occurred at <5 km, and 15.3% were >5 km from a front. For all stations, the values were 51.8% and 48.2%, respectively. Such tight spatial correlations with frontal zones are best seen by compar- ing the abundance of C. pauciradiatus in the transects with that in the surveys. In both legs of the 1987 survey a com- bined mean of 207 (under 10 m2) larval C. pauciradiatus were caught at 14 grid stations. Mean abundance averaged 2— 3 fold higher in frontal zones, for 70% of these larvae ( 160) were taken at nine stations that were on or near a front. Only five stations with 47 larvae (30%) were not associated with an identifiable frontal structure. In the same 1987 survey, 6 dedicated transects across the Loop Current caught a combined mean of 693 larvae (number under 10 m2). Larval C. pauci- radiatus were caught at 22 bongo sta- tions and another 91 were taken in neuston tows. If these numbers are com- bined, 94.5% were caught along frontal zones. Results were similar in 1988. During the grid survey, 13 stations contained 343 C. pa uciradiatus, only one of which was not associated with the front. This station accounted for only 1.7% of the larvae. In the one transect, 330 C. pauciradiatus larvae (under 10 m2) were found at 9 of 16 stations. Further examination of these tran- sects across the Loop Current and the grid stations shows that C. pauci- radiatus occur primarily on the cold side of the temperature gradient. When the temperature gradient is well defined and C. pauciradiatus are present across the front, abundance is highly corre- lated with bulk plankton standing stocks (displacement volume maxima). The pattern can be clearly seen in transect III and in the transect com- pleted in 1988 (Figs. 15 and 16). In transect III, C. pauciradiatus were not present until sea surface temperature (SST) started to increase, and their abundance peaked just before an SST maximum. Bulk plankton displacement vol- Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 257 umes were also at or near maxima at the same stations where C. pauci- radiatus were abundant. In the 1988 transect, with stations 8 km apart, the patterns of abundance clearly showed that C. pauciradiatus prefer the fron- tal interface between the Gulf common water over the continental margin and the subtropical underwater of the Loop Current. Larvae were not present until the SST began to increase, rose to a peak after the initial temperature in- crease, then declined sharply in the warmer waters. Bulk plankton displace- ment volumes did not follow an identi- cal pattern but nonetheless peaked in mid-transect, just before those stations where C. pauciradiatus were most abundant. Such patterns may be a spa- tial consequence of the fact that this transect had stations 8 km apart rather than 3.2 km apart. Discussion In this study, the abundance and dis- tribution of larval C. pauciradiatus were examined over five yearly surveys and seven transects. Analyses of survey data and transects depict a species whose adult spawning grounds and lar- val habitat are tied to sharp tempera- ture gradients associated with the Loop Current in the eastern Gulf and to an- ticyclonic-cyclonic rings in the western Gulf of Mexico. Larval C. pauciradiatus were most abundant near a tempera- ture front. It is apparent from these data and the transects made in 1987 and 1988 that this frontal environment is the preferred habitat and probable spawning area for the adult population of C. pauciradiatus in the Gulf of Mexico. Salenkov (1989) found that ar- eas of highest density of juvenile and adult C. pauciradiatus in the tropical Atlantic Ocean were situated in zones of high production such as the edge of cyclonic gyres, the equatorial countercurrent, and the upwelling regions of the Sierra Leone Ridge. In general, fish are often found aggregated at fronts (Brandt and Wadley, 1981; Nero et ah, 1990). Cur- rently there are two competing theories to explain this relationship: 1) that fish have thermal require- ments and are attracted to temperature gradients, and 2) that fish are attracted to fronts because of the increased concentration of prey (Brandt, 1993). A third explanation may be that spawning in a frontal area may also provide optimal conditions for survival and growth of larvae and that increased concentra- 258 Fishery Bulletin 95(2), 1997 tion of prey may be beneficial for both larvae and adults. The concentration of biomass at a front may be caused by advection (Olson and Backus, 1985) or by new production. Claustre et al. ( 1994) found evidence that suggested that the increased biomass found along a frontal region is due to new production. In the Mediterranean they found that frontal stations had phytoplankton biomass levels much higher than those at adjacent zones. These areas of high biomass were dominated by diatoms as opposed to flagellates and cyanobacteria found in typical Atlantic and Medi- terranean waters. They concluded that the high biomass levels found at the front are not the result of purely pas- sive accumulation but are the result of physically driven new production. The Loop Current and the anticy- clonic-cyclonic gyres found in the Gulf of Mexico provide an extensive (and dynamic) frontal habitat, and new production, coupled with coastal produc- tion advected off the shelf by ring-ring dipoles (Biggs and Muller-Karger, 1994), may play an important role in maintain- ing the productivity of these areas. It follows that stability and position of these mesoscale physical features may have a profound impact upon the spawn- ing success of C. pauciradiatus and on subsequent recruitment to the stock. Examination of the survey and transect data indicates considerable within- and among-year variation both in the position, shape, and intensity of the dominant physical oceanographic features as well as in the abundance of larvae and plankton along the front. Variation in the distribution of larvae along the Loop Current is evident and is the result of the interaction of physical and biological processes. As Loop water flows north, it makes an anticyclonic turn to the east and south. The meanders and eddy separations that result can be thought of as forcing mechanisms for ecology and population structure through divergence and upwelling; likewise convergence re- sults in passive accumulation of plankton and larvae and in the formation or disper- sion of micro patches of prey. Cold-core submesoscale (~50 km) cy- clonic rings that form along the northern edge of the Loop Current would also be expected to affect both the physical and biological component. In the eastern Gulf, these closed cyclonic domes apparently form as a cold perturbation on the north- ern boundary of the Loop Current and move south along the Florida shelf (Vukovich and Maul, 1985; Vukovich, © - 5 larvae © - 175 larvae Figure 1 0 NOAA ship Oregon II cruise 173, leg 2, 12 May-26 May 1988. Symbols as in Figure 2. Number of larvae under 10 m2 ranged from 5 to 58 individuals per station. -94 -93 -92 -91 -90 -89 -88 Figure 1 1 NOAA ship Oregon II cruise 173, leg 2,12 May-26 May 1988. Tempera- ture (°C) of northern Gulf of Mexico at 200 meters. Symbols as in Fig- ure 2. Number of larvae under 10 m2 ranged from 5 to 58 individuals per station. Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 259 -96 1986). Large filaments of warm Loop Current water are advected north as much as 300 km in the flow confluence of cold-ring and Loop Current interactions. These cyclonic rings were noted in every survey and the biologically fine structure of one of these was sampled in transect V. Analysis of satellite SST images shows that at least one and often two cold per- turbations are generally present along the northern edge of the Loop Current and along the west Florida shelf. The formation and circulation patterns are not completely under- stood, but other authors have ex- amined similar features else- where. In the western North At- lantic, Pollard and Regier (1990) described the structure and vari- ability of the upper 300-m of fronts. They found that small-scale eddies, tens of kilometers in width, may have vertical velocities as large as tens of meters a day and may approach 50 m/d. Trantor et al. (1983) suggested that in the Tasman Sea, an upwelling-down- welling circulation cell existed at the interface between a cyclonic crescent of cool water and an anti- cyclonic ring. They reported high concentrations of surface chloro- phyll, surface nitrate, and the copepod Calinoides carcinatus of- ten associated with upwellings along the edge of a warm core eddy in the cool crescent. Both of these mechanisms act to inject nutrients into the photic zone. However, whether there is a direct effect of these cold-core eddies on larvae and zooplankton of Gulf of Mexico stocks is not clear. Plankton dis- placement volume and larval abun- dances are larger, especially in the area between the ring and Loop Current. Transect V, which apparently bisected a cyclonic ring, had plankton displacement volumes higher than those of any of the other five tran sects (202 mL/1,000 m). The cold-core rings found to the south are usu- ally associated with increased abundances of both zoo- plankton and larvae. Maul et al. (1984) found that a -94 -92 -90 -88 -86 -84 Figure 1 3 Diagram of transects across the Loop Current in 1987 leg 2. cold ring that persisted for several months off the Dry Tortugas was associated with a 3-fold increase in catch per unit of effort of Atlantic bluefin tuna. A cyclonic ring was present in this position in 1983, 1984, and 1988. In fact, in 1983 the highest catches of C. pauciradiatus ( 188 under 10m2) were found near this feature along with elevated plankton displacement vol- ume and larval abundance. 260 Fishery Bulletin 95(2), 1997 Western Gulf of Mexico The situation in the western Gulf of Mexico is less clear owing to the complexity of the physical regime on mesoscales of 10-100 km, coupled with the fact that fewer ichthyoplankton stations were made in this region. In this region there was no systematic effort to define the features that were present and to sample densely enough in a way that defined coarse scale (<10 km) distributions of larvae. Because surface thermal fronts associated with the warm-core rings are much more diffuse, they were difficult to characterize on the scale of this survey. Several research efforts have been directed to this area in an attempt to understand the complexities of the interaction among large anticyclonic rings generated by the Loop Current, cold dome cyclonic rings, and the continental shelf in the western Gulf of Mexico (Lewis, 1992). Only a few research scientists have dealt with biological compo- nents, but they suggest that the cyclonic cir- culation regions, similar to shelf waters, have a level of primary productivity much greater than that in surrounding oceanic waters (Biggs, 1988). Wormuth ( 1982) found 1.5-3 times more bulk plankton volume in cyclonic rings than in warm-core rings. By comparison, the near surface waters of warm-core rings are oligotrophic and depleted in nutrients; therefore only near the ring edge were there sig- nificant concentrations of nitrate at 100 m and el- evated primary production in the upper 100 m (Biggs, 1992). Because large anti- cyclonic rings are olig- otrophic, it is unlikely that mobile oceanic predators, such as C. pauciradiatus or scombrids, would be found within its interior where prey is presumably scarce. Despite the paucity of ichthyoplankton stations west of 89° in the Gulf, trends are evident. Fore- most of these is that larval C. pauciradiatus appear more frequently in collec- tions along the edges of both anticyclonic and cy- clonic rings than in tows made inside warm eddies or over the adjacent conti- nental margin. This find- -96 -94 -92 -90 -88 -86 -84 Figure 14 Diagram of transect across the Loop Current in 1988. Q) Q. E CD Sea surface temperature Cupiceps (bongo) Cupiceps (neuston) 35 30 25 20 15 10 5 0 O c "O Figure 1 5 Plots of transects across the Loop Current in 1987. Figure shows SST and number of C. pauciradiatus in bongo nets (under 10 m2) and total number of larvae in the neuston net. Stations were two nautical miles apart. Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 261 ing can be seen in 1984 when there were two warm anticyclonic rings separated by a cold cyclonic ring at 26.5°N, 93.5°W (Fig. 3). Cubiceps pauciradiatus were distributed primarily around the edge of this cyclone and the leading edge of the incoming warm- core ring to the east. In 1988 the situation was similar. Most C. pauciradiatus larvae were found west of 89°W. Al- though at 100 m, the Gulf waters west of the Missis- sippi River seem fairly homogeneous, at 200 m there was considerable structure evident in the water col- umn (Figs. 8 and 10): in leg 1, the edge of a warm- core ring was evident at 26.5°N, 93°W with cooler water to the east (Fig. 9); by leg 2 there was a cold- core ring centered at 26.5°N, 93°W, approximately A Station number Figure 1 6 Transect in 1988 south along 86° west, where (A) depicts depth of 15°C, 20°C, and 22°C isotherm and SST, (B) indicates SST and C. pauciradiatus at biological stations, and (C) shows total larvae under 10 m2 (bongo) and neuston for the same stations as B. 83 km in diameter as defined by the 14°C isotherm at 200 m (Fig. 11). These cold-core cyclonic rings may have a life span of 6 months or more, and it is not unusual for there to be a weak temperature signa- ture in the upper 100 m (Hamilton, 1992). In both legs of the 1988 survey west of 89°W, C. pa uciradiatus were distributed around the edges of the two cold- core rings and in the cooler waters to the east of these features. The basic pattern was the same in other years, but oceanographic features could not be as well defined as they were in 1984. First, the large anticyclonic rings often pass south of the survey area and thus are incompletely sampled. Second, after 1984, the number of stations in the western Gulf was reduced, and therefore the physical features could not be as densely sampled as they were the first two years. For a species such as C. pauciradiatus , the western Gulf of Mexico may at times provide a va- riety of frontal habitats, such as those that oc- curred in 1984. In other years the oceanographic conditions may be less favorable. Abundance This year-to-year change in the number and posi- tion of mesoscale oceanographic features occurred in both the eastern and western Gulf of Mexico, both within years and between years; the abun- dance of C. pauciradiatus changed similarly. (Fig. 18). Cubiceps pauciradiatus larvae were abundant in 1983 but declined thereafter. These changes in abundance are the result of natural mortality be- cause there is no fishery for this species; owing to their pelagic nature, they are taken only occasion- ally as bycatch by longliners. lies and Sinclair (1982) and Sinclair (1988) ar- gued that the existence of a population “depends on the ability of the larvae to remain aggregated during the first few months of life,” and that abso- lute abundance was a function of the physical oceanographic processes of the spawning areas. Population abundance depended on the horizon- tal size scale of the physical system underlying larval retention. Rothschild et al. (1989) suggested that the physical environment underlies the pro- cesses acting on recruitment variability. Cubiceps pauciradiatus do not spawn at a specific geographic location as do herring populations, but instead have spawning sites that appear to be tied to dynamic oceanographic features, namely to the Loop Current and its associated rings. Larvae are spawned at the frontal zones regardless of geographic position. The physical oceanographic processes acting on the spawning sites of C. pauciradiatus change im- 262 Fishery Bulletin 95(2), 1 997 mensely from year to year and within time scales on the order of weeks. Figure 1 shows the range in vari- ability of the northern perimeter of the Loop Current over the period studied. Not only do the size and north- south position of the front vary, but stability, length, shape, and intensity of the frontal system vary as well. Major changes in position may occur within time scales of weeks. In 1986 and 1987, the Loop Current was positioned south of 26°N during the first leg of the survey. In both years the front pushed north be- fore the second leg. In 1987 it moved almost 100 km north in 2 weeks. In other years, 1983, 1984, and 1988, the Loop Current was already at 27°N when the survey began. Over the years studied, the length of the northern perimeter of the Loop Current front ranged from 880 km in 1988 to 182 km in 1986 (as measured along the 22°C isotherm in the area sampled). Length of this front in each year is sum- marized in Table 2. However, length and position in itself says little about the frontal interface and bio- logical response to hydrodynamic processes. Larval C. pauciradiatus were most numerous in both the eastern and western Gulf of Mexico in 1983. In this year the Loop Current had pushed north in Table 2 Length of the Loop Current as measured at the 22° iso- therm at 100 m and number of C. pauciradiatus larvae caught at grid stations (under 10 m2). Date Leg Length in kilometers No. of larvae under 10 m2 1983 647 1,445 1984 332 258 1986 1 182 179 1986 2 299 124 1987 1 564 170 1987 2 681 44 1988 1 697 219 1988 2 880 307 the winter and despite shedding a ring, had been stable for almost 6 months prior to the survey. Plank- ton displacement volumes were also high, averaging 100 mL/1,000 m3 in the eastern Gulf and 130 mL/ 1,000 m3 in the western Gulf. By remaining in a relatively stable position, the biological components have time to respond to physi- Lamkin: The Loop Current and abundance of larval Cubiceps pauciradiatus 263 cal input of upwelling regions, and adults will be con- centrated by increased abundances of prey (Atkinson and Targett,1983). Turbulent mixing is increased at frontal areas; therefore, formation and dissipation of prey patches will also be affected. A recent model by Davis et al. ( 1991) shows that formation and abun- dance of microscale patches (< 10 m) of prey can change the growth rate of larvae by up to 25%. Fron- tal regions, with convergent and divergent zones that result in biological gradients such as those found in the transects across the Loop Current, could be ex- pected to lead to patchiness both across and along the front. This model predicts that even small varia- tion in growth rates due to turbulence and patchi- ness can lead to large fluctuations in recruitment. In contrast, the years following 1983 were charac- terized by a less stable Loop Current structure, and fish abundances were considerably reduced. In 1984 the Loop Current did not move north of 27°N until late March, only one month before the survey began (Fig 3). In 1986 the Loop was positioned well to the south, and little frontal habitat was available in the Gulf of Mexico to the spawning population (Figs. 4 and 5). The pattern was similar in 1987 and 1988 (Figs. 6-10). In both years the 22°C isotherm at 100 m began well to the south of 27°N, before pushing north in the spring. It is not clear what factors are the important ones in driving such variations in abundance. Variance of fish populations is a natural occurrence and the sub- ject of many studies on recruitment. However, the pattern of abundance during these five years was not unique to C. pauciradiatus. Aggregation of blue- fin tuna (Thunnus thynnus), and billfish (Istio- phoridae) are also closely tied to the location of ther- mal fronts (Roffer, et al., 1994). These unrelated pelagic species showed similar trends in larval abun- dance with peaks in 1983 and with decreases there- after. Thunnus thynnus larval abundance closely par- alleled C. pauciradiatus abundance, even showing an increase in 1988. Ariomma melanum, another stromateoid, showed similar trends in larval abun- dance (Fig. 18). These fish are benthopelagic over the continental margin, and most adults are taken in bottom trawls at depths of 225-480 m. This cer- tainly indicates that the variation in the Loop Cur- rent position and stability may impact the abundance of a wide range of species and not just large pelagic predators such as bluefin and billfish. Although stability and size of the frontal system are important to frontal species, other physical-bio- logical interactions take place within the system on a variety of scales. The importance of each interac- tion will vary on time scales ranging from months to days because of the inherent nature of frontal sys- tems. Rothschild et al. (1989) states that it is neces- sary to consider the “mechanisms of population sta- bilization at each phase of the life history.” Although it has sometimes been possible to tie population fluc- tuations to a certain physical event (Harris et al., 1992 ), it is more often the case that we are faced with a much more multidimensional problem. That large-scale oceanographic processes have a major influence on the abundance of fish stocks has been recognized by a variety of authors. Harris et al. (1988) reported on several species and presented evidence that large-scale changes in the distribution of southern bluefin tuna result from large-scale changes is SST. Koslow ( 1984) argued that large-scale physical forcing rather than ecological and biologi- cal interactions is the dominant factor controlling the recruitment of several northwest Atlantic fisheries. Basin-scale circulation patterns may be the driving 264 Fishery Bulletin 95(2), 1997 influence on the distribution of larval C. pauci- radiatus and other pelagic species. In conclusion, this study indicates that larval C. pauciradiatus are a frontal species, concentrated at temperature fronts throughout the Gulf of Mexico. This is an important concept that needs to be recog- nized. Fronts and eddies are fundamental to the world oceans, and they are the only coherent feature in the Gulf of Mexico. The Loop Current itself acts as a zoogeographic barrier separating the oceanic and shelf species (Richards et al., 1993). Although it is not surprising that certain species have evolved to take advantage of this environment, it means that frontal species must be recognized as such in order to sample and manage these stocks effectively. Spawning-stock biomass estimates are an impor- tant consideration in the management of pelagic spe- cies. These are inferred from the abundance of lar- vae taken at fixed stations (CalCofi, SEAMAP). If the target species is tied by life history parameters to a frontal system, then its apparent abundance will be affected by the extent of the frontal system found within the sampled area. For instance, if the frontal system within the area sampled is extensive, higher numbers of larvae are expected. Likewise, if only a small portion of the frontal system is sampled, then numbers are expected to be low, as was the case in 1986. Thus, the lower abundance estimates do not necessarily mean that there were fewer fish spawned that year but, rather, may indicate that they were spawned along the front outside the area sampled. Accurate abundance estimates are a problem in the Gulf of Mexico because only the northern and eastern Gulf are sampled. The boundaries of the Loop Current pass through the Exclusive Economic Zone of the United States, Mexico, and Cuba, and the large anticyclonic rings often pass south of the area sampled. In addition, there is considerable variation in abundance along a temperature front. It is impor- tant to note that the presence of a frontal region in itself does not necessarily constitute a favorable habi- tat, but favorable spatial and temporal patterns of the front may determine the abundance of the lar- vae on a basin scale. Literature cited Ahlstrom, E. H., J. L. Butler, and B. Y. Sumida. 1976. Pelagic stromateoid fishes ( Pisces, Periformes ) of the eastern Pacific kinds, distributions, and early life histo- ries and observations on five of these from the Northwest Atlantic. Bull. Mar. Sci. 26:285-402. Ahlstrom, E. H., and J. R. Thrailkill. 1960. Plankton volume loss with time of preserva- tion. CalCOFI Rep. 9:57-63. Alverson, F. G. 1963. The food of yellowfin and skipjack tunas in the east- ern tropical Pacific Ocean. Inter-Am. Trop. Tuna Comm. Bull. 7:295-396. Atkinson, L. P., and T. E. Targett. 1983. Upwelling along the 60-m isobath from Cape Canaveral to Cape Hatteras and its relationship to fish distribution. Deep-Sea Res. 30a:22 1-226. Behringer, D. W., R. L. Molinari, and J. F. Festa. 1977. The variability of anticyclonic current patterns in the Gulf of Mexico. J. Geophys. Res. 82:5469-5476. Biggs, D. C. 1992. Nutrients, plankton and productivity in a warm-core ring in the western Gulf of Mexico. J. Geophys. Res. 97:2143-2153. Biggs, D. C., and F. E. Muller-Karger. 1994. Ship and satellite observations of chlorophyll stocks in interacting cyclone-anticyclone eddy pairs in the west- ern Gulf of Mexico. J. Geophys. Res. 99:7371-7384 Biggs, D. C., A. C. Vastano, R. A. Ossinger, A. Gil-zurrta, and A. Perez-Franco. 1988. Multidisciplinary study of warm and cold-core rings in the Gulf of Mexico. 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Vertical distributions of Pteropods and zooplankton biomass in the upper 200m of the western Gulf of Mexico. Trans. Am. Geophy. Union. 63:89-90. 267 Abstract .—The concept that de- pleted populations of marine fishes can be revitalized by releasing cultured fish is being tested in Hawaii. In this study we evaluated effects of interaction be- tween release season and size-at-re- lease on recapture rates of cultured striped mullet, Mugil cephalus, re- leased into Kaneohe Bay, Hawaii. Over 90,000 cultured M. cephalus finger- lings, ranging in size from 45 to 130 mm total length, were tagged with binary coded-wire tags. Half were released in spring, the remainder in summer. In both seasons, releases were made in three replicate lots. In each replicate, five size intervals of fish were released at two nursery habitats in Kaneohe Bay. Monthly cast-net collections were made in 6 nursery habitats over a 45- week period to monitor recapture rates, growth, and dispersal of cultured fish. Recapture rate was directly affected by the seasonal timing of releases. Greatest recovery of the smallest fish released (individuals <60 mm) occurred following spring releases and coincided with peak recruitment of similar-size wildM. cephalus juveniles. In contrast, recovery of fish that were <60 mm at release was very poor after summer releases. Overall survival was similar at both release sites. We hypothesize that survival of released cultured fish will be greater when releases are timed so that fish size-at-release coincides with modes in the size structure of wild stocks. To optimize effectiveness of stock enhancement as a fishery-man- agement tool, pilot release-recapture experiments should be conducted to evaluate effects of release season on size-dependent recovery of released animals. Manuscript accepted 9 September 1996. Fishery Bulletin 95:267-279 (1997). Influence of release season on size-dependent survival of cultured striped mullet, Mugil cephalus, in a Hawaiian estuary Kenneth M. Leber The Oceanic Institute Makapuu Point, Waimanalo, Hawaii 96795 Present Address: Mote Marine Laboratory 1 600 Ken Thompson Parkway, Sarasota, Florida 34236 E-mail address: KLeber@marinelab.sarasota.fl.as H. Lee BSankenship Steve M. Arce Washington Department of Fish and Wildlife 600 Capitol Way North, Mail Stop 43 1 35 Olympia, Washington 98501-1091 Nathan R Brennan Tennessee Cooperative Fishery-Research Unit Tennessee Technological University PO. Box 5144, Cookeville, Tennessee 38505 With world fisheries yields in steady decline (FAO, 1992, 1994; WRI, 1996), renewed interest in stock enhancement based on marine hatchery-releases is growing world- wide. This interest follows the dem- onstrated impact of stock enhance- ment in freshwater systems (e.g. Foerster, 1936; Solazzi et al., 1991) and is coupled with rapidly expand- ing marine aquaculture technology (Colura et al., 1976; Roberts et al., 1978; 0iestad et al., 1985; Lee and Tamaru, 1988; Eda et al., 1990; Fores et al., 1990; Tilseth and Blom, 1992; Honma, 1993; Main and Rosenfeld, 1994; Ostrowski et al., 1996). An experimental and careful ap- proach is needed to ensure that hatchery releases in marine sys- tems result, at best, in successful supplementation or replenishment of marine fish populations, or, at least, in a better understanding of system uncertainty (Peterman, 1991; Blankenship and Leber, 1995). This approach should involve an initial research phase with pilot releases to explore the effectiveness of release strategies. Before initiat- ing a test release to evaluate stock- enhancement potential in Hawaiian coastal environments, initial re- search was focused on a series of release experiments to determine which release strategies yielded greater survival of hatchery fish in the wild. This approach provided a more powerful field test of the ma- rine stock-enhancement concept by using prior knowledge about the effects of 1) fish size-at-release, 2) release habitat, and 3) release sea- son on growth and survival (Cowx, 1994; Blankenship and Leber, 1995; Leber et al., 1996). Evidence is mounting that release habitat, season, and size-at-release, can substantially affect success of marine hatchery releases (e.g. Tsukamoto et al., 1989; Svasand 268 Fishery Bulletin 95(2), 1997 and Kristiansen, 1990; Stoner, 1994; Leber, 1995; Willis et al., 1995). Pilot releases have shown that survival rates following hatchery releases of striped mullet, Mugil cephalus, in Hawaii (Leber and Arce, 1996; Leber et al.1) and of queen conch, Strombus gigas, in the Caribbean (Stoner, 1994) were strongly affected by release habitat. Pilot releases with M. cephalus have also shown differential survival based on size-at-release. Pilot releases conducted during summer and fall in Maunalua Bay, Hawaii, (south- ern exposure) and during summer in Kaneohe Bay (eastern, windward exposure) have shown poor sur- vival of cultured M. cephalus smaller than 70 mm total length (TL) at the time of release, compared with survival of larger-size individuals (e.g. 70 to 130 mm TL, Leber, 1995). In this study, we document a substantial effect of the seasonal timing of releases upon size-at-release-dependent recapture rates (number recaptured /number released) of cultured M. cephalus. Materials and methods Hatchery releases Striped mullet were spawned at The Oceanic Insti- tute in 1991 and reared to fingerling size. Batches of striped mullet eggs were hatched approximately ev- ery 5-6 weeks over a 5-month period and reared through three stages in cylindrical tanks. Larvae from each batch were hatched and cultured in 5,000- L conical-bottom tanks for 45 days. Stage- 1 juveniles (i.e. postlarvae 45 days old, 20 mm total length [TL]) were transferred to 8,000-L tanks and reared for 40 days to stage-2 juveniles (i.e. the age and size at which we typically transfer fish out of nursery tanks into larger growout tanks, 85 days old, around 40 mm TL). Stage-2 juveniles were transferred to 30,000-L tanks and reared to tagging size (45 to 130 mm TL). A factorial-design release-recapture experiment was performed to compare interactive effects of re- lease season and fish size-at-release upon growth and survival of about 90,000 cultured striped mullet in the wild. During the period 5 May through 17 May 1991, and again from 12 July through 26 July 1991, 1 Leber, K. M., D. A. Sterritt, R. N. Cantrell, and R. T. Nishimoto. In press. Contribution of hatchery-released striped mullet, Mugil cephalus , to the recreational fishery in Hilo Bay, Hawaii. In K. Lowe (ed.), Proceedings of the first biennial symposium for the Main Hawaiian Islands Marine Resources Investigation. Technical Rep. 96-01. Hawaii Depart- ment of Land and Natural Resources, Division of Aquatic Re- sources, Honolulu, HI. juvenile striped mullet, ranging in size from 45 to 130 mm TL, were harvested from culture tanks and transferred to 40,000-L holding tanks. These fish were graded into five size groups, tagged, then re- leased into Kaneohe Bay; half were released in May, the other half in July. To identify experimental treatment conditions, all released fish were tagged with binary coded- wire tags (Jefferts et al., 1963). Tags identified release season, release site, size-at-release (SAR), release lot (date), and number of fish per treatment condition. Fish were tagged in batches, with a different code for each season-site and SAR-lot combination (2x2x5x3=60 batch codes). The five size groups released were 45- 60 mm; 60-70 mm; 70-85 mm; 85-110 mm; and 110- 130 mm TL. Tags were implanted in the snout area with an automatic injector with head molds designed specifi- cally for striped mullet. Previous studies have shown a coded- wire tag retention rate of 97% for striped mullet over a 6-month period (Leber, 1995). To verify tag-retention rates in this study, at least 5% of the fish tagged for each release lot were randomly subsampled prior to each release. The subsamples were retained in tanks for up to 6 months to check tag retention. Subsampled fish were not released. Release statistics During May and July 1991, 90,817 juvenile striped mullet were tagged and released into Kaneohe Bay. Numbers of fish released varied among size groups but were held nearly constant among release lots and between release sites and seasons (Table 1). At least 7,500 tagged fish were released in each of 12 release lots. There was size variation in all batches of mul- let reared for this study. However, the primary dif- ference among size-at-release groups was fish age. For each season and SAR treatment combination, the experiment was replicated at two sites in Kaneohe Bay, and within each site, three replicate release lots were made (Table 1). The release lots were introduced into the bay over a 3-week period during both seasons (spring and summer). In each season, releases were made simultaneously at the inlets of two primary striped mullet nursery habi- tats, Kahaluu Stream and Kaneohe Stream. Kahaluu Stream is located in the north end of Kaneohe Bay (Fig. 1). This tributary is fed by several stream sys- tems that originate in the Ko’olau mountain range. Kahaluu Stream expands into a lagoon about 300 m upstream. The mouth of Kaneohe Stream is 11.6 km southeast of Kahaluu Stream. Kaneohe Stream is also a Ko’olau mountain drainage system. Selection of release habitats in the vicinity of fresh-water tribu- Leber et at.: Influence of release season on size-dependent survival of Mugil cephalus 269 Figure 1 Map of the study area in Kaneohe Bay. Releases were conducted near the mouths of Kahaluu Stream and Kaneohe Stream. Recapture collections were conducted in streams throughout the Bay and on reef flats in the vicinity of stream mouths. Table 1 Summary statistics for 90,817 striped mullet, Mugil cephalus, tagged and released in the 1991 pilot experiment to evaluate release-season effects on hatchery releases in Kaneohe Bay. Unique batch codes were used to identify fish from each cell in the matrix. Spring release lot 1 occurred on 3 May, lot 2 on 10 May, and lot 3 on 17 May. Summer release lot 1 occurred on 12 July, lot 2 on 19 July, and lot 3 on 26 July. Release season Release site Size at release Spring Total Summer Total Release lot Release lot 1 2 3 1 2 3 Kahaluu 45-60 mm 2,090 2,090 2,088 6,268 2,081 2,058 2,084 6,223 Stream 60-70 mm 2,090 2,089 2,087 6,266 2,082 2,090 2,085 6,257 70-85 mm 2,054 2,090 2,090 6,234 2,084 2,088 2,035 6,257 85-110 mm 1,119 959 990 3,068 1,128 956 1,296 3,380 110-130 mm 150 323 386 859 151 323 76 550 Subtotal 7,503 7,551 7,641 22,695 7,526 7,515 7,626 22,667 Kaneohe 45-60 mm 2,065 2,087 2,089 6,241 2,281 2,088 2,001 6,370 Stream 60-70 mm 2,070 2,089 2,088 6,247 2,044 2,086 2,086 6,216 70-85 mm 2,088 2,090 2,088 6,266 2,047 2,088 2,090 6,225 85-110 mm 1,127 958 990 3,075 1,152 959 1,298 3,409 110-130 mm 147 323 386 856 151 323 76 550 Subtotal 7,497 7,547 7,641 22,685 7,675 7,544 7,551 22,770 Grand total 15,000 15,098 15,282 45,380 15,201 15,059 15,177 45,437 270 Fishery Bulletin 95(2), 1 997 taries was based upon results from earlier releases (Leber, 1995; Leber and Arce, 1996; Leber et al.1) where release habitat appeared to be critical to survival. All releases were conducted at about noon or early afternoon. The successive weekly release lots spanned the rising tide (lot 1 on a low tide; lot 2 on a rising tide; lot 3 on a low tide in both seasons). Releases were made near the shoreline in water from 0.5 to 1.5 m deep. There was a wider range of salini- ties at the southernmost site (Kaneohe Stream; Table 2). Monitoring Beginning 21 May 1991, we monitored abundances of hatchery-released and wild Mugil cephalus in Kaneohe Bay monthly for 11 months by sampling with cast nets. Recaptured tagged fish were removed from collections and returned to the laboratory for tag analysis. The first field collection after spring and summer releases began 2 weeks after the middle release lot (lot 2) was planted. Each monthly collection was conducted over ap- proximately a 2-week period. Collections were made at six nursery sites (sampling stations) within Kaneohe Bay. Collections were made for about an 8- hr period during the day at each sampling station. Stations were established in the vicinity of docu- mented striped mullet nursery habitats at various tributaries located throughout the bay (Leber, 1995; six streams in Fig. 1: Waiahole, Kaalaea, Kahaluu, Heeia, Keaahala, and Kaneohe Streams). To standardize collection effort, at each station two substations were sampled — one substation was es- tablished upstream, the other near the mouth of the tributary. Within substations, 15 cast net throws were made. To broaden the range of microhabitats and fish size-ranges sampled, two sizes of cast nets were employed. Ten of the 15 casts per substation were made with a 5-m diameter, 10-mm mesh net, and 5 casts were made with a 3-m diameter, 6-mm mesh net. Thus, a total of 180 casts were made each month. Placement of net samples was stratified over ob- served schools of striped mullet juveniles. Completely random sampling in preliminary collections yielded few wild striped mullet and very few tagged individuals. Striped mullet schooled in fairly low densities within these clear-water nursery habitats, and our stratified- random collections targeted those schools. Neverthe- less, the sample data used to determine proportions of tagged versus untagged mullet were randomly distrib- uted because we had no a-priori indication that schools, once sighted, contained tagged individuals. All striped mullet sampled were measured and checked for tag presence with a field-sampling de- tector (Northwest Marine Technology, Inc., Shaw Is- land, WA). Tagged fish were placed on ice and re- turned to the laboratory where the tags were recov- ered, and each fish was weighed and measured. Untagged fish were held at the field site in oxygen- ated water and then released after the 30 cast-net samples were completed. Treatment identifications were made on the basis of the tags retrieved from recaptured fish. In the labora- Tafofe 2 Physical data recorded at the two release sites in Kaneohe Bay, Kahaluu Stream and Kaneohe Stream, for each release lot (release date) of striped mullet, Mugil cephalus. IN = incoming. Season Temperature (°C) Salinity ( %c ) and release site (stream) Release date Tide stage Secchi (cm) Depth (cm) Top Bottom Top Bottom Spring Kahaluu 5/03/91 IN 0.2' 51 59 33 32 11 12 Kaneohe 5/03/91 IN 0.5' 110 120 27 27 6 32 Kahaluu 5/10/91 IN 0.8' 70 75 29 26.5 15 27 Kaneohe 5/10/91 IN 1.6' 92 92 26 27 4 35 Kahaluu 5/17/91 IN 0.0' 25 40 29 29 24 26 Kaneohe 5/17/91 IN 0.0' 55 80 28 28.2 3 15 Summer Kahaluu 7/12/91 IN 0.8' 57 57 27.5 28 11 28 Kaneohe 7/12/91 IN 0.8' 75 122 27 27 11 35 Kahaluu 7/19/91 IN 1.6' 85 100 25.3 27 10 19 Kaneohe 7/19/91 IN 1.7' 115 115 26 27 4 35 Kahaluu 7/26/91 IN 0.7' 40 70 27.6 28 12 20 Kaneohe 7/26/91 IN 0.9' 65 90 26.2 26.5 6 34 Leber et al.: Influence of release season on size-dependent survival of Mugil cephalus 271 tory, tags were located and extracted with a field- sampling detector. Tags were decoded by using a bin- ocular microscope (at 40x). To verify tag codes, each tag was read twice (once each by two different re- search assistants). Data were analyzed with Systat (Wilkinson, 1990). A randomized-block factorial analysis of variance (ANOVA) was used to compare means. Systat Basic was used to write tag decoding algorithms. For each recaptured fish, the algorithms identified batch size, release date (lot), release site, size-at-release, and release season from the tag codes identified in the laboratory. An error-check algorithm was also writ- ten to help identify errors that may have been made in reading tag codes. Variance estimates are ex- pressed throughout as standard errors (with n=number of release lots). Results Tag retention Tag retention in 4,799 individuals subsampled and held in tanks for six months averaged 98.6% (0.4% SE). With one exception (92.4%), all retention rates within release lots exceeded 97%. No significant tag loss was observed in any group later than 1 month after tagging. This is a normal tag loss rate for coded- wire tags (Blankenship, 1990). Recapture summary Of the fish released, 2,511 cultured striped mullet were recaptured in monthly cast-net samples at nurs- ery habitats. Based on the 98.6% average tag reten- tion rate, the number of cultured fish recaptured can be extrapolated to 2,546, or 2.8% of the fish released. About 6.6% (166) of the tags taken from the 2,511 recaptured fish were lost during extraction. Total number of tagged fish in samples decreased over the 11 -month monitoring (Table 3) but was fairly constant during the last 7 months of the study (when numbers of tagged fish ranged from 49 to 134 indi- viduals). Total number of tagged fish collected was greater at Kaneohe Stream. However, this pattern varied considerably from month to month, and most of those fish were collected within 1 month after the May and July releases. Tagged fish represented between 8% and 48% of the striped mullet captured in monthly samples (from all stations combined; Table 3). Percentage of cul- tured fish in samples was greatest at Kaneohe Stream, where contribution rates declined from 76% following the May release to 41% by the end of the study. Although numbers of tagged fish collected at Kahaluu Stream were often similar to those for Kaneohe Stream, there were always greater numbers of wild fish in collections at Kahaluu Stream (Table 3). Impact of release season Recapture rates and contribution rates When size- at-release was not considered, the contribution of cultured fish to recruitment appeared to be unaf- fected by release season. Release season had no sig- nificant effect on mean recapture rates over time (ANOVA, P>0.54, data from all size-at-release inter- vals combined). After 3 months in the wild, mean numbers of cultured fish in samples varied between Table 3 Numbers of wild and hatchery-released striped mullet, Mugil cephalus, recovered in cast-net samples made in Kaneohe Bay. Proportions of hatchery fish were determined by the presence of a coded wire tag. 1991 1992 Standard Collection site Source May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Total Mean Error All stations in Wild 985 1,099 1,439 453 659 722 952 550 824 864 310 8,857 805.2 95.7 Kaneohe Bay Hatchery 912 194 551 184 101 117 134 76 116 77 49 2,511 228.3 79.9 % Hatchery 48.1 15.0 27.7 28.9 13.3 14.0 12.3 12.1 12.3 8.2 13.6 18.7 3.5 Kahaluu Stream Wild 265 201 303 137 260 318 331 158 350 184 134 2,641 240.1 24.3 (Release site in Hatchery 184 122 237 25 20 56 91 25 46 15 18 839 76.3 22.7 N. Kaneohe Bay) % Hatchery 41.0 37.8 43.9 15.4 7.1 15.0 21.6 13.7 11.6 7.5 11.8 20.6 4.1 Kaneohe Stream Wild 207 87 264 88 44 85 115 52 85 87 41 1,155 105.0 20.9 (Release site in Hatchery 653 64 270 135 43 52 35 45 55 45 28 1,425 129.5 56.5 S. Kaneohe Bay) % Hatchery 75.9 42.4 50.6 60.5 49.4 38.0 23.3 46.4 39.3 34.1 40.6 45.5 4.2 272 Fishery Bulletin 95(2), 1997 about 10 and 27 individuals per release lot through- out 36 weeks (Fig.2). However, as shown below, re- capture rate was in fact dependent upon the interac- tive effect of release season and size-at-release. (Note: for evaluating the effect of release-season, data can be compared only through weeks 35 or 36 following releases, the length of time fish were monitored af- ter summer releases; by the end of the study, fish released in the spring had been in the wild for an average of 45 weeks, 10 weeks longer than those re- leased in summer.) Dispersal patterns There were no clear seasonal trends in dispersal patterns. Cultured striped mul- let showed a strong tendency to remain in the vicin- ity of release sites, regardless of release season or size-at-release. Few of the 2,511 tagged fish recov- ered in samples had moved into other nursery habi- tats in the bay. The only significant movements ob- served were from release habitats into the streams located immediately to the north of each release site (Table 4). This pattern was repeated after spring and summer releases. There were isolated cases of fish moving from one release habitat to the other, as well as movement from release habitats into other nurs- ery habitats in the bay. But the magnitude of dis- persal out of release habitats and beyond the streams immediately north of those sites was negligible. Over- all, 90.8% ± 3.1% (SE) of the cultured fish collected through 36 weeks in the wild were recovered at the nursery habitats into which they had been released. Growth Growth after spring releases was similar to growth following summer releases. Length in- crease following releases is plotted in Figure 3 for fish from the 70-85 mm treatment group, which was representative of all 5 size-at-release groups. There was little change in mean length during winter months (from September 1991 through February 1992; weeks 20^45 following spring releases in Fig. 3). Release season effect on recapture frequencies among size-at-release groups Recapture frequen- cies ([number recaptured / number released] x 100% ) within size-at-release intervals revealed an obvious Figure 2 Mean number of tagged cultured fish in samples following spring and summer releases into Kaneohe Bay. Data are means per release lot (± standard error [SE]; n = 6 lots per season [3 at each release site]). Table 4 Movement patterns following 1991 releases in Kaneohe Bay. Release season and release site are identified for tagged fish recov- ered at the various collection (recapture) sites throughout the Bay. Recapture sites (and distances travelled) are ordered geo- graphically within collection dates, from the northernmost site (Waiahole Stream) to the southernmost site (Kaneohe Stream) at which tagged fish were collected (see Fig. 1). Totals for spring releases represent those through week 36; totals for summer releases are those through week 35. To compare results between release seasons over a similar time frame, data are excluded for weeks 40 and 45 after spring releases. Release season and recapture site Kahaluu Stream Kaneohe Stream Release season and recapture site Kahaluu Stream Kaneohe Stream n Distance (km) n Distance (km) n Distance (km) n Distance (km) Spring release Summer release Waiahole i 3.05 0 15.00 Waiahole 0 3.05 0 15.00 Kaalaea 92 0.98 0 12.59 Kaalaea 14 0.98 0 12.59 Kahaluu 509 0 1 12.04 Kahaluu 298 0 0 12.04 Heeia 0 5.55 1 5.88 Heeia 0 5.55 0 5.88 Keaahala 0 10.61 31 1.08 Keaahala 1 10.61 57 1.08 Kaneohe 1 11.58 947 0 Kaneohe 0 11.58 392 0 Total 603 980 Total 313 449 Leber et a I.: Influence of release season on size-dependent survival of Mugil cephalus 273 and direct relationship between size-at-release and recapture rate (Fig. 4) — when fish were released in summer, recapture frequency was almost directly 0 10 20 30 40 50 Weeks after release Figure 3 Mean total length (± SE) of cultured fish recaptured in collections made following spring and summer releases into Kaneohe Bay. Data are for the 70-85 mm size-at-release interval. Length was averaged within replicate release lots. Standard errors were based on replication established by release lots (n= 6 lots per season [3 at each release site], not total number of individuals recaptured). 2° Figure 4 Recapture frequencies of tagged cultured Mugil cephalus recaptured in cast-net samples after summer releases into Kaneohe Bay. Data are presented for each of the five size intervals released (size-at-release: 1=40-60 mm total length, 2=60-70 mm, 3=70-85 mm, 4=85-110 mm, and 5=110-130 mm). Data are given as percent recaptured fish of the total fish released per size-at-release interval. proportional to size-at-release within 1 month after release. This pattern was evident throughout the rest of the study. In contrast, size-at-release had much less effect on recapture frequencies for fish released 10 weeks earlier, in the spring (Fig. 5). Recapture frequencies of small tagged fish ( <70 mm TL) were clearly greater throughout collections made following spring releases than in those after summer releases. After 45 weeks in the wild, fish from the smallest size classes released in spring re- mained abundant in net samples. The relative im- pact derived from the smallest fish released in spring (45-60 mm) corresponded to impacts of some of the larger sizes released. In contrast, on the majority of collection dates following summer releases, not a single individual (released in summer) was collected from the 45-mm to 60-mm size-at-release group. After a few months in the wild, the larger fish released ( >85 mm) generally were more abundant in samples when they were liberated in summer rather than in spring. To compare recapture frequencies statistically among size-at-release intervals, values per release lot were summed across weeks for the period between 16 and 36 weeks after releases. After summer re- leases, mean recapture frequencies of fish <70 mm when released were substantially less than frequen- cies for fish > 85 mm when released (Fig. 6; ANOVA, P < 0.001 in a posteriori orthogonal contrasts [Sokal and Rohlf, 1981] of intervals 1 and 2 combined ver- sus intervals 4 and 5 combined). 274 Fishery Bulletin 95(2), I 997 However, with the data from spring releases, mean recapture frequency of the smallest fish released (45 to 60 mm) was statistically similar to frequencies of some of the larger fish released (70 to 85 mm and those >110 mm) (Fig. 7; P= 0.33). Fish from groups 2 and 4 (60 to 70 mm and 85 to 110 mm when released) had marginally greater recapture frequencies than those for small fish (P <0.03; spring releases). Fish from the two largest size intervals (fish >85 mm) released in summer exhibited mean recapture fre- quencies about twice as high as those for any size fish from spring releases (P<0.02). Interaction between size-at-release effects and re- lease season effects was statistically significant (P=0.01, season x size interaction term, Table 5). A significant interaction term indicates dependence of one factor upon the other; in this case, size-at-release affected recapture rate (P<0.001), but the degree of that effect depended upon release season. Size structures of released cultured Mugil cephalus and wild recruits A comparison of the sizes of fish in cast-net scamples revealed that similar-size indi- viduals schooled together. One month after spring releases, most of the smaller tagged striped mullet collected were schooling with relatively large num- Size-at-release interval Figure 6 Mean recapture frequencies (± SE, «= 6 lots) for the five sizes of fish released into Kaneohe Bay during summer releases (see Fig. 4 for description of fish size-at-release). Data are mean recapture frequencies per release lot ([num- ber recaptured/number released] x 100%) summed over collections made between 16 and 36 weeks after release. See Figure 4 for description of fish size-at-release codes. Letters above bars indicate results of multiple compari- sons of means; size-at-release intervals that share the same letter were not significantly different. bers of wild M. cephalus similar in size to the tagged individuals. However, the larger cultured fish re- leased found relatively few counterparts in size among wild individuals at that time (Fig. 8). The size structure of cultured fish released in spring was clearly out of phase with the wild recruitment pulse at that time. Whereas we had timed spring releases Size-at-release interval Figure 7 Recapture frequencies (± SE, n=6 lots) for the five sizes of fish released into Kaneohe Bay during spring releases (see Fig. 4 for description of fish size-at-release). Data are mean recapture frequencies per release lot ([number recaptured/ number released] x 100%) summed over collections made between 16 and 36 weeks after release. Letters above bars indicate results of multiple comparisons of means; size-at- release intervals that share the same letter were not sig- nificantly different. Table 5 ANOVA table (randomized-block design, lots=blocking vari- able) for evaluation of release season and size-at-release effects on recapture frequencies after 4 months in the wild. Data (means per release lot) were combined here over the 20-week period following 4 months in the wild (weeks 16 to 36). Recapture frequencies are percent of the total num- ber of fish released that were recovered during this pe- riod; these proportions were arc-sin square-root trans- formed prior to analysis. Source of variation Sum of squares df Mean square F-ratio P Release lot 0.009 2 0.004 4.309 0.030 Release season 0.000 1 0.000 0.021 0.886 Size-at-release 0.030 4 0.007 7.173 0.001 Season x size 0.019 4 0.005 4.577 0.010 Error 0.019 18 0.001 Leber et al.: Influence of release season on size-dependent survival of Mugil cephalus 275 to coincide with peak abundances of young-of-the- year recruits in these nursery habitats, the modal size of cultured fish led that of the wild recruitment pulse by around 30 mm at one month after spring releases. In contrast, the size structures of wild young-of-the-year and cultured fish were nearly iden- tical 1 month after summer releases (Fig. 9). Discussion Recapture rates and release impact Release impact on striped mullet abundance was comparable to contributions from experimental re- leases of cod in Norway (e.g. Kristiansen and Svasand, 1990; Nordeide et al., 1994), red drum in Florida (Willis et al., 1995), and to proportions of cultured flounder in commercial landings in Japan (Kitada et al., 1992). Cultured striped mullet amounted to no less than 7% of the fish in monthly samples throughout the 11-month study period at both release sites. By the end of this study, cultured fish represented about 12% of the striped mullet sampled at Kahaluu Stream, over 40% of those sampled in Kaneohe Stream, and 13.6% of the total collected in Kaneohe Bay. There was clearly an improvement in this study in recapture frequencies compared with initial re- leases into the Bay in 1990 (Leber, 1995). The im- provement was largely due to adjusting release strat- egy in this study to avoid releases outside of streams, the nursery habitats preferred by striped mullet. Recovery rates (number recaptured/number released) x 100%) of fish released at Kahaluu Stream in 1991 (this study) were similar to rates following a release of 10,000 fish at the same site in 1990; whereas, con- siderably fewer striped mullet were recovered follow- ing 1990 releases of 30,000 fish into more marine conditions near Coconut Island in the southern por- tion of the bay (authors’ unpubl. data for juveniles; and see Leber and Arce, 1996, for data on adults). Temporal changes in abundance of released fish Reduction in abundance of cultured fish over time at release sites was likely a result of 1) mortality, 2) emigration from nursery habitats into adjacent reef habitats in the bay, and 3 ) sampling bias as fish grew to larger sizes and moved out of shallow water. Mortal- ity appeared to be more important than emigration as the cause for reduction over time in recapture rates. Juvenile striped mullet have a relatively strong affinity for brackish water during the nursery stage of their life cycle (Major, 1978; Blaber, 1987). After earlier pilot releases, when cultured striped mullet were released into more marine conditions (surface salinities >25 ppt), they schooled in both directions along the shore and rapidly occupied nearly all striped mullet nursery habitats (streams and tribu- taries) in those bay systems (Leber, 1995). In con- trast, when striped mullet were released into habi- tats with lower surface salinities, as in this study, the majority of individuals recaptured were caught at or near the release site (Leber et al., 1995, 1996). Had emigration out of release habitats remained high in this study after fish had had time to accli- OJ CD a: Total length (mm) Figure 8 Size structures of wild and cultured Mugil cephalus col- lected in samples made about 1 month following spring releases into Kaneohe Bay. ai cr - Wild fish in August r n=453 ..all - 20 40 60 80 10C 120 140 160 180 200 220 240 260 Cultured fish in August i n=184 . ,_jiS illl __ 20 40 60 80 100 120 140 160 180 200 220 240 260 Total length (mm) Figure 9 Size structures of wild and cultured Mugil cephalus col- lected in samples made about 1 month following summer releases into Kaneohe Bay. 276 Fishery Bulletin 95(2), 1997 mate in the wild, then cultured individuals should have occupied several of the other tributaries sampled. However, few tagged fish were collected farther than 1 km away from either release site, and the difference in proportions of cultured fish retrieved outside of release sites, compared with proportions collected at release sites, did not increase over time. These data (Table 4) provide circumstantial evidence that, following 2 weeks of acclimation in the wild, cultured striped mullet then tended to stay at or near the stream they occupied for the duration of the study. Strong site fidelity (during the juvenile stage) has also been documented in marine nursery habitats following hatchery releases of lobster (Bannister and Howard, 1991; Latrouite and Lorec, 1991) and cod (e.g. Nordeide et al., 1994). Impact of release season Fish size-at-release is clearly an important media- tor of the effect of hatchery releases on stock abun- dance (Hager and Noble, 1976; Bilton et al., 1982; Tsukamoto et al., 1989; Liu, 1990; Svasand and Kristiansen, 1990; Ray et al., 1994; Leber, 1995; Wahl et al., 1995; Willis et al., 1995). At all of the release sites tested in Hawaii, size-at-release has been an important factor affecting recapture probability of cultured striped mullet (Leber, 1995; Leber et al.1). In previous studies with striped mullet, where re- leases were conducted in summer and fall, recapture rate was directly related to size of fish at the time of release. As expected (Leber, 1995), in this study recapture rates after summer releases of small fish (individu- als <60 mm long) approached zero and were an or- der of magnitude less than recapture rates of the larger fish released. Thus, when releases are made in summer in Kaneohe Bay, small (<60 mm) cultured striped mullet do not significantly affect juvenile re- cruitment in Kaneohe Bay. It is important to note that the fish in the different size intervals released were produced from multiple rearings and that the smallest fish released in summer were not merely the slowest growing individuals; rather, size-at-re- lease was related primarily to age. A new finding revealed by this study was that the seasonal timing of striped mullet releases can sub- stantially alter size-at-release effect on recapture rate. Compared with recapture rates after summer releases, recovery of the smallest individuals released was significantly greater when releases were timed to coincide with peak recruitment of small wild indi- viduals (in the spring). This was the first evidence that releases of relatively small (45 to 60 mm TL) individuals could make any lasting contribution to striped mullet abundances in nursery habitats on Oahu. Subsequently, Leber and Arce (1996) showed that some of the small fish released in spring did survive to adult size and contribute to the commer- cial fishery catch in Kaneohe Bay. The latter study also revealed that the smallest individuals in sum- mer releases from this study apparently suffered to- tal mortality. Because of the obvious economic im- portance of our findings, we replicated part of this study in a follow up study, with spring releases of the same size groups studied here; the results were identical — small fish (<60 mm) did contribute to ju- venile recruitment when releases were made in spring (Leber et al., 1996). It is not clear how one is to interpret the lack of a strong correlation between size-at-release and recap- ture rates following the spring releases. On the ba- sis of cast-net samples alone, we cannot rule out the possibility that a direct relation existed between size- at-release and survival after spring releases. Cast nets are biased in favor of collecting small individu- als (Leber et al.1). Thus, a weak size-at-release ef- fect following spring releases could be masked by sampling bias. Indeed, for fish from the spring re- leases, data from subsequent samples of adult cul- tured fish caught in the Kaneohe Bay mullet fishery revealed a (nonsignificant) trend towards a direct size-at-release effect (Leber and Arce, 1996). As in this study of juveniles, the data for adults revealed a highly significant effect of size-at-release on recov- ery rates when releases were made in summer. On the basis of this study and on subsequent data on adult recruitment to the commercial fishery (Leber and Arce, 1996), striped mullet < 60 mm should not be released during summer in Kaneohe Bay. How- ever, early (spring) releases of 45-60 mm striped mullet can make a contribution both to juvenile re- cruitment (this study) and to adult recruitment (Leber and Arce, 1996). Maximum recovery from summer releases will occur when individuals are >85 mm at the time of release. To determine optimal size- at-release, an economic analysis is needed to evalu- ate benefits and costs of releasing larger individuals. Bilton et al. ( 1982) showed an interaction between release timing and size of juvenile coho salmon, Oncorhynchus kisutch, released in British Columbia. In that study, returns would be maximized from early release of large juveniles. The effect of the seasonal timing of releases on size-dependent recapture rates may not be universal (e.g. Willis et al., 1995); never- theless, release season could be a key factor in suc- cessful enhancement of many marine species. What processes could account for the seasonal change in size-at-release dependent recapture rates? Size structures of cultured and wild fish suggest that Leber et al.: Influence of release season on size-dependent survival of Mugi! cephalus 277 schooling behavior of striped mullet may partly con- trol the release-season effect. Schools of juvenile striped mullet are usually aggregated according to size (Leber, 1995). Because of the difference in size structures between wild and cultured fish in the spring, schools of larger striped mullet, after spring releases, contained mostly cultured fish and few wild fish. We hypothesize that at the time of spring re- leases, the large individuals were more susceptible than smaller ones to mortality from predation. We reason that, because the smallest fish released in spring had merged with relatively large numbers of small wild striped mullet, the smallest fish should have been afforded greater refuge from predators than that provided the large fish in our spring re- leases, because there were more small wild fish than large ones (i.e. refuge effect from schooling behav- ior; e.g. Parrish, 1989, 1992; deVries, 1990; Ranta et al., 1994). This pattern was reversed following summer re- leases, when size structures of the larger cultured and wild individuals were equivalent. By summer, most wild juveniles had grown larger than the size range of the smallest cultured individuals released. Thus, few small wild juveniles were available to form schools with small cultured fish and thus the advan- tage of refuge that such schooling behavior would provide to small cultured fish was reduced. The results of this study are consistent with the hypothesis that size-selective predation is a primary mechanism controlling recapture rates following hatchery releases in Kaneohe Bay (Leber, 1995). Al- though, after summer releases, large wild fish were not as abundant as small wild fish in the spring (thus reducing the advantage gained by cultured fish from schooling with large wild fish), larger cultured fish would have the added advantage of size in escape from predators. Whatever the cause(s) of size-at-re- lease impact on recovery rates, it was clear from this study that release season can influence the underly- ing mechanism. Conclusions The importance of conducting test releases to evalu- ate release strategies prior to conducting full-scale hatchery releases cannot be overemphasized. This study documented that release season can have a significant effect upon recovery of cultured striped mullet in the wild by affecting size-at-release depen- dent recapture rates. To optimize the impact of full- scale releases, marine stock-enhancement programs should perform test releases to evaluate interaction of release season with size-at-release effects. We hypothesize that survival of cultured fish will be greater when releases are timed so that size-at- release coincides with modes in population size struc- tures of wild stocks. A corollary to this is that the fewer cultured fish there are in a particular size in- terval at the time of release, the lower survival will be for wild fish in that interval. These results need to be related to the hatchery costs of rearing fingerlings to various sizes and also to the increased production allowed by releasing small fingerlings in the spring, because spring re- leases would make nursery tanks or ponds available to grow more fish for summer releases. Although the mechanism underlying the direct relationship between survival and size-at-release is not well understood, it is clear that in Hawaii, fish size-at-release can determine release success follow- ing summer releases of striped mullet. Based on this study, critical release size (CSAR, the size-at-release below which probability of survival approaches zero; Leber, 1995) for enhancing striped mullet in Kaneohe Bay appears to be lower when releases are made in spring (CSAR <45 mm) than when releases are made in summer (CSAR <60 mm). Acknowledgments We thank Johann Bell, Laurie Peterson, and the anonymous reviewers for their comments on and improvements to the manuscript. Thanks to Dave Sterritt for help with graphics, data management, and field work. We thank Ryan Takushi and the Oce- anic Institute finfish program for their dedicated assistance with fish production for this study; Anton Morano and Dan Thompson for help with tagging, field collections, and tag decoding; and Joyce Gay for her assistance in preparing the tables. This paper was funded by a grant from the National Oceanic and Atmospheric Administration (NOAA). Literature cited Bannister, R. C. A., and A. E. Howard. 1991. A large-scale experiment to enhance a stock of lob- ster ( Homarus gammarus L.) on the English east coast. ICES Mar. Sci. Symp. 192:99-107 Bilton, H. T., D. F. Alderdice, and J. T. Schmute. 1982. Influence of time and size at release of juvenile coho salmon ( Oncorhynchus kisutch) on returns at maturity. Can. J. Fish. Aquat. Sci. 39:426-447. Blaber, S. J. M. 1987. F actors affecting recruitment and survival of mugilids in estuaries and coastal waters of Southeastern Africa. Am. Fish. Soc. 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Press, New York, NY, 400 p. 280 Abstract .—The catch equation used in virtual population analysis (VPA), and most annual age-structured meth- ods, assumes a constant fishing mor- tality rate (F) throughout the year even though many, if not most, fisheries are seasonal. Breaking this assumption of a constant F creates a bias in the re- sulting population-size estimates when the observed catch is used as input in VPA. The bias can be reduced by chang- ing the time step in the analysis to quarters or months, as has been sug- gested in the past, but this change is not always easy or practical. This pa- per presents an alternative method for reducing the bias: correction of the catch values to meet the assumption of a constant fishing mortality rate. A simple algorithm is presented that gives the number of fish t hat would have been caught from a given population if the observed fishing mortality rate had been spread evenly throughout the year. An iterative process improves the required guess for the population size such that the bias is eliminated. Manuscript accepted 12 September 1996. Fishery Bulletin 95:280-292 (1997). Correcting annual catches from seasonal fisheries for use in virtual population analysis Christopher M. Legault Nelson M. Ehrhardt Division of Marine Biology and Fisheries Rosenstiel School of Marine and Atmospheric Science University of Miami 4600 Rickenbacker Causeway, Miami, Florida 33149 E-mail address: clegault@rsmas.miami.edu Many, if not most, fisheries operate during only part of the year. The seasonal nature of fisheries is caused by quota and regulatory limitations, weather conditions, and fish availability among other rea- sons. The catch equation used in virtual population analysis (VPA) and in other annual age-structured analyses, assumes that a constant fishing mortality rate is applied con- tinuously throughout the year. Un- der this assumption, a bias will be introduced into the analysis when the fishery is in fact seasonal. With the catch equation, the total catch is assumed to be distributed through- out the year, such that it follows the exponential decline of the popula- tion. The resulting total mortality rate inferred from the decline of the population with the catch equation is different from what actually oc- curred in the population. For ex- ample, given a total catch of 5,000 fish distributed unevenly during the first half ofthe year (Fig. 1, top panel), the population numbers at the end of the year would be negatively biased with the assumption inherent in the catch equation (Fig. 1, lower panel). This bias has been examined in the past and found to be at a low level for most situations; exceptions occur for heavily exploited fisheries that occur during either the first or last quarter of the year. The fact that seasonal catches cause the es- timated exploitation rate to be bi- ased was described by Youngs (1976). The impact of seasonal catches on population-size esti- mates from cohort analysis was ex- plored by Ulltang (1977), who rec- ommended using smaller time units than a year to overcome the errors. Sims (1982) used both analytic methods and simulation to demon- strate the effects of seasonal catches on cohort analysis, concluding that the relative errors in population- size estimates are not severe unless the natural mortality rate is large or the fishery is heavily exploited, or both. The traditional recommen- dation for seasonal fisheries is to change the time scale from year to quarter or month so that the fish- ing mortality rate will be approxi- mately constant within the time unit. The conversion from annual to monthly or to some other time step is not always simple, either in the coding of programs or in the collec- tion of data. For example, the cre- ation of adequate age-length keys for ageing the catch under monthly or even quarterly time steps could require prohibitively expensive sampling schemes and would be technically challenging. A more recent approach to deal with the problem of seasonal fish- eries is the generalization of the equations used in virtual population analysis. An attempt to remove the Legault and Ehrhardt: Correcting annual catches from seasonal fisheries for use in virtual population analysis 281 2 4 6 8 10 12 Figure 1 Monthly catches from a total catch of 5,000 fish, given an initial population of 10,000, under a seasonal fishery and under a constant fishing mortality rate (top panel). The resulting population decline by month under the two conditions is shown in the lower panel. bias caused by seasonal fisheries was made by MacCall (1986) who provided a family of approxima- tions to virtual population analysis based on Pope’s ( 1972) cohort analysis. Hiramatsu ( 1995) generalized the equations to allow for a constant catch rate within a season, and Mertz and Myers (1996) reformulated the equations to allow for any seasonal pattern of catches. Most current software available for virtual population analysis and other age-structured analy- ses are designed for constant fishing mortality rates and annual time steps, however. Reformulating the basic equations used in virtual population analysis may not be practical for situations where a given al- gorithm is used that is already quite complex. An alternative to changing the time scale or modi- fying the equations for the analysis of a seasonal fish- 282 Fishery Bulletin 95(2), 1 997 ery is to correct the catch values to reflect the as- sumption of a constant fishing mortality rate (F). The catch matrix for VPA would no longer contain the observed numbers of fish caught, but rather the num- bers of fish that would have been caught under the assumption of an annual F. This paper presents a simple method for this conversion along with ex- amples of the reduction of bias due to the method and a discussion of further applications. The Fortran source code and the executable program for this cor- rection process are available from the authors. Methods The algorithm for correcting annual catches from seasonal fisheries to meet the assumption of a constant fishing mortality rate during the year is as follows: Let i = 1, 2, . . ., K index time intervals (not neces- sarily of equal length) during the year; A /. = the length of time in years for interval i; M = annual natural mortality rate; C( = observed catch in numbers during interval i; N' = population numbers at the start of inter- val i) F = fishing mortality rate during interval i\ and Fa = annual fishing mortality rate. For each year, age cell in the VPA catch matrix: 1 Assume a value for NK+V 2 For each time interval progressing backwards from if to 1. 2a Solve for F given C-, Ni+V M, and A ti from the catch equation: C. Nl+1eMAt'+F' Ft(l- e~MAI~F‘) MM, + F 2b Compute AT. given Ni+V M, AC and F from the exponential decline equation: Nt=Nl+1el 3 Compute FA that reduces N1 to NK+1 given M as Fa=-M- In Nk+i N, 4 Compute annual catch (CA) under FA, given Nx and M from catch equation: N1FA(l-e~M~F'A) A M + Fa CA is the corrected catch to be used in VPA for the given year and age. Once all years and ages in the catch matrix have been corrected, the population abundance matrix can be estimated through virtual population analysis with the corrected catch values in place of the observed catches. The resulting popu- lation abundances at the end of the year can be used as the assumed values for NK+1 in step 1 and the process repeated to generate a recorrected catch matrix. Note that the observed catches are still used in step 2a of the algorithm; it is only the NK+1 values that change from computing the corrected to com- puting the recorrected catch. The recorrected catch matrix can again be used in virtual population analy- sis to estimate the population abundance matrix, and this iterative procedure can be repeated until the corrected catches do not change value. This iterative process will produce population num- bers from virtual population analysis that are con- sistent with the assumption of a constant fishing mortality rate during the year. Each annual catch in the VPA matrix is treated individually for the cor- rection and then all the corrected catches used as input for VPA. The purpose for the iteration is to give a more solid basis for the choice of NK+] for each ob- served catch (step 1 in algorithm) because the cor- rected catch value depends on the choice ofiVA+1 (Fig. 2). Guessing too high a value for NK+l results in an underestimation of the corrected catch and vice versa, although, in general, the magnitude of bias is less for choosing NK+1 too large than too small. The tim- ing of the catch also impacts the amount of bias in the corrected catch; earlier catches are slightly less biased than later catches (Fig. 2). The high biases found with low guesses for NK+1 correspond to ex- tremely high values of the fishing mortality rate (Fig. 3, top panel) and are due to the catch removing a large portion of the population (>90%). When the catch is not removing such a large proportion of the population, a wide range of guesses for NK+1 will re- sult in similar corrected catches (Fig. 3, bottom panel). The direction of the change between observed and corrected catch depends more upon the time of the catch than the NK+1 though (Fig. 4). It should be noted that the apparent linear relationship between corrected catch and time of the catch shown in Fig- ure 4 is due to the values of NK+1 and M used in the example and will not always occur. The use of VPA results for values of NK] ensures a reasonable cor- rected catch value. Once a value of NK+1 is chosen for an age cell of a given year in the VPA catch matrix, either from a Legault and Ehrhardt: Correcting annual catches from seasonal fisheries for use in virtual population analysis 283 Assumed N(K+\) Figure 2 Percent bias in corrected catch due to incorrect choice of NK+l for 3 different cases of when the catch occurs. In this example, the true NK+1 is 10,000, the observed catch is 5,000, and M is 0.3 annually. guess or from results of VPA, the corrected catch can be computed from the observed catch, the time se- quence of the accumulation of this observed catch, and the natural mortality rate. Each cell in the catch matrix can have its own timing pattern. For example, if two gears operate in the fishery during different times of the year and target different-size fish, the timing of the catch will be different among ages. The observed catch for each time interval (C-) is used to solve for the fishing mortality rate during the inter- val (Fj ), given the population numbers at the start of the next interval (Af+1), the annual natural mortal- ity (M), and the length of the time interval (At-) (step 2a of the algorithm). The fishing mortality rate can- not be solved for directly in the equation and thus a search routine or iterative solution must be em- ployed. A simple bisection algorithm will suffice, al- though quicker methods are available (see e.g. Press et. al., 1989). Once the fishing mortality rate for the time interval (F ) is estimated, the population size at the start of the time interval (Af ) can be computed directly with the equation in step 2b of the algorithm. Thus the natural and fishing mortality rates are as- sumed constant during each time interval, and the year should be split into time intervals accordingly. In most cases, monthly time steps should be suffi- cient unless the fishing season is extremely short and intense or the natural mortality rate is extremely high (or both situations occur). The algorithm progresses backwards in time, from 31 December to 1 January, to minimize the propaga- tion of errors, in the same manner that virtual popu- lation analysis follows a cohort backwards in time (Pope, 1972). When all the time intervals are com- pleted, the corresponding annual fishing mortality rate (FA) can be computed from the equation given in step 3 of the algorithm. The population size at the start of time interval K+l is equivalent to the popu- 284 Fishery Bulletin 95(2), 1997 Assumed N(K+ 1) Figure 3 Annual fishing mortality rate (FA) and corrected catch for 3 different cases when the catch occurs under the same conditions as those in Figure 2. lation size for that cohort at the start of the next year and thus the annual F will reduce N1 to NK+V The annual F is then applied in the catch equation to generate the corrected catch (CA) (step 4 in the algorithm). The resulting catch is distributed throughout the year according to a constant F and thus reflects a smooth population abundance decline (see Figs. 5 and 6). In both Figures 5 and 6, the sum of the observed monthly catches are different from the sum of the corrected monthly catches, whereas the observed and corrected population numbers fol- low different paths to the same endpoint. Figures 1 and 5 have the same observed catch, but the popula- tion numbers (and resulting fishing mortality rates) under the assumption inherent in the catch equa- tion are different owing to the corrected catch value used in Figure 5. It is exactly the nonalignment of endpoints in Figure 1 that causes the bias in virtual Legault and Ehrhardf: Correcting annual catches from seasonal fisheries for use in virtual population analysis 285 6,000 — + + 5,500 — + xT + u-< — o + 6 a + o 5,000 — + s ln ' (J2j- ) + r2j In ( Jxj ) + (rK, + r3j .) In ( 1 - J u ) 7=1 +A +r3j)ln(l-J2j)-{rlj +r2j +r3j)\na-JljJ3jj\; We used a weighted linear regression approach, as suggested by Wetherall (1982) for multiple re- leases, for an exploratory analysis of the data. The results indicated that p; did not vary with tag type, but that L did. The regression approach assumed that the error terms were independent and normally distibuted. We believed that these assumptions may not be valid and that it would be more appropriate to use a maximum-likelihood procedure for the analy- sis. We also decided to assume that p is independent of tag type. Because the linear regression approach was used only for an exploratory analysis of the data, we neither describe it nor present the results from using it in this paper. We developed a new model and used maximum- likelihood principles to estimate the parameters, fol- lowing the suggestions of Wetherall (1982). We com- bined recoveries from the three release periods and estimated confidence bounds for the parameters ( p,Lv and L2 ) by bootstrapping ( Efron and Tibshirani, 1993 ). The probability that a tag of type i is shed by the jth recovery period is J,j = 1- pe L,tj . (2) Then the probability that a recovered tag-bearing fish has only tag type 1 during the jth recovery period is p _ j) 1J '-"'.A, where T = number of recovery periods; when i = 1 or 2, rtj = number of fish recovered with only a type i tag duringyth recovery period; and when i =3, r = number of fish recovered with both tags. We used the NLIN procedure (SAS Institute Inc., 1990) with the Gauss-Newton method, which re- quires derivatives of the log likelihood with respect to the parameters, to estimate the parameters of the model. The derivatives are 8J£ 8p XK /{P ~ eLlt‘ ) + r2j /(P - eL'tj ) + 7=1 A +r2j +2r3j)/p- (ri; + r2) +r3j)( 1/ p- 1 /(div)eL'+L2)tj)], jr = Xhi A /eL'tj ~ p)~{r^ + r37 Xj - ° ^i 7=1 (rt 7 + r2j + r3j)tj e~L'tj(pe ^ - 1)/ div J, T = S[rnA /(e^ - P] - (r2 7 “ r37)f7 - 2 7=1 A +r2 J +r3j'>tje L‘‘J(Pe L>‘J ~ Lenarz and Shaw: Estimates of tag loss from double-tagged Anoplopoma fimbria 295 Figure 1 Estimated distribution function of initial tag-retention rate, p , for sable- fish from a 2,000 replicate bootstrap. Intersections of the vertical lines with the distribution function mark the estimated 90% confidence band. 1 7 * L-it . Ijnpt . ( Li b Z/o ) t . where dw - e J + e J - pe . We employed Mathematica (Wolfram, 1991) as an aid in deriving the derivatives. We programmed a parametric bootstrap with 2,000 replicates in SAS to estimate confidence limits and bias. Since the bias estimates were very low, we used the uncorrected percentile method to estimate 90% confidence limits (Efron and Tibshirani, 1993). Results The SWFSC double tagged 229 fish during its egg- production survey cruise in early 1987. These fish were caught by bottom trawl and represented what was left over after needs for extensive biological samples were satisfied. The AFSC double tagged 10,316 fish during its sablefish abundance-indexing surveys in the fall of 1986, 1987, and 1988. The fish were caught by fish traps and represented a signifi- cant portion of the catches by the AFSC. There were five recoveries of trawl-caught fish and 1,552 recov- eries of trap-caught fish through the end of March 1995. Because there was an insufficient number of recoveries from trawl-caught fish to allow for exami- nation of recoveries by release gear types, we com- bined trawl and trap releases of tagged sablefish. We used recoveries of tag-bearing fish that were at lib- erty for no more than six years so that each release would have the same number of full years at liberty. Recoveries of tag-bearing fish were summarized by year of release and years at liberty (Table 1). Bootstrap estimates of the averages and medians of the parameters, p and L , were very close to the maximum-likelihood estimates, indicating that the estimation procedure was unbiased (Table 2). The bootstrap-estimated distribution functions indicated that the density functions were unimodal, smooth, and symmetrical (Figs. 1 and 2). The 90% confidence band for p does not overlap with 1 (Fig. 1), indicat- ing that although initial shedding is low, it is greater than 0. The 90% confidence bands for L1 and L9 do not overlap (Fig. 2), indicating that the instantaneous shedding rate is greater for posterior tags than for anterior tags. The model provided an excellent fit to the observed pattern of tag recoveries (Fig. 3). Discussion The double-tagging experiment with sablefish re- vealed that both immediate ( 1-p) and long-term in- stantaneous (L • ) tag loss rates were low and that long- term loss rates were higher for the posterior tagging position. The model fitted the recovery data very well, indicating that loss rates did not change with time at liberty during the first six years. Loss rates may have been higher for tags from the first release year because the ratio of single to double tag recoveries was higher than that during the other years (Table 1). Since tags and tagging procedures were identical in all three years, we assumed that any differences in loss rates were random. Fishermen may have occasionally reported only one tag from recaptures of fish bearing two tags (Laurs et al., 1976; Wetherall, 1982). A reward was given for each tag returned to encourage complete re- porting of tags, and single tags were checked to deter- mine if the other tag of the pair had been reported at 296 Fishery Bulletin 95(2), 1 997 L Figure 2 Estimated distribution functions of instantaneous tag-shedding rates, L;, of anterior and posterior tags for sablefish from a 2,000 replicate bootstrap. Intersections of the vertical lines with the distribution func- tions mark the estimated 90% confidence bands. Figure 3 Observed and expected double- and single-tagged (anterior and poste- rior tags separately) recoveries of sablefish. Expected values were cal- culated from maximum-likelihood parameter estimates. another time. Although we believe that most, if not all, reports of single tag recaptures were accurate, misreporting may have caused underestimation of p. Tag-loss rates in this study are similar to those of Beamish and McFarlane (1988) for sablefish. They used two types of tags (anchor and suture) and did not find a significant difference in the rate of loss by tag type. From a line fitted by eye through the data, they found a loss rate of approximately 10% during the first year and 2% per year thereafter. Examination of Figure 2 of their paper indicated that p was about 0.95. We present tag-loss rates from sablefish and other species in Table 3. Values were taken from the lit- erature and standardized, as much as was feasible within limitations, owing to the variety of models used and plethora of reporting styles. The median es- timate of L was 0.15, and the range was 0.00 to 3.93. Estimates of L for most species were higher than that for sablefish. The distribution ofL estimates had a rela- tively long upper tail. Only a few of the other studies provided estimates of p, and the estimates for sablefish were in the middle of the range of the other estimates. Although tag-shedding rates for sablefish were low, it still appears worthwhile to double tag. During the six-year recovery period, 128 sablefish were recov- ered with only a posterior tag. Thus, by double tag- Lenarz and Shaw: Estimates of tag loss from double-tagged Anoplopoma fimbria 297 Table 1 Double-tag releases and recoveries of sablefish, Anoplo- poma fimbria, during first six years at liberty. Number of releases are shown in parentheses. Recoveries Single tag Years at liberty (Midpoint) Both tags Anterior Posterior Total 1986 releases (2,652) 0.5 116 21 12 49 1.5 77 10 13 100 2.5 29 8 6 43 3.5 37 11 5 53 4.5 16 18 3 37 5.5 31 17 8 56 Total 306 85 47 438 1987 releases (1,872) 0.5 74 3 5 82 1.5 16 4 1 21 2.5 19 7 2 28 3.5 19 3 4 26 4.5 11 5 2 18 5.5 11 6 1 18 Total 150 28 15 193 1988 releases (6,021) 0.5 272 16 11 299 1.5 159 34 14 207 2.5 98 23 12 133 3.5 86 26 14 126 4.5 37 4 11 52 5.5 26 16 4 46 Total 678 119 66 863 Total releases (10,545) 0.5 462 40 28 530 1.5 252 48 28 328 2.5 146 38 20 204 3.5 142 40 23 205 4.5 64 27 16 107 5.5 68 39 13 120 Total 1,134 232 128 1,494 Table 2 Maximum-likelihood estimates of rates of immediate tag retention ( p ) and tag-shedding rates for anterior tags ( L p and posterior tags ( L 2)for sablefish. Also shown are esti- mates of the averages, medians, standard deviations, and ranges of the rates from 2,000 bootstrap replicates. Parameter P ^2 Maximum-likelihood estimate 0.9516 0.0304 0.0694 Bootstrap average 0.9517 0.0304 0.0693 Median 0.9519 0.0302 0.0694 Standard deviation 0.0098 0.0062 0.0075 Minimum 0.9176 0.0108 0.0457 Maximum 0.9855 0.0515 0.0968 ging is necessary to estimate tag-loss rates. Thus we recommend that double tagging be considered, when feasible, for at least a portion of any tagging study. The number of fish released in our study was not affected by double tagging. It is possible, however, that in some situations double tagging could increase the time required to process fish so as to decrease the number of fish released. The tradeoff between the po- tential reduction in number of fish released and the potential increase in number of fish recovered should be considered when designing a tagging program. In summary, analysis of returns from double-tag re- leases indicates that initial shedding of tags was 0.048. The long-term instantaneous rates of shedding were 0.030 and 0.069 for the anterior and posterior positions, respectively. Because there was a difference in the long- term instantaneous rates and because fish released with single tags are only tagged in the anterior posi- tion, corrections made for single-tagging experiments should be done only with the anterior tag loss rates. ging the fish, the total recoveries appeared to be in- creased by 9%. The cost of the double tagging was low compared to the cost that would have been in- curred by increasing time at sea by 9%. The parameter estimates of this study indicated that by the middle of the sixth recovery period, 19% of the anterior tags ( J 1>6) and 35% of the posterior tags ( J 2 6) had been shed, and 7% of the fish had lost both tags (( J 1>6) ( J 2 6)). Thus, even though shed- ding rates are low for sablefish, these rates are suffi- ciently high to affect analysis of tag-return data from this long-lived species. Tag-shedding rates were high enough in many of the reviewed studies to warrant incorporation of tag- loss rates in analysis of tag-return data. Double tag- Acknowledgments We owe considerable thanks to the many people who tagged and recovered sablefish. In particular, we are indebted to Norm Parks for the role he played in or- ganizing and implementing the trap surveys. We thank Steve Ralston for helping us with SAS and Mark Wilkins, Steve Ralston, and two anonymous reviewers for their constructive reviews of the paper. Literature cited Baglin, R. E., Jr., M. I. Farber, W. H. Lenarz, and J. M. Mason Jr. 1980. Shedding rates of plastic and metal darttags from 298 Fishery Bulletin 95(2), 1 997 Table 3 List of immediate ( 1- p ) and long-term instantaneous (L) tag-loss rates found in the literature. Some authors did not estimate p . Species (and tag type) Authors Immediate 1 ~P Annual L Plaice (silver wire) Gulland, 1963 0.162 Plaice (stainless steel) Gulland, 1963 0.025 Pacific yellowfm tuna Chapman et al., 1965 0.814 Pacific yellowfm tuna Bayliff and Mobrand, 1972 0.087 0.278 Southern bluefin tuna Hynd, 1969 0.26 Southern bluefin tuna Kirkwood, 1981 0.205 Southern bluefin tuna (60s and 70’s) Hampton and Kirkwood, 1990 0.173-0.301 Southern bluefin tuna (80s) Hampton and Kirkwood, 1990 0.056 Atlantic bluefin tuna Lenarz et al., 1973 0.027 0.310 Atlantic bluefin tuna Baglin et al., 1980 0.042 0.186 North Pacific albacore Laurs et al. 1976 0.12 0.086-0.098 Australian salmon Kirkwood and Walker, 1984 0.29 Stripey sea perch (dart) Whitelaw and Sainsbury, 1986 2.116 Stripey sea perch (anchor) Whitelaw and Sainsbury, 1986 0.415 Sablefish Beamish and McFarlane, 1988 0.05 0.020 Sablefish (anterior) This study 0.048 0.030 Sablefish (posterior) This study 0.048 0.069 Rig (anterior ) Francis, 1989 0.039 Rig (posterior ) Francis, 1989 0.013 Largemouth bass (anterior) Hightower and Gilbert, 1984 3.977 Largemouth bass (posterior) Hightower and Gilbert, 1984 1.370 Striped bass (anchor) Waldman et al., 1991 0.229 Striped bass (internal anchor) Waldman et al., 1991 0.004 White bass Muoneke, 1992 0 0.285 Lingcod Smith et al., 1990 0.137 Black rockfish Lai and Culver, 1991 0.131 Brown trout Faragher and Gordon, 1992 0.181 Rainbow trout Faragher and Gordon, 1992 0.201 Cutthroat trout (coded wire) Blankenship and Tipping, 1993 0.000 Cutthroat trout (visible impl) Blankenship and Tipping, 1993 0.035 Northern pike (anchor) Pierce and Tomcko, 1993 0.015 Northern pike (Dennison) Pierce and Tomcko, 1993 0.015 White sturgeon (anterior) Rien et al., 1991 0.041 White sturgeon (posterior) Rien et al., 1991 0.128 Channel catfish (spaghetti) Timmons and Howell, 1995 0.286 Channel catfish (anchor) Timmons and Howell, 1995 0.252 Blue catfish (spaghetti) Timmons and Howell, 1995 0.177 Blue catfish (anchor) Timmons and Howell, 1995 0.083 Smallmouth buffalo (spaghetti) Timmons and Howell, 1995 0.489 Smallmouth buffalo (anchor) Timmons and Howell, 1995 0.036 Bigmouth buffalo (spaghetti) Timmons and Howell, 1995 0.611 Bigmouth buffalo (anchor) Timmons and Howell, 1995 0.000 Paddlefish (spaghetti) Timmons and Howell, 1995 0.036 Paddlefish (anchor) Timmons and Howell, 1995 0.022 Atlantic bluefin tuna, Thunnus thynnus. 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Addison Wesley, Redwood City, CA, 961 p. 300 Aerial survey of giant bluefin tuna, Thunnus thynnus, in the Great Bahama Bank, Straits of Florida, 1 995 Molly Lutcavage* Scott Kraus Edgerton Research Laboratory New England Aquarium, Central Wharf, Boston, Massachusetts 02110 *E-mail address: mlutcavg@neaq.org. Wayne Hoggard Southeast Fisheries Science Center National Marine Fisheries Service, NOAA 3209 Frederic Street, Pascagoula, Mississippi 39568 Abstract .—Aerial surveys were con- ducted daily from 19 May to 9 June 1995 to document the apparent abun- dance and migration behavior of giant bluefin tuna, Thunnus thynnus , over the Great Bahama Bank region of the Straits of Florida. Our objectives were to conduct an aerial assessment of gi- ant bluefin tuna in this region and to compare our results with previous aerial surveys conducted in the 1950’s and 1970’s. Two professional bluefin spotter pilots flew 70-nmi transect sur- veys along “Tuna Alley” as well as sur- veys into adjacent areas in search of bluefin tuna. The present study area was broader than that surveyed in the 1970’s, which consisted of repeated flight tracks, each 1 nmi, across Tuna Alley at a point just south of South Cat Cay. Spotter aircraft carried a data ac- quisition system consisting of a global positioning system (GPS), a laptop com- puter, and a 35-mm camera to photo- graph schools. A total of 839 giant blue- fin tuna were documented, within range of totals counted in the 1974-76 surveys (368-3,125 bluefin tuna). Single fish and loosely aggregated schools of up to 100 fish were seen trav- elling steadily north along the western flank of the Great Bahama Bank. They did not engage in feeding, smashing, or cartwheeling behaviors that are exhib- ited in New England waters. All blue- fin tuna appeared to be “large giants,” weighing an estimated 227 kg and over. There is little information document- ing the origins and previous locations of giant bluefin tuna travelling along the Great Bahama Bank; therefore the use of direct counts of bluefin tuna in this region as an index of spawning biomass would require further documentation. Manuscript accepted 27 September 1996. Fishery Bulletin 95:300-310 ( 1997). In the 1950’s, and later in 1974-76, the U.S. National Marine Fisheries Service conducted aerial surveys for bluefin tuna, Thunnus thynnus, mi- grating along the Great Bahama Bank region (Rivas, 1954, 1978). It is generally believed that large blue- fin tuna travel along the Straits of Florida from late April through mid- June on their way to feeding grounds at higher latitudes where they are usually resident from June through October. The bluefin tuna found on the Great Bahama Bank are giants (over 185 cm/107 kg) and are be- lieved to have recently spawned in the Gulf of Mexico or in the Straits of Florida (Rivas, 1978; Mather et al., 1995). Sport fishermen since the 1930’s and researchers alike believe that these fish are members of the seasonal assemblage occurring off New England and maritime Canada (Farrington, 1939; Rivas, 1954; Mather et al., 1995). Fish tagged and released on the Great Bahama Bank have been recovered prima- rily in the northeastern U.S., Cana- dian, and Norwegian waters. Recreational fishermen and re- searchers have identified a narrow region of the Great Bahama Bank off South Cat Cay as Tuna Alley (Fig. 1) because travelling schools seem to concentrate in this region and are easily visible by air (Rivas, 1954, 1978; Anonymous1). In three surveys conducted from May through June 1974—76, survey aircraft flew a 1-mi long transect across Tuna Al- ley, at 25°31'N and 79°18'W, for about 60 minutes (Rivas, 1978). Flights were conducted on days when weather was suitable for fly- ing for a total transect effort rang- ing from 38 to 52 hours per survey period (Rivas, 1978). The number of bluefin tuna encountered was mul- tiplied by the number of minutes in a day to derive a daily abundance estimate. This estimate was then multiplied by the assumed 50-d migration interval to derive an es- timate of spawning population size. Over the three-year survey period this estimate ranged from 9,630 to 99,360 fish. Rivas ( 1978) linked the presence and apparent abundance of bluefin tuna in the area with envi- ronmental factors, such as increased wind speed and (less strongly) with 1 Anonymous. 1975. A study of the appli- cation of remote sensing techniques for detection and enumeration of giant blue- fin tuna. Southeast Fish. Sci. Center, Natl. Mar. Fish. Serv., NOAA, Miami FL. Contribution rep. 437, 48 p. Lutcavage et al.: Aerial survey of Thunnus thynnus in the Straits of Florida 301 Figure 1 View of the study region showing the location of “Tuna Alley” along the western margin of the Great Bahama Bank, Straits of Florida. wind direction, lunar phase, and tide. He tentatively concluded that the difference in magnitude of the an- nual population estimates might be attributed to dif- ferences in wind speed across Tuna Alley and, conse- quently, to changes in the visibility of bluefin tuna to aircraft and fishing vessels. The decline of North Atlantic bluefin tuna stocks since the 1970’s has heightened efforts to obtain more accurate indices of abundance, particularly for spawning biomass. Despite documented changes in bluefin tuna stocks and commercial fishing practices, there have been no aerial surveys or direct assess- ments of giant bluefin tuna transiting the Great Bahama Bank for over 20 years. From 19 May to 9 June 1995, we conducted an aerial survey of giant bluefin tuna transiting the Great Bahama Bank re- gion in the general vicinity of the Bimini islands and sand cays. Our objectives were to document their apparent abundance and behavior and to compare the results of the present study with those obtained 302 Fishery Bulletin 95(2), 1 997 in previous aerial surveys conducted by the National Marine Fisheries Service. Methods Bluefin tuna were sighted and counted by two tuna spotter pilots each having over 20 years of experi- ence in the commercial bluefin tuna, yellowfin, and tropical tunas purse-seine fisheries. It is standard practice for spotters to identify species and to esti- mate average size, weight, and total tonnage before a set is made. The two spotter pilots, having partici- pated in the 1994 New England bluefin tuna aerial survey (Lutcavage and Kraus, 1995), flew a single- engine aircraft (Supercub, tailnumber 344Z, and Cessna 172, tailnumber 270Q) that had viewing ac- cess from both sides. Flights originated from Execu- tive Airport, Fort Lauderdale, FL, and required ap- proximately a 40-55 min transit to reach the Great Bahama Bank area near Bimini. The two pilots be- gan spotting fish when they reached the Florida Straits. The survey was targeted to occur between 11:00-13:00 h, similar to the time of day covered by the 1974-76 surveys. The data acquisition system (Tunalog, Cascadia Research, Inc.) consisted of a glo- bal positioning system (GPS), a laptop computer with mouse (for event marking), and a 35-mm camera, identical to that used in the New England bluefin tuna spotter survey (Lutcavage and Kraus, 1995), to photograph schools. Position was automatically logged every 15 seconds, and daily flight tracts were reconstructed and bluefin tuna positions plotted with OPCPLOT, version 7.0. Each day the transect aircraft (Supercub 344Z, except on 28 May) surveyed a zigzag transect line of approximately 70 nmi in length along Tuna Alley, beginning at a southernmost point near 24°45'N and following a zigzag pattern north to approximately 25°48’N. The starting point was set far enough south to incorporate the southernmost limit of the pre- sumed migration route on the Great Bahama Bank where bluefin tuna are visible from the air (Rivas, 1954; Mather et al., 1995). On the first survey day (19 May) the transect aircraft 344Z carried an ob- server (Hoggard) to establish and verify survey pro- tocol. The starting point of the transect was stag- gered slightly so that daily transects were not iden- tical. Surveys were conducted at an altitude of 750- 1,000 feet and at a true airspeed of 80 knots. The transect legs forming the zigzag were flown to points approximately 3 nmi west of Tuna Alley and were bounded on the east by the shallows of the Great Bahama Bank. The transect was repeated unless rain squalls and strong winds greatly reduced visibility. The spotter conducting the transect noted any blue- fin tuna encountered during transit to the starting point. The “discovery” aircraft Cessna 270Q did not fly dedicated transects, (except on 28 May). Its mission was to search Tuna Alley and adjacent areas between N. Bimini and Orange Cay in order to identify the general limits of bluefin tuna travel patterns, and to locate, photograph, and observe the behavior of any bluefin tuna encountered. The spotter was free to determine his own search patterns and carried an observer on six survey days. There were two aircraft present in the study area on 14 out of 17 survey days. Pilots remained in radio contact with one another, except for the period of time when the Supercub 344Z was conducting the transect. At the beginning of each survey and at the end of each transect leg pilots re- corded their estimation of wind strength and direc- tion, visibility, cloud cover, and water color. During surveys they were instructed to mark the location of all sighted bluefin tuna with the mouse event marker and to document them with photographs when pos- sible. Radio contact with local sport fishing boats targeting bluefin tuna allowed the spotters and ob- server to collect general information on sea surface tem- perature, sizes of landed fish, and additional sightings. Results Spotters flew a total of 11,910 nmi ( 158 hr, including time in transit), encountering bluefin tuna on 10 out of 17 survey days. Approximately 7,126 nmi (115 h) were flown over the Great Bahama Bank. Of these, 1,502 nmi were trackline distance (usually 2 transects/day). Spotters documented 53 bluefin tuna schools overall and estimated a total count of 839 fish (Table 1). No bluefin tuna were sighted on any transits over the Florida Straits; turtles, sharks, delphinids, and flying fish, however, were sighted on numerous occasions. Most bluefin tuna sightings oc- curred north of 24°30'N, and within the presumed migratory route identified by Rivas (1954) and Mather et al. (1995). Other sightings on or adjacent to the Great Bahama Bank near Tuna Alley included loggerhead sea turtles ( Caretta caretta ), unidentified dolphins, tiger ( Galeocerdo cuvier) and other sharks, a single sperm whale ( Physeter macrocephalus), schools of skipjack tuna ( Katsuwonus pelamis ), Ber- muda chub ( Kyphosus sectatrix ), permit ( Trachinotus falcatus), and other unidentified fish. Sightings ranged from individual bluefin tuna to loosely aggregated schools from 20 to 100 individu- als, all judged by spotters to be large giants (> 226 kg, or about 196 cm), similar in size to giants landed Lutcavage et a I.: Aerial survey of Thunnus thynnus in the Straits of Florida 303 Table 1 Giant bluefin tuna aerial survey, 19 May-9 June 1995, over the Great Bahama Bank. Sea water temperatures provided by charter boats. Wind speed and direction estimated by pilots based on sea state. Sea water temp (°C ) Date Total no. of bluefin Total no. of sightings Winds (knots) North end South end 19 May 0 0 S 15-20 21 May 0 0 SSW 10-15 22 May 0 0 WNW <10 WNW 10-15 23 May 0 0 WNW 8 26.4 25 May 0 0 ENE 10-15 26 May 0 0 CALM 26.7 28 May 8 1 ENE 8 SSE 10-12 28.6 29 May 8 2 E 15 E 12-15 29.2 30 May 45 4 CALM ESE 8-10 28.9 31 May 75 9 CALM SE <10 28.9 1 June 181 3 E 10-12 ESE 15-20 2 June 125 9 ESE 20 SE 20-30 29 3 June 149 10 SSE 20 ESE 25-30 (squalls) 27.9 4 June 186 8 ESE 12-15 S/E 25+ (squalls) 28.5 7 June 59 5 WSW 15 W 5-8 29 8 June 1 1 NNW 8-10 WSW 8 (squalls) 9 June 1 1 CALM Totals 839 53 by anglers (Beare2). Bluefin tuna were first observed in Tuna Alley on 24 May from a sport fishing boat but were not observed from the air until 28 May. Sightings peaked in the first week of June (Table 1) and declined gradually to the last survey day (9 June) when only one giant was seen. According to interviews conducted with charter boat captains, aerial sightings were con- sistent with the timing and general location of bluefin tuna sightings by recreational vessels. However, aerial sightings were more extensive and covered a much broader area than that covered by charter boats, which tended to limit their fishing on Tuna Alley to a strip of approximately 12 nmi between Bimini and Victory Cay. The last bluefin tuna was sighted in Tuna Alley on 11 June, and all fishing ended by 12 June. Surface seawa- ter temperatures taken by charter boats during the survey ranged from 26 to 29°C, and the prevailing winds were primarily from the E/SE sectors (Table 1). A general account of sightings per unit of effort (SPUE) and search mileage is given in Table 2. Daily transects were conducted by the Supercub 344Z on all but one survey day (28 May), and the 4 June transect was abandoned because of squalls at the starting point (Table 3). Three out of 53 sightings (with counts of 100, 6, and 1 bluefin tuna, respec- tively) occurred on transect. 2 Beare, Captain D. 1995. 2462 Lighthouse Point, FL 33064. Personal, commun. Although our analyses of environmental conditions occurring during the survey are limited, some gen- eral conclusions can be drawn. Tropical storm Allison in the Gulf of Mexico generated strong winds and squalls that affected the survey region beginning on 1 June. General SPUE was highest from 1 to 4 June, associated with strongest winds, although fish were also seen on completely calm days with light and variable winds. Peak sightings occurred in the six days following the new moon on May 29. Although the majority of search effort occurred between 11:00 and 13:00 h, the largest school of an estimated 100 bluefin tuna was sighted on 1 June at 09:53 h. On this day pilots had an early start because wind con- ditions (ESE 15-20 kn) were expected to be espe- cially suitable for the appearance of bluefin tuna. According to interviews with charterboat captains, a total of no more than 10-20 bluefin tuna were sighted over the survey period on Tuna Alley by rec- reational vessels before survey aircraft had arrived in the morning at the study site. Discussion The total number of bluefin tuna seen in the 1995 survey (839) was generally within the range of blue- fin tuna counted in the 1974-76 surveys (368-3,125). Upon examination of school positions for possible re- 304 Fishery Bulletin 95(2), 1997 Table 2 A summary of sightings of giant bluefin tuna during an aerial survey, 19 May-9 June 1995 over the Great Bahama Bank. Aircraft T/D Date Start time Total time (h) Est. time on Banks Total nmi Nmi. on banks Number of sights No. of bluefin Sight, per 100 nmi Bluefin per 100 nmi 344Z rp* 19 May 9:14:0 4.9 3.4 340 230 0 0 0.00 0.00 344Z T 21 May 9:15: 0 3.7 2.2 249 139 0 0 0.00 0.0 270Q D 22 May 9:48: 0 6.2 4.7 441 331 0 0 0.00 0.0 344Z T 22 May 9:47:15 5.9 4.4 388 278 0 0 0.00 0.0 270Q D* 23 May 9:17:25 6.6 5.1 424 314 0 0 0.00 0.0 344Z T 23 May 9:18:15 6.5 5.0 440 330 0 0 0.00 0.0 270Q D 25 May 9:52:15 6.1 4.6 438 328 0 0 0.00 0.0 344Z T 25 May 9:42:30 6.4 4.9 406 296 0 0 0.00 0.0 270Q D 26 May 9:23:15 3.6 2.1 268 158 0 0 0.00 0.0 344Z T* 26 May 9:23:45 3.6 2.1 244 134 0 0 0.00 0.0 270Q T 28 May 9:46: 0 7.5 6.0 540 430 1 8 0.23 1.9 344Z T 29 May 9: 9:15 5.9 4.4 383 273 2 9 0.73 3.3 270Q D 30 May 9:21:45 5.7 4.2 364 254 3 37 1.18 14.6 344Z T 30 May 9:20: 0 5.7 4.2 372 262 1 8 0.38 3.1 270Q D 31 May 9:23:15 5.6 4.1 345 235 3 30 1.27 12.7 344Z rp* 31 May 9:17:15 5.7 4.2 374 264 6 45 2.27 17.0 270Q D 1 June 7:13: 0 5.9 4.4 348 238 2 81 0.84 34.1 344Z T 1 June 7:13:30 6.0 4.5 399 289 1 100 0.35 34.7 270Q D 2 June 8:51:15 5.5 4.0 316 206 6 93 2.91 45.2 344Z T 2 June 8:44:30 5.6 4.1 341 231 3 32 1.30 13.8 270Q D 3 June 9:11:30 5.1 3.6 297 187 4 84 2.14 44.9 344Z T 3 June 9: 5:15 5.2 3.7 298 188 6 65 3.19 34.6 270Q D* 4 June 9:54:15 3.2 1.7 206 96 4 90 4.18 94.0 344Z rp** 4 June 9:53:15 3.2 1.7 188 78 4 96 5.10 122.4 270Q D* 7 June 9: 8:15 6.5 5.0 464 354 2 29 0.57 8.2 344Z T 7 June 9: 2:15 7.1 5.6 445 335 3 30 0.90 9.0 270Q D* 8 June 9:34:30 5.9 4.4 425 315 1 1 0.32 0.3 344Z T 8 June 9:38:15 5.8 4.3 384 274 0 0 0.00 0.0 270Q D 9 June 9:54:15 4.0 2.5 268 158 1 1 0.63 0.6 344Z T 9 June 9:54:15 4.2 2.6 262 152 0 0 0.00 0.0 Totals 162.9 117.9 10,656 7,356 53 839 Abbreviations: nmi=nautical miles; T=transect aircraft; D=discovery aircraft; *=Observer present. Great Bahama Bank mileage was estimated as total flight miles minus 110 (distance over land/Straits of Florida). Estimated time on Bank is total time minus 1.5 h. Transect aircraft mileage includes survey miles spent off transect. **=Transect abandoned due to squalls. dundant counts, a school of 100 fish recorded by Supercub 344Z and one of 80 fish recorded by Cessna 270Q (Table 4) were judged to be the same, giving an adjusted estimated total count of 759 bluefin tuna. Bluefin tuna were most abundant adjacent to the region west of and between South Bimini and Castle Rock, with sighting concentrations near Victory and Gun Cays (Fig. 2A) similar to distributions described in the past by anglers (Farrington, 1939) and noted by Rivas (1954; 1978). The Cessna 270Q’s search area included broad search tracks extending to North Rock and west of Tuna Alley (Fig. 2B), but no bluefin tuna were sighted in these areas. In comparison with the 1970’s surveys, the two survey aircraft produced a 2-3 fold increase in effort hours but had only 33-45% of the number of observa- tion days in comparison with the 1974-76 surveys, which began almost three weeks earlier and ran 7-11 days later in June (Table 5). In the 1974 and 1975 sur- veys, no bluefin tuna were observed after 11 June and 2 June, respectively. Although it is possible that blue- fin tuna entered the region without being detected by recreational vessels, nevertheless, according to aerial and charter boat sightings, the 1995 migration period of about 20 days was considerably shorter than the pre- sumed 50 day migration period noted in the 1950’s and the 1970’s (Rivas, 1978). Although SPUE values are not strictly comparable in the present and 1974-76 sur- veys because of differences in survey protocols and plat- forms (Table 5), there are resemblances in the general appearance and behavior of giant bluefin tuna. In the present survey the majority of bluefin tuna (50 out of 53 sightings) were documented off transect; therefore the longitudinal transect along the Great Lutcavage et at: Aerial survey of Thunnus thynnus in the Straits of Florida 305 Table 3 Transect of the giant bluefin tuna aerial survey, 19 May-9 June 1995 over Great Bahama Bank. Date Start time End time Trackline nmi Sighting Bluefin tuna Sightings per 100 nmi Bluefin tuna per 100 nmi 19 May 10:51:0 11:50:0 s 0 0 0 0 19 May 12:35:0 13:15:0s 0 0 0 0 21 May' 11:07:30 12:09:15 72 0 0 0 0 22 May 11:28:0 12:42:45 80 0 0 0 0 22 May 13:33:0 14:50:45 75 0 0 0 0 23 May 10:57:15 12:11:0 77 0 0 0 0 25 May 11:44:0 12:46:0 71 0 0 0 0 25 May 13:40:15 14:43:15 70 0 0 0 0 26 May 11:06:30 12:05:45 71 0 0 0 0 28 May 11:46:45 12:47:29 80 0 0 0 0 29 May 11:06:45 12:15:14 74 1 1 1.3 1 29 May 13:04:29 04:15:00 74 1 1 1.3 1 30 May 11:06:30 12:11:15 71 0 0 0 0 30 May 13:05:0 14:10:30 71 0 0 0 0 31 May 11:11:30 12:10:45 69 1 1 1.46 1 31 May 13:07:15 14:07:15 69 0 0 0 0 1 June 09:06:15 10:06:0 68 1 100 1.47 147 1 June 10:59:45 11:58:15 66 0 0 0 0 2 June 11:13:0 12:08:30 68 0 0 0 0 3 June 11:24:45 12:22:45 69 0 0 0 0 4 June2 7 June 11:28:50 12:52:45 95 0 0 0 0 7 June 13:19:15 13:46:15 28 1 6 3.54 21 8 June 11:25:0 12:31:0 70 0 0 0 0 8 June 13:25:0 14:40:30 78 0 0 0 0 9 June 11:45:0 13:09:15 84 0 0 0 0 Totals 1,650 5 109 1 Trackline altered to avoid local storm squalls. 2 Transect abandoned because of squalls. All transects, except that flown on 28 May, were conducted by Supercub 344Z. 3 We experienced GPS problems on 19 May 1995; therefore times were estimated, not actual. Bahama Bank and Tuna Alley may be less effective than other survey methods. On days when fish were present on the banks, general sightings per search mile (i.e. schools or bluefin tuna per nmi of spotter pilot search effort) were within the same order of mag- nitude for the Cessna 270Q and the Supercub 344Z (which spent nearly half its search time off transect). In general, the schooling behavior of bluefin tuna travelling adjacent to the Great Bahama Bank dif- fered substantially from what we have observed in the Gulf of Maine aerial surveys (Lutcavage and Kraus, 1995). On the Great Bahama Bank, giant bluefin tuna were much less tightly aggregated and did not exhibit cartwheeling and milling formations or smashing behaviors that indicate feeding, al- though they are said to “smash” on rare occasion far- ther offshore (Mather et al., 1995). In contrast with prolonged surface “shows” and the appearance of densely packed schools in New England, the Great Bahama Bank schools spent very little time at the surface, making it difficult for pilots to photograph the school in entirety. As in previous surveys, schools were most readily detected and successfully photo- graphed while swimming over white sand in shal- low water. Photographs of schools in the deeper blue water usually depicted only a few fish visible at the surface. Because of the lack of color contrast between the tuna and the water and because of their deeper position in the water column, these schools were more difficult to detect and photograph, but experienced spotters use several cues including color contrast and surface disturbance to identify bluefin tuna. Singles and loosely aggregated groups swam steadily north at an estimated speed of 6-8 knots, similar to speeds reported by Mather et al. (1995), with the exception of one school, which we followed for 36 minutes in the air (Fig. 3). As two fishing boats approached from opposite sides, the school of ten fish changed spatial conformation several times, turned west, and disappeared into deeper water. 306 Fishery Bulletin 95(2), 1997 Figure 2 (A) Example of aircraft transit (Supercub 344Z, 25 May 1995) showing tracks across the Straits of Florida and two zigzag transects along Tuna Alley. Inset depicts the location of giant blue- fin tuna sightings. For reference, arrows denote the location of the 1 mi flight tract conducted in the 1974-76 surveys (Rivas, 1978) (B) Combined search tracks of the discovery aircraft, Cessna 270Q. In general, spotters estimated that all the bluefin tuna they had encountered were large giants rang- ing from 227 kg to over 295 kg (approximate length: 225-250 cm straight fork length [SFL]). Three fish landed by anglers during the survey period ranged from 264 to 280 cm SFL (250-317 kg), equivalent to Lutcavage et al.: Aerial survey of Thunnus thynnus in the Straits of Florida 307 Table 4 Giant bluefin tuna aerial survey, 19 May-9 June 1995, over the Great Bahama Bank. Sighting positions. Aircraft Date Time Latitude Longitude Count 270Q 28 May 16:10:30 N25:24.60 W079: 14.93 8 344Z 29 May 10:50:42 N25:03.50 W079:09.68 8 344Z 29 May 11:16:24 N24:52.97 W079:10.30 1 344Z 30 May 12:28:0 N25:24.73 W079:14.15 8 270Q 30 May 11:17:20 N25:25.47 W079:14.42 15 270Q 30 May 12:32:19 N25:26.04 W079:14.45 10 270Q 30 May 12:39:48 N25:25.87 W079:14.43 12 270Q 31 May 11:10:1 N25:36.93 W079:18.94 10 344Z 31 May 12:47:27 N25:05.97 W079:09.72 1 344Z 31 May 10:52:30 N25:05.76 W079:09.71 8 270Q 31 May 13:7:15 N25:27.35 W079:15.17 10 344Z 31 May 10:28:42 N25:29.94 W079:17.04 7 270Q 31 May 10:35:13 N25:33.75 W079: 18.40 10 344Z 31 May 10:17:24 N25:34.92 W079:19.50 25 344Z 31 May 11:20:0 N24:54.18 W079:11.33 1 270Q 1 June 08:30:29 N25:32.09 W079:18.06 1 344Z 1 June 09:52:42 N25:28.17 W079:15.65 100 270Q 1 June 09:53:40 N25:28.53 W079:15.94 80 270Q 2 June 10:34:41 N25:29.17 W079:16.36 7 270Q 2 June 12:39:10 N25:30.49 W079:17.37 7 270Q 2 June 12:2:38 N25:29.70 W079:17.02 27 270Q 2 June 11:14:47 N25:29.63 W079:16.73 17 270Q 2 June 13:5:52 N25:32.93 W079:18.48 27 344Z 2 June 12:23:29 N25:32.52 W079:18.22 12 270Q 2 June 12:15:29 N25:28.30 W079:16.24 8 344Z 2 June 13:14:06 N25:31.65 W79:18.18 10 344Z 2 June 10:26:12 N25:28.10 W079:15.54 10 344Z 3 June 12:33:00 N25:34.15 W079:19.16 12 344Z 3 June 12:56:00 N25:22.33 W079:12.31 14 344Z 3 June 12:43:15 N25:29.69 W079:17.21 3 344Z 3 June 10:54:59 N25:07.61 W079:10.14 6 344Z 3 June 11:15:47 N24:51.24 W079:10.59 5 344Z 3 June 10:37:44 N25:20.97 W079:12.35 25 270Q 3 June 12:55:51 N25:30.92 W079:17.93 5 270Q 3 June 12:26:57 N25:30.55 W079:17.38 35 270Q 3 June 11:59:11 N25:24.77 W079:14.62 17 270Q 3 June 13:10:5 N25:25.24 W079:14.60 27 344Z 4 June 12:09:59 N25:30.59 W079:17.58 15 270Q 4 June 12:1:59 N25:28.80 W079:16.36 35 270Q 4 June 11:35:26 N25:27.84 W079:15.54 13 270Q 4 June 11:24:0 N25:29.77 W079:17.07 35 344Z 4 June 11:18:0 N25:27.88 W079:15.51 25 344Z 4 June 11:14:45 N25:28.34 W079:16.13 30 270Q 4 June 12:7:30 N25:27.25 W079:14.91 7 344Z 4 June 11:48:28 N25:27.53 W079:15.31 26 270Q 7 June 11:2:6 N25:28.58 W079: 15.90 25 270Q 7 June 14:10:22 N25:30.77 W079:17.70 4 344Z 7 June 14:40:00 N25:27.00 W079: 14.92 12 344Z 7 June 11:08:30 N25:28.97 W079:16.35 12 344Z 7 June 13:28:29 N25:24.24 W079:14.37 6 270Q 8 June 14:40:00 N25:27.82 W079:15.29 1 270Q 9 June 12:08:14 N25:08.88 W079:11.53 1 308 Fishery Bulletin 95(2), 1 997 the highest range of values given in the length his- togram of fish captured previously in the Bahamas from 1939 to 1966 (Mather et al., 1995). Rivas (1976) noted that the mean length of Bahama bluefin tuna increased by 20-25 cm over a 20-yr period dating back to the 1950’s. Bluefin tuna documented in New England aerial surveys spanned a much broader range of size classes and include small medium ( 145- 178 cm SFL, 61<107 kg), large medium ( 178-196 cm, 107<141 kg), and a broader range within the giant bluefin tuna size class (>196 cm, >141 kg). As in previous Bahamas surveys, the majority of sightings occurred under conditions of strong winds, but 121 out of 839 (759 adjusted total) bluefin tuna were sighted under calm or variable wind conditions. Experienced tuna guides emphasized that bluefin tuna do not appear on the Bank until winds are of sufficient strength from the southern sector or when Table 5 Comparison of giant bluefin tuna aerial surveys conducted in the Straits of Florida and the Great Bahama Bank region. 19747 1975' 19767 1995 Survey dates 9 May-16 June 1 May-16 June 2 May-20 June 19 May-9 June Survey type 1-mi transect across Tuna Alley 1-mi transect across Tuna Alley 1-mi transect across Tuna Alley 70-nmi transect and discovery fits. Aircraft used not given not given not given 2, single engine Time of day (h) 11:00-13:00 12:00-14:00 09:30-14:00 11:00-13:00 Total observation days 37 46 42 17 Total survey hours 37.7 48.6 51.6 117.9 Date of first sighting 9 May 1 May 6 May 28 May Date of last sighting 11 June 2 June 15 June 9 June Total bluefin 3,125 368 1,120 839 1 1974-76 surveys are taken from Rivas, 1978. Lutcavage et al.: Aerial survey of Thunnus thynnus in the Straits of Florida 309 the Gulf Stream’s edge intercepts the Bank (or both), producing stronger northerly flow. Previous reports have also noted the bluefin tuna’s apparent avoid- ance of the “dirty water” tidal flow from the Bank, which varied a good deal over the survey period. However, on at least two occasions we observed blue- fin tuna in turbid water. In the present study, the period of highest sightings occurred in the six days following the new moon. Although aerial sightings were not given in relation to lunar phase for the 1974-76 surveys, this period coincided with the low- est catch per boat day for 11 Cat Cay bluefin tuna tournaments from 1941 to 1960 (Rivas, 1978). It is possible that the apparent relation of strong winds with appearance of bluefin tuna on Tuna Al- ley may be driven by oceanographic conditions oc- curring in adjacent staging areas. In general, flow over the Great Bahama Bank in the Bimini area is weak and driven by wind and tide (Lee3). Although the Bank constitutes a topographic wall, it is not associated with strong upwelling. Much stronger flow and upwelling occurs where the Loop Current leav- ing the Gulf of Mexico impinges on the north coast of Cuba. The dynamics of eddy systems near Cay Sal Bank and northern Cuba could conceivably influence travel routes of bluefin tuna, a concept that is rein- forced by the reports of giant bluefin tuna on Cay Sal Bank and the Old Bahama Channel by anglers and fish spotters (Rivas, 1954; 1978; Mather et al., 1995), and one that would explain the large variabil- ity in numbers of bluefin tuna sighted on Tuna Alley from year to year (e.g. an order of magnitude differ- ence in sightings between 1974 and 1975 (Rivas, 1978). During the survey, surface sea water temperatures in the Straits of Florida and adjacent to the Great Bahamas Bank, obtained from advanced high-reso- lution radiometer ( AVHRR) satellite imagery, ranged from 26° to 30°C, nearly 10°C higher than the mean sea surface temperature associated with bluefin tuna schools in the New England region (Lutcavage et al.4). However, our opportunity to examine additional en- vironmental conditions that might have influenced bluefin tuna occurrence on the Great Bahama Bank region in 1995 was limited. Sea surface temperatures across the Straits of Florida are somewhat uniform in the late spring and summer, and to our knowl- edge there were no current meters or buoy data re- 3 Lee, T. 1995. Rosenstiel School of Marine and Atmospheric Science, Univ. Miami, Miami, FL. Personal commun. 4 Lutcavage, M., J. Goldstein, and S. Kraus. 1996. Sustaining tuna fisheries — issues and answers. Proceedings of the 47th tuna conference; Lake Arrowhead, CA, 20-23 May 1996. fleeting the precise boundary of the Gulf Stream edge. This information, along with tide and wind stress records, might have provided a more specific rela- tion between environmental conditions and the ap- pearance of bluefin tuna on the Great Bahama Bank. There are numerous reports of giant bluefin tuna in other areas of the Bahamas and Straits of Florida beyond Tuna Alley, particularly to the east and north- east off Walkers Cay, the Abacos, and also in deep water regions west and southwest of the Great Bahama Banks and off Cuba (Rivas, 1978; Mather et al., 1995; Murray5). Recent longline captures also corroborate the presence of bluefin tuna in the east- ern areas, well before and concurrent with the as- sumed migration period of fish that transit Tuna Al- ley (Turner6). In addition, giant bluefin tuna were landed in the first week of June in the Gulf of Maine, nearly coincident with our first sightings on the Great Bahama Bank. At present there is little information that would identify whether fish travelling in this region are members of the same assemblage, and further, whether they had recently exited the Gulf of Mexico, or had travelled from areas to the south and east, or from the Windward Passage, as suggested by Mather et al. (1995). Mather et al. (1995) reported that the Great Bahama Bank migration area continues along the western edge of the Little Bahama Bank, and they sighted giant bluefin tuna travelling north between the Great and Little Bahama Banks in May-June 1968. It is clear that without complementary oceano- graphic surveys, the sporadic appearance and dif- fuse aggregation behavior of giant bluefin tuna on the Great Bahama Bank present serious problems for direct aerial assessment in this region. Although fish can be seen and enumerated under suitable con- ditions during daylight hours, there is no way of de- termining how many fish transit the Straits of Florida in deeper water, or determining their pres- ence and abundance in other regions of the Bahama islands. Lacking this information, the use of direct counts of bluefin tuna in this region as an index of spawning biomass seems unwarranted. Alternatively, aerial surveys on the Great Bahama Bank, in con- junction with direct sampling of landings, may pro- vide an index of regional abundance and informa- tion on the size classes and reproductive status of blue- fin tuna transiting the area. In the future, examina- tion of oceanographic conditions occurring in the Loop 5 Murray, Captain E. 1996. 8101 Nashua Dr., Palm Beach Gardens, FL 33418. Personal commun. 6 Turner, S. 1995. Southeast Fish. Sci. Center, Natl. Mar. Fish. Serv, NOAA, Miami, FL. Unpubl. data. 310 Fishery Bulletin 95(2), 1997 Current, Cay Sal Bank, and north Cuban coast might provide information that could be used to forecast the appearance and relative abundance of bluefin tuna across the western Bahama Banks and the Straits of Florida. A direct hydroacoustic count of all bluefin tuna transiting the Straits of Florida would provide additional information on the numbers of bluefin tuna exiting the Gulf of Mexico, and possi- bly, regions of the Caribbean. Acknowledgments Spotter pilots George Purmont (Little Compton, RI) and Dave Thompson (Chatsworth, NJ) provided their considerable skills and aircraft for this survey. We thank Gerry Scott, Steve Turner, and Rosemary Sullivant for logistical support, Dennis Lee for blue- fin tuna longline capture locations, and Captains Edward Murray, Danny Beare, and Richard Wright for helpful discussions on the Bahamas bluefin tuna sport fishery. Literature cited Farrington, S. K., Jr. 1939. Atlantic game fishing. Garden City Publ. Co., Inc. NewYork, NY, 298 p. Lutcavage, M., and S. Kraus. 1995. The feasibility of direct photographic assessment of giant bluefin tuna in New England waters. Fish. Bull. 93(3):495-503. Mather, F. J., Ill, J. M. Mason Jr., and A. C. Jones. 1995. Historical document: life history and fisheries of At- lantic bluefin tuna. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-SEFSC-370, 165 p. Rivas, L. R. 1954. Preliminary report on the spawning of the western North Atlantic bluefin tuna ( Thunnus thynnus) in the Straits of Florida. Bull. Mar. Sci. 4:302-321. 1976. Variation in sex ratio, size differences between sexes, and change in size and age composition in Western North Atlantic giant bluefin tuna ( Thunnus thynnus). Inter- national Commission for the Conservation of Atlantic Tunas, Madrid, Collective Volume of Scientific Papers 5(2):297-301. 1978. Aerial surveys leading to 1974-1976 estimates of the numbers of spawning giant bluefin tuna (Thunnus thynnus) migrating past the western Bahamas. Int. Comm. Conserv. Atl. Tunas Coll. Vol. Sci., paper 7, p. 301-312. Abstract.- Populations of Atlantic hagfish, Myxine glutinosa (L.), are found throughout the Gulf of Maine in soft-bottom substrates at depths greater than 50 m. This report presents data on the sizes, weights, morphometric characters, and reproductive states for specimens collected at a study site ap- proximately 50 km offshore in the Gulf of Maine. Limited comparisons with data from specimens collected else- where suggest that this data set is rep- resentative of hagfish populations within the inner Gulf of Maine. The small number of eggs produced (less than 30 per female), the large number of animals without macroscopically vis- ible gonadal tissue (25% of the popula- tion), and the small number of males ( <6% of the population ), gravid females (<1%), and postovulatory females (<5%) suggest that hagfish have limited re- productive potential. This raises serious questions about the long-term viability of the New England eelskin fishery. A population profile for Atlantic hagfish, Myxine glutinosa (L.), in the Gulf of Maine. Part I: Morphometries and reproductive state Frederic Martini* John B. Heiser** Michael R Lesser*** Shoals Marine Laboratory, G-14 Stimson Hall Cornell University, Ithaca, New York 14853 ‘Present address: 5071 Hana Hwy, Haiku, Hawaii 96708 E-mail address: martini@maui.net “Present address: Section of Ecology and Systematics Cornell University, Ithaca, New York 1 4853 “'Present address: University of New Hampshire Department of Zoology and Center for Marine Biology Durham, New Hampshire 03824 Manuscript accepted 12 November 1996. Fishery Bulletin 95:311-320 (1997). The hagfishes, or Myxinoidea, are worldwide in distribution, with 59 species recognized at present (Fern- holm1). Hagfishes are noteworthy from an evolutionary standpoint because they represent the oldest extant clade among the craniates. A better understanding of their ana- tomical and physiological charac- ters may thus reveal information about an early stage in vertebrate evolution. Although eel-like in general body form, hagfish lack jaws, paired fins, vertebrae, bone, and a variety of other gnathostome characteristics. All known species of hagfish live in close association with the bottom, resting on the substrate or occupy- ing burrows within soft sediments (Gustafson, 1935; Adam and Strahan, 1963, a and b; Foss, 1963; Fern- holm, 1974; Neira, 1982; Martin and Heiser, 1989; Cailliet et ah, 1992; Barss, 1993). They are gener- ally described as predators on inver- tebrates and as opportunistic scav- engers on both invertebrate and vertebrate remains. There are two major groups of living hagfishes united under the family Myxinidae: the Eptatretinae, typified by the genus Eptatretus (30-35 species), and the Myxininae, typified by the genus Myxine (19 species) but also including the genera Nemamyxine, Neomyxine, and Notomyxine (Nel- son, 1994). The characteristics of the Myxine appear to be more de- rived than those of the Eptatretus. For example, hagfishes of the genus Eptatretus have multiple efferent gill openings on each side of the pharynx, vestigial eyes beneath a pale skin patch, and traces of a cephalic lateral line complex. In contrast, hagfishes of the genus Myxine have a single common effer- ent duct opening on each side of the pharynx, even smaller eyes covered by undifferentiated integument, and no traces of any lateral line components. In general, the genus Eptatretus has a more widespread distribution than the genus Myxine , whose center of diversity appears to be the New World, where 14 of 19 1 Femholm, B. 1996. Box 50007, S-104 05, Stockholm, Sweden. Personal commun. 312 Fishery Bulletin 95(2), 1997 species have been identified (Wisner and McMillan, 1995). Only one myxinid, Myxine glutinosa L., is found on both sides of the Atlantic Ocean, and this is the only hagfish reported within the Gulf of Maine. There are several reasons why Myxine glutinosa is an im- portant species for the Gulf of Maine: 1 The substantial numbers present and their ongo- ing energetic requirements suggest that they play a significant role in the benthic ecosystem through- out the Gulf of Maine (Lesser et al., in press). 2 This species has both direct and indirect effects on commercial fisheries in the Gulf of Maine. In areas of abundance their opportunistic feeding habits can reduce the value of the catches made by longline or fixed gillnet fisheries. Hagfish have been known to feed on restrained or moribund cod, herring, haddock, hake, mackerel, spiny dogfish, and mackerel sharks caught in fisheries gear (Gustafson, 1934; Bigelow and Schroeder, 1953; Strahan, 1963). Equally important, feeding stud- ies by Shelton (1978) suggest that hagfish preda- tion could have a significant impact on Pandalus borealis populations within the Gulf of Maine. 3 Myxine glutinosa populations are now targeted by American and Canadian fishermen in the Gulf of Maine to meet the South Korean demand for “eelskin” used to manufacture expensive leather goods. In 1990, the sale of eelskin leather goods, all produced from hagfish skin, brought South Korea revenues of approximately US$100 million (Gorbman et al., 1990). The value of eelskin prod- ucts imported into the U.S. alone in 1992 was US$70 million (Melvin and Osborn2). So large is this market that Korean processors, unable to supply the demand from overexploited eastern Asian fisheries, have begun sampling and purchas- ing hagfish from several other regions, including North and South America (Gorbman et al., 1990). During 1993 and 1994, Gulf of Maine fishermen harvested roughly 1600 metric tons (3.6 million pounds) of hagfish, and there were unknown ef- fects on the ecology of the region (Kuenstner, 1996). Part 1 of this report presents morphological data and a population profile generated in a study of a hagfish population in the Gulf of Maine. Part 2 of this report, published separately, will relate these and other data to the proposal made by Wisner and McMillan ( 1995) to reserve M. glutinosa for the east- 2 Melvin, E. F., and S. A. Osborn. 1992. Development of the west coast fishery for Pacific hagfish. Seattle, WA. Natl. Mar. Fish. Serv., NOAA. Final Rep. NA90AA-H-SK142. ern Atlantic, and to give western Atlantic popula- tions, including those of the Gulf of Maine, separate status as Myxine limosa. Materials and methods The primary study site was adjacent to a small rock ledge known locally as “the Nipper” (near 42°57'N, 70°17'W). This site is within the Bigelow Bight, ap- proximately 25 km west of Jeffrey’s Ledge and 50 km east of the New Hampshire coast. The Bigelow Bight and Jeffrey’s Ledge are both important groundfishing areas. Hagfish in the study area in- habit a superficial zone of fine, organic sediment cov- ering a layer of grainy clay that overlies a thick layer of silty clay. Individual hagfish are usually found in shallow, sinusoidal, temporary burrows, with nose and barbels exposed to passing currents. The bot- tom temperature year-round is 4-6°C, and the sa- linity is 32 ppt or higher at all times. The superficial biotic community includes representatives from sev- eral families of tube worms, Cerianthid anemones, tunicates, sponges, and shrimp (Pandalus borealis). Comparable habitats that could support hagfish populations cover 60-70% of the floor of the Gulf of Maine (National Ocean Service3). Hagfish were collected with baited traps set on the bottom in depths of 130-150 m. The traps consisted of garbage cans with holes punched in the side and with an internal screen that tunneled hagfish toward the enclosed bait. The baited traps were left on the bottom for periods of 30 minutes to 1 hour and then retrieved. The animals were then placed in seawa- ter chilled to approximately 4°C for transport to the Shoals Marine Laboratory on Appledore Island, Maine. After being held in refrigerated aquaria for a period of hours to days, animals were sacrificed and measurements were taken. The aquarium complex was monitored daily, and animals dying in captivity were measured immediately, prior to disposal. Mor- phometric data were collected from fresh specimens from the primary site between June 1989 and Au- gust 1992. Methods of measuring and counting followed those of Fernholm and Hubbs (1981) and McMillan and Wisner (1984). All measurements were recorded in millimeters. For descriptive purposes, after total length (TL) was recorded, the body axis was divided into 3 regions (snout-pcd, trunk, and tail regions; n=143) or 4 regions (prebranehial, branchial, trunk, 3 National Ocean Service, Coast and Geodetic Survey. 1995. Gulf of Maine and George’s Bank, Chart No. 13009. Na- tional Ocean Service, Silver Springs, MD. Martini et at: Population profile of Gulf of Maine Myxine glutinosa 313 Table 1 Morphological measurements for the sample population ofAtantic hagfish, Myxine glutinosa. Character Mean SD Range %TL' SD Range n Total length (mm) 509 104 195-724 100 202 Snout-pcd2 135 27 54-200 27.0 1.6 24-37 143 Prebranchial 82 20 34-130 17.0 1.9 67 Branchial 46 14 16-75 9.3 2.1 5-21 67 Trunk 311 72 107-459 61.4 3.8 42-83 143 Tail 64 14 25-106 12.6 1.1 9-17 143 Width 14 7 4-35 2.7 1.1 2-6 91 Depth (trunk) 22 7 8-35 4.2 0.7 2-7 87 Depth (cloaca) 19 5 6-28 3.7 0.5 2-5 198 Depth (tail) 20 5 8-30 4.0 0.5 2-5 97 Weight (g) 136 67 8-290 80 Total cusps 35 2 28-40 97 Multicusps outer 2 0.3 1-3 97 inner 2 0.1 1-2 97 Unicusps outer 7 0.7 5-9 97 inner 7 0.7 6-9 97 Total slime pores (left side) 114 7 91-128 %TL3 SD Range 94 Snout-pcd 33 4 20-45 29 3.0 21-40 94 Trunk 67 4 51-77 59 2.9 51-69 94 Tail 13 2 8-19 11 1.3 9-15 94 1 %TL = Percentage of total length. 2 pcd = pharyngocutaneous duct. 3 % TP = Percentage of total slime pore counts. and tail regions; n- 67). The snout-pcd measurement extends from the tip of the snout to the anterior margin of the pharyngocutaneous duct (pcd), the trunk continues to the anterior margin of the cloaca, and the caudal region extends from that point to the tip of the tail. The sum of these measurements is equal to the total length. For one series of animals, prebranchial and bran- chial measurements were taken within the snout- pcd length. The prebranchial region extends from the tip of the snout caudally to the rostral margin of the first gill pouch, the branchial region extends from that point to the anterior margin of the pharyn- gocutaneous duct. Width is the maximum width of the trunk; depth (trunk) is the body height, exclusive of fin fold, at that site. Cloacal depth, measured at mid-cloaca, excludes the dorsal fin fold, whereas tail depth spans the entire tail, including dorsal and ventral fin folds. Cusp counts (unicusps and multicusps on outer and inner rows) were recorded for the left side; then the right side was counted to obtain the total cusp count. When slime pores were counted, the first two axial regions were combined and recorded as the snout- pcd count, which corresponds to the prebranchial count of Wisner and McMillan (1995). This distinc- tion was made to maintain consistency with the length data, where prebranchial and branchial lengths together constitute the snout-pcd length. Reproductive state was determined by visual inspec- tion, rather than histological analysis. If present, ova were measured and the maximum length recorded. Results Table 1 summarizes pertinent morphometric data for this population of Myxine glu tinosa. Hagfish species in general show remarkable variation in number of gill pouches. Table 2 presents data on the total num- ber of gill pouches in our sample population. The range of gill pouch data (range:10-14, n= 94) is greater than that reported for M. glutinosa in the eastern Atlantic by Fernholm and Hubbs (1981) (range: 11-13, n- 8). Despite the size of the landings (over 1,400 metric tons in 1994 [Kuenstner, 1996]), there are relatively few available data concerning the lengths and 314 Fishery Bulletin 95(2), 1 997 Table 2 Number of gill pouches in Myxine glutinosa (n= 94). No. of gill pouches (total) 10 11 12 13 14 Incidence (%) 1.1 1.1 74.5 7.4 15.9 weights of the harvested hagfish. This may in part reflect the effort required to immobilize and weigh individual hagfish. To address this problem we re- viewed the morphometric data for a relatively quick and reliable method of estimating sizes and weights in the field. The easiest and most accurate method found involved measuring the depth of the body at the cloaca, excluding the dorsal fin fold. The fin fold was excluded to make the measurement easier to perform at sea with unanesthetized animals. (A cali- per measurement of cloacal depth can be taken quickly, with minimal stress to the animal.) Figure 1 presents the relationship between total length and weight for a sample population of n- 83. Figure 2A is a length histogram for the entire sample population (n=306) which comprised 202 animals whose lengths were measured directly (see Table 1) and 104 lengths calculated on the basis of cloacal depth using the formula shown in Figure 2B. Figure 3A is a weight histogram for the entire sample population which comprised 80 direct mea- surements (see Table 1), 122 weights calculated on the basis of total length (see Fig. 1), and 104 weights calculated on the basis of cloacal depth with the for- mula shown in Figure 3B. Note the preponderance of adult specimens and the absence of juveniles smaller than 195 mm (7.6 in.) at this collection site. No smaller individuals have been seen with ROV’s or manned submersibles in this area, either on the soft bottom or over the asso- ciated rocky ledges (Martini and Heiser, 1989; 1991). Data on 1,172 animals from other locations in the Gulf of Maine (details below) indicate animals as small as 170 mm TL. However, the size of M. glutinosa at hatching has been estimated to be ap- proximately 50 mm (Fernholm, 1969), and there has long been a general consensus that hagfishes, includ- ing Myxine, do not have a larval stage (Putnam, 1874; Dean, 1900; Worthington, 1905; Walvig, 1963). The absence of animals of 50-170 mm TL from traps at widespread locations and in visual surveys of bait stations suggests that newly hatched M. glutinosa may target different feeding resources from those targeted by older animals. Juveniles may, for ex- ample, feed solely on invertebrates within the sub- strate. No data are available concerning the reproductive cycle and behavior of M. glutinosa. The sampled population contained a mixture of sexually imma- ture and sexually mature individuals (Fig. 4). The following patterns can be recognized: 1 Individuals shorter than 400 mm TL are sexually immature. These animals either lack macroscopi- cally visible gonads altogether or have granular tissue in the gonadal mesentery that cannot be identified as either testicular or ovarian in nature. 2 Approximately 59% of the population is classified as females on the basis of egg development. Testicu- lar tissue is usually rudimentary in these animals. Figure 1 Total length (mm) versus body weight (g) in a sample population of Atlantic hagfish, Myxine glutinosa (n=80). Martini et al.: Population profile of Gulf of Maine Myxine glutinosa 315 3 Males represent a very small percentage of the population (less than 6%). 4 Roughly 25% of the adult population does not have macroscopically identifiable gonadal tissue; the presence of large numbers of sterile individuals has also been reported for populations in the east- ern North Atlantic (Schreiner, 1955; Jespersen, 1975). 5 The overlap in sizes between males and females suggests neither protandry nor protogyny. Regression analyses were performed on morpho- logical data sets to detect significant trends. No re- lationships were found between total slime pores, snout-pcd slime pores, or tail slime pores versus to- tal length. This finding indicates that the number of slime pores is fixed for each individual and that ad- ditional slime pores are not added as growth occurs. However, with growth, the prebranchial region forms a significantly smaller percentage of the total length. The feeding apparatus, consisting of the tooth cusp plates and the dental muscle complex (Dawson, 1960), is therefore relatively large in smaller indi- viduals. No data are available concerning the life span or growth rates for this species. To determine whether or not our data were repre- sentative of the Gulf of Maine as a whole, we began by comparing the morphological data from our study site with data from eight specimens collected at Stellwagen Bank in Massachusetts Bay (42°20'N, 70°17'W), roughly 36 km from our primary study site. The size range (460-600 mm TL; average: 523 mm Cloacal depth (mm) Figure 2 Total length (TLKmm) in the sample population ofAtlantic hagfish. (A) A frequency histogram for total length (rc=306). This graph in- cludes direct measurements (n=202) and lengths calculated from the equation in part (B) (n=104). For these data: Mean = 511 mm, SD = 93 mm, range = 195-724 mm. (B) The relationship between cloacal depth and total length (n= 83). /-2=0.835, P=0.0001. 316 Fishery Bulletin 95(2), 1 997 Weight (g) ' 1 ' 1 > 1 ■ 1 i 1 1 1 1 1 T , 1 , 5 7.5 1 0 12.5 1 5 17.5 20 22.5 25 27 5 30 Cloacal depth (mm) Figure 3 Body weight (g) in the sample population of Atlantic hagfish. (A) A fre- quency histogram for body weight (rc=306). This graph includes direct measurements {n= 80), weights calculated from the equation in Figure 1 (n=22), and weights calculated from the equation in part (B) (rc=04). (B) The relationship between cloacal depth and body weight (n=53). r2 =0.864, P=0.0001 TL) was comparable to that found at the Nipper (range: 195-724 mm TL; average: 509 mm TL). Al- though the small size of the Stellwagen sample con- strains the power of statistical comparison, the only statistically significant differences found between these groups were that the maximum depth and width (in percent of body length) of animals from Stellwagen Bank were greater than those from the original study site. This may reflect differences in the substrate and food availability between the two locations, or it may be an artifact of the small sample size. We next compared our length distribution data with catch statistics collected between 19 May and 28 July 1994 by the New England Fisheries Devel- opment Association (Kuenstner, 1996). The close agreement between the data sets (Fig. 2A vs. Fig. 5) suggests that the sampling reported here is repre- sentative of hagfish populations throughout the Gulf of Maine. Discussion The gonads in hagfishes develop within a mesenterial fold located to the right of the dorsal mesentery that supports the gut. The anterior 2/3 of the gonad may develop into ovarian tissue, and the posterior 1/3 may develop into testicular tissue. Details of sexual dif- ferentiation are known for only a few species, nota- bly the Pacific hagfish, Eptatretus stouti. Gorbman (1990) reported that E. stouti are protogynous her- Martini et at.: Population profile of Gulf of Maine Myxine glutinosa 317 Figure 4 A scattergram of the reproductive state of the population of Atlantic hagfish as a function of total body length (n = 122). Key: -3 = male, swol- len testicular follicles; -2 = male, testicular follicles containing fluid; -1 = male, testicular tissue present; 0 = no macroscopically visible go- nadal tissue; 1 = female, eggs <5 mm; 2 = female, eggs 5-9 mm; 3 = female, eggs 10-14 mm; 4 = female, eggs 15-19 mm; 5 = female, eggs >20 mm; 6 = female, shelled eggs in coelom; and 7 = female, postovulatory follicles. 0 100 200 300 400 500 600 700 800 900 1000 Total length (mm) Figure 5 A length (total length) histogram for Atlantic hagfish, prepared from data provided by the New England Fisheries Development Association (n=l,172). For these data: Mean=529 mm, SD=104 mm, range=170- 950 mm. Compare with Figure 2A, P=0.000. maphrodites: sexually immature animals are found at some stage of female differ- entiation, and mature animals are usually differentiated as either males or females. Mature females are longer than 200 mm TL and males are longer than 280 mm TL. The largest animals are usually females. The incidence of hermaphroditism in ani- mals over 230 mm TL is very low (0.3% [Gorbman, 1990]), but there is evidence that this condition may persist through- out the life of the individual (Johnson, 1994). In our study of M. glutinosa, animals at any size above 400 mm TL, the minimum size at which gonadal tissues become mac- roscopically identifiable, may have no dis- cernible gonads or possess an immature ovary and immature testis, a mature ovary and immature testis, an immature ovary and a mature testis, or a mature ovary only. Animals with only mature testes were not seen, and only one animal was ob- served with what appeared to be a mature ovary and a mature testis. There was no apparent relation between total length and sex of the individual, nor between length and the lack of visible gonadal tissue. An incidence of sterility of 25% in animals over 400 mm TL is higher than the 13% incidence reported by Schreiner ( 1955) for mature eastern Atlantic M. glutinosa. The sex ratio of females to males in many Eptatretus species has been reported to be skewed, from slightly to strongly in favor of females. For example, Johnson (1994) reported a sex ratio for E. deani of 2.58:1 and for E. stouti a sex ratio that gradually decreased from 1.8:1 at small sizes to roughly 1:1 for animals near 380 mm TL. Because sizes and sexes are un- evenly distributed over the depth range where E. stouti is abundant (100-400 m), the sex ratio can vary widely depending on the depth of and season at the collec- tion site. This may explain the broad range of sex ratios (0.58:1 to 4.38:1) reported for E. stouti above 200 mm TL collected from a single area in British Columbia (Leaman4). The sex ratio of females to males in our sample of M. glutinosa was highly skewed, at 9.8:1. This highly 4 Leaman, B. M., ed. 1992. Groundfish stock assessments for the west coast of Canada in 1991 and recommended yield op- tions for 1992. Biological Sciences Branch, Dep. Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia. skewed sex ratio is typical for the species as a whole. The paucity of males in populations on both sides of the Atlantic has long been recognized, but it remains unexplained (Schreiner and Schreiner, 1904; Conel, 1931; Holmgren, 1946; Schreiner, 1955; Walvig, 1963; Cunningham, 1886-87). Males whose testes contain mature spermatozoa are even more unusual. Jespersen ( 1975) collected 1,000 specimens at a fjord 318 Fishery Bulletin 95(2), 1997 reputed to contain a relatively high proportion of males. Of 200 animals identified as male, only one contained a testis with motile sperm. Holmgren (1946) suggested that either ripe males may have a different distribution or that the ripe males do not feed. The latter suggestion appears more plausible in view of the broad areas sampled by investigators over the last 100 years. Among hagfish, only Eptatretus burgeri has been shown to have an annual breeding cycle (Fernholm, 1975; Patzner, 1977; Tsuneki et al., 1983). Our data, collected during the summer months (June-August), indicate that there is no correlation between the size of a female and the size of the eggs within the ovary. Thus at any given time, one can collect females with ova at any stage of maturation. This is consistent with the contention that M. glutinosa, like most other hagfishes studied, have no specific breeding season (Cunningham, 1886-87; Nansen, 1887; Walvig, 1963). The location of egg deposition also remains a mys- tery. Over the last 150 years, fewer than 200 eggs of My xine glutinosa have been recovered. Only 4 of these eggs were fertilized, and none of the embryos were in an ideal state of preservation when examined. A trawled and damaged embryo, described by Dean (1899), has been the only report of a fertilized hag- fish egg recovered in the western Atlantic. The great majority of the Myxine eggs — fertilized or not — de- scribed in the literature were collected in the east- ern Atlantic, primarily from the nets of trawlers working soft bottom substrates. Three embryos of M. glutinosa , in somewhat better condition than Dean’s specimen, served as the basis for papers by Holmgren (1946) and Fernholm (1969). Despite concerted ef- forts, no egg clusters were seen during winter and summer ROV surveys or summer submersible dives in an area supporting a large hagfish population, nor on the adjacent ledges (Martini and Heiser, 1989). It is not known where or when mating takes place, nor how males locate females (or vice versa). Al- though at least one species ( Eptatretus burgeri) has an annual reproductive season and migrates to re- productive sites that are used year after year (Tsuneki et al., 1983), such is not the case for M. glutinosa. The population at our primary study site appears to remain in place throughout the year; ROV work in June— Sep- tember and December-January did not reveal any ob- vious differences in abundance at our study site.5 How- ever, these observations need to be supported by addi- tional collections and tagging studies. 5 This conclusion is based on visual surveys only. Weather condi- tions and equipment problems made it impossible to collect specimens during the winter trips. Although our picture of reproduction in this spe- cies remains incomplete, it is clear that the popula- tion reproduces very slowly. Of 122 animals surveyed in the summers of 1989-90, only five were females with postovulatory follicles, and only one had fully developed shelled eggs loose in the coelom. In ani- mals with postovulatory follicles, the remaining ova- rian tissue did not contain eggs in advanced stages of development; there must therefore be a signifi- cant time period between reproductive cycles for a given individual. The time required for a female M. glutinosa to produce a clutch of eggs is not known, but it is probably longer than a year (Patzner and Adam, 1981). This makes good sense because the synthesis of large (25 mm x 10 mm) yolky eggs is a substantial energetic investment. It appears likely that the reproductive potential of the population as a whole is relatively low, because 1) many of the in- dividuals have no discernible gonads, 2) each ma- ture female produces only 20-30 eggs at a time, and 3) a relatively small proportion of the females con- tain mature eggs at any given time. This low re- productive potential has obvious implications for the development of a sustainable fishery for these animals. Given the evidence, there is considerable risk that the Gulf of Maine industry will prove to be another boom-and-bust fishery. The processors accept only fish greater than 350 mm in length (Kuenstner, 1996), which corresponds to a weight of more than 43 g (see Fig. 1). Because the average weight for Gulf of Maine specimens greater than 350 mm was 140 g, the fishing years of 1993-94 probably represented a harvest of roughly 11 million individual hagfish. The actual impact on the population is considerably greater, however, because 1) smaller hagfish are caught in the traps and are discarded into the sur- face waters and 2) hagfish of all sizes escape from the trap as it ascends. Except in winter, when the fishery is relatively inactive, hagfish released or es- caping in this manner are unlikely to survive. The oceanographic conditions where these hagfish are collected are extremely stable, with summer tem- peratures of 4-6°C and a salinity of 33-34 ppt. Hag- fish held in aquaria at the Shoals Marine Labora- tory survive at 0-4°C but become increasingly agi- tated and soon die if the temperature rises above 10°C. Surface temperatures in the inner Gulf of Maine reach 16-18°C or more in the top 25-50 m during July and August, and at least one other warm- water mass covers the cold bottom water ( Appollonio and Mann, 1995). When suddenly exposed to salini- ties below 31 ppt, individuals will struggle violently, produce copious slime, and then become moribund (Martini et al., pers. obs., and Adam and Strahan, 1963a). Surface salinities are often below 30 ppt in Martini et al.: Population profile of Gulf of Maine Myxine glutinosa 319 surface waters of the Gulf of Maine (Bigelow, 1914). This combination of factors suggests that hagfish released at the surface or escaping from a trap within superficial water layers are unlikely to reach the bottom alive.6 On some commercial hagfishing trips, up to 70% of the catch (by weight) was discarded as unmarket- able (Gryska7; the average for late 1995 was esti- mated at 41.1% (Kuenstner, 1996). The number of escaping animals cannot be estimated. It is there- fore possible that the number of individuals removed from the environment may be twice the number landed onshore. Although the hagfish population present in the Gulf of Maine as a whole might well support such a harvest for a time, this level of fish- ing pressure could not be sustained. Because the fish- ing effort is not randomly distributed throughout the Gulf of Maine, the populations at sites targeted by this fishery can be expected to decline much more precipitously. There are already anecdotal reports suggesting that after only two years the catch per trap set has declined, and the average size of caught hagfish is decreasing (Hall-Arber, 1996). It is not known what effects a decline in hagfish abundance will have on benthic ecology. However, from a regulatory perspective it is obviously difficult to set politically viable quotas or guidelines for a fish- ery when virtually nothing definitive is known about 1) the size of the population, 2) reproductive poten- tial, 3) individual growth rates, or 4) longevity. There is therefore an urgent need for increased research on the basic biology and ecology of this interesting species. Acknowledgments This study was funded in part by a grant from the National Oceanic and Atmospheric Administration’s National Undersea Research Center at the Univer- sity of Connecticutt, Avery Pt., CT. The authors would like to thank the crew and staff of the RV Seward Johnson and the Johnson SeaLink submersible for their assistance with this project. Time aboard the research vessel John M. Kingsbury and logistical support at sea and ashore were generously provided by the Shoals Marine Laboratory, Cornell Univer- sity, Ithaca, NY. The collection of animals at Stellwagen Bank was performed by Ed Lyman and 6 For release of live hagfish, the Shoals Marine Laboratory uses special gear that holds the animals in a volume of chilled, full- salinity sea water until the apparatus contacts the bottom. 7 Gryska, A. 1994. New England Fish. Development Assoc., 451 D St., Boston, MA. Personal obs. the staff and students on the research vessel West- ward, operated by the Sea Education Association of Woods Hole, MA. Alexander Gryska and Susan Kuenstner of the New England Fisheries Develop- ment Association provided statistics and other tech- nical information concerning the hagfish fishery, in- cluding the length data presented in Figure 5. Literature cited Adam, H., and Strahan, R. 1963a. Notes on the habitat, aquarium maintenance, and experimental use of hagfishes. In A. Brodal and R. Fange ( eds. ), The biology of Myxine, p. 33-41. Universitets- forlaget, Oslo. 1963b. Systematics and geographical distribution of Myxinoids. In A. Brodal and R. Fange (eds.), The biology of Myxine, p. 1-8. Universitetsforlaget, Oslo. Appollonio, S., and K. Mann 1995. From Cape Cod to the Bay of Fundy: an environmen- tal atlas of the Gulf of Maine, P. W. Conkling (ed.). Mass. Inst. Tech., Boston, MA, 83 p. Barss, W. H. 1993. Pacific hagfish, Eptatretus stouti, and black hagfish, E. deani : the Oregon fishery and port sampling observa- tions, 1988-1992. Mar. Fish. Rev. 55( 4): 19—30. Bigelow, H. 1914. Explorations in the Gulf of Maine, July and August 1912, by the U. S. Fisheries schooner Grampus', oceanog- raphy and notes on the plankton. Bull. Mus. Comp. Zool. 58:31-134. Bigelow, H. B., and W. C. Schroeder. 1953. Fishes of the Gulf of Maine. U.S. Gov. Printing Of- fice, Washington, D.C., 577 p. Cailliet, G. M., M. McNulty, and L. Lewis. 1992. Habitat analysis of Pacific hagfish ( Eptatretus stouti) in Monterey Bay, using the ROV Ventana. In proceedings of the western groundfish conference. Union, WA. Conel, J. 1931. The genital system of the Myxinoidea: a study based on notes and drawings of those organs in Bdellostoma made by Bashford Dean. In E. W. Gudger (ed.), Bashford Dean Memorial Vol.: archaic fishes, 64-101 p. Am. Mus. Nat. Hist., New York, NY. Cunningham, J. T. ( 1 886-87 ) . On the structure and development of the repro- ductive elements in Myxine glutinosa. Q. J. Microsc. Sci. 27: 49-76. Dawson, J. A. 1960. The oral cavity, the ‘jaws,’ and the horny teeth of Myxine glutinosa. In A. Brodal and R. Fange (eds.), The biology of Myxine, p. 231-255. Universitetsforlaget, Oslo. Dean, B. 1899. On the embryology of Bdellostoma stouti. In Fest. Schrift. fur Carl von Kupffer, p. 221-277. G. Fischer, Jena. 1900. The egg of the hag-fish, Myxine glutinosa. N.Y. Acad. Sci. 2:33-45. Fernholm, B. 1969. A third embryo of Myxine: considerations on hypo- physial ontogeny and phylogeny. Acta Zool. (Stockh.) 50:169-177. 1974. Diurnal variations in behavior of the hagfish, Epta- tretus burgeri. Mar. Biol. 27:351-356. 320 Fishery Bulletin 95(2), 1997 1975. Ovulation and eggs of the hagfish, Eptatretus burgeri. Acta Zool. (Stockh.) 56:199-204. Fernholm, B., and C. L. Hubbs. 1981. Western Atlantic hagfishes of the genus Eptatretus (Myxinidae) with description of two new species. Fish. Bull. 79:69-83. Foss, G. 1963. Myxine in its natural habitat. In A. Brodal and R. Fange (eds.). The biology of Myxine, p. 42-46. Universi- tetsforlaget, Oslo. Gorbman, A. 1990. Sex differentiation in the hagfish, Eptatretus stouti. Gen. Compar. Endocrinol. 77:309-323. Gorbman, A., H. Kobayashi, Y. Honma, and M. Matsuyama. 1990. The Hagfishery of Japan. Fisheries 15:12-18. Gustafson, G. 1935. On the biology of Myxine glutinosa L. Ark. Zool. 28:1-8. Hall-Arber, M. 1996. Workshop probes hagfish processing potential in US: fishery discards worry fishermen. Commercial Fisheries Arews.T8B-19B. Stonington, ME. Holmgren, N. 1946. On two embryos of Myxine glutinosa. Acta Zool. XXVII (Stockh. ):l-90. Jespersen, A. 1975. Fine structure of spermiogenesis in eastern Pacific species of hagfish (Myxinidae). Acta Zool. (Stockh.) 56:189-198. Johnson, E. W. 1994. Aspects of the biology of Pacific ( Eptatretus stouti) and Black (Eptatretus deani ) hagfishes from Monterey Bay, California. M.S. thesis, School of Natural Sciences, Cali- fornia State Univ., Fresno, CA, 130 p. Kuenstner, S. E. 1996. Harvesting the value-added potential of Atlantic hagfish. New England Fisheries Development Assoc., Boston, MA, 46 p. Lesser, M., F. H. Martini, and J. B. Heiser. In press. Ecology of hagfish, Myxine glutinosa, L., in the Gulf of Maine. I: Metabolic rates and energetics. J. Exp. Mar. Biol. Ecol. Martini, F., and J. B. Heiser. 1991. A population profile for a community of hagfish, Myxine glutinosa. Am. Zool. 31:35A. 1989. Field observations on the Atlantic hagfish, Myxine glutinosa, in the Gulf of Maine. Am. Zool. 29:38A. McMillan, C. B., and R. L. Wisner. 1984. Three new species of seven-gilled hagfishes (Myxinidae, Eptatretus) from the Pacific Ocean. Proc. Calif. Acad. Sci. 43:249-267. Nansen, F. 1887. A protandric hermaphrodite ( Myxine glutinosa, L.) amongst the Vertebrates. Bergen Mus. Aarsber. 7:1-34. Neira, F. J. C. 1982. Behavioral aspects of Polistotrema decatrema. Bre- nesia 19-20, 181-182. Nelson, J. S. 1994. Fishes of the world, 3rd ed. John Wiley and Sons, New York, NY, 688 p. Patzner, R. A. 1977. Cyclical changes in the testis of the hagfish Eptatretus burgeri (Cyclostomata). Acta Zool. (Stockh.) 58:223-226. Patzner, R. A., and H. Adam. (1981). Changes in the weight of the liver and the rela- tionship to reproduction in the hagfish Myxine glutinosa (Cyclostomata). J. Mar. Biol. Assoc. U. K. 61:461-464. Putnam, F. W. 1874. Notes on the genus Myxine. Proc. Boston Soc. Nat. Hist. 16:127-135. Schreiner, A., and K. A. Schreiner. 1904. Uber die Entwicklung der mannlichen Geschlechts- zellen von Myxine glutinosa (L.) Vermehrungsperiode, Reifungsperiode, und Reifungsteilungen. Arch. Biol. 21:183-355. Schreiner, K. E. 1955. Studies on the gonad of Myxine glutinosa L. Univ. Bergen, Bergen. Shelton, R. G. J. 1978. On the feeding of the hagfish Myxine glutinosa in the North Sea. J. Mar. Biol. Assoc. U. K. 58:81-86. Strahan, R. 1963. The behavior of Myxinoids. Acta Zoologica 44:1-30. Tsuneki, K., M. Ouji, and H. Saito. 1983. Seasonal migration and gonadal changes in the hag- fish, Eptatretus burgeri. Jpn. J. Ichthyol. 29:429-440. Walvig, F. 1963. Gonads and the formation of sexual cells. In A. Brodal and R. Fange (eds.), The biology of Myxine, p. 530- 580. Universitatsforlaget, Oslo. Wisner, R. L., and C. B. McMillan. 1995. Review of the new world hagfishes of the genus Myxine (Agnatha, Myxinidae) with descriptions of nine new species. Fish. Bull. 93:530-550. Worthington, J. 1905. Contribution to our knowledge of the Myxinoids. Am. Nat. 39:625-663. 321 Abstract .-^The reproductive biol- ogy and sexual maturity of Atka mack- erel ( Pleurogrammus monop terygius) in Alaskan waters were examined with data collected from commercial fishing vessels and National Marine Fisheries Service research surveys. The female reproductive system and ovarian devel- opment over time were described by using histological methods. The repro- ductive cycle is characterized by a pe- riod of slow development from J anuary until May, a rapid growth period of vi- tellogenesis in June, and a protracted spawning period, July until October, during which three batches of eggs are spawned on average. Length and age at maturity were cal- culated and compared for different sub- areas of the Aleutian Islands and Gulf of Alaska region. Size at 50% maturity was significantly different among the subareas, decreasing from east to west. Lengths at 50% maturity were 38.24, 35.91, 33.55, and 33.64 cm in the Gulf of Alaska, eastern Aleutian Islands, central Aleutian Islands, and western Aleutian Islands, respectively. Age at maturity was not significantly differ- ent by area; Atka mackerel were found to reach 50% maturity at 3.6 years. Therefore, it was assumed that differ- ent sizes at sexual maturity were re- flections of different growth rates in the respective geographic subareas. Manuscript accepted 24 September 1996. Fishery Bulletin 95:321-333 ( 1997). The reproductive cycle and sexual maturity of Atka mackerel, Pleurogrammus monopterygius, in Alaska Waters Susanne F. McDermott University of Washington Schooi of Fisheries Seattle, Washington 98115 E-mail address: smcdermo@fish.washington.edu Sandra A. Lowe National Marine Fisheries Service Alaska Fisheries Science Center 7600 Sand point Way, NE Seattle, Washington 981 15-0700 Atka mackerel, Pleurogrammus monopterygius, is a member of the greenling family (Hexagrammidae). It is distributed in Alaskan and Russian waters from the Gulf of Alaska to Kamchatka and is most abundant in the North Pacific Ocean, southern Bering Sea, and along the Aleutian Archipelago (Rutenberg, 1962). It has been of increasing commercial importance to the United States, with Alaskan catches averaging about 80,000 metric tons (t) in the last 3 years (valued at $14 million [ex-vessel] in 1993). Recent information suggests that Atka mackerel play an important role in the Aleutian Islands and Gulf of Alaska ecosystems as forage for other groundfish, seabirds, and marine mammals, including the Steller sea lion ( Eumetopias jubatus) which has been listed as a threat- ened species under the U.S. Endan- gered Species Act (Kajimura, 1984; Livingston et al., 1993; NMFS1). Despite the value of the species to commercial fisheries and other piscivores, many aspects of its life history and ecology are poorly un- derstood. Furthermore, information and data available suggest behav- iors and distribution patterns unique among Alaska groundfish. During much of the year, Atka mackerel are pelagic but migrate annually from the lower edge of the continental shelf to shallow coastal waters where they spawn demersally. In eastern Kamchatka waters, the spawning migration begins at the end of May and peaks in the middle of June (Zolotov, 1993). Spawning peaks June through September, but may occur intermittently through- out the year (Gorbunova, 1962; Zolotov, 1993). Atka mackerel spawn their eggs in rock crevices or among stones, which are guarded by brightly colored males until hatch- ing occurs (Gorbunova, 1962; Zolo- tov, 1993). Females are reported to spawn an average of three batches per season with at least a 2-week hiatus between subsequent spawn- ings (Zolotov, 1993). Batches of eggs in different phases of development were found inside one nest, suggest- ing a promiscuous mating system 1 NMFS. 1995. Status review of the U.S. Steller sea lion ( Eumetopias jubatus) population. Natl. Mar. Mamm. Labora- tory, Natl. Mar. Fish. Serv., NOAA, 7600 Sand Point Way NE, Seattle, WA 98115, 61 p. 322 Fishery Bulletin 95(2), 1997 with polygyny in the males and polyandry in the fe- males (Zolotov, 1993). The adhesive eggs hatch in 40- 45 days, releasing planktonic larvae that have been found up to 800 km from shore (Gorbunova,. 1962). Preliminary analyses of fishery and survey data sug- gest evidence of sex segregation during the spawn- ing period. Males presumably remained on the spawning grounds guarding the nests, whereas fe- males were found in exploitable concentrations far- ther offshore in high current areas such as island passes.2 The Atka mackerel resource in the Aleutian Islands appears to be in excess of 0.5 million t.3 Owing to a lack of a strong market for the product, and insuffi- cient biological information that prompted conser- vative catch recommendations, it was lightly ex- ploited through the 1980’s. Catch recommendations depend on an accurate knowledge of abundance which is based on the biology, distribution, and popu- lation dynamics of the species. The expansion of the fishery has greatly intensified the need for accurate estimates of life history parameters. However, to date most of the life history information available on Atka mackerel has been obtained in Russian waters (Gorbunova, 1962; Rutenberg, 1962; Zolotov, 1993); there is little or no information on the reproductive cycle, behavior, and ecology of Atka mackerel in U.S. waters. Because its distribution appears to be closely related to its reproductive life history, information on the reproductive cycle and spawning behavior of Atka mackerel off Alaska could lead to a better un- derstanding of its localized movement patterns. This information is necessary to improve surveys for bio- mass estimates which will result in more accurate stock assessments and provide better long-term man- agement of the fisheries. Of particular importance are parameters governing the reproductive potential of the stock, i.e. maturity at age, which is a direct input into the stock assessment model and is required to estimate female spawner biomass. This paper presents the results of a study that was undertaken to examine the reproductive biology of Atka mackerel. Female gonads and otoliths were collected, gonads examined histologically, egg stages and maturity stages defined, and the reproductive cycle was described. Ages were estimated from otoliths. The gonad somatic index (GSI) and the mean 2 Fritz, L. W. 1995. Alaska Fish. Sci. Center, Natl. Mar. Fish. Serv., Seattle, WA 98115. Personal commun. 3 Lowe, S. A., and L. W. Fritz. 1995. Atka Mackerel. In Stock Assessment and Fishery Evaluation Report for the Groundfish Resources of the Bering Sea/ Aleutian Island Regions as Pro- jected for 1996. North Pacific Management Council, P.O. Box 103136, Anchorage, AK 99510. egg stage per month were used as indicators of ova- rian development over time. Population parameters such as length and age at 50 % maturity were deter- mined and compared between different geographi- cal areas. Methods Few opportunities existed for the collection of bio- logical samples of Atka mackerel other than aboard commercial fishing boats or National Marine Fish- eries Service (NMFS) research surveys. Conse- quently, sample collection was restricted to periods when the fishery was open and when the NMFS sur- veys were conducted. Data and sample collection The data and samples analyzed in this study were collected from 1992 through 1994 in 1) the Gulf of Alaska and 2) the Aleutian Island Region by observ- ers and research scientists aboard commercial fish- ing vessels and NMFS research boats, respectively (Fig. 1). For purposes of collection and analysis, the study region was subdivided into four geographical subareas: western Aleutians, central Aleutians, east- ern Aleutians, and the Gulf of Alaska. The total number of gonad samples collected was 978. Monthly sample sizes ranged from a low of 30 in August to a high of 196 in June (Table 1 ). Otoliths were also collected from 537 of the sampled fish. Overall sampling effort by area was fairly even. How- ever, sampling effort in each area by month was strongly dependent on the location of the seasonal fishing effort in winter, spring, and fall. Winter samples were available only from the eastern Aleu- tians, whereas spring and summer sampling took place in all areas. The only fall samples taken were in October from the Gulf of Alaska. Samples collected on research cruises were taken from June through August throughout most of the areas. Since the sam- pling scheme for samples on commercial vessels did not differ from the sampling scheme on research boats, all data were combined. However, commercial catches were obtained by directly targeting certain locations or schools, whereas the survey catches were obtained by sampling at randomly stratified stations. Therefore the age and size composition of the com- mercial catch may reflect a more uniform popula- tion structure because most commercial boats tar- get schools of adult fish. There were insufficient samples to distinguish annual differences, therefore all samples were pooled by month and subarea for the determination of length McDermott and Lowe. Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 323 Haul locations for Atka mackerel ovary samples; 541 = eastern Aleutians, 542 = central Aleutians, 543 = western Aleutians, 610 and 620 = Gulf of Alaska. Table 1 Number of samples of Atka mackerel, Pleurogrammus monopterygius, collected from 1992 to 1994 by month and area. Month Eastern Aleutians Central Aleutians Western Aleutians Gulf of Alaska Total January 85 0 0 0 85 February 55 0 0 0 55 March 68 1 9 71 149 April 0 12 71 0 83 May 0 52 45 0 97 June 62 0 72 62 196 July 0 84 83 16 183 August 0 20 10 0 30 September 0 38 0 0 38 October 0 0 0 62 62 Total 270 207 290 211 978 and age at maturity and pooled by month only for the description of the reproductive cycle (Table 1). Atka mackerel were collected from subsamples of individual trawl tows. Collections were stratified by size of individual fish. No more than five fish per sex in each 1-cm size group were collected within each subarea during a sampling cruise. Each selected fish was measured to the nearest centimeter and weighed to the nearest 0.1 kg. In most cases, the stomach was emptied before weighing the individual fish. The ovaries were excised and placed in labeled cloth bags in a 10% buffered formalin solution. Sodium acetate (20 g per liter of formalin solution) was used as a buffer. Weights of fresh ovaries were taken and re- corded to the nearest gram for 254 specimens col- lected during the 1994 bottom trawl survey of the Aleutian Islands. In addition to the ovary samples, otoliths were collected opportunistically from sampled specimens. Ages were determined from otoliths by the Alaska Fisheries Science Center Age and Growth Unit using the surface-reading and break- and-burn technique (Chilton and Beamish, 1982). 324 Fishery Bulletin 95(2), 1997 Table 2 Definition of oocyte stages of Atka mackerel based on major histological characteristics. Oocyte stage Mean oocyte size (pm) (range) Major histological characteristics 1 Early perinucleus 55 (30-80) Small oocyte with hematoxylin-positive cytoplasm. Nucleoli on the outer margin of nucleus. 2 Late perinucleus 147 (117-176 ) Oocyte becoming larger, cytoplasm lighter, nucleoli still present. 3 Cortical alveoli 230 (216-255 ) Cortical alveoli present as a ring on the outer margin of the cytoplasm. Cortical alveoli appear as white droplets since they do not stain in H&E. The zona radiata can be seen developing as a thin, pink layer. Cytoplasm in center of oocyte appears granular. 4 Oil droplet stage 490 (313-628) Oil droplets appear first on inner margin of cytoplasm and then start to fill out the inner half of the oocyte. Zona radiata thickens, nucleoli are still present in nucleus, granulosa cells in tight circle around zona radiata. 5 Yolk globule stage 677 (549-843 ) Eosin-positive yolk droplets appear between the inner layer of oil vesicles and the outer layer of cortical alveoli, giving the oocyte a three- layered appearance. Vacuoles appear in oil droplet or cortical alveoli layer. Cytoplasm around nucleus granular, staining eosin-positive. With further development, yolk droplet zone and oil droplet zone may fuse together. Zona radiata thickens and oocyte increases in size. 6 Migratory nucleus 944 (686-1,294 ) Yolk platelets form by the fusion of smaller yolk droplets. Oil droplet and yolk platelet zone have fused, with cortical alveoli still on the margin of the oocyte. Nucleus in the center loses its shape (nuclear membrane gets dissolved) and migrates towards the micropyle. 7 Early hydration 1,277 (999-1529 ) Zona radiata thickens to almost twice the thickness characterizing migratory nucleus stage. Oocyte increases rapidly in size. Yolk fuses to uniform, pink mass (H&E stain) in center of oocyte, with still some large yolk platelets surrounding it. The margin of the cytoplasm does not stain, nucleus is no longer visible. 8 Late hydration 1,932 (1,646-2,195 ) Yolk fused to one mass in the center of oocyte, surrounded by nonstaining area. Oocyte still within follicle. 9 Ovulation Same as in stage 8 Oocyte same as in stage 8, but oocyte no longer inside follicle and usually found within lumen of ovary. Histological preparation After storage for several months in formalin, the ovary pairs were reweighed and sections from the middle of one ovary were taken and processed for histological examination. The tissue samples were embedded in Paraplast and sectioned with a micro- tome to a thickness of 5 pm. All samples were rou- tinely stained with hematoxylin and eosin (H&E). Selected samples were stained with Periodic Acid Schiff reagent (PAS) to identify carbohydrate com- plexes in cortical alveoli while other samples were sectioned frozen and stained with Sudan black in order to demonstrate the presence of oil droplets (Galigher and Kozloff, 1971). Oocytes in each ovary were subsequently classified into histological oocyte stages (Table 2). Postovulatory follicles and atretic oocytes were also recorded and classified according to the categories defined by Hunter and Macewicz (1985). Mean oocyte stage per month Each ovary was classified to the most advanced oo- cyte stage present using the histological criteria sum- marized in Table 2. Mean oocyte stage per month was determined by summing the individual speci- men’s oocyte stages (most advanced) by month and dividing the sum by the number of specimens col- lected in that month as follows: McDermott and Lowe: Reproductive cycle and sexual maturity of Pteurogrammus monopterygius 325 with geographical area as a factor. The variance for the estimated L50 or AgebQ was calculated using the delta method (Seber, 1982): where e. 'ij nj mean oocyte stage in month j\ the most advanced oocyte stage of speci- men i in month j; and number of specimens in month j. The estimated variance is2) of the mean egg stage was determined using the formula: Sef Z(e»-^)2 ni inl - 1) S ( Age50 , L50 ) — S2ia) 2 aS{a)S((3)r a2S2(f3 ) PA where S2 iL50] Age50) a P r Sid) Sip) variance of the length or age at 50% maturity; estimate of a; estimate of (3; correlation coefficient; standard error of a ; and standard error of /? . Calculating gonad somatic index Size measurement of oocytes Oocyte diameters were measured from histologically prepared ovary sections using a compound micro- scope with an ocular micrometer. Random measure- ments were taken by measuring oocytes along mul- tiple transect lines across the section. Only oocytes that touched the transect line and which had been sec- tioned through the nucleus were measured. For each oocyte stage a minimum of 15 oocytes per fish were measured from at least two individuals. Length and age at 50% maturity To minimize confusion between immature and rest- ing fish (mature females with oocytes smaller than oocyte stage 4; see Table 2), only samples in which the oocytes of mature fish were in advanced oocyte stages (oocyte stages 4-9, Table 2) were used for the calculation of length and age at maturity, except for some samples collected in the Gulf of Alaska as dis- cussed below. The proportion of fish mature at length or age was estimated by fitting a logistic model to the observed proportion mature. The logistic equation used was: y_ 1 l + e-(“+/5r) where Y = proportion mature at length or age x; a, p = model parameters to be estimated; and x = fork length (cm). Length or age at 50% maturity (L50; Age50) was cal- culated as -a/p. The statistical program used was S- plus (Venables and Ripley, 1994). A general linear model with a binomial error distribution was applied Relative reproductive effort was expressed as a go- nad somatic index (GSI), defined as the ratio of go- nad weight to somatic body weight. In all cases the gonads were weighed after they had been preserved in formalin. For the samples that did not have weights for fresh gonads, fresh gonad weight was estimated with a linear regression using the samples for which both fresh weight and formalin-preserved weight of the gonads had been measured (ft =254). The regression line was forced through the origin using: y = cx, where y = fresh weight of ovary; x = formalin preserved weight of ovary; and c - constant. The GSI was calculated as: GSI = — x 100, B where GSI = gonad somatic index; G = fresh gonad weight; and B = somatic body weight (stomach empty, gonads removed). Results Definition of oocyte stages and maturity stages Oocyte development was classified into nine oocyte stages based on major histological characteristics (Fig. 2, Table 2). The oocyte stages were then used to determine maturity stages. 326 Fishery Bulletin 95(2), 1 997 Figure 2 McDermott and Lowe: Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 327 Figure 2 Histological cross sections of Atka mackerel ovaries stained with hematoxylin and eosin stain, except E and F: (A) cross section of ovary with early perinucleus oocyte (egg stage 1) (x 200); (B) cross section of ovary with late perinucleus oocyte (egg stage 2) (x 79); (C) cross section of ovary with late perinucleus oocyte (egg stage 2) and cortical alveoli stage (egg stage 3) (x 79); (D) cross section of ovary with oil droplet oocyte (egg stage 4), both cortical alveoli and oil droplets appear as clear droplets (x 200); (E) cross section of ovary with oil droplet oocyte (egg stage 4), oil droplets are staining deep black, cortical alveoli are clear (Sudan black) (x 200); (F) cross section of ovary with oil droplet oocyte (egg stage 4), cortical alveoli are staining PAS positive, oil droplets appear clear (PAS) (x 200); (G) cross section of ovary with vitellogenic oocyte (egg stage 5), yolk is staining eosin-positive (x 79); (H) cross section of ovary with early migratory nucleus stage (egg stage 6), yolk droplets fuse to yolk platelets (x 79); (I) cross section of ovary with late migratory nucleus oocyte, nuclear wall is disintegrated (x 200); (J) cross section of ovary with early hydrated oocyte (egg stage 7) (x 79); (K) cross section of ovary with late hydrated oocyte (egg stage 8) (x 79); (L) cross section of ovary with atretic hydrated oocyte (x 79); (M) cross section through ovary showing alpha atresia in a yolked oocyte (x 79); (N) cross section through ovary with post-ovulatory follicle (x 79). Roman numberals I-VIII=oocyte stages 1-8. AO=atretic oocyte; CA=cortical alveoli; HAO=hydrated atretic oo- cyte; MN=migratory nucleus; OD=oil droplets; POF=postovulatory follicle; and YG=yolk globules. 328 Fishery Bulletin 95(2), 1997 In order to define maturity stages, the most ad- vanced oocyte stage in each specimen was used (Table 3). In most cases, oocytes in all stages up to the most advanced stage observed were present. For some of the spawning fish, however, stage 5 oocytes (vitel- logenic) were absent. Since Atka mackerel are batch spawners (Zolotov, 1993), the number of advanced oocytes (egg stage 5 and larger) decreased with the number of batches spawned. For the Aleutian Islands region, the ovaries of the mature females were far enough advanced to distin- guish mature from immature fish by the presence of advanced oocyte stages (stages 5-9). Because of the timing of the collection of samples for the Gulf of Alaska, some of the maturity classification was done by comparing GSI values. Certain Gulf of Alaska samples showed a GSI that was almost an order of magnitude smaller than the GSI of the mature fish even though the oocytes appeared to be in a similar oocyte stage (stage 4, cortical alveoli and oil globules present). Because the GSI value was not continuous but showed a distinctive gap and because there was no evidence of yolk in the presumably immature ova- ries, fish that belonged in the group with the lower GSI value were classified as immature. This GSI value coincided with the GSI value of the immature fish in the Aleutian Island region, and the age at maturity calculated also coincided with the age at maturity determined for the samples in the Aleutian Island region. However, until year-round samples for the Gulf of Alaska can be obtained, the possibility of the presumably immature fish spawning later in the year cannot be excluded. Reproductive cycle Since data were not available throughout the year in all of the areas, the data were pooled and com- pared by month only. Mean oocyte stage did not in- crease substantially from January until June, when most females possessed ovaries with stage 5 oocytes (vitellogenesis) (Fig. 3). Mean oocyte stage started to increase rapidly in June, peaked in August, and declined slightly in September. The mean GSI value Table 3 Definition of maturity stages of Atka mackerel. Maturity stage Description Most advanced oocyte stages Stage 1: Immature Ovary small with small oocytes. Oogonial nests, early and late perinucleus stages and cortical alveoli stage present. In some ovaries early oil droplet stage present. Oocyte stages 1-3; early oocyte stage 4 Stage 2: Developing Ovary increasing in size. Oocytes show oil droplets in advanced stage. Ovary wall thickens. Vascularization increases. Oocyte stage 4 Stage 3: Vitellogenesis Large visible eggs undergoing yolk development. Yolk globules present in oocytes. Wide range in oocyte diameter since oocytes from stage 1 through stage 5 are present. Oocyte stage 5 Stage 4: Early hydration Most advanced yolked oocytes are in migratory-nucleus and early hydration stage. Yolk is not completely coalesced in hydrated oocytes. Oocytes are present in stages 1-7 Oocyte stages 6 and 7 Stage 5: Spawning Large oocytes visible at 2 mm. In advanced oocytes, yolk is completely coalesced. Number of yolked oocytes decreases as multiple batches are spawned. After first spawning, postovulatory follicles (POF) are present. In ovaries of fish that have spawned more than one batch, different stages of POF are distinguishable. In some cases proportion of vitellogenic oocytes decreases with the increase of hydrated oocytes. Ovaries are highly vascularized. Ovulated oocytes are found free-flowing in the center of the ovary. Oocyte stages 8 and 9 Stage 6: Spent Ovary appears flaccid and highly vascularized. Ovary shows abundance of late POF, and atretic hydrated oocytes. Healthy oocytes are all in early developing stage. Oocyte stage 3 and 4, presence of post- ovulatory follicles and atretic hydrated oocytes. Atresia of oocyte stages 5-8. McDermott and Lowe: Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 329 reflected a similar pattern (Fig. 3), although a slight increase was noted from February through June, re- flecting slow growth of oocytes during that time. The rapid increase from June through August suggests a rapid period of oocyte growth during vitellogenesis and hydration. The decrease in GSI after August reflects the loss of ovary weight due to spawning single batches, but it should be noted that the GSI in October is still higher than the GSI value in June, which suggests, that the fish might be still spawning their last batches. High variance in GSI and mean egg stage during the spawning period (July though October) could be attrib- uted to batch spawning since most females were not spawning synchronously and the ovary weight and oo- cyte stages differed accordingly. Examination of maturity stages over time indi- cated the same cycle with a long period of initial oo- cyte development, the appearance of vitellogenesis in June, and peak spawning in August (Fig. 4). Vi- tellogenesis progressed rather rapidly and was ob- served almost exclusively in June. However, vitellogenic eggs were found throughout most of the early hydration stage and during the spawning in some ovaries. It should be mentioned that in one cruise a few spawning fish were found in the central Aleutians in March and April 1992. While this was an oddity not observed in other years, it suggests that under certain circumstances fish can spawn as early (or late) as March. In general, Atka mackerel develop their oocytes slowly in the oil droplet phase from at least January until May, with a gradual increase in oocyte size and ovary weight. Vitellogenesis starts in June and early migratory nucleus and early hydration is observed in July. Spawning individuals were observed from July until October with the peak in August, when the highest mean egg stage and GSI values were observed. Fish with ovaries having hydrated eggs in October were clearly spawning their last batch as they had many atretic hydrated oocytes, no vitel- logenic, and few early hydrated oocytes present. Spent ovaries were found in September and October. Since no samples were collected in November and December, it is not clear how long the spawning season could last. However, by January all fish collected were in the early developing phase for the next year’s cycle. Size and age at 50% maturity Size at 50% maturity ranged from 33 cm to 38 cm (Table 4). However, subarea was a highly significant factor (P<0.001), exhibiting a cline in the size at 50% maturity from east to west (Table 4, Fig. 5). Samples from the eastern ar- eas reached 50% maturity at larger sizes. Length at 50% maturity in the Gulf of Alaska was 38.24 cm, while in the eastern Aleutian subarea the fish matured at 35.91 cm. Samples from the central and western Aleutian subareas matured at essentially the same size, 33.55 cm and 33.64 cm, respectively . Age at maturity was not significantly different among the different Aleutian subareas (P=0.66) or between the Gulf of Alaska versus the Aleutian subareas combined (P=0.69), therefore the data were pooled. The age at 50% maturity (all areas combined) was 3.6 years for Atka mackerel (Table 4, Fig. 6). Discussion Figure 3 Mean oocyte stage and mean gonad somatic index (GSI) by month for ma- ture Atka mackerel. In order to make inferences about popu- lation parameters and biology, it is nec- essary to take representative samples of the population’s true age and size 330 Fishery Bulletin 95(2), 1 997 composition and sex ratio throughout their distribu- tion. Due to the opportunistic nature of obtaining samples, only the portion of the population available to fisheries and research vessels was sampled. These are likely the larger animals, i.e. the mature individu- als of the population. Therefore the results of length and age at maturity could be overestimated. Addition- ally, during the time of sex segregation the samples may be biased towards more females, since the males may be unavailable for sampling. Until the population distribution over time and space is better understood, it will be difficult to design sampling schemes that will yield unbiased population parameters. Egg stage development for Atka mackerel is simi- lar to that described for the masked greenling (Hexagrammos octagrammos ) (Munehara et al., 1987; Munehara and Shimazaki, 1989). Oogenesis in Atka mackerel exhibited the following sequence: Table 4 Length and age at maturity for Atka mackerel. Area a S(a) P S(p) L 50% 95% Cl (low) 95% Cl (upper) Var (L 50%) Gulf of Alaska -27.16 3.69 0.71 0.09 38.24 36.27 40.21 1.00 Eastern Aleutians -25.50 3.57 0.71 0.09 35.91 33.94 37.90 1.01 Central Aleutians -23.83 3.67 0.71 0.09 33.55 31.12 36.57 1.54 Western Aleutians -23.89 3.68 0.71 0.09 33.64 31.20 36.69 1.55 Area a S(a) P S(p) Age 50% 95% Cl (low) 95% Cl (upper) Var (Age 50%) Areas combined -7.33 0.87 2.03 0.22 3.60 3.40 3.81 0.01 McDermott and Lowe: Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 331 the formation of cortical alveoli, followed by oil drop- lets, yolk accumulation, nuclear migration, and hy- dration (Fig. 2). The appearance of oil droplets after the formation of cortical alveoli and the coalescence of yolk before or during nuclear migration are fea- tures that have also been described for masked green- ling (Munehara et ah, 1987). Another feature that is similar to the masked greenling is that hydrated atretic oocytes were reabsorbed very slowly and could be found in the ovary for over 1 year. The early maturation of the Atka mackerel ovary is characterized by an accumulation of oil droplets in the developing oocytes with a gradual increase in oocyte size over several months from January until May. Vitellogenesis is completed within 1 month, similar to the duration of vitellogenesis in masked greenling, but uncommonly short for most subarctic fishes (Munehara and Shimazaki, 1989). The spawning period for Atka mackerel is extended and can last up to 4 months, from July through Oc- tober. This relatively long spawn- ing period can be attributed to the duration of time spent be- tween the spawning of indi- vidual batches. Zolotov (1993) stated that the greater number of batches and the longer inter- val between their spawning cor- responds to the longer duration of the reproductive period of Atka mackerel as compared with the arabesque greenling ( Pleurogrammus azonus). The average spawning duration of 3 months found in this study is in agreement with the spawning duration reported for Atka mackerel in Kamchatka waters (Zolotov, 1993). However, the beginning of spawning in Alaska waters was observed in July last- ing until October, whereas the spawning period in Kamchatka waters was described as start- ing in June and lasting until Sep- tember (Zolotov, 1993). These dif- ferences may be attributed to year-to-year variations; however, different oceanographic condi- tions in Alaska versus Kam- chatka waters may also be a contributing factor. There are not enough data to substantiate a seasonal cline in the timing of the reproductive cycle ranging from Kamchatkan waters to the Gulf of Alaska. The observation of a few spawning fish in March and April indicates that spawn- ing times may be more variable than previously assumed. Data from several years were pooled in this study as the number of samples was insufficient to dis- tinguish annual differences. 0 3 03 E c o o Q. O CL 1 1.5 2 2.5 3 3.5 4 4 5 5 5.5 6 6 5 7 7.5 8 8 5 9 9 5 10 Age (yr) Figure 6 Age at maturity (all geographical areas pooled) for Atka mackerel. 332 Fishery Bulletin 95(2), 1997 Reproductive timing and parameters can vary from year to year; this might be reflected in increased vari- ance and range of the results in this study. The spawning period during late summer and fall for Atka mackerel is earlier than that observed for the masked greenling (September through October; Munehara and Shimazaki, 1989), but later in the year than most other Alaska groundfish of commer- cial importance. Sablefish (Anoplopoma fimbria), Pacific halibut ( Hippoglossus stenolepis), arrowtooth flounder ( Atheresthes stomias), and flathead sole (Hippoglossoides elassodon) are reported to spawn in winter and early spring in the Gulf of Alaska, whereas walleye pollock ( Theragra chalcogramma). Pacific cod ( Gadus macrocephalus), Pacific ocean perch (Sebastes alutus), and rock sole ( Pleuronectes bilineatus) were reported to have their spawning peak from spring to early summer in the Gulf of Alaska (NPFMC4). The life history feature of sum- mer and fall spawning for Atka mackerel and other greenling species might be an adaptation to spawn- ing large demersal eggs, the larvae of which enter the plankton at a larger size than larvae from pe- lagic eggs (Kendall and Dunn, 1985). The differences by subarea for length at 50% ma- turity can be attributed to different growth rates by subarea, given that no age-at-maturity differences among geographical areas were found. Length-at-age curves in each management area revealed an increas- ing size at age from west to east (Lowe and Fritz3). The reasons for these different growth rates are un- known. It is not clear whether more favorable condi- tions such as food availability, or a more favorable temperature regime are contributing to a higher growth rate in the Gulf of Alaska and eastern Aleu- tian Islands subarea, or whether there are genetic differences in the populations. However, initial ge- netic studies suggest that there is little or no stock differentiation in Alaska (Winans5). Analysis of fisheries data (Fritz2) indicated that in late summer and fall commercial hauls in many locations had a larger proportion of females than males. Sex segregation coincided with the spawning season (July through October) and supported the hypothesis that males were unavailable to the fish- ery, presumably guarding nests close to shore. Seg- regation of the Atka mackerel population by sex dur- ing the spawning season could also affect the results of summer trawl surveys used to assess population 4 NPFMC. 1994. Fishery Management plan for the Alaskan Gulf of Alaska groundfish fishery. North Pacific Fishery Man- agement Council, P.O. Box 103136, Anchorage, AK 99510. 5 Winans, G. Northwest Fish. Sci. Center, Natl. Mar. Fish. Serv., NOAA, Seattle, WA 98112. Personal commun. size if only a portion of the population is surveyed. More information on the location of nesting sites, behavior, and spawning distribution is necessary to understand the implications of population segrega- tion on resource assessments. Future research should be conducted on a more long-term basis for collection of maturity informa- tion, and larval and juvenile biology and distribu- tion. Time and area gaps should be filled to under- stand the spatial and seasonal distribution patterns linked to spawning, crucial for assessing and man- aging this species appropriately. Acknowledgments The authors would like to thank the following scien- tists from the Alaska Fisheries Science Center: F. Morado for the generous use of his laboratory space and his helpful advice in histological matters, L. W. Fritz and J. N. lanelli for their helpful suggestions and comments, D. Nichol and A. R. Hollowed for re- viewing the manuscript, and D. Anderl (Age and Growth Unit) for the ageing of Atka mackerel otoliths. This study would not have been possible without the help from the Observer Program and the research scientists of the Resource Assessment and Conservation Engineering division in obtaining samples in the field. We greatly appreciated the sup- port and advice of R. D. Gunderson (University of Washington) through all phases of this work. Literature Cited Chilton, D. E., and R. J. Beamish. 1982. Age determination methods for fishes studied by the groundfish program at the Pacific Biological station. Can. Spec. Pub. Fish. Aquat. Sci. 60, 102 p. Galigher, A. E., and E. N. Kozloff. 1971. Essentials of practical microtechnique. Lea and Febiger, Philadelphia, PA, 531 p. Gorbunova, N. N. 1962. Spawning and development of greenlings (family Hexagrammidae). Tr. Inst. Okeanol., Akad. Nauk SSSR 59:118-182. In Russian. (Trans, by Isr. Program Sci. Trans., 1970, p. 1-103. In T. S. Rass ( ed. ), Greenlings: taxonomy, biology, interoceanic transplantation. Available from U.S. Dep. Commer., Natl. Tech. Inf. Serv., Springfield, VA, as TT 69-55097. Gunderson, D. R., and P. H. Dygert. 1988. Reproductive effort as a predictor of natural mortal- ity rate. J. Cons. Cons. Int. Explor. Mer. 44:200-209. Hunter, J. R., and B. J. Macewicz. 1985. Rates of atresia in the ovary of captive and wild north- ern anchovy, Engraulis mordax. Fish. Bull. 83( 2 ): 119 — 136. Kajimura, H. 1984. Opportunistic feeding of the northern fur seal, McDermott and Lowe: Reproductive cycle and sexual maturity of Pleurogrammus monopterygius 333 Callorhinus ursinus, in the eastern north Pacific Ocean and eastern Bering Sea. U.S. Dep. Commer., NOAATech. Rep. NMFS SSRF-779, 49 p. Kendall, A. W., Jr., and J. R. Dunn. 1985. Ichthyoplankton of the continental shelf near Kodiak Island, Alaska. U.S. Dep. Commer., NOAA Tech. Rep. NMFS 20, 89 p. Livingston, P. A., A. Ward, G. M. Lang, and M-S. Yang. 1993. Groundfish food habits and predation on commer- cially important prey species in the eastern Bering Sea from 1987 to 1989. U.S. Dep. Commer., NOAA Tech. Memo. NMFS-AFSC-11, 192 p. Munehara, H., K. Shimazaki, and S. Mishima. 1987. The process of oogenesis in masked greenling, Hexagrammos octogrammus. Bull. Fac. Fish. Hokkaido Univ. 38( 11:27-33. Munehara, H., and K. Shimazaki. 1989. Annual maturation changes in the ovaries of masked greenling Hexagrammos octogrammus . Nippon Suisan Gakkaishi 55(31:423-429. Rutenberg, E. P. 1962. Survey of the fishes of family Hexagrammidae. In Rus- sian. (Trans, by Isr. Program Sci. Trans., 1970, p. 1-103. In T. S. Rass (ed.l, Greenlings: taxonomy, biology, interoceanic transplantation. Available from U.S. Dep. Commer., Natl. Tech. Inf. Serv., Springfield, VA, as TT 69-55097. Seber, G. A. F. 1982. The estimate of animal abundance and related parameters. Charles Griffin and Company, London, 654 p. Venables, W. N., and B. D. Ripley. 1994. Modern applied statistics with S-plus. Springer Verlag, NY, 462 p. Zolotov, O. G. 1993. Notes on the reproductive biology of Pleurogrammus monopterygius in Kamchatkan waters. J. Ichthyol. 33(41:25-37. 334 Prey occurrence in pantropical spotted dolphins, Stenella attenuata, from the eastern tropical Pacific Abstract .—Identified prey of pan- tropical spotted dolphins, Stenella attenuata, include 56 species offish and 36 species of cephalopods. Species iden- tifications were made from fish otoliths and cephalopod beaks recovered from 428 stomachs collected throughout the eastern tropical Pacific between 1989 and 1991. The most frequently found fish were lanternfish (family Mycto- phidae) at 40%, and the most frequently found cephalopods were flying squids (family Ommastrephidae) at 65%. The dominance of these primarily mesope- lagic prey species and a significantly higher stomach fullness index for stom- achs collected during the morning hours 60% was calculated for 42.0% of the animals collected between 0600 and 0900 h, whereas only 1.0% of the sample collected between 1501 and 1800 h had a SFI >60% (Fig. 4). Reproductive condition The mean number of squid differed significantly be- tween pregnant and lactating females (Student’s t- test=-2.65, P=0.010); however, no significant differ- ence was found in the mean number of fish consumed (Student’s (-test=0.25, P-0.803). When stratified by time-of-day, the mean SFI was significantly higher for lactating dolphins during all time periods (X2=46.98, df=6, P<0.0001; Table 4). Discussion Mesopelagic prey were found to dominate the diet of pantropical spotted dolphins; myctophid fish, and enoploteuthid and ommastrephid squid accounted for 69% of all prey consumed (Table 1). These mesope- lagic species are associated with the deep scattering layer and most undergo diel vertical migration, mov- ing into the upper 200 m at dusk to feed and retreat- ing to depth at dawn to avoid predation (Gibbs and Roper, 1971; Clarke, 1973, 1978; Wisner, 1974; Roper Table 4 The average stomach fullness (%) for lactating and preg- nant spotted dolphins throughout the day. The day is di- vided into three hour increments from 0600 h to 1800 h. ‘No.’ is equal to the number of stomachs in each time category. Average stomach fullness Lactating Pregnant Time (h) % No. % No. 0600-0900 52.7 15 38.6 6 0901-1200 37.3 18 23.1 10 1201-1500 27.2 15 15.2 7 1501-1800 26.1 9 10.9 14 Overall average 44.3 57 19.5 37 and Young, 1975; Roper et al., 1984; Smith and Heemstra, 1986). The SFI, which we found to be high- est in the morning hours (i.e. 0600-0900 h, Fig. 4), suggests that pantropical spotted dolphins feed dur- ing the night when these prey are nearest to the sur- face. In fact, Shomura and Hida (1965) hypothesized that the spotted dolphin caught off Hawaii fed just before dawn, prior to descent of the deep scattering layer, because fresh mesopelagic prey were found in its stomach (enoploteuthid squid and myctophid fishes). Evidence of nighttime feeding by pantropical spotted dolphins has also been presented by Scott (1991), who reported that the highest proportion of undigested prey was recovered from spotted dolphin stomachs collected between 0700 and 0930 h and that 342 Fishery Bulletin 95(2), 1 997 Dophin length class (cm) Figure 2 Mean estimated length of squid (A) and fish (B) with 2 SE shown for each 5 cm dolphin length class. Significant dif- ferences were found in the size of prey consumed with in- creasing dolphin length for both squid (r2=0. 791, P<0.0001 ) and fish (r2=0.648, PcO.OOl). no fresh prey was recovered from stomachs collected after 1200 h. More recently, the collection of data on dive patterns has provided additional evidence of nighttime feeding by spotted dolphins. The dive pat- terns show marked diurnal changes and both deeper and longer dives at night. In particular, the dawn- dusk diving patterns suggest that dolphins were fol- lowing the ascent and descent of the deep scattering layer (Scott et al.7). The capture of prey by dolphins in the deep scat- tering layer may be facilitated by abundance of prey, schooling size of prey, and bioluminescence of prey (Clarke, 1973; Crawford, 1981; Clarke, 1986b). The top three prey families in our study (Myctophidae, Enoploteuthidae, Ommastrephidae) are all abundant in the ETP and have bioluminescent organs (Clarke, 1973, 1978; Wisner, 1974; Okutani, 1974; Clarke, 1977; Crawford, 1981; Roper et al., 1984; Harman and Young, 1985; Clarke, 1986b). Myctophids, which accounted for 49.7% of all prey recovered in our sample, are the most abundant deep scattering layer species, representing 25% of the biomass of all me- sopelagic fishes (Karnella, 1987). Myctophids are small in size, school in large numbers, and have bi- oluminescent organs, all characteristics that might facilitate detection by dolphins (Crawford, 1981). Prey size There does appear to be some selectivity of prey by size because larger dolphins tend to eat larger prey (Fig. 2). Our results support the supposition that because most dolphins have been observed to con- sume their prey whole, the size of prey ingested is limited by the size that can be swallowed (Fiscus and Kajimura, 1981; Fiscus and Jones, 1990; Wolff4). The size of prey in our study indicated that prey consumed by juveniles through adults ranged from 2.4 to 320.9 mm (Okutani, 1974, Butler, 1979; Uchida, 1981; Roper et al., 1984; Clarke, 1986a; Smith and Heemstra, 1986; Karnella, 1987; Murata and Hayase, 1993; Welch and Morris, 1993). Most of our size ranges corresponded with those reported by Wolff,4 who examined the squid prey of spotted dolphins from the Perrin et al. (1973) study, except for A. affinis, which had a narrower size range in our study, and O. banksii, which encompassed a broader size range. Wolff4 also showed an increase in the size of squid consumed by spotted dolphins; there was a 30-mm mantle length increase for squid found in dolphins from 140 to 205 cm in length. He also found that a broader size range of squid was consumed for larger dolphins, which was also the case in our study. Geographic and seasonal variability The composition of prey species consumed by pantropical spotted dolphins changed both tempo- rally and spatially (Fig. 3), suggesting that they are opportunistic feeders, as is the case with many other 7 Scott, M. D., S. J. Chivers, H. Rhinehart, M. Garcia, R. Lindsey, R. L. Olson, W. Armstrong, and D. A. Bratten. 1997. Move- ments and diving behavior of pelagic spotted dolphins. Inter- Am. Tropical Tuna Comm., c/o Scripps Institution of Oceanog- raphy, Univ. Calif., San Diego, CA 92037. Robertson and Chivers: Prey of Stenella attenuate 343 Prey species Fish 1 = Symbolophorus spp 2 = M aurolaternatum 3 = L. parvicauda 4 = D. spiendidus 5 = C pauciradiatus Cephalopods 6=0. bartrami 7 = 0. banksii 8 = A. affinis 9 = M. dentata 10 = /-. dislocata □ West 0 South ■ Northeast 520 390 260 130 0 1 23456789 10 Prey species Figure 3 The percent number of the 10 most numerous prey species by season: winter and summer, and geographic area: northeast (/i=159), south (n= 72), and west (n=198). See text for full name of the species listed. dolphin species (Brown and Norris, 1956; Ross, 1979; Jones, 1981; Fiscus, 1982; Gaskin, 1982; Leather- wood et al., 1983; Evans, 1987; Young and Cockcroft, 1995). Geographic and seasonal changes in prey com- position could be a result of migration of prey into or out of an area, prey spawning seasons, or simply dis- tributional boundaries of prey. It has been suggested that the movements of dolphin may correspond to the movement or availability of prey (Jones, 1981; Reilly, 1990; Young and Cockcroft, 1994). There is evidence that the distribution of pantropical spotted dolphin shifts westward along the 10°N latitude as the summer season progresses, and it has been hy- pothesized that this change in distribution is due to changes in prey distribution re- sulting from the equatorial cur- rents (Au and Perryman, 1985; Reilly, 1990). Unfortunately, in- formation on precise distribu- tions and seasonal movements of identified prey species in the ETP is limited; therefore this hypothesis cannot be addressed properly (Clarke, 1973; Oku- tani, 1974; Wisner, 1974; Clarke, 1977; Roper et al., 1984; Clarke, 1986, a and b). Reproductive condition Changes in diet composition between lactating and pregnant dolphins have been documented in a number of species (Perez and Mooney, 1986; Bernard and Hohn, 1989; Recchia and Read, 1989; Cockcroft and Ross, 1990; Young and Cockcroft, 1994, 1995). The physiological energy required to maintain lactation is quite high for mammals and may require a change in diet composition to include food with a higher caloric content (Clutton-Brock et al., 1982; Perez and Mooney, 1986; Recchia and Read, 1989; Iverson, 1993). In fact, Bernard and Hohn ( 1989) presented evi- dence for a shift in diet between pregnant and lactating pantropical spotted dolphins and suggested that it was due to the physiological demands of lactation. They found a higher proportion of flying fish (family Exocoetidae) in the diet of lactating females and a higher proportion of ommastrephid squid in the diet of pregnant females. We tested the same hypothesis for our sample, but our results were different. Although the proportion of fish (family Myctophidae) in the diet was higher than the proportion of squid (family Omma- strephidae) for both pregnant and lactating females, the proportion of squid was significantly higher in the diet of lactating females. Fish may provide most of the caloric intake for both lactating and pregnant females because both lanternfish and flying fish have a high caloric and lipid content in comparison with squid (Childress and Nygaard, 1973; Sidwell et al., 344 Fishery Bulletin 95(2), 1997 ■ 61-100% FULL ^ 3 1 -60% FULL □ 0-30% FULL Time (h) Figure 4 Percent of the sample in each of the stomach fullness catego- ries: 0-30%, 31-60%, and 61-100% for spotted dolphins col- lected between 0600-1800 h: 0600-0900 h (n=106), 0901-1200 h (n= 87), 1201-1500 h (n = 134), and 1501-1800 h (ra = 101). The SFI was scaled to 100% by using the stomach of maxi- mum fullness in the sample (see text for details). 1974; Sidwell, 1981; Croxall and Prince, 1982). A possible explanation for increased consumption of squid by lactating spotted dolphins is that milk pro- duction increases the demand for metabolic water (Croxall and Prince, 1982; Young and Cockcroft, 1994) and most squid have a higher water content than fish (Sidwell, 1981; Croxall and Prince, 1982). Ceta- cean milk has been estimated to be 40-60% water (Eichelberger et al., 1940; Gregory et al., 1955; Slijper, 1966; Best, 1982; Lockyer, 1984). Rather than by a change in diet composition, the high metabolic demands of lactation could be met by increasing the amount of food consumed (Baldwin, 1978; Millar, 1979; Clutton-Brock et al., 1982; Yasui and Gaskin, 1986; Perez and Mooney, 1986; Kastelein et al., 1993). Estimates of food intake for lactating versus nonlactating females have been estimated to increase by 75-86% for lactating minke whales (Balaenoptera acutorostrata ) and fin whales ( Balaen - optera physalus) (Lockyer, 1978, 1981), by 500 g for lactating harbor porpoise ( Phocoena phocoena) (Recchia and Read, 1989), and by 30% for captive Commerson’s dolphins ( Cephalorhynchus com- mersoni) (Kastelein et al., 1993). For spotted dol- phins, Bernard and Hohn (1989) reported a SFI that was 24% higher for lactating females (Mann- Whitney, 0.050.100 are shown. using each set as a sampling unit. The percent num- bers, for set as the sampling unit and specimen as the sampling unit, were then compared by using a Mann-Whitney rank test, and no significant differ- ence was found in the rank order of prey families between the set and specimen methods (Zar, 1984, p. 141-143; P= 0.05, Fig. 5). Therefore, we conclude that for our data set, there was no bias introduced by using multiple specimens collected from the same set. Summary Based on the analysis and identification of fish otoliths and cephalopod beaks, our results provide evidence that pantropical spotted dolphins feed pri- marily at night on mesopelagic fish and squid. The dominant prey species belong to the families Myctophidae, Enoploteuthidae, and Ommastrephi- dae. Composition of the diet differed by season and area; thus pantropical spotted dolphins are likely op- portunistic feeders. Prey included a wide range of sizes of both fish and squid, with the largest prey consumed by the largest dolphins, and the smallest prey consumed in the largest numbers. Furthermore, the diet of female dolphins differed by reproductive condition. Lactating females consumed more food and a higher proportion of squid than did pregnant females. Acknowledgments We would like to thank the biological technicians (NMFS, Southwest Region) who collected the stom- achs for this study, F. G. Hochberg for assistance in identifying the cephalopod beaks (Santa Barbara Museum of Natural History), R. Lavenberg for the use of the otolith reference collection at the Los An- geles County Museum of Natural History, and R. Lindsey ( Inter- American Tropical Tuna Commission [IATTC]) and R. Rassmussen (Southwest Fisheries Science Center [SWFSC]) for time-of-day data. We would also like to thank J. Carretta and R. Pitman (SWFSC) for their assistance with identifying myctophid and flying fish otoliths. We are grateful to M. Henshaw (SWFSC), W. F. Perrin (SWFSC), R. Olson (IATTC), M. Scott (IATTC), and two anony- mous reviewers for their helpful comments and me- ticulous reviews of the manuscript. 346 Fishery Bulletin 95(2), 1 997 Literature cited Akin, P. A., K. M. Peltier, and R. B. Miller. 1993. 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Later, vertebral cen- tra were examined for opaque band for- mation, and there were positive results in two individuals. Outer margin analy- sis of centra from captive and field-col- lected guitarfish indicated that opaque bands formed between August and De- cember. Guitarfish were aged to 11 years, and growth appeared to be best represented by a linear growth equa- tion, TL = 43.33 + 6.90.x, where TL = total length and x = estimated age in years. Analysis of reproductive tracts showed that female guitarfish matured at 99 cm (estimated age at seven years). Clasper length and width indicated that males matured at 90-100 cm (es- timated age at eight years). Manuscript accepted 5 October 1996. Fishery Bulletin 95:349-359 (1997). Age, growth, and sexual maturity of shovelnose guitarfish, Rhinobatos productus (Ayres) Maryellen Timmons* Richard N. Bray** California State University Long Beach Long Beach, California 90840 *Present address: Marine Extension, University of Georgia 30 Ocean Science Circle, Savannah, Georgia 3141! ** Present address Department of Biology California State University San Marcos San Marcos, California 92096 E-mail address (for Maryellen Timmons): mare@uga.cc.uga.edu Recently, a Federal fishery manage- ment plan was initiated for some large coastal and pelagic species of sharks of the eastern seaboard of the United States (NMFS1). Regu- lations on shark fisheries are impor- tant not only because they affect fisheries but because they set an example to be followed by other coastal areas. Many species of elas- mobranchs are highly migratory; thus regulations are necessary on a broader scale if they are to be ef- fective management tools. Elasmobranchs tend to have slow growth and low fecundity (Holden, 1973); thus, overexploitation of a species is possible. Fortunately, re- cent collection of age, growth, and reproductive data on elasmobranchs has helped provide some of the baseline information necessary to manage many species. The shovelnose guitarfish, Rhino- batos productus , is a common coastal ray found in temperate wa- ters along the Pacific coast of the United States from Baja California to San Francisco (Miller and Lea, 1972). Although not a highly prized commercial catch, it is edible and is often found in fish markets la- beled as generic “shark steak” and sold on piers in Santa Barbara, Cali- fornia, as “fish n’ chips.” Guitarfish is not sold as “guitarfish” on restau- rant menus; however it may become a popular fare in the future as a substitute for shark. Furthermore, dried guitarfish are sold in large numbers as curios in shell shops from central California to Baja Cali- fornia. The majority of guitarfish sold for human consumption are the larger, mature individuals; how- ever, curio and shell shops tend to sell all sizes, especially newborn pups. Congeners of Rh inobatos are particularly targeted for commer- cial sale in other areas of the world including Peru (Tresierra et al., 1989) and Brazil (Lessa and Vooren, 1986). Currently, in southern Cali- fornia, commercial landings of gui- tarfish are grouped under benthic shark species and not recorded as guitarfish.2 Most literature on R. productus is contained in field guides and California Fish and Game publications (Roedel, 1953; Miller and Lea, 1972; Lane and Hill, 1975; Eschmeyer et al. 1983; Tal- ent, 1982, 1985) usually with no more than brief mention of some of 1 NMFS. 1993. Fishery management plan for sharks of the Atlantic Ocean. Prepared for the U.S. Dep. Commer., Natl. Mar. Fish. Serv., NOAA, 167 p. 2 Vojkovich, M. 1994. Dep. Fish and Game, Long Beach, C A 90807 . Personal commun . 350 Fishery Bulletin 95(2), 1 997 its life history aspects, such as maximum size and food preferences. One particular aspect of guitarfish behavior is that large numbers of them are often found in shallow embayments, such as Elkhorn Slough and Mugu Lagoon, California, and Almejas Bay, Baja California, Mexico. In these areas, they are easily captured with a seine net and are thus particularly susceptible to fishing pressure. Because elasmobranchs tend to be exploited be- fore regulatory measures are in effect (Pratt and Casey, 1990), it is necessary to determine age and growth relationships and size at sexual maturity of R. pi'oductus prior to increases in fishing pressure. The results of this study provide basic information for management of guitarfish, should it become more popular as a food item. We have incorporated the following methods of age determination into this study of the age, growth, and sexual maturity of guitarfish: 1) a laboratory analy- sis of the vertebral bands and their outer margin state (translucent or opaque) in order to assign ages to individuals; 2) a study of growth in captivity to verify estimated growth from the laboratory analy- sis; and 3) a determination of age at sexual matu- rity. The main focus of this age and growth study is based on an examination of vertebral centra and their use in ageing guitarfish. Methods Age and growth Seventy-five guitarfish were collected between No- vember 1988 and January 1991 from the waters be- tween Seal and Redondo Beaches, California (Fig. 1). Guitarfish were captured by hook and line, gill net, otter trawl, long line, or beach seine, and then frozen. Lengths were measured with a tape measure to the nearest centimeter over the contour of the dor- sal portion of the guitarfish and included total length (TL), disc width (DW), first dorsal fin length (ID), and second dorsal fin length (2D) (Fig. 2). The con- tour measurement over the dorsal portion provided a more precise measurement of the first and second dorsal fin lengths. This method will increase the to- tal length measurement and should be taken into consideration if comparisons are made with lengths of guitarfish in this study. The only portion of the guitarfish that is available in fish markets is the trunk and tail or loin region, which includes the two dorsal fins. Therefore, we included the measurement of the distance from the origin of the first dorsal fin to the origin of the sec- ond dorsal fin (2D) to facilitate future predictions of Study sites where guitarfish, Rhinobatos productus, were col- lected along the coast of southern California. A = Redondo Beach, B = Palos Verdes, C = San Pedro, D = Long Beach, E =Belmont Shores, and F = Seal Beach. total length from market fish. Damp weight was measured for all guitarfish with a spring balance. Ten vertebrae were removed from each guitarfish just anterior to the first dorsal fin for analysis. The larger vertebrae were located just posterior to the eyes; however, they were not used because removal of these vertebrae would have interfered with dissection of the female reproductive tract. Each guitarfish was assigned a code number and this became the only identifying feature for each guitarfish for the remain- der of the study. Vertebrae were cleaned by placing them in a dermestid beetle colony. The beetles con- sumed almost all muscle and connective tissue; the only remaining tissue was a cone-shaped membrane (membrane elastica externa) on the centrum that was easily removed from the dry vertebrae with fine for- ceps. Cleaned and dried vertebral centra were viewed whole with a Wilde dissecting scope with transmit- ted light within a dark field. Ten vertebrae from each guitarfish were examined to determine consistency of band formation within an individual. If all verte- brae for an individual guitarfish contained the same number of bands, then two of those vertebrae were used for three separate readings. Those having vari- able band counts or unreadable vertebrae among the ten vertebrae were discarded. Opaque bands present beyond the birth mark were counted (Fig. 3). Rings within bands were not always discernible as sepa- rate rings; therefore, bands were determined to be the most useful increment. The birth mark was de- fined here as the centermost opaque portion (first band) of the centra. It was present in the smallest of the guitarfish and was in the same position in all larger specimens. This birth mark is similar in place- Timmons and Bray: Age, growth, and sexual maturity of Rhinobatos productus 351 Figure 3 Examples of band formations in the vertebra of a 125.5-cm guitarfish (A) and a 28.8-cm guitarfish (B). The birth mark (b) appeared in all three vertebrae. Band formations (c) were poor towards the outer edge of the centra of the larger individual (A), and this individual was not used in the age study. Poor band formations were indicative of individual guitarfish with deformed vertebral columns. 352 Fishery Bulletin 95(2), 1997 ment to that found by Cailliet et al. (1983) in the blue shark, Prionace glauca, and Casey et al. (1985) in the sandbar shark, Carcharhinus plumbeus. Di- ameters of vertebral centra were measured with an ocular micrometer at 12x magnification. Each verte- bra was read three times, one month apart, and those vertebrae in which all three readings agreed were used in the final analysis. To determine periodicity of band formation, the condition of the outermost band was recorded as either translucent, opaque, or undetermined. Ages were assigned to guitarfish on the basis of the number of opaque bands. Statistical analyses included least squares regres- sion analysis to provide predictive equations for es- timates of TL from centrum diameter, TL from band counts, TL from second dorsal fin length (2D), and age from TL. Regression parameters were obtained with SAS PC software (SAS, 1985). Male and female growth curves were constructed from von Bertalanffy’s growth curve equation (von Bertalanffy, 1938): Lt = (l-e-*u“(o>), where Lt = total length at time t\ = maximum theoretical length of species; k = growth constant; t0 = theoretical age at zero length; and t = estimated age. The von Bertalanffy growth equation was fitted by using FISHPARM (FISHPARM software [Prager et al., 1989]) to estimate the growth constant k and was compared to a linear least squares regression by us- ing the same data. Growth rate of guitarfish in captivity The main purpose of the captivity study was to de- termine if Terramycin (manufactured by Pfizer Ag- ricultural Division) produced a readable time mark in vertebral centra of guitarfish. The study was de- signed to maintain guitarfish in captivity for at least one year to determine the temporal periodicity of band formation and growth rate of guitarfish in captivity. Over a two-year period, 13 guitarfish (five males and eight females) were taken live and placed in an outdoor saltwater tank at California State Univer- sity, Long Beach, California. Before guitarfish were introduced into the tank, we repeatedly measured TL, DW, ID, and 2D until we obtained consistent, repeatable measurements. Guitarfish were first weighed, and then injected with Terramycin (dos- age=0.5 mg/kg). Terramycin was injected with tuber- culin-type syringes in the epaxial musculature, within two centimeters of the skin surface. Guitar- fish were fed every other day a diet of anchovy, mack- erel, mud shrimp, ghost shrimp, and squid. When a guitarfish died in captivity, it was used for vertebral and reproductive analysis. Vertebral growth (beyond the time mark) was measured with the aid of a Wilde dissecting scope and ultraviolet flashlight (Fig. 4). Because time marks could be seen only under ultraviolet light and the opaque band for- mation could not be seen under ultraviolet light, transmitted light was used immediately after the ultraviolet light to compare the time mark with the opaque band position. This method allowed deter- mination of whether a translucent or opaque band had formed after the time mark. Reproductive maturity Thirty-six female guitarfish were dissected for ex- amination of their reproductive tract. Mature indi- viduals were categorized into one of three visual stages: Stage 1 — shell gland not differentiated from uteri, uteri empty, small follicles present; Stage 2 — shell gland and characteristic diagonal white band pattern within it forming, large Graafian follicles present, uteri thick; and Stage 3 — uteri full, large Graafian follicles present. Immature individuals had no visible egg follicles, uteri were thin and transpar- ent, and shell glands consisted only of a slight bulge in the upper portion of the uteri. These stages were distinct; any female guitarfish, upon dissection, could be categorized by using these criteria. No dissections were made for male guitarfish. The maturity of male guitarfish was determined by measuring the clasper width and length and by comparing the clasper length to total length, as well as by visual examination. Results Age and growth Growth of the vertebrae was proportional to the growth of the guitarfish, as evidenced by the signifi- cant positive relation between centrum diameter and total length for females and males (females: r2=0.98, n- 27, P=0.0001; males: r2=0.96, n=31, P=0.0001; Fig. 5). The number of bands per vertebra correlated strongly (r=0.92, n= 42, P=0.0001) with the diameter of the centra, indicating that individuals having more bands had larger centra. Similarly, the number of opaque bands present in any individual was higher in larger guitarfish; the regressions were significant for females and males (females: r2=0.95, rc = 19, P=0.0001; males: r2=0.78, n=24,P=0.0001). APearson correlation matrix analysis of total length, centrum Timmons and Bray: Age, growth, and sexual maturity of Rhinobatos productus 353 diameter, and number of opaque bands further em- phasized the strong relation between the three vari- ables for males and females combined (opaque bands and TL: r=0.92, n= 43, P=0.0001; centrum diameter and TL: r=0.99, n=60, P=0.0001; for opaque bands and centrum diameter see above). Growth zones formed at approximately the same time each year, as was evident from examination of centra of two captive guitarlish. One of these guitar- fish was injected with Terramycin in December 1989 and in July 1990 and was held in captivity for 13 months. The first Terramycin mark (closest to the focus of the centrum) was found at the peripheral edge of an opaque band (Fig. 6A). We do not know if the opaque band had formed prior to the injection or during the same month as the injection because the Terramycin may have diffused into the opaque band region. It was clear, however, that the mark was the same distance (0.07 cm) from the outer margin of the centrum as the peripheral edge of the opaque band. From the periphery of the opaque band to the outer margin of the centrum, a translucent band was present; and the outer margin of the centrum con- tained the other Terramycin mark. The predicted growth (0.029 cm) of this centrum (with the formula in Fig. 5) was lower than the actual growth of 0.07 cm and shows that individuals probably vary in growth, especially in more optimal laboratory condi- tions. A second guitarfish was first injected in Octo- ber 1989 and again in July 1990. This guitarfish lived for 14 months and had completely formed one opaque band during this period (Fig. 6B). This opaque band was formed after the October injection, and a trans- lucent band was present beyond the opaque band to the outer margin of the centrum. The outer margin of the centrum showed the second injection mark at the periphery of the translucent zone at the time of death in January 1991. For this guitarfish, the pre- dictive equation (Fig. 5) estimated centrum growth to be 0.062 cm; it was actually 0.053 cm. In both gui- tarfish, opaque band formation occurred sometime between the months of October and December fol- 354 Fishery Bulletin 95(2), 1997 lowed by translucent band formation. The remain- ing eleven guitarfish were held in captivity for six months or less. Eight of the eleven were injected with Terramycin and did not show any growth beyond the Terramycin mark on the vertebrae and each had grown less than one centimeter in total length. Three control guitarfish (no injections) lived 10, 50, and 72 days in captivity and showed no gain in length. Analysis of the outer edges of the centra with re- gard to periodicity of band formation provides fur- ther evidence for opaque band formation between October and December. Eight out of 17 guitarfish collected between October and November had opaque outer margins, whereas none of the 34 other guitar- fish collected during the other months (excluding August) had opaque outer margins. Three guitarfish caught in August showed opaque formation on the outer margins. A two-way test of independence indi- cated rejection of the null hypothesis that opaque band formation was independent of month (group l=January-June, group 2=August-November; G(adjusted)= 18.94, df=l, P=0. 00003). Therefore, it appears that opaque bands form from late summer (August) into fall (November). There was no pro- nounced relation between outer margin width and months of the year, probably indicating variation of growth within individual guitarfish. Another way to interpret these findings is to suggest that guitarfish lay down opaque bands bi-yearly (every other year). If this is the case, then the guitarfish were twice as old. However, the band formations found in the two captive guitarfish led us to assume that opaque bands were formed once per year. Assigning ages under the assumption of the an- nual formation of one opaque and one translucent band, we found that both males and females ranged in age from one to 11 years. Females ranged from 25 to 130 cm TL. Males ranged from 23 to 114 cm TL. Percent agreement in band counts from three sepa- rate readings of two vertebrae from each guitarfish showed 73.8% in total agreement (43 guitarfish), 16.4% disagreement ± 1 band (10 guitarfish), 6.5% disagreement ± 2 bands (4 guitarfish), and 3.3% dis- D= 0.415 cm D- 0.420 cm Figure 6 Examples of opaque band formation indicating seasonal formation of bands (no photograph was available). The dia- grams of centra represent rays in the captive study that were injected twice with Terramycin. Tx = first injection, T2 = second injection, TZ = translucent zone, and D = cen- trum diameter. Gray areas indicate opaque band areas and are not to scale. Example A was held alive in captivity for 13 months, and example B was alive for 14 months. Timmons and Bray: Age, growth, and sexual maturity of Rhmobatos productus 355 agreement ± 3 bands (2 guitarfish). Only bands in total agreement (from 43 guitarfish) were used in the final analysis. The linear model best represented growth of com- bined sexes of guitarfish because the coefficient of determination was 0.90 for the linear regression, and 0.81 for the nonlinear von Bertalanffy curve (Fig. 7: Table 1). For females only, the linear regression and the von Bertalanffy curve produced similar values Estimated age (yr) Figure 7 Growth of the guitarfish predicted by a linear regression (dotted line). The solid line represents a von Bertalanffy growth curve for the data. Equations are those used to cal- culate the regression. for the coefficient of determination (r2=0.95, and r2=0.94, respectively). For males, the linear regres- sion also appeared to be a better predictor (r2=0.78) than the von Bertalanffy curve (r2=0.70; Table 1). Residuals for both the linear regression and the von Bertalanffy model (females and males) clustered evenly about both sides of the prediction lines. There was no reason to suggest any violation of homo- scedasticity in either model. If it is necessary to predict the total length of a specimen from fish markets using only the tail re- gion of the guitarfish, the following equation is sug- gested (Fig. 8): Table 1 A comparison of linear regression parameters and von Bertalanffy parameters for male (n= 24) and female (n=19) guitarfish and for combined sexes (n= 43). Linear regression parameters von Bertalanffy parameters Y-intercept Slope r2 k t0 r2 Female 34.02 8.29 0.95 594 0.016 -3.80 0.94 Male 47.00 6.32 0.78 142 0.095 -3.942 0.70 Female and male 43.33 6.90 0.90 228 0.047 -4.030 0.81 356 Fishery Bulletin 95(2), 1997 TL = 6.01 + 5.56(2,0), where TL = estimated total length of the guitarfish; and 2 D = second dorsal length (when 2D >3.5 cm and <20 cm). Reproductive maturity The smallest sexually mature female guitarfish was 99 cm TL and was estimated to be seven years old, based on vertebral band counts. Developing ovaries were present in 26 specimens from 40 to 99 cm TL. These individuals showed no evidence of previous birthing or egg follicles: uteri were thin walled and shell glands were not distinguishable from surround- ing oviducts. Immature female guitarfish accounted for the majority of specimens taken (27 of 36). A well-developed shell gland (nidimental gland) was present in mature shovelnose. Females with full uteri contained a case as described by Cox (1963) for Rhinobatos. In four individuals with full uteri, no developing embryos were seen in any of the speci- mens. These specimens contained either four or five yolks within the right or left egg case and, with the exception of one specimen, had nine total yolks per mature female. These four fish were captured in Feb- ruary (one), April (one), and June (two). Male guitarfish reached maturity between 90 and 100 cm TL. At maturity there was an abrupt increase in clasper length and claspers extended well beyond the pelvic fin (Fig. 9). Claspers of mature males were at least 13 cm in length, and clasper width at matu- rity was at least 1 cm. A well-developed spur was present on both claspers in mature males and was not present in immature males (Fig. 10). Immature male squaloid sharks also lack spines (Applegate, 1967). Twelve of the 38 sampled were mature and 26 were immature. Discussion Age and growth The shovelnose guitarfish is best described as a slow- growing species typified by linear growth after par- turition. Our total estimated age range (one to 11 years) for R. productus was the same that Lessa (1982) found fori?, horkelii. Her specimens were also in the same size range as R. productus (20 to 120 cm). Rossouw (1984) found ages 0 to 6 years in R. annulatus, and his largest specimen was 99.3 cm. Age estimates in this study were based on the as- sumption that one opaque and one translucent band • Mature • - o Immature n = 38 • - • • - ^ ° o %o° o o© 1 <9 1 1 -I 1 1 0 20 40 60 80 100 120 140 Total length (cm) Figure 9 Relation of clasper length with total length for males. Clasper length beyond 10 cm indicates a mature individual. Timmons and Bray: Age, growth, and sexual maturity of Rhinobatos productus 357 are formed annually. One verification procedure, ex- amination of individuals held in captivity, provided support for the outer margin analysis; however, this analysis was based on only two specimens. The sec- ond verification procedure, outer margin analysis of field-caught specimens, indicated that band forma- tion was dependent on season; however, there was no correlation between width of the outer margin and month. Early band formations at the margin can be difficult to detect with whole centra. To avoid this difficulty we tried sectioning the centra; however, we were unable to obtain readable sectioned centra. Oth- ers, such as Tanaka ( 1990 ) and Gruber and Stout ( 1983 ) have had success in sectioning vertebrae to view band formations. Therefore, we do not consider our verifica- tion procedure to be complete. It is evident that guitar- fish have linear growth which might be somatic and not correlated with age of the guitarfish, as was sug- gested by Natanson et al. (1984) for Squatina cali- fornica. Further studies should be attempted to answer this question. Specifically, we suggest more tagging and injection studies to validate laboratory data. Reproductive maturity We encountered a problem collecting large ( >90 cm) females; it has been suggested by Baxter ( 1980) and Lane and Hill ( 1975) that individuals of this size are uncommon. Our largest female was 130 cm. In Almejas, Baja California Sur, Mexico, Villavicencio- Garayzar (1993) reported that his largest captured female R. productus was 137 cm. Females in the present study were mature at >99 cm TL, whereas Villavicencio-Garayzar (1993) suggested that matu- rity of R. productus was at >70 cm TL. The youngest free-living guitarfish obtained was 23 cm TL, and it appears that the estimate of 15 cm (Eschmeyer et al., 1983) for newborn pups might be low. Melouk (1949) reported 16-cm specimens of R. halavi that still had sizable yolk attachments in utero. It is pos- sible that Eschmeyer’s measurements of 15 cm were taken from expelled premature pups. Expulsion of embryos can occur from stressed females (Pratt and Casey, 1990). Another possibility is that mortality is high in postpartum pups and many do not survive. Perhaps the smallest specimens that we sampled were first-year survivors. Rossouw ( 1984 ) suggested that the average length of Rhinobatos annulatus at birth was 23 cm TL and Dubois (1981) stated that embryos of R. productus at parturition were 23 cm. Villavicencio- Garayzar ( 1993) reported a free-swimming R. productus at 24 cm and suggested neonates are 20-24 cm. The first year class we collected (presumably represented as the smallest guitarfish we obtained) did not have any bands present beyond the birth mark. Many of the young guitarfish were captured by otter trawls in the Belmont Shores area in Long Beach, CA.; it appears that this is a nursery ground for guitarfish. Our estimates of nine offspring per female were also the mean number of offspring found by Villavicencio-Garayzar (1993) for Rhinobatos productus in Almejas, Baja California Sur, Mexico. He found thati?. productus females had a minimum of six pups and a maximum of 16. Additionally, Villavicencio-Garayzar (1995) found that Zapterix exasperata females contained a minimum of 4 and a maximum of 11 embryos (the most common numbers of embryos per individual were between 6 and 9). Males showed the same size at maturity as males sampled by Dubois (1981). His males were all ma- ture when TL exceeded 92 cm. No males in his study had clasper lengths in the range of 11 to 15 cm, indi- cating a definite size break in clasper length between immature and mature males. Our male guitarfish showed this same break between clasper lengths of 11 and 13 cm, and all males in our study were ma- ture when TL exceeded 100 cm. Our smallest ma- ture male was 91 cm. Both of our studies indicated a lack of individuals with clasper lengths in the 10-13 cm range, and Martin and Cailliet (1988) found a similar break in clasper lengths (between approxi- mately 22-37 cm) in Myliobatis californica. This in- dicated to us that sexual maturity occurred within a distinct size range (TL) for males. Visual examina- tions of the claspers confirmed maturity; they were well developed and occasionally contained semen. Villavicencio-Garayzar ( 1993) found male Rhinobatos productus with sperm in their vasa deferentia at 63, 68, and 69 cm TL, but did not indicate a length at first maturity. For Zapterix exasperata , Villavicencio- Garayzar (1995) found males at 69 cm with semen. Information from this research will provide a start- ing point for persons who may be interested in regu- lating guitarfish catch in the future. The informa- tion on size at first maturity for both males and fe- males and the equation for estimating total length of guitarfish from tails sold to markets by fisherman will be useful management tools. Although the age estimates of the guitarfish are preliminary, total length (TL) at sexual maturity is most valuable. This information provides a starting point for evaluation of possible future size limitations for catches of gui- tarfish. We suggest further studies in order to attempt to age guitarfish over its entire population range. Acknowledgments We would like to thank the Orange County Depart- ment of Fish and Game and the Department of Biol- 358 Fishery Bulletin 95(2), 1 997 ogy at CSULB for providing funds for this project. The following businesses kindly provided specimens: Ron’s Bait and Tackle in Redondo Beach, Terminal Island Seafood Company, and the sports fishermen of 22nd Street Landing in San Pedro. Maria Vohevic from the California Department of Fish and Game in Long Beach provided information on catches of guitarfish. Additionally, the following persons pro- vided support: Tim Dorsey of the Seal Beach Life- guards, Chris Lowe, Joe Sisneros, Kirk McCoy, Marsha Schindler, Alisa Shulman, Shelly Moore, Adel Rajab, Matthew Timney, the Young Scholars instruc- tors, Dave Soltz, Alan Miller, Cindy, Jessica, and Andrew Bray, and Peg and Ed Timmons. This re- search was part of an M.S. thesis, presented by the senior author at California State University Long Beach, Long Beach, California. Literature cited Applegate, S. P. 1967. A survey of shark hard parts. In P. W. Gilbert, R. F. Mathewson, and D. P. Rail (eds.). Sharks, skates, and rays. Johns Hopkins Press, Baltimore, MD, 624 p. Baxter, J. L. 1980. Inshore fishes of California. Dep. Fish Game, Sac- ramento, CA, 72 p. Cailliet, G. M., L. K. Martin, P. Kusher, P. Wolf, and B. A. Welden. 1983. 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Culquichicon, and T. Alvarado. 1989. Artisanal fishing at Constante cove (Piura, Peru): status quo and perspectives. Simp. Int. Recursos vivos Pesquerias Pacifico Sudeste, Vina del Mar (Chile), 9 May 1988. In Rev. Com. Perm. Pac. Sur, p. 485-494. Villavicencio-Garayzar, C. J. 1993. Biologia reproductiva de Rhinobatos productus (Pi- sces: Rhinobatidae), en Bahia Almejas, Baja, California Sur, Mexico. Rev. Biol. Trop. 41(3):777-782. 1995. Biologia reproductiva de la Guitarra Pinta, Zapterix exasperata (Pisces: Rhinobatidae), en Bahia Almejas, Baja California Sur, Mexico. Cienc. Mar. 21(2):141— 153. von Bertalanffy, L. 1938. A quantitative theory of organic growth (inquiries on growth laws. 2). Hum. Biol. 10:181-213. 360 Food habits and energy values of prey of striped marlin, Tetrapturus audax, off the coast of Mexico Leonardo A. Abitia-Cardenas* Felipe Galvan-Magana Departamento de Pesqueriasy Biologia Marina. Centro Interdisciplinario de Ciencias Marinas, IPN. Apdo. Postal 592. La Paz, Baja California Sur, Mexico. C.P 23000 *E-mail address. Iabitia@vmredipn.ipn.mx Jesus Rodriguez-Romero Centro de Investigaciones Biologicas del Noroeste, S.C. Apdo. Postal 128. La Paz, Baja California Sur, Mexico. C.P 23000. The waters off the tip of the Baja California peninsula are good fish- ing grounds for striped marlin, Tetrapturus audax (Squire and Suzuki, 1990) because they offer a shallow thermocline and an abun- dant food supply (Hanamoto, 1974). Although striped marlin are an important game fish, few biological studies have been done on them. Most trophic studies on marlin spe- cies have simply identified and de- termined the relative importance of food consumed in a given geo- graphic region and were based on few samples (Morrow, 1952; Hubbs and Wisner, 1953; Yabuta, 1953; La Monte, 1955; de Sylva, 1962; Will- iams, 1967; Koga, 1968). Only two studies have been done off the coast of Mexico in the Pa- cific Ocean. Evans and Wares (1972) described the stomach con- tents of striped marlin caught at three locations off southern Califor- nia and Mexico (San Diego, Ma- zatlan, and Buenavista) from 1967 to 1969. They found in Buenavista, the site closest to our study area, that the food for marlin consisted mainly of squid and fish, particu- larly red-eye round herring (Etru- meus teres ) and chub mackerel (Scomber japonieus). In the second study, Eldrige and Wares (1974) de- scribed food habits, seasonal abun- dance, and parasites of striped mar- lin caught in 1970 near the same lo- cations. The differences found, in comparison with the first study were the absence of S. japonieus and a greater importance for three fish spe- cies: E. teres , black skipjack (Euthyn- nus lineatus ), and oceanic puffer (Lagocephalus lagoeephalus). This paper provides information on food habits and energy content of the principal prey consumed by striped marlin in waters off the coast of the Baja California penin- sula, Mexico. Materials and methods Striped marlin were caught by trolling with live chub mackerel, S. japonieus, and jacks, Caranx spp., as bait or by jigs used by the sport fishing fleet. All fish were collected at approximately 22° 53'N, 109°54'W (Fig. 1) near Cabo San Lucas, Baja California Sur (B.C.S.), Mexico. Stomachs were sampled in port, May 1988 to December 1989, by personnel of the Centro Inter- disciplinario de Ciencias Marinas (CICIMAR), La Paz, B.C.S. Each fish was weighed to the nearest kg and its length (eye fork length) measured to the nearest cm. Stom- ach contents were removed and fixed in 10% formalin. Prey were identified to the lowest possible taxon. Vertebral characteristics (e.g. number, position) were used to identify fish with the help of taxo- nomic keys (Clothier, 1950; Monod, 1968; Miller and Jorgensen, 1973). The fish collection of CICIMAR was also used for comparison and valida- tion of identifications. For complete, undigested fish, the keys of Jordan and Evermann (1896-1900), Meek and Hildebrand (1923-28), Miller and Lea (1972), and Thomson et al. (1979) were used for identification. Crustacean prey were identified from exoskeleton remains with keys pro- vided by Garth and Stephenson (1966) and Brusca (1980). Cephalo- pods were identified from mandibles with the keys of Clarke ( 1962, 1986), Iverson and Pinkas ( 1971), and Wolff (1982, 1984). The stomach contents were enu- merated (A0 and the volume (VO mea- sured to the nearest mL. These two measures and frequency of occur- rence (FO) were combined to calcu- late the index of relative importance (IRI) of Pinkas et al. (1971) as IRI = (%N + %V) %FO. IRI is a commonly used measure that provides a more representative summary of dietary composition (Caillet et al., 1986). A multivariate analysis of vari- ance (MANOVA) was made on IRI values to examine differences in the relative importance of prey by sea- son and between species. The treat- ment included only five seasons because the data in two seasons (summer and fall 1989) had too few values for statistical analysis (Table 1). The data were standard- ized following the formula Manuscript accepted 4 November 1996. Fishery Bulletin 95:360-368 (1997). NOTE Abitia-Cardenas et al : Food habits and energy values of prey of Tetrapturus audax 361 Figure 1 Map showing the location of the study area off the tip of Baja California. xt-X /SD, where: x; = the absolute IRI value of each prey species; X = the mean value of the IRI; and SD = standard deviation. The caloric content of each prey, based on three samples obtained from stomach contents, was mea- sured with a Parr 1241 adiabatic calorimeter and expressed as calories per gram of dry weight, wet weight, and ash-free dry weight following Phillipson (1964). One-way analysis of variance was used to evaluate differences between ash-free dry weight caloric values of particular prey. Also a post-hoc test T-method (Sokal and Rohlf, 1981) was used to com- pare the means of dry-weight caloric values. The calories provided by each prey species were calculated by multiplying the values (calories/g wet weight) of each prey by the sum of their total contri- bution (weight) in the diet. To convert prey volumes to calories we assumed a density of 1.0 g/mL. Results Food habits Striped marlin (403) were sampled. The mean pos- torbital length was 177 ± 15 cm (standard deviation) and the mean weight was 58.4 ± 12.8 kg. Of those specimens sampled, 27 (6.7%) had empty stomachs and 26 (6.5%) had regurgitated their stomach con- tents. A total of 33 prey taxa were identified that comprised fish, cephalopods, and crustaceans. Only 17 prey types could be identified to species (Table 2). The most important prey by volume were fish (86.2%), including S. japonicus (25.7%), California pilchard, Sardinops caeruleus, (18.8%), and E. teres (10.2%). Cephalopods made up 12.8% of the total volume, and jumbo flying squid, Dosidicus gigas, was particularly important (11.3%). Crustaceans, mainly red crab, Pleuroncodes planipes, represented only 1% of the total volume. A total of 2,679 organisms were enumerated, 68.6% of which were fish, 21.3% cephalopods, and 10.2% crustaceans. The dominant fish prey by number were S. caeruleus (18.9%), S. japonicus (14.3%), and Pa- cific hake, Merluccius productus, (9.6%). The cepha- lopod D. gigas represented 14.9%, and Argonauta spp. 3.0% of the total stomach contents by number. Pleuroncodes planipes was the most abundant crus- tacean, representing 7.2% of the total number of food items. In frequency of occurrence, fish were the most im- portant food in the diet of striped marlin (93.4%), particularly S. japonicus (45.4%), S. caeruleus (27 .7%), and E. teres (12.6%). Cephalopods occurred in 32.9 % of the samples; and D. gigas was the most common species (28.3%). Crustaceans, mainly P. planipes , occurred in 6.3% of samples. According to the IRI, fish were the most impor- tant prey (80.7%) of striped marlin, followed by cephalopods (18.5%), and crustaceans (0.8%). Scomber japonicus , S. caeruleus x and D. gigas were the most important fish prey (Fig. 2). Relative importance of several prey varied season- ally (Table 1). During 1988, fish were the most im- portant prey in spring and fall, cephalopods the most important prey in summer. In spring 1988, S. caeruleus was the most important fish in the diet, followed by S. japonicus and E. teres. In summer 1988, the most important species was D. gigas, fol- lowed by the fish Selar crumenophtalmus , S. japonicus, and E. teres. In fall 1988, the highest IRI values were for S. japonicus, D. gigas, E. teres, and M. productus. During 1989, fish were the most important prey in all seasons, followed by cephalopods and crusta- ceans. In winter, the dominant species were S. japonicus, M. productus x and S. caeruleus. In spring, S. japonicus, D. gigas, S. caeruleus x and E. teres were the most important species. In summer, Caranx caballus was the most important prey. In fall, the highest IRI values were for S. caeruleus, S. japonicus, and Decapterus hypodus. The MANOVA showed no significant differences among seasons in the IRI val- ues of food groups consumed (F=1.96; df=4; P=0.11). However, when we considered taxa consumed (33 recorded), we found significant differences (F- 17.6; df= 32; P<0.005), probably caused by the greater 362 Fishery Bulletin 95(2), 1 997 Table 1 Summary of food categories in stomach contents of striped marlin from Cabo San Lucas, B.C.S., Mexico, expressed as percentages based on frequency of occurrence (FO), number (n), volume (Vol.), and index of relative importance (IRI). Prey FO % FO n % n Vol. % Vol. IRI % IRI Mollusca Cephalopoda Teuthoidea Enoploteuthidae Abraliopsis affinis 12 3.43 46 1.72 1,254 0.65 8.13 0.19 Ommastrephidae Dosidicus gigas 99 28.3 399 14.9 21,866 11.3 740.37 17.8 Stenoteuthis oualaniensis 15 4.28 34 1.27 688 0.35 6.93 0.17 Octopoda Octopodidae Octopus spp. Argonautidae 4 1.14 11 0.41 131 0.07 0.55 0.01 Argonauta spp. 13 3.71 80 2.99 819 0.42 12.65 0.3 Total 570 21.29 24,758 12.79 768.63 18.47 Arthropoda Crustacea Amphipoda 2 0.57 22 0.82 13.5 0.01 0.47 0.01 Isopoda Stomatopoda 3 0.86 8 0.3 3 0 0.26 0 Squillidae Squilla spp. 1 0.28 1 0.04 15.1 0 0.01 0 Euphausiacea Decapoda Galatheidae 3 0.86 48 1.79 11 0.05 1.55 0.04 Pleuroncodes planipes 14 4 193 7.2 1,929 0.99 32.76 0.79 Total 272 10.15 1,971.6 1.05 35.05 0.84 Chordata Osteichthyes Clupeiformes Clupeidae 30 8.57 12 0.44 3,206 1.65 17.99 0.43 Etrumeus teres 44 12.57 199 7.42 19,681 10.16 220.98 5.31 Ophistonema libertate 10 2.86 27 1.01 4,985 2.57 10.24 0.25 Sardinops caeruleus Gadiformes 97 27.7 507 18.92 36,492 18.83 1,046.05 25.15 Merluccidae Merluccius productus Cyprinodontiformes Belonidae 19 5.43 257 9.59 16,619 8.58 98.66 2.37 Strongylura spp. Syngnathiformes Fistulariidae 1 0.28 1 0.04 340 0.17 0.06 0 Fistularia spp. Scorpaeniformes Triglidae 16 4.57 38 1.42 5,065 2.61 18.42 0.44 Prionotus spp. Perciformes 1 0.28 1 0.04 10 0.01 0.01 0 Serranidae 1 0.28 2 0.07 245 0.13 0.06 0 Carangidae 10 2.86 15 0.56 2,111 1.09 4.72 0.11 Caranx caballus 11 3.14 15 0.56 1,988.5 1.02 4.96 0.12 Caranx hippos 9 2.57 10 0.37 796 0.41 2 0.05 Decapterus hypodus 18 5.14 87 3.25 8,365 4.32 38.91 0.93 Selar crumenophthalmus 16 4.57 31 1.16 3,337 1.72 13.16 0.32 Coryphaenidae continued on next page NOTE Abitia-Cardenas et al.: Food habits and energy values of prey of Tetrapturus audax 363 Table 1 (continued} Prey FO % FO n % n Vol. % Vol IRI % IRI Coryphaena hippurus Mugilidae 1 0.28 i 0.04 180 0.09 0.04 0 Mugil spp. 1 0.28 i 0.04 290 0.15 0.05 0 Sphyraenidae Sphyraena ensis Scombridae 1 0.28 2 0.07 680 0.35 0.12 0 Auxis spp. 10 2.85 83 3.09 7,870.5 4.06 20.38 0.49 Scomber japonicus Tetraodontiformes 159 45.43 382 14.26 49,778.5 25.69 1,814.93 43.63 Balistidae Batistes polylepis 18 5.14 164 6.12 4,844.5 2.5 44.31 1.06 Xanthichthys mento Diodontidae 1 0.28 1 0.04 no 0.06 0.03 0 Diodon spp. 1 0.28 1 0.04 27 0.01 0.01 0 Total 1,837 68.55 167,021 86.18 3,356.09 80.66 Unidentified organic matter 1 0.28 145 0.07 0.02 0 Percent index of relative importance G> CL 1 Scomber japonicus 2 Sardinops caeruleus 3 Etrumeus teres 4 Dosidicus gigas 5 Merluccius productus 6 Auxis spp. 7 Decapterus hypodus 8 Fistularia spp. 9 Batistes polylepis 1 0 Selar crumenophthalmus 1 1 Pteurocodes planipes Percent frequency of occurrence Figure 2 The major prey species found in the stomachs of striped marlin pre- sented as percentages of number of individuals, volume, frequency of occurrence, and IRI. number of five prey species: D. gigas, S. japonicus, S. caeruleus, E. teres, and M. productus. Calorimetric analysis The energy content of the most important prey of striped marlin as wet, dry, and ash- free dry weights, is given in Table 3. Val- ues ranged from 3.42 kcal/g dry weight for red crab, P. planipes, to 6.14 kcal/g dry weight for the cornet fish, Fistularia spp. The ANOVA showed that the caloric val- ues of the 11 most important prey were significantly different (,F=904.3; df=10; P=2.3E-26). When the means of the caloric values were compared by P-method, a sig- nificant difference was obtained (a=0.05) (Fig. 3). Caloric percentages of the 11 major prey types (Fig. 4), indicate two species, S. ja- ponicus (32.4% ) and S. caeruleus (21.2%), contributed 53.7% of the total calories to the diet of striped marlin. Discussion Food habits Previous studies have shown that striped marlin mainly consume prey that school near the surface. Such prey are generally 364 Fishery Bulletin 95(2), 1997 fish of the families Engraulidae (Hubbs and Wisner, 1953; de Sylva, 1962; Evans and Wares, 1972; Holts and Bedford, 1990), Clupeidae (Hubbs and Wisner, 1953; Koga, 1968), Scombridae (Backer, 1966; Evans and Wares, 1972), Scomberesocidae (Morrow, 1952; Hubbs and Wisner, 1953), and Carangidae (de Sylva, 1962; Backer, 1966; Evans and Wares, 1972), and some cephalopods (Morrow, 1952; Yabuta, 1953; La Monte, 1955; de Sylva, 1962; Williams, 1967; Eldrige and Wares, 1974). We also found that striped marlin feed on demer- sal species, such as M. productus and searobins, Prionotus spp, as well as on benthic species, such as mantis shrimp, Squilla spp. Other authors have found occasional prey from benthic or reef habitats in striped marlin (Morrow, 1952; Backer, 1966; Wil- liams, 1967; Evans and Wares, 1972; Eldrige and Wares, 1974); thus, it appears that striped marlin move to the bottom to prey on benthic organisms. Our results show the importance of seasonal prey availability off Cabo San Lucas. During spring 1988, S. caeruleus was the main prey of striped marlin, whereas in fall and winter, S. japonicus was more important. The latter is probably more abundant in Table 2 Seasonal absolute values of the index of relative importance (IRI) of the stomach contents of striped marlin from Cabo San Lucas, B.C.S., Mexico (WI = Winter, SP = Spring, SU = Summer, FA = Fall). Species 1988 SP 73 = 55 1988 SU 73 = 34 1988 FA 73 = 92 1989 WI 73 = 56 1989 SP 73 = 67 1989 SU 73 = 11 1989 FA 73 = 35 Cephalopoda Abraliopsis affinis 0 0 6.13 160.62 0.82 0 0 DosicLicus gigas 21.08 2,637.22 480.66 59.48 1,031.04 0 672.83 Stenoteuthis oualaniensis 12.21 8.58 24.28 0.97 1.79 0 0 Octopus spp. 1 1.99 0.31 0 2.49 0 0 Argonauta spp. 4.14 42.34 38.08 2.04 0 0 0 Crustacea Amphipoda 0 0 0 0 0 0 33.57 Isopoda 0 0 1.05 0 0 0 0 Squilla spp. 0 0 0 0 0.45 0 0 Euphausiacea 10.32 21.49 0 0 3.04 0 0 Pleuroncodes planipes 22.56 3.09 104.10 16.99 13.50 628.17 0 Osteichthyes Clupeidae 49.08 27.96 9.59 18.31 33.27 0 0 Etrumeus teres 368.43 346.57 357.95 67.47 369.22 0 51.76 Sardinops caeruleus 8,049.39 30.58 102.05 473.74 739.77 644.94 2,072.04 Opisthonema libertate 0 0 0 0 295.76 0 0 Merluccius productus 0 0 203.19 855.85 49.07 0 0 Strortgylura spp. 0 0 0 2.27 0 0 0 Fistularia spp. 0 0 83.03 40.63 0 0 51.76 Prionotus spp. 0 0 0 0.50 0 0 0 Serranidae 0 0 0.81 0 0 0 0 Carangidae 0 74.97 1.97 27.56 1.55 0 0 Caranx caballus 25.30 27.81 4.17 0 0 780.10 0 Caranx hippos 4.48 0 0.59 2.86 0 0 19.88 Decapterus hvpodus 54.50 58.56 1.62 0 0 0 1,036.23 Selar crumenophthalmus 1.60 446.19 28.31 2.17 0 0 0 Coryphaena hippurus 0 0 0 1.41 0 0 0 Mugil spp. 1.87 0 0 0 0 0 0 Sphyraena ensis 0 0 0 0 0 586.04 0 Auxis spp. 2.69 8.76 131.51 22.63 0 0 0 Scomber japonicus 1,299.15 351.06 1,957.42 4,324.37 2,117.20 0 1,073.73 Balistes polylepis 0 0 115.06 0 0 0 864.20 Xanthichthys mento 0 0 0.37 0 0 0 0 Diodon spp. 0 2.09 0 0 0 0 0 Unidentified organic matter 0 0 0.07 0 0.03 0 0 NOTE Abitia-Cardenas et al. : Food habits and energy values of prey of Tetrapturus audax 365 the area, as happens in waters off southern Califor- nia where fall and winter catches present large num- bers of chub mackerel (Roedel, 1952). Both S. japonicus and S. caeruleus were found in some stom- achs, but this finding is not surprising because S. japonicus is abundant off Baja California and in the Gulf of California (MacCall, 1973), where mixed populations of S. japonicus and S. caeruleus are of- ten found (Kramer, 1969). During summer, the greater numbers of the jumbo squid D. gigas in striped marlin stomachs are not surprising because this squid is very common in waters from 200 to 2,000 m in depth off Cabo San Lucas (Sato, 1976). This species, from subtropical and tropical waters, undergoes long, large seasonal migrations. The presence of D. gigas can be associated with tropical water masses at the en- trance of the Gulf of California (25° to 29°C) and with the occurrence of prey species (pilchards and macker- els) in this area (Erhardt et al., 1986). Our results, compared with those of studies in other areas, showed similar types of prey consumed by striped marlin. Previous studies found that striped mar- lin commonly feed on clupeids, scombrids, jacks, and cephalopods. Striped marlin in New Zealand ate saury and squid (Mor- row, 1952). Baker ( 1966), in the same area, found that jacks and cephalopods were the main prey. In Peru and Chile, La Monte ( 1955) and de Sylva ( 1962) found cephalo- pods, engraulids, and jacks in the stom- ach contents of striped marlin. In East Africa, Williams (1967) found cornet fish (Fistularia sp.), bullet mackerel (Auxis thazard), and unidentified squid. Fish of the families Alepisauridae and Clupeidae are common in the Tasman Sea (Koga, 1968). Around the Bonin Islands, striped marlin ate Gempylus sp., Pseudoscopelus sp Alepisaurus sp., Ostracion sp., cepha- lopods, and crustaceans (Yabuta, 1953). In the eastern Pacific Ocean, Hubbs and Wisner (1953) found that striped marlin consumed saury, anchovy, and sardine. Evans and Wares (1972) and Eldrige and Wares (1974) found that the most important prey of striped marlin off Buenavista, Mexico, included the fish E. teres, Euthynnus lineatus, Lago- cephalus lagocephalus, and S. japonicus, as well as the squid D. gigas. These findings are similar to those of our study, even though the relative impor- tance of the main species dif- fered; e.g. in our study S. japon- icus and S. caeruleus were more important than E. teres, and squid were less important. These results indicate that the prey composition of striped mar- lin probably has not changed drastically off the coast of 6500 6000 5500 5000 4000 3500 3000 A Maximum value ■ Minimum value • Average value Figure 3 Comparison of group caloric values (cal/g dry wt ) of dominant prey: 1 = D. gigas, 2 = P. planipes, 3 = E. teres, 4 = S. caeruleus, 5 = M. productus, 6 = Fistularia sp., 7 = D. hypodus, 8 = S. crumenophtalmus, 9 = Auxis spp., 10 = S. japonicus, and 11 = B. polylepis. Scomber japonicus (32.4) Sardinops caeruleus (21 .2) Selar crumenophthalmus (1 .7) Auxis spp. (4.6) r Balistes polylepis (2 2) Decaplerus hypodus (4.5) Dosidicus gigas (1 0.5) Etrumeus teres (10. 7) Fistularia spp. (4.3) Pleuroncodes planipes (0.5) Merluccius productus (1 .4) Figure 4 Caloric contribution (expressed as a percent) of the eleven dominant prey types of striped marlin. 366 Fishery Bulletin 95(2), 1997 Table 3 Mean and standard deviation (SD) caloric values, water content, and ash content of prey in the diet of striped marlin. Kcal/g Kcal/g Kcal/g Prey % Water SD % Ash SD wet wt SD dry wt SD ash-free dry wt SD Cephalopoda Dosidicus gigas 70.02 0.97 2.95 0.04 Crustacea Pleuroncodes planipes 72.66 0.05 4.67 0.03 Osteichthyes Etrumeus teres 64.34 1.10 3.78 0.04 Sardinops caeruleus 65.92 0.38 2.71 0.01 Merluccius productus 68.92 1.01 5.60 0.01 Fistularia spp. 64.44 0.72 13.05 0.07 Decapterus hypodus 64.95 0.34 6.09 0.03 Selar crumenophthalmus 68.64 0.60 7.00 0.01 Auxis spp. 66.31 0.83 1.53 0.03 Scomber japonicus 63.90 0.10 3.16 0.03 Batistes polylepis 69.38 0.54 2.83 0.15 1.57 0.08 5.24 1.20 5.40 0.13 0.94 0.01 3.42 0.11 3.59 0.01 1.80 0.05 5.06 0.03 5.26 0.01 1.77 0.02 5.19 0.09 5.33 0.01 1.47 0.06 4.74 0.57 5.02 0.06 2.18 0.04 6.14 0.11 7.06 0.01 1.79 0.01 5.11 0.12 5.44 0.01 1.53 0.03 4.87 0.02 5.24 0.00 1.92 0.05 5.69 0.28 5.78 0.03 2.16 0.01 5.99 0.01 6.19 0.00 1.48 0.02 4.83 0.14 4.97 0.01 Mexico in the last two decades. Cabo San Lucas ap- pears to be an area with stable prey populations, probably the result of prevailing oceanographic con- ditions (Roden and Groves, 1959; Alvarez, 1983). In waters off Baja California, the thermocline is generally shallow and there is a correspondingly high standing crop of zooplankton (Brandhorst, 1958). Laevastu and Rosa (1963) suggested that the shal- low thermocline promotes a high standing crop of zooplankton and thus increases the production of small foraging organisms, which in turn may result in the aggregation of top predators. It is likely that the seasonal shifts in good fishing areas for striped marlin coincide with shallow thermocline areas. Feeding ecology, however, may play a major role in determining the distribution and abundance of striped marlin in some areas. Calorimetric analysis Of the eleven most important prey analyzed, P. planipes had a significantly low caloric content, com- mon in crustaceans (Golley, 1961; Slobodkin and Richman, 1961; Thayer et al., 1973). Paine (1964) concluded that the presence of calcium carbonate and calcium phosphate in cuticle and valves was the cause of their low caloric value. We found our results agree well with values from other studies. Thayer et al. (1973) found a caloric value of 5.74 kcal/g dry weight and 1.05 kcal/g wet weight for the squid Loligo brevis. For crustaceans, caloric values ranged between 2.12 and 6.03 kcal/g dry weight (average value: 5.74 kcal/g dry weight, range: 0.80-1.48 kcal/g wet weight). They also found fish contained 4.39 to 6.0 kcal/g dry weight and 0.67 to 1.57 kcal/g wet weight. Cortes and Gruber (1990) estimated the energy content of prey of lemon shark, Negaprion brevirostris, and found caloric values of 4.81 kcal/g dry weight and 0.68 kcal/g wet weight for cephalopods, Octopus spp. Crustaceans of the genus Callinectes yielded 3.2 kcal/g dry weight and 1.04 kcal/g wet weight. For fish, Cortes and Gruber found values that ranged from 3.38 to 4.73 kcal/g dry weight and 0.96 to 1.86 kcal/g wet weight. Our results show that pelagic fishes and cephalo- pods yielded more than 80% of the caloric content in the diet of striped marlin. However, if we take into account that more than 70% of the stomachs were less than full and that the predatory capacity of striped marlin allows them to consume large quan- tities of prey in a short time, as is the case with yel- lowfin tuna, Thunnus albacares (Olson and Boggs, 1986), a pelagic species with feeding habits similar to those of marlin in the eastern Pacific Ocean, we believe the estimated caloric values underestimated actual energy intake. In summary, we consider that striped marlin is a generalist as a predator and has a high predatory capacity, foraging mainly on schools of epipelagic organisms in neritic and oceanic zones. Acknowledgments We wish to thank Robert J. Olson of the Inter-Ameri- can Tropical Tuna Commission, David B. Holts of the NOTE Abitia-Cardenas et al. : Food habits and energy values of prey of Tetrapturus audax 367 Southwest Fisheries Science Center, and Sergio Martinez of CICIMAR for the statistical analysis. 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Lab. 1(5), 6 p. 369 Note on plankton and cold-core rings in the Gulf of Mexico Douglas C. Biggs* Robert A. Zimmerman Department of Oceanography Texas A&M University, College Station, Texas 77843-3146 *E-mail address: dbiggs@ocean.tamu.edu Rebeca Gasca Eduardo Suarez-Morales Ivan Castellanos ECOSUR-Unidad Chetumal A.P 424, Chetumal, QR 77000, Mexico Robert R. Leben Colorado Center for Astrodynamics Research University of Colorado, Boulder, Colorado 80309 Data from ship and aircraft hydro- graphic surveys, supplemented with data from current meter moor- ings and drifters, have demon- strated that one or more cyclonic circulation features, 100-200 km in diameter, are often present in the Gulf of Mexico. In the eastern Gulf, these cold-core rings (CCR’s) occur in close association with the Loop Current (LC) (Lee et ah, 1994), and in the central and western Gulf, they are companions of the anticy- clonic eddies that are shed during northern excursions of the LC (Hamilton, 1992). Cyclone-anticy- clone dipoles and cyclone-anticy- clone-cyclone triads have been de- scribed (Lewis and Kirwan, 1985; Rouse et al., 1994; Vidal et aL, 1994). Temperature-salinity relationships document that cyclones and anti- cyclones in the Gulf of Mexico form from the same water types, but con- vergence flow within the anticy- clones causes the surface waters of these gyres to be regions of low pro- duction. The upper 100 m are de- pleted in nitrate and chlorophyll concentrations, primary productiv- ity, and zooplankton biomass are generally extremely low (Biggs, 1992). In contrast, the companion cyclones are mesoscale regions of divergence flow. From nutrient- chlorophyll data collected during several cruises when Gulf of Mexico CCR’s were tracked, Biggs et al. (1988) hypothesized that cyclones were regions of locally high primary productivity which could support elevated stocks of zooplankton. In March 1993, a CCR was de- tected by remote sensing of the western central Gulf of Mexico, and the opportunity arose to study its hydrographic and biological signa- ture as RV Gyre transited this fea- ture while proceeding along a TOPEX ground track. This meso- scale cyclonic circulation was vis- ible in remote sensing data as a region of surface temperatures 1- 2°C cooler than the adjacent oce- anic surface waters (Fig. 1; Table 1) and as an elliptical local depres- sion in sea surface height (SSH) (-15 to -20 dyn cm of SSH anomaly; see Table 1). Expendable bathy- thermographs (XBT’s), dropped to profile isotherm depths in the up- per 760 m, resolved strong doming of subsurface isotherms within this CCR (Fig. 2), and Gyre’s hull- mounted 153 kHz acoustic Doppler current profiler (ADCP) confirmed that cyclonic near-surface currents were associated with this feature. Both the amplitude and direction of these ADCP-measured currents were found to be in close agreement with those computed from the along-track horizontal geopotential gradient in relation to a reference level of 800 db: a low of 88 dyn cm in the interior of the CCR, versus >102 dyn cm to the north and south (Table 1). This -14 cm SSH gradi- ent between interior and periphery of the CCR over a distance of 100 km should have driven a cyclonic transport (relative to 800 db) of 6- 7 Sverdrup, with mixed-layer ve- locities of 40-60 cm/s (see p. 165- 166 in Texas A&M1). Altimeter-de- rived dynamic height anomalies from TOPEX Cycle 18, which flew over the ship’s track just as Gyre completed the XBT survey, showed agreement with the hydrographic estimates to better than 2 cm re- sidual mean squares difference with respect to a corrected along- track mean surface, which is within the generally accepted error range for altimeter measurements (Leben et al, 1993). A comparison with TOPEX Cycle 17 data from 10 days earlier also demonstrates the pres- ence of this cylone (see Fig. 2 in Leben et al, 1993). Time constraints did not allow us to divert the ship to make a more detailed hydrographic survey of the CCR or to stop to make time-series measurements. However, we were 1 Texas A&M University. 1993. Ship-of- opportunity hydrographic data from PUV Gyre cruise 93G-03. Tech. Rep. 93-04-T, Dep. Oceanography, TAMU, College Sta- tion, TX, 216 p. Available from NTIS, Springfield, VA: PB94-123957. Manuscript accepted 26 November 1996. Fishery Bulletin 95:369-375 (1997). 370 Fishery Bulletin 95(2), 1 997 able to slow the vessel eight times for 15-20 min periods for net tows in order to collect zooplankton at three stations within and five outside the CCR for comparison with ADCP acoustic backscatter inten- sity data logged while these tows were made. The ADCP acoustic backscatter intensity data were col- lected according to the methods described by Flagg and Smith (1989) and Zhou et al. (1994). Figure 1 Analysis of NOAA-12 advanced very-high-resolution radiometer satellite imagery of sea surface temperatures (SST) processed by Louisiana State University. Circles give the location of the 33 XBT stations; the numbers inside these circles designate where zooplankton tows 1-8 were made. Lighter grey shades represent lower SST; the CCR is visible as a region of locally cooler surface temperatures encircling XBT’s 10-17 (zooplankton tows 2-4). Arrowheads show the expected counter-clockwise circulation in CCR periphery. NOTE Biggs et a I.: Plankton and cold-core rings in the Gulf of Mexico 371 The plankton tows were made with an open net of 333-pm mesh and 1 m in diameter, at every 3rd XBT site beginning at 27°00'N. The net was outfitted with an impeller-type flowmeter (General Oceanics) that allowed the volume of water fished to be determined upon recovery of the net. In 15-20 min oblique tows to 100 m depth, volumes of water fished ranged from 450 to 800 m3. Collections were preserved in a 4% formaldehyde solution buffered with borax, and then bulk sample displacement volume was measured according to the method of Ahlstrom and Thrailkill (1960). Four of the 8 tows were made during day- light hours and the other 4 were made at night. Tow 1 was made in daylight outside and to the northwest of the CCR, whereas tows 2-4 were at night, within the CCR. Tows 5-7 were daylight tows outside and to the southeast of the CCR, and tow 8 the following night was also outside and to the southeast of the CCR. Because only nighttime tows were able to be made in the CCR, we chose to enumerate the taxo- nomic composition of all eight samples for three groups of macrozooplankton that are well known to exhibit diel vertical migration (Gasca et al., 1995). Each sample was split 1:4 with a Folsom plankton splitter and then euphausiids, thecosome pteropods, and siphonophores were enumerated to species at the Centro de Investigaciones Quintana Roo. | i i i i LAT: 27 40' 26 40' 25 40' 24 40' 23 40' Figure 2 Along-track plot of the doming of 15°C and 8°C isotherms, show- ing the cyclone as a 150-km wide region where 8°C <500 m. Results Zooplankton biovolume averaged 2.4-fold higher in nighttime tows than in daytime tows (90 ver- sus 38 mL/1,000 m3; see Table 2). This eleva- tion of stock at night reflects greatly increased numbers of euphausiids at night, for most of the euphausiid species present in the western Gulf of Mexico perform vertical migratory pat- terns during a day-night cycle. During the night hours, these euphausiid species can be collected in the upper 200 m (Mauchline, 1980). How- ever, the numbers and kinds of euphausiids present at night inside versus outside the CCR were quite different: 56% of the number of eu- phausiids inside the CCR were species of the genus Euphausia, whereas 63% of the individu- als in the night tow outside the CCR belonged to two species of the smaller-size genus Stylocheiron. Moreover, euphausiid species of the genus Euphausia at night were, on aver- age, 1.8-fold more abundant within the CCR than outside (321 individuals/1,000 m3 inside, Table 1 Surface temperature, mixed layer (ML) depth, temperature at 100 m, and the calculated dynamic height (relative to 800 db) for stations where plankton tows were made. Plankton Tow Local Time XBT station Temp (°C) at surface Depth (m) to reach surface temp, minus 1°C (= ML depth) Temp (°C) at 100 m Dynamic height (cm) 1 15:52-16:09 7 23.24 54 19.7 102 2 19:23-19:38 10 22.34 72 19.2 97 3 23:11-23:27 13 21.50 75 17.8 88 4 03:08-03:23 16 21.75 71 19.0 94 5 07:08-07:19 19 22.50 99 21.4 106 6 12:11-12:26 22 23.62 98 22.4 115 7 16:08-16:25 25 23.39 112 22.5 113 8 21:17-21:34 28 23.36 95 21.9 111 372 Fishery Bulletin 95(2), 1 997 versus 179 individuals/1,000 m3 outside), the most abundant being E. tenera, E. mutica, and E. americana. Species richness of euphausiids was also greater inside the CCR than outside: five species (Euphausia pseudogibba, E. brevis, Nematoscelis atlan- tica, Thysanopoda monoacantha, and Nematobrachion flexipes) were found only in collections made inside the CCR. Details are available from the authors in a 3- page table. We speculate that the presense of the later three mesopelagic euphausiid species within the CCR, but not recorded outside the CCR, reflects the extension of their upper vertical distribution limits: where cold water domed shallower than 100 m, these mesopelagic species reached up into the zone where our nets collected samples at night. We have calculated a mean acoustic backscatter intensity (ABI) 0-200 m by time-averaging ADCP data from the 15-20 minute periods when net tows were made. The ADCP was calibrated, as explained by Zimmerman (1993), with mean ABI expressed as dB re(Mx47t)-1. We also computed the integrated ABI (I ABI): the amount of backscatter that was greater than the grand mean of -74 dB for the upper 200 m, bin by bin, from the 8-12 m bin to the 96-100 m bin) to provide a summary number for comparison with wet displacement volume of zooplankton collected from 0 to 100 m in the net tows. Figure 3 summarizes the mean ABI during the ensembles when net collections were being made. These acoustic data have been corrected for sound attenuation with depth, which was modeled from the T/Z relationship at each XBT station. Sub- surface regions of locally intensified return (locally higher backscatter) are presumed to be local concen- trations of biological scatterers. Although these regions of locally enhanced backscatter were concentrated into the vertical range of 60-100 m during the day, they reached closer to the surface and occurred over a greater vertical range of the water column at night. The ABI data, however, are not sufficient to distinguish whether at night euphausiids were more abundant and more species rich within the CCR than without. Discussion In the decade since lies and Sinclair (1982) recog- nized the existence of larval retention zones caused by oceanographic features, the relations between stocks of phytoplankton, zooplankton, larval nekton, and frontal zones have been an area of intense re- search. For example, it is now well known that local aggregations of phytoplankton can develop along and within week-long meanders and eddies in the Gulf Stream (Lee et al., 1991) and that elevated fish stocks often co-occur in these frontal disturbances (Atkinson and Targett, 1983). In the Gulf of Mexico, frontal zones at the periphery of meanders and eddies that are seaward of the continental margin are typically expressed as sharp gradients in temperature. These may have secondary expression as gradients in sa- linity, particularly in local convergences that can entrain low-salinity water and transport it off shelf as plumes or jets. For example, Biggs and Muller- Karger ( 1994) reported that some cyclone-anticyclone geometries in the Gulf of Mexico create flow confluence zones that can transport high-chlorophyll shelf water seaward several hundreds of kilometers. Sharp frontal zones may also be created during peri- ods of northern extensions of the Loop Current. Lamkin ( 1997) found a significant positive correlation between the abundance of larval nomeid fish and the location of the northern edge of the Loop Current by analyzing NOAA annual icthyoplankton survey data from 1983 to 1988. Lamkin’s data indicate that Cubiceps pauciradiatus, in particular, is a species whose adult spawning grounds and larval habitat are tied to sharp temperature gradients. Peak lar- val abundance was found close to the frontal inter- face, and peak abundance occurred just above the region of peak sea surface temperature (SST) gradi- ent. Lamkin went on to speculate that the extent of the frontal systems in the Gulf of Mexico would be expected to impact annual recruitment of a species that is tied to a frontal habitat. Table 2 Comparison of net-collected with acoustic characterization of zooplankton stocks. See text for explanation of how Acoustic Back- scatter was calculated. CCR = cold-core ring; IABI = integrated acoustic backscatter intensity. Plankton Tow (0-100-0 m) Total wet displaced volume (mL/1,000 m3± SD) Acoustic Backscatter (db) (IABI, 10-100 m ±SD) Euphausiids Pteropods Siphonophores (numbers per 1,000 m3 ±SD) 1 (day: NW of CCR) 36 46.3 112 93 212 2-4 (night: inside CCR) 90 ±12 87.7 ±11.5 574 ±138 204 ±19 580 ±78 5-7 (day: SE of CCR) 39 ±6 36.2 ±31.2 154 ±56 104 ±48 453 ±253 8 (night: SE of CCR) 67 82.7 806 198 840 NOTE Biggs et al.: Plankton and cold-core rings in the Gulf of Mexico 373 On shorter time scales, the biological implications of thermal fronts in the Gulf of Mexico are widely recognized by fishermen: many of them now direct their boats to selected fishing areas where SST im- agery shows sharp temperature gradients over short (<10 km) distances. Skipjack, blackfin tuna, sword- fish, and blue marlin have been reported by fisher- men to be locally abundant in these frontal zones (Roffer2). Also, on seven research cruises of the GulfCet program 1992-94, there were frequent sightings of family groups of sperm whales, Physeter catodon, and periodic sightings of pods of killer whales, Orcinus orca, in association with thermal fronts over the continental slope of the northern Gulf of Mexico (Davis and Fargion3). Clearly, populations of apex predators like these are not likely to be sus- tained by low or infrequent episodes of enhanced secondary productivity. One explanation for the fact that elevated stocks of biomass were not found in the CCR during the March 1993 transect is that biomass may “grow in” only when cyclones are “spun up” into surface waters. That is, if “new” ni- trate is but episodically injected into the photic zone of cyclones, there may be lag times of days or weeks between what we hypothesize should be pulses of new pro- duction and secondary production. Alter- natively, as these cyclones spin up, nitrate levels may be slowly domed and then de- crease as the ring spins down and loses its cold-core surface expression. Because Gulf of Mexico cyclones contain water of the same temperature-salinity properties as the rest of the Gulf of Mexico, only when they are well “spun up” will they have colder surface as well as colder inte- rior temperatures. In fact, the cyclone of the present study was one of the few that has been visible in SST as well as in al- timeter imagery; it may have spun up to have locally cool surface temperatures in response to cyclonically favorable wind curl from the passage of a strong atmo- spheric cold front. This strong “norther” passed through Texas and out across the Gulf of Mexico 36 hours before the cruise; the cloud banks that stretch NE to SW along the trailing edge of this norther can be seen in Figure 1. Rapid (hours-to-days scale) and intense cyclogenesis has been reported to occur after cold front passage in the northern Gulf of Mexico, especially when the cold fronts stall over deep water off the edge of the continental margin (Lewis and Hsu, 1992). .c Q. CD Q o 20 40 60 80 100 120 140 160 180 200 -85.0 -80.0 -75.0 -70.0 -65.0 Acoustic backscatter intensity (db re(m x n)~') Figure 3 Acoustic backscatter intensity versus depth for time-averaged ADCP (acoustic Doppler current profiler) records (mean of three ensembles of 5-min duration each) that were concurrent with times of plankton net tows: (A) three nighttime tows in the cold-core ring (CCR); (B) night tow southeast of CCR (solid line) and mean of four day tows outside CCR (dashed line). The region 10-100 m where acoustic backscatter intensity (ABI) >-74 db is shaded; the inset at top left summarizes this integrated ABI > -74 db (IABI). 2 Roffer, M. 1994. Ocean Fishing Forecasting Ser- vice, Miami, FL. Personal commun. 3 Davis, R. W., and G. A. Fargion (eds.). 1996. Dis- tribution and abundance of cetaceans in the north- central and western Gulf of Mexico. Outer Con- tinental Shelf Study (OCS) Study MMS 96-0027. U.S. Dep. Interior, Minerals Manage. Serv., Gulf of Mexico OCS Region, New Orleans, LA, 357 p. 374 Fishery Bulletin 95(2), 1997 Clearly, we need additional information on how and when the biological productivity of Gulf of Mexico cyclones may “spin up.” As a corollary, however, we need to remember that Gulf of Mexico cyclones are analogous but not homologous to Gulf Stream cold- core rings. As a consequence of their cyclonic nature, Gulf of Mexico cyclones are regions of elevated near- surface nutrients but unlike Gulf Stream cold-core rings, they are not regions of biological expatriation. Studies of the fauna within Gulf Stream cold-core rings have documented that because these rings are “oases” of temperate slope water that are transported into an oligotrophic subtropical central gyre, some of their resident fauna succumb to thermal stress as the cold-core of temperate origin dissipates by mix- ing with the surrounding subtropical water (Wiebe et al., 1976; Boyd et al., 1978). In contrast, popula- tions of plankton and nekton in Gulf of Mexico cy- clones should be sustained (rather than stressed) by mixing with surrounding subtropical water and so persist as local aggregations of enhanced food sup- ply for apex predators that feed on krill-size food. Acknowledgments Shiptime for this cruise was provided by Texas A&M University for graduate student training and re- search. We thank John Wormuth and graduate stu- dents Luiz Fernandes, Marilyn Yeager, and Wen- tseng Lo for making the meter net hauls with us. We also thank Larry Rouse and Nan Walker at the Coastal Studies Institute at Louisiana State Univer- sity in Baton Rouge, LA, for providing the SST im- age of 9 March, and Tom Berger at Science Applica- tions International Corporation in Raleigh, NC, and Don Johnson at the Naval Research Lab in Stennis Space Center, MS, for providing the XBT’s. Literature cited Ahlstrom, E. H., and J. R. Thrailkill. 1960. Plankton volume loss with time of preservation. CalCOFI Rep. 9:57-63. Atkinson, L. P., and T. E. Targett. 1983. Upwelling along the 60-m isobath from Cape Canaveral to Cape Hatteras and its relationship to fish distribution. Deep Sea Res. 30: 221-226. Biggs, D. C. 1992. Nutrients, plankton, and productivity in a warm-core ring in the western Gulf of Mexico. J. Geophys. Res. 97:2143-2154. Biggs, D. C., and F. E. Muller-Karger. 1994. Ship and satellite observations of chlroophyll stocks in interacting cyclone-anticyclone eddy pairs in the west- ern Gulf of Mexico. J. Geophys. Res. 99:7371-7384. Biggs, D. C., A. C. Vastano, R. A. Ossinger, A. Gil-Zurita, and A. Perez-Franco. 1988. Multidisciplinary study of warm- and cold-core rings in the Gulf of Mexico. Memorias Soc. Ciencias Naturales de Venezuela 48 (3):11— 31. Boyd, S. H., P. H. Wiebe, and J. L. Cox. 1978. Limits of Nematoscelis megalops in the NW Atlantic in relation to Gulf Stream cold core rings. II: Physiological and biochemical effects of expatriation. J. Mar. Res. 36:143-159. Flagg, C. N., and S. L. Smith. 1989. On the use of the acoustic Doppler current profiler to measure zooplankton abundance. Deep-Sea Res. 36:455- 474. Gasca, R., E. Suarez, and I. Castellanos. 1995. Biomass zooplancticas en aguas superficiales del Golfo de Mexico durante verano e invierno de 1991. Caribb. J. Sci. 31:128-140. Hamilton, P. 1992. Lower continental slope cyclonic eddies in the cen- tral Gulf of Mexico. J. Geophys. Res. 97: 2185-2200. lies, T. D., and M. Sinclair. 1982. Atlantic herring: stock discreetness and abun- dance. Science (Washington, D.C.) 215: 627-633. Lamkin, J. 1997. The Loop Current and the abundance of larval C. pauciradiatus in the Gulf of Mexico: Evidence for physi- cal-biological interaction. Fish. Bull. 95:251-267. Leben, R. R., G. H. Born, D. C. Biggs, D. R. Johnson, and N. D. Walker. 1993. Verification of TOPEX altimetry in the Gulf of Mexico. TOPEX/Poseidon Res. News. 1:3-6. Lee, T. N., M. E. Clarke, E. Williams, A. F. Szymant, and T. Berger. 1994. Evolution of the Tortugas Gyre and its influence on re- cruitment in the Florida Keys. Bull. Mar. Sci. 54:621-646. Lee, T. N., J. A. Yoder, and L. P. Atkinson. 1991. Gulf Stream frontal eddy influence on productivity of the southeast U.S. continental shelf. J. Geophys. Res. 96:22191-22205. Lewis, J. K., and S. A. Hsu. 1992. Mesoscale air-sea interactions related to tropical and extratropical storms in the Gulf of Mexico. J. Geophys. Res. 97: 2215-2228. Lewis, J. K., and A. D. Kirwan. 1985. Some observations of ring topography and ring-ring interactions in the Gulf of Mexico. J. Geophys. Res. 90: 9017-9028. Mauehline, J. 1980. The biology of euphausiids. Adv. Mar. Biol. 18:373- 677. Rouse, L. J., Jr., N. D. Walker, D. C. Biggs, and R. R. Leben. 1994. Cyclonic-antieyclonic eddy pair interactions with the continental margin of the western Gulf of Mexico. Earth- Oceans-Space Transactions of the Am. Geophysical Union 75(16), p. 213. Vidal, V. M. V., F. V. Vidal, A. F. Hernandez, E. Meza, and J. M. Perez-Molero. 1994. Baroclinic flows, transports, and kinematic proper- ties in a cyclonic-anticyclonic-cyclonic ring triad in the Gulf of Mexico. J. Geophys. Res. 99:7571-7597. Wiebe, P. H., E. M. Hulburt, E. J. Carpenter, A. E. Jahn, G. P. Knap III, S. H. Boyd, P. B. Ortner, and J. L. Cox. 1976. Gulf Stream cold-core rings: large-scale interaction sites for open-ocean plankton communities. Deep-Sea Res. 23: 695-710. NOTE Biggs et al .: Plankton and cold-core rings in the Gulf of Mexico 375 Zimmerman, R. A. 1993. Bioacoustic surveys of planktonic sound scatterers and their diel and seasonal variability in the Northwest Gulf of Mexico. MS thesis, Dep. Oceanography, Texas A&M Univ., College Station, TX, 82 p. Zhou, M., W. Nordhausen, and M. Huntley. 1994. ADCP measurements of the distribution and abun- dance of euphausiids near the Antarctic Peninsula in winter. Deep-Sea Res. 41:1425-1445. 376 Physical environment and recruitment variability of Atlantic herring, Clupea harengus, in the Gulf of Maine Mark A. Lazzari David K. Stevenson Stephen M. Ezzy Marine Resources Laboratory, Maine Department of Marine Resources RO. Box 8, West Boothbay Harbor, Maine 04575 E-mail address: mrmlazz@state.me.us Recruitment is generally recog- nized as a complex ecological pro- cess determined by the interrela- tion of many biological and environ- mental variables and has always been one of the most difficult terms to estimate in fisheries science (Russell, 1931). Methods for fore- casting fisheries yields with time- series analyses (Saila et al., 1980; Mendelssohn, 1980), surplus pro- duction models (Schaaf et al., 1975), and models employed to re- late recruitment to egg production (Koslow et al., 1987), larval abun- dance (Lett and Kohler, 1976; Lough et al., 1981; Smith, 1981), or spawning stock size (Sissenwine, 1984) have had limited success. Factors dominating recruitment appear to operate on local scales (Cohen et al., 1991); changes in physical factors operating through marine food webs are a major force affecting the abundance of fish stocks (Mann, 1993). Clupea harengus , are an impor- tant component of the fisheries of the Northwest Atlantic and show great variability in recruitment. Spawning usually begins in the eastern part of the Gulf of Maine (Fig. 1) during August (Graham, 1982; Stevenson et al., 1989) and over the Nova Scotian shelf (Mc- Kenzie, 1964), and occurs as late as November or December (Graham, 1982; Lazzari and Stevenson, 1992). Eggs are deposited on the bottom (Boyar et al., 1973; Caddy and lies, 1973; Stevenson and Knowles, 1988) and hatch in one to two weeks depending on temperature. Larvae are transported to estuaries and embayments along the central and western Maine coast (Graham, 1982; Graham and Townsend, 1985) or remain offshore for the winter (Townsend, 1992). The planktonic larval stage lasts until spring when larvae undergo meta- morphosis into the juvenile form. Recruitment to the fishery occurs primarily in the following spring (at age 2) when juveniles reach a size appropriate for canning (150- 200 mm). The recruitment success of her- ring may be associated with vari- ous physical environmental factors, including sea surface temperature (SST) (Sutcliffe et al., 1977; Cush- ing, 1982; Anthony and Fogarty, 1985; Murawski, 1993), residual surface currents (Norcross and Shaw, 1984), winds (Corten1; Chris- tensen et al.2), or atmospheric-pres- sure gradients (Carruthers, 1938), or a combination of the last two. Theoretical models for predicting variations in juvenile herring pro- duction in the Gulf of Maine were developed by using sea surface tem- perature from the late-larval to early-juvenile period (Anthony and Fogarty, 1985), first quarter ( Janu- ary-March) sunshine (Ezzy, 1988), and by using either food supply and spawning distribution when year- class strength was established dur- ing the larval stage or predation for those years when year-class strength was established in the brit stage (Campbell and Graham, 1991). In addition, several hypotheses concerning wind events or larval dispersal may help us to under- stand herring recruitment in the Gulf of Maine. Ridgway (1975) pro- posed a conceptual model of recruit- ment variability based on changes in the dispersal of herring larvae by ocean currents from spawning areas to nursery areas. Water col- umn stability and its impact on the availability of food resources for larval fish at some critical life stage also has been proposed to affect re- cruitment (Lasker, 1975). Periodic winds that produce moderate tur- bulence may enhance larval sur- vival by increasing the probability of encounter between larvae and their prey (Sundby et al., 1989; MacKenzie et al., 1994). The purpose of this study was to associate physical environmental factors with size estimates of age-2 herring of the coastal Atlantic her- ring stock in order to identify the important environmental factors underlying recruitment variablity and to examine the importance of the wind and dispersal hypotheses 1 Corten, A. 1984. The recruitment fail- ure of herring in the central and northern North Sea in the years 1974-78 and the mid-1970s hydrographic anomaly. ICES Mini-Symposium. Council Meeting 1984/ Gen., 12 p. 2 Christensen, V., M. Heath, T. Kiorboe, P. Munk, H. Paulsen, and K. Richard- son. 1985. Investigations on the rela- tionship of herring larvae, plankton pro- duction and hydrography at Aberdeen Bank, Buchan Area, September 1984. ICES Council Meeting 985/L, 23 p. Manuscript accepted 7 November 1996. Fishery Bulletin 95:376-385 (1997). NOTE Lazzari et al.: Physical environment and recruitment variability of Ciupea harengus 377 Figure 1 Map of the Gulf of Maine showing the spawning grounds of Atlantic herring, Ciupea harengus. as subjects for future research. Through examina- tion of time-series data and the use of exploratory correlations, contingency tables, and /-tests, a search was made for those environmental factors that re- lated positively or negatively with size estimates of age-2 herring populations. Methods The method of exploratory correlation (Sutcliffe et al., 1977; Hayman, 1978) was used to determine re- lationships between monthly means of environmen- tal factors along the Maine coast. Records were ana- lyzed for the larval year 1 August through 31 July for years 1965 through 1990 (Table 1). The herring recruitment index used was the virtual population analysis (VPA) estimation of two- year-olds from 1967 to 1991 in the coastal Atlantic stock (NEFC, ! Fig. 2). For purposes of our analysis, we assumed predation to be constant and that spawning stock biomass was not a major factor affecting re- cruitment. Sea surface tempera- ture (SST) records were supplied by the Maine Department of Ma- rine Resources Laboratory, Booth- bay Harbor, ME. Sunshine was measured as percent possible sun- shine from observations of cloud cover conditions at the Portland, ME, airport. Long-term sunshine, atmospheric pressure, wind direc- tion, and velocity data records were compiled for Portland as 12 monthly averages per year and archived by the National Climatic Data Center. Wind speed and direction were measured at the Portland airport, 8 km inland, where an anemometer is situated 7 m above the ground at an elevation of 25 m. Storm fre- quency was also complied as days with mean winds in excess of 5 m/s. Daily average wind speed and di- rection were further analyzed for the period August- December when the influence of wind-driven surface currents on the dispersal of newly hatched herring larvae would be greatest. The daily resultant wind direction (a vector variable) was separated into one of four directions on the basis of compass headings of northeasterly (1-90°), southeasterly (91-180°), southwesterly (181-270°), and northwesterly (27 1— 3 NEFC (Northeast Fisheries Center). 1991. Assessment of the coastal Atlantic herring stock. Thirteenth Northeast Regional Stock Assessment Workshop. Northeast Fish. Sci. Center, Natl. Mar. Fish. Serv., NOAA, 111 p. Table 1 The location and origin of environmental factors tested for association with the age-2 Atlantic herring abundance estimates. ME DMR = Maine Department of Marine Resources; NCDC = National Climatic Data Center; NEFC = Northeast Fisheries Center. Environmental factor Location Years Source Sea surface temperature Boothbay Harbor 1965-90 ME DMR Wind speed and direction Portland, ME 1965-90 NCDC, Asheville, NC Storm index (no. of days > 5 m/s) Portland 1965-90 ME DMR Sunshine Portland 1965-90 NCDC, Asheville, NC Barometric pressure Portland 1965-90 NCDC, Asheville, NC Herring abundance estimates Gulf of Maine 1967-91 NEFC, 1991 378 Fishery Bulletin 95(2), 1 997 2,200). Wind direction at both locations showed a definite seasonal trend from May into September when more southerly winds predominated. Monthly mean wind speeds for the period 1961-90 between Portland and Boston were always significantly greater at Boston in all months ((-test, P<0.001, n- 30), except in January and Octo- ber. In addition, significant Pearson correlations (P<0.001, n=30) were found for all monthly means of solar radiation (r2=0.80), total sky cover (r2=0.79), air temperature (r2=0.95), relative humidity (r2=0.68), and precipitation (r2=0.91) between Port- land and Boston for the same period. The null hypothesis of independence was rejected for the age-2 Atlantic herring abundance estimates and ten environmental factors with 3x3 contingency tables. These environmental factors were November storms, March sunshine, and October sea surface temperature, October and first quarter (January- March) barometric pressure, December and August- September Lasker events, the number of days of southwesterly winds in September, the total num- ber of days of southeasterly winds between August and December, and the number of storm days with southeasterly winds in November. Five environmental factors were associated with either high or low age-2 abundance in the 2x2 con- tingency table analyses (Table 2). Low abundance was associated with reduced sunshine in March and with fewer days of southeasterly winds from August to December. Associations with high age-2 abundance Table 2 Probabilities of 2 x 2 contingency table (Fisher’s exact chi- square test) results for the environmental factors associ- ated with the age-2 Atlantic herring estimates, ns = not significant. 2x2 contingency tables Low age-2 High age-2 estimate estimate Environmental factor High factor Low factor High factor Low factor November storms ns ns 0.020 ns March sunshine ns 0.012 ns ns September SW wind ns ns 0.002 ns Aug-Dec SE wind ns 0.005 ns ns November SE storms ns ns 0.026 ns occurred with more November storms, with more days of southwesterly winds in September, and with more days of southeastern storms in November. Comparison of environmental factors (mean val- ues) between the four best and four worst recruit- ment years revealed only March sunshine and three southern wind direction factors were significantly different. The amount of monthly March sunshine and the number of days of southwesterly winds in September, the number of days of southeasterly winds from August to December, and the number of storm days with southeasterly winds in November were all significantly higher during good recruitment years than during lower recruitment years (Table 3). The four strongest year classes (1966, 1970, 1983, 1988) were associated with greater than 50% of the possible March sunshine and were associated in eleven out of twelve cases with above average Sep- tember southwestern, August-December southeast- ern winds and November southeastern storms (Fig. 3). In eight cases, conditions were greater than 50% above normal for the entire period. The only excep- tion during the four years of above average recruit- ment occurred in 1970 when August-December southeastern winds were extremely low. The four worst year classes (1971, 1972, 1974, 1978) were associated with less than half of the pos- sible March sunshine and with average or below av- erage September southwesterly, August-December southeasterly winds and November southeasterly storms in 9 of 12 cases. In eight of these cases, condi- tions were at least 25% below normal for the entire period. However, two below average year classes (1979 and 1984) were produced despite the fact that all three of the significant wind factors were above 380 Fishery Bulletin 95(2), 1 997 CZZD AGE 2 ■ SEPSW --A--TOTSE --K--NOVSES O MAR SUN Year class Figure 3 Percent anomalies of the number of days of September southwesterly wind (SEPSW), num- ber of days of southeasterly wind from August through December (TOTSE), and number of storm days in November with southeasterly winds (NOVSES) and March sunshine (MAR SPIN ) compared with anomalies of the 1965-89 age-2 Atlantic herring abundance estimates. Table 3 Mean and standard deviation (in parentheses) and f-test results assuming unequal variances for the environmental factors associated with the high and low age-2 Atlantic herring estimates, ns = not significant. Environmental factor Low age-2 estimate (n= 4) High age-2 estimate (n= 4) t-test probability November storms 7.20 (1.64) 7.00(5.23) ns March sunshine 40.75 (6.94) 57.00 (3.16) 0.012 September SW wind 11.40(2.79) 17.00 (1.63 0.008 Aug-Dec SE wind 14.40(2.30) 19.75 (3.30) 0.039 November SE storms 0.20 (0.451) 1.75 (0.50) 0.003 normal in those years. Average year classes were much more common during the 25-yr period and, in 14 of these 17 years, August-December southeast- erly winds were also below average. September southwesterly winds and November southeasterly storms, on the other hand, were below average dur- ing about half of these years. Discussion In our study, we observed associations of sunshine, wind direction, and velocity with the number of age- 2 herring estimated to recruit to the coastal Atlantic stock. Strong year classes were produced in years with more days of southerly fall (September) winds and storms (November), and weak year classes were produced in years with less sunshine in March and fewer days of southeasterly fall (August-December) winds. However, because southwest wind velocities are lower than velocities from other directions in the Gulf of Maine in fall, it is not possible to differenti- ate between the effects of wind direction and strength in this study. We believe that our results, without specifically addressing how wind events influence herring larval survival, show that recruitment suc- cess and, therefore, larval survival are related to wind events. In general, wind-driven transport and tur- NOTE Lazzari et a!.: Physical environment and recruitment variability of Clupea harengus 381 bulence are two processes hypothesized to affect the survival of marine fish larvae (Lasker, 1975; Norcross and Shaw, 1984), but strong evidence linking larval survival to wind conditions remains inconclusive. Wind-related transport is believed to influence the recruitment of many species of marine invertebrates (Roughgarden et ah, 1988; Farrell et al., 1991) and fishes (Bailey, 1981; Heath, 1989). For Atlantic herring in the Gulf of Maine, the east- ern Maine-Grand Manan Island spawning ground presents a unique case in how southwesterly winds enhance larval transport and survival. Bigelow (1927) found that winds from the southwest tend to “build up” surface waters in the Bay of Fundy caus- ing an “overflow” in the shape of a westerly drift that increases the flow of the coastal current along the eastern Maine coast, i.e. against the prevailing winds. Herring larvae depend on these currents for dispersal to more productive nursery areas (Graham, 1982; Graham and Townsend, 1985; Townsend et ah, 1986, 1987) because the area of extensive tidal activity in the Bay of Fundy and off eastern Maine (Garrett et al., 1978) leads to pronounced vertical mixing and less stratification of the water column off eastern coastal Maine (Yentsch and Garfield, 1981). As a re- sult, primary production is much lower in the north- eastern Gulf of Maine because the mixed layer ex- tends deeper than the critical depth for plankton production (Townsend et al., 1987). Zooplankton prey organisms that support larval growth and survival are extremely rare on this spawning ground in the fall, only reaching adequate densities about 100 km “downstream” from the spawning ground (Townsend et al., 1986, 1987); therefore, increased dispersal of recently hatched larvae that originate on the east- ern Maine-Grand Manan Island spawning ground is a mechanism that could enhance the recruitment of juveniles to the coastal herring stock. This study generally supports the Campbell and Graham (1991) theory that release of larvae from the eastern Maine-Grand Manan spawning ground is mediated by wind events that generate horizontal flows and can carry larvae out of retention areas into the counterclockwise residual flow that moves from northeast to southwest along the Maine coast. Bigelow and Schroeder (1953) thought that this cir- culation is set in motion by wind and freshwater in- flow and that it influences the availability of two- year-old herring because the fish follow the drifting planktonic animals on which they feed. Furthermore, Chenoweth et al. ( 1989) observed that larvae hatched in this spawning area at the same time had differ- ent horizontal displacements away from the spawn- ing area; some larvae remained in the area for up to a month after spawning, whereas others were trans- ported up to 100 km southwestward down the coast during the same time period. Townsend (1992) at- tributes this variable release of larval herring from the eastern Maine retention area to the intrusion of slope water into Jordan Basin (a deep offshore basin located in the northeastern Gulf of Maine), the tim- ing of hatching (to coincide with lunar periodicity and the intensity of tidal mixing), and the location of egg beds in relation to the front between the area of tidal mixing and more stratified water offshore, where the geostrophic flow that would pull larvae out of the retention area is greatest (Brooks and Townsend, 1989). In addition, Brooks (1990) found a relation between wind stress and currents that suggested the action of a density-modulated coastal upwelling mechanism in which the deep inward currents over Lindenkohl sill respond directly to northeastward alongshore wind stress at times of weak stratifica- tion, such as occurs in fall. Once entrained within the coastal current, the lar- vae are dispersed to an overwintering area that has not been conclusively determined as yet. Greater advection of larvae from the eastern Maine-Grand Manan Island spawning area to the southwest as hypothesized by Graham ( 1982) would distribute lar- vae among more coastal overwintering areas. This distribution would improve recruitment success by lessening density-dependent mortality within the estuarine and nearshore waters shallower than 100 m that act as a nursery area and would establish a carrying capacity for larvae on the Maine coast for a given year. Research has shown that larvae from this spawning ground reach at least as far south as the Sheepscot River in mid-coastal Maine (Graham, 1982; Graham and Townsend, 1985; Stevenson et al., 1989). However, recent research shows that larval herring overwintering “offshore” may have a higher survival rate than those wintering in nearshore wa- ters (Townsend et al., 1989; Townsend, 1992) and that dispersal associated with southerly winds could en- hance offshore transport. In either case, dispersal of larvae away from the eastern Maine-Grand Manan Island spawning area is critical for good larval sur- vival (Graham, 1982; Campbell and Graham, 1991; Townsend, 1992). Wind-induced effects on transport have been shown to affect the distribution and re- cruitment of other marine fishes (Stevenson, 1962; Checkley et al., 1988; Fechhelm and Griffiths, 1990; Koutsikopoulos et al., 1991; Castillo et al., 1993). Therefore, we propose that more southwesterly wind conditions in September increased dispersal of eastern Maine-Grand Manan Island larvae in 1966, 1970, 1983, and 1988, setting up an initial situation of high larval survival, which, when combined with more southerly wind conditions through December 382 Fishery Bulletin 95(2), 1 997 and more sunshine in March, resulted in the success of these year classes. A continuation of a more sum- mer-like (southwesterly) wind pattern through Sep- tember may result in better larval herring survival in the Gulf of Maine during these years. Average fre- quencies of southwest winds off Nova Scotia can vary between 10% and 39% for the months of July-Sep- tember, 1955-1980 (Hudon, 1994), and summer (June-August) wind stress over the eastern conti- nental shelf is generally toward the northeast and about 0.25 dyn/cm; whereas in fall (September-No- vember), wind stress shifts toward the southeast and can be twice as strong (Saunders, 1977). Effects of turbulent mixing on food encounter rates must be considered because southwesterly winds are lower in velocity in the Gulf of Maine during the early (August-Becember) larval phase. The effects of tur- bulence on the availability of zooplankton prey for larvae are related to those biological processes (pri- mary production) that are disturbed by physical pro- cesses, i.e. turbulence generated by wind mixing (Rothschild and Osborn, 1988; Sundby et al., 1989; MacKenzie and Leggett, 1991). Recently, the overall probability of larval feeding has been described as a dome-shaped function of turbulent velocity with maximum feeding, depending on turbulence level and behavioral characteristics of predator and prey (MacKenzie et al., 1994). Calmer wind conditions through September when most first-feeding larvae are present, could enhance larval survival and re- sult in the success of these year classes. Strong re- cruitment to walleye pollock, Theragramma chalco- grarnma, stocks in the Gulf of Alaska (Megrey et al., 1994) and Bering Sea (Bailey et al., 1986) has been linked to initially calm wind conditions and is asso- ciated with calm periods preceded and succeeded by periods of stronger mixing (Bailey and Macklin, 1994). For Atlantic herring in the Gulf of Maine, these conditions would result from periods of calm south- westerly winds in conjunction with stronger south- easterly winds. However, strong mixing can disrupt layers of prey (Lasker, 1975; Wroblewski and Richman, 1987; Owen 1989) and has also been linked to reduced growth of Atlantic herring larvae (Heath, 1989). Three other environmental variables, the amount of March sunlight, August-December southeasterly winds, and November southeasterly storms, were related to herring year-class size. Dispersal associ- ated with the latter two southeasterly wind factors could result in a positive effect on recruitment by transporting larvae spawned in the western Gulf of Maine and on Jeffreys Ledge toward inshore larval overwintering and juvenile nursery areas along the Maine coast (Lazzari and Stevenson, 1992). Because herring larvae feed on zooplankton, spring phy- toplankton production and, ultimately, sunshine should be positively related to larval survival. The timing of plankton blooms in the Gulf of Maine was highly influenced by the amount of sunshine avail- able early in the year (Townsend and Spinrad, 1986). Therefore reduced sunshine in March would have a detrimental effect on the spring bloom, resulting in fewer food resources for herring larvae and reducing recruitment as seen in Ezzy’s (1988) model using first quarter sunshine. Recruitment of animals with planktonic stages is a complex process; we would not expect any single factor affecting the early larval stage to dominate the entire survival process (Wooster and Bailey, 1989; Campbell and Graham, 1991 ). In our study, although more days of southerly winds were generally associ- ated with higher than expected age-2 recruitment, this was not always the case. For example, in two of the six years (1976 and 1984, Fig. 3) when south- western winds averaged > 25% higher than normal, strong year classes were not produced. Other factors may have reduced year class size (e.g. predation on larvae or age-1 juveniles) during these periods of lower abundance, or some other conditions may not have been suitable for prey production or feeding. We would have been surprised if the relation of any environmental factor and recruitment had always been consistent, because a high larval survival rate appears to be a necessary, but not sufficient, condi- tion for strong recruitment. Year-class strength can instead be determined by conditions that prevail during the juvenile life stage in some years (Campbell and Graham, 1991; Bailey and Spring, 1992). The environment does not act alone in affecting recruit- ment success; biotic effects, competitive interaction between species, and the removal of adults caused by fishing mortality, should be considered (Drink- water, 1987). The results of our analyses to date are interesting and worth expanding, with more research and analyses, to other Atlantic herring stocks to de- termine the effects of the environment, particularly the wind-driven transport of larvae, on their recruit- ment variability Acknowledgments The authors thank several individuals who main- tained the Environmental Monitoring Project at the Maine Marine Resources Laboratory including L. Churchill, D. Smith, and W. Welch. Special thanks to D. Libby for invaluable computer assistance and to K. Friedland, National Marine Fisheries Service who collaborated with us on the stock assessment. NOTE Lazzari et al.: Physical environment and recruitment variability of Gupea harengus 383 This manuscript benefited from the comments of D. Campbell, S. Chenoweth, and several anonymous reviewers. Literature cited Anthony, V.C., and M. J. Fogarty. 1985. 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Bigelow, H. B. 1927. Physical oceanography of the Gulf of Maine. Fish. Bull. 40:511-1027. Bigelow, H. B., and W. C. Schroeder. 1953. Fishes of the Gulf of Maine. Fish. Bull. 53:1-577. Boyar, H. C., R. R. Marak, F. E. Perkins, and R. A. Clifford. 1973. Seasonal distribution and growth of larval herring, Clupea harengus, in the Georges Bank-Gulf of Maine area from 1963 to 1970. J. Cons. Int. Explor. Mer 35:36-51. Brooks, B.A. 1990. Currents an Lindenkohl sill in the southern Gulf of Maine. J. Geophys. Res. 95:22173-22192. Brooks, D. A., and D. W. Townsend. 1989. Variability of the coastal current and nutrient path- ways in the Gulf of Maine. J. Mar. Res. 47:303-321. Caddy, J. F., and T. D. lies. 1973. Underwater observations on herring spawning grounds on Georges Bank. ICNAF Res. Bull. 10:131-139. Campbell, D. E., and J. J. Graham. 1991. Herring recruitment in Maine coastal waters: an eco- logical model. Can. J. Fish. Aquat. Sci. 48:448-471. Carruthers, J. N. 1938. 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Climate and Fisheries. Academic Press, New York, NY, 373 p. Drinkwater, K. F. 1987. “Sutcliffe revisited”: previously published correlations between fish stocks and environmental indices and their recent performance. In R. I. Perry and K. T. Frank (eds.), Environmental effects on recruitment to Canadian Atlan- tic fish stocks, p. 41-61. Can. Tech. Rep. Fish. Aquat. Sci. 1556. Ezzy, S. M. 1988. Environmental effects on recruitment variability of Atlantic herring, Clupea harengus, in the Gulf of Maine. M.S. thesis, Univ. Maine, Orono, Maine, ME, 118 P- Farrell, T. M., D. Bracher, and J. Roughgarden. 1991. Cross-shelf transport causes recruitment to intertidal populations in central California. Limnol. Oceanogr. 36:279-288. Fechhelm, R. G., and W. B. Griffiths. 1990. Effect of wind on the recruitment of Canadian Arctic cisco (Coregonus autumnalis) into the central Alaska Beau- fort Sea. Can. J. Fish. Aquat. Sci. 47:2164-2171. Garrett, C. J. R., J. R. Keeley, and D. A. Greenburg. 1978. 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Principal areas of vertical mixing in the Gulf of Maine, with reference to the total productivity of the area. In J. F. R. Gower (ed.), Oceanography from space, p. 303- 312. Plenum, New York, NY. Zar, J. H. 1984. Biostatistical analysis. Prentice-Hall Inc., Engle- wood Cliffs, NJ, 620 p. 386 In vitro digestibility of some prey species of dolphins Kesko Sekiguchi* Peter B. Best Mammal Research Institute University of Pretoria, Pretoria 0002, South Africa Mailing address: Division of Natural Science, International Christian University 3-10-2 Osawa, Mitaka-City, Tokyo 181, Japan *E-mail address: keikos@icu.ac.jp Studies of dolphin (Cetacea, Odon- toceti) food habits are conducted by examining stomach contents be- cause it is difficult to observe feed- ing behavior directly. It is rare, however, to find prey items intact in stomachs; often only fragments of muscle and some hard parts re- main. Identification of prey species and estimation of their original size are usually carried out with trace remains, such as cephalopod beaks (Clarke, 1980) and fish otoliths (Fitch and Brownell, 1968), because of their species-specific shapes and allometric relationships with body size (Clarke, 1962; Jobling and Breiby, 1986). There are several problems with using cephalopod beaks and fish otoliths in dietary studies. Otoliths are composed of calcium carbonate and can be eroded by stomach ac- ids (McMahon and Tash, 1979; da Silva and Neilson, 1985; Murie and Lavigne, 1985, 1986; Jobling and Breiby, 1986; Harvey, 1989). Reduc- tion in otolith size depends on the length of time they are exposed to stomach acids. Because otoliths are located inside the skull, the length of time they are exposed to acids may differ depending on the over- all digestibility of the fish species concerned. Some species are iden- tifiable even after their otoliths have been eroded and reduced in size. For such species, it may be difficult to tell if the otolith is of a reduced or original size (McMahon and Tash, 1979). Because estima- tion of fish prey size is usually based on a regression between otolith size and the weight or length of the prey, any reduction in otolith size that is not detected may cause prey size to be underestimated. The use of cephalopod beaks may create different problems. Although Bigg and Fawcett (1985) reported that soft-bodied squids ( Loligo opalescens ) decreased in weight faster than herring ( Clupea haren- gus pallasi) in an artificial diges- tion solution, cephalopod beaks were not dissolved by gastric acids. Cephalopod beaks may, therefore, accumulate in cetacean stomachs. It has been observed that some marine mammals occasionally re- gurgitate squid beaks (Clarke, 1980; Pitcher, 1980). Cephalopod beaks present in a stomach may, consequently, represent the re- mains of more than one meal and thus may result in overestimations of the proportion of squid to fish in the predator’s diet. Bigg and Perez ( 1985) introduced the “modified volume” method to avoid the problem of the accumu- lation of cephalopod beaks. This method uses the frequency of occur- rence of nontrace remains to calcu- late the ratio between cephalopods and fish in a meal. However, if all prey remnants come from the same meal, any difference in digestibil- ity between prey items will affect the relative frequency of occurrence of nontrace remains when the stom- ach is examined. As an extreme case, prey items that are digested very rapidly would not be repre- sented by “nontrace remains” in the stomach soon after feeding. Differentials in digestion rates between Loligo squid and herring in an artificial digestion solution, as demonstrated by Bigg and Fawcett ( 1985), may apply to other prey species. For example, Jackson et al. (1987) could not detect differ- ences in the rates that fish and squid were completely digested in vitro but noted that exoskeletons of intact crustaceans resisted diges- tion. Thus, it is possible that diges- tion rates for each prey species, or prey type, could be used as “correc- tion factors” in dietary analysis. The present study investigates the differences in digestion rates of major prey species of dolphins in artificial digestion solutions. In addition, digestion rates of differ- ent sizes of the same prey species are considered. Digestion rates are then calculated to establish the basis for a revised method of di- etary analysis. Materials and methods The following fish and squid spe- cies were used in a set of six experi- ments: 1) 5 lanternfishes (Mycto- phidae), 5 large and 5 small Cape anchovies ( Engraulis capensis, Engraulidae); 2) 5 large and 5 small round herrings ( Etrumeus white- headi, Clupeidae); 3) 5 large and 5 small pilchards ( Sardinops sagax, Clupeidae); 4) 5 hakes (Merluccius sp., Merlucciidae) and 5 chokka squids (Loligo vulgaris reynaudii, Loliginidae); 5) 5 maasbankers (horse mackerel) ( Trachurus tra- ehurus capensis, Carangidae) and Manuscript accepted 1 November 1996. Fishery Bulletin 95:386-393 (1997). NOTE Sekiguchi and Best: In vitro digestibility of some prey species of dolphin 387 5 red squids (Todaropsis eblanae, Ommastrephidae); and 6) 5 pelagic gobies (Sufflogobius bibarbatus, Gobiidae) and 5 lanternfishes. These taxa are com- monly found in stomachs of dolphins (including com- mon dolphins, Delphinus delphis, dusky dolphins, Lagenorhynchus obscurus, and Heaviside’s dolphins, Cephalorhynchus heavisidii ) along the west coast of southern Africa (Sekiguchi et al., 1992). Table 1 shows the sizes of sample species used: all were collected in trawls by the RV Africana, November 1987 or Janu- ary 1988, and frozen at -20°C. For the first experiment, the procedure followed that of Jackson et al. (1987). Four liters of a diges- tion solution of 0.15% HC1, 0.05% Na9C03 (buffer) and 1.0% pepsin (pepsin A powder, BDH Chemicals Ltd.) were adjusted to an initial pH of 2.30, near the midpoint of the range of that recorded for cetacean stomachs (pH=1.4 to 3.0, Ishihara, 1960; pH=1.8 to 3.0, Smith, 1972; pH=1.5 to 3.5, Jobling and Breiby, 1986 ). A Beckman expanded scale pH meter was used to monitor pH. The solution was then divided into 240-mL portions in each of seven 600-mL beakers, and 1,150 mL portions in each of two 5-L beakers. The beakers were placed in two water baths con- tinuously agitated (rocked) 20-30 times per minute at 38°C. Each fish was put in a small fiber glass bag (mesh size 0.5 x 0.5 mm) and then suspended in the solution. Four samples were placed in each of the 5- L beakers and a single sample in each of the 600-mL beakers. The pH for each beaker was maintained between 1.90 and 3.37; pH increased with time and was adjusted by adding HC1. Owing to the effort required to maintain pH in in- dividual beakers, one large PVC container (40 x 28 x 20 cm) made specifically to fit in the water bath was used in subsequent experiments. Ten liters of diges- tion solution, consisting of 0.50-0.56%- HC1, 0.27- 0.29% Na2C03, and 1.0% pepsin, were maintained at 36.0 to 39.i°C in the PVC container. A Beckman expanded-scale pH meter was placed in the corner of the container to monitor pH constantly. The pH was maintained between 2.25 and 2.51 by occasional Table 1 The species used in the artificial digestion experiments; their total length (TL, cm) or dorsal mantle length (DML, cm) and corresponding weight (WT, g). (L=large size and S=small size groups.) Sample species Length (cm ) and weight (g) Cape anchovy (L) TL 13.6 12.7 12.7 12.8 11.7 WT 19.48 17.28 21.25 18.09 16.45 Cape anchovy (S) TL 9.9 9.6 9.6 9.4 9.6 WT 8.25 8.05 8.01 7.81 7.56 Round herring (L) TL 19.5 18.7 19.5 21.0 19.8 WT 68.95 63.49 79.03 96.27 79.47 Round herring (S) TL 14.4 14.8 14.8 14.8 15.2 WT 28.28 33.47 34.88 36.77 34.69 Pilchard (L) TL 20.8 20.0 20.8 20.0 20.3 WT 108.70 104.55 105.27 103.11 103.72 Pilchard (S) TL 13.7 13.0 14.0 14.1 13.5 WT 31.0 27.88 30.59 30.89 30.65 Hake TL 17.0 17.4 17.3 17.5 16.5 WT 46.1 51.2 48.4 52.2 41.5 Maasbanker TL 18.8 19.4 18.9 19.1 16.5 WT 79.5 83.3 80.2 80.4 53.6 Goby TL 8.7 7.8 8.2 8.5 8.6 WT 10.2 7.1 8.6 8.9 10.0 Lanternfish TL 5.3 5.1 3.5 3.9 4.5 4.4 4.4 4.2 4.1 4.6 WT 1.85 1.64 0.61 0.93 1.16 1.1 1.1 0.8 1.0 1.0 Chokka squid DML 16.0 18.5 16.5 15.8 15.8 WT 118.3 152.9 120.7 104.4 98.8 Red squid DML 10.4 9.6 10.7 9.9 9.2 WT 60.6 55.8 61.2 49.1 40.8 388 Fishery Bulletin 95(2), 1 997 addition of 10% HC1 (45 to 655 mL per experiment in total). The water bath rocked the container about 40 times per minute. As in the first experiment, each sample item was placed in a small mesh bag and suspended in the digestion solution. Every hour, each bag was lifted from the container, all excess liquid was wiped off with a paper towel, and the bag with sample weighed to the nearest 0.1 g. The physical appearance of each sample was also recorded. Weighings were made at 1- h intervals until the sample mass (i.e. measured weight minus weight of the empty mesh bag) had decreased to 5-10% of its original mass. To compare digestion rates between each species, the mean time to reach 20% of original weight (T.,0) was calculated for each sample species. This percentage was chosen because the rate of decline in mass decreased when the sample reached this point. This decrease probably resulted from inaccuracies in weighing smaller masses as well as from the accumulation of less digestible remains. The T.,0 values for different size groups of the same prey species were compared first with a Ltest. Then, one-way ANOVA and the Newman- Keuls test were applied to compare all sample species (Zar, 1974). A digestion rate ratio was calculated from the T20 values for each species, expressed as a proportion of that for lanternfish. Results 100 100 c to E CD cc 100 Time (h) Figure 1 The rate of digestion of fish and squid species in an artificial digestion solution, expressed as the percentage of their original weight remain- ing at hourly intervals. The slope shown is the digestion rate to the mean time to 20% of the initial weights (T20). (A) 10 lanternfish Samples were digested almost completely in the pepsin solution. Although digestion rates were quite different among species, the sequences of digestion of particular tissues were similar among species (Table 2). Although the head of a fish usually dis- integrated when about half the body had been di- gested, otoliths were not always visible through the mesh bag at this stage. In the case of hake and maasbanker, the dorsal surface of the head began to be digested at an earlier stage (15% digested at 2-3 h for hake, 5-6 h for maasbanker) than that found for other fish species. Otoliths became visible (through the mesh bag) at 5-8 h for hake and at 19 h for maasbanker. Hake otoliths fell through the mesh at 9-13 h. Most otoliths were dissolved completely when the experiments with hake terminated at 20 h (Myctophidae sp.), (B) 5 maasbanker (Trachurus t. capensis), (C) 5 hake (Merluccius sp.), (D) 5 pelagic goby (Sufflogobius bibarbatus), (E) 5 chokka squids ( Loligo v. reynaudii), and (Fl 5 red squids ( Todaropsis eblanae). (except one otolith), and at 27 h (except for five otoliths) with maasbanker. Some otoliths of reduced size were recovered in experiments involving other species (i.e. 16 from anchovy, 8 from herring, and 10 from goby). All squid beaks recovered at the termi- nation of the experiments showed no obvious signs of having been digested. All samples decreased in weight over time (h), each species having different rates of digestion (Figs. 1 and 2). Lanternfish were digested very quickly, and were almost completely gone within 9 hours. Hake NOTE Sekiguchi and Best. In vitro digestibility of some prey species of dolphin 389 Table 2 The generalized sequence of digestion for fish and squid in the artificial digestion experiments. Weight remaining (%) Squid Fish 95-85 Begins to lose skin and viscera Abdomen breaks up; begins to lose skin and viscera 85-60 Loses fins; muscle reduced Most of skin and viscera are gone; loses eyes and tail; head begins to be digested 60—40 Tentacles featureless; mantle splits, exposing the pen Head is gone; muscle reduced; releases otoliths 40-30 Flesh reduced further Muscle disintegrates; backbone exposed 30-10 Beaks, eyes, and pen released Muscle reduced further <10 Beaks, eyes, part of pen, and a little flesh remain Pieces of muscle and skin, and some vertebrae remain and goby were also digested quickly and reduced to less than 10% of their original weight within 15 hours. Most species, however, took longer for com- plete digestion (about 20 h); maas- banker took as long as 27 hours to be reduced to less than 10% of its origi- nal weight. Table 3 lists the T20 values for each sample species. The T.1() values var- ied between species sampled (from 5.68 h to 21.35 h), but for most spe- cies, the T20 value was roughly 13 h. Among the 12 species and size groups sampled, maasbanker had the slow- est rate of digestion and lanternfish the highest, being digested about 3.8 times faster than maasbanker. Red squid was digested faster than most fish species except lanternfish, whereas the digestion rate of chokka squid was slower than that of large round herring, hake, goby, red squid, and lanternfish. There appeared to be differences between digestion rates of different sizes of the same species (Table 3). Smaller anchovy and pilchard were digested about 1.2 times faster than larger ones, but round herrings showed the opposite pattern. How- ever, for anchovy and round herring, the T20 values for large and small fish were not significantly different (£=1.65, df=8,P=0.1381, for large fish; £=1.05, df=8, P=0.3256, for small fish). The two size groups of pilchard had significantly different T20 values 390 Fishery Bulletin 95(2), 1 997 Table 3 The list of calculated mean times and standard deviations for each species in the artificial digestion experiments when remaining weights reach 20% of the original weight (T20). The digestion rate ratio shows the T20 value for each species in relation to that of lantern fish. (L=large size and S=small size groups — see Table 1). Sample species n Time (h) to reach 20% of original wt. (T20) Digestion rate ratio Mean SD Maasbanker Trachurus t. capensis 5 21.35 3.09 3.76 Cape anchovy (L)1 Engraulis capensis 5 15.67 2.81 2.76 Pilchard (L) Sardinops sagax 5 14.82 0.80 2.61 Round herring (SP Etrumeus whiteheadi 5 13.35 3.33 2.35 Cape anchovy (S)J Engraulis capensis 5 12.85 2.60 2.26 Pilchard (S) Sardinops sagax 5 12.57 0.62 2.21 Chokka squid Loligo v. reynaudii 5 11.82 1.04 2.08 Round herring (L)2 Etrumeus whiteheadi 5 11.54 1.96 2.03 Hake Merluccius sp. 5 11.36 0.98 2.00 Goby Sufflogobius bibarbatus 5 9.88 0.39 1.74 Red squid Todaropsis eblanae 5 8.44 0.70 1.49 Lanternfish Myctophidae 8 5.68 0.66 1.00 1 T20 for large and small anchovy = 14.26 ± 2.95 h. 2 T20 for large and small round herring = 12.45 ± 2.75 h. (f=5.02, df=8, P=0.001). Because there was no sig- nificant difference between the two size groups of anchovy and round herring, data were combined for one-way ANOVA on all sample species. The T.,0 values for 10 sample groups (maasbanker, large and small pilchard, anchovy, round herring, hake, goby, lanternfish, chokka squid, and red squid) showed a significant difference (one-way ANOVA, F= 27.3, total df=62, P<0.0001). The Newman-Keuls test indicated maasbanker, goby, red squid, and lanternfish had different T.,0 values from other spe- cies (P<0.05). Discussion Compared with previous digestion experiments, com- plete digestion of samples took longer than expected (Figs. 1 and 2). Bigg and Fawcett (1985) reported that whole herring and squid were digested within 10 h in an artificial solution of 1% HC1 and 1% pepsin. Jackson et al. (1987) found that about 10-15 h were required to digest whole anchovies in vitro (pH=1.25- 1.35). These time differences are probably the result of differences in acidity of the digestion solutions. In the present experiments, the solutions had a pH of ~2.3. The pH of the solution used by Bigg and Fawcett ( 1985) can be calculated as about 1.1. Therefore, their solu- tion was far more acidic than ours, resulting in more rapid digestion of fish and squid tissues. As noted, there was a general tendency for the di- gestion rate to decline when the remaining weight was less than 20% of the original weight. This was more pronounced for cephalopods than fish (Figs. 1 and 2). Bigg and Fawcett (1985, Fig. 16.1) reported similar trends: declines in rates of digestion can be caused by the accumulation of less digestible mate- rial, i.e. squid beaks and pens (Table 2; also Table 16.3 in Bigg and Fawcett, 1985). Although their procedure was different from that used in the present study, the digestion experiment ofNordpy et al. (1993) for herring (Clupea harengus ) also showed a rapid decline in digestion rate after about 70% of “dry matter disappearance” (DMD), and stated that the maximum DMD of herring is about 80%. The digestion rate decline at 80% in the present study may also be related to the digestibility of prey species of dolphins, or cetaceans in general. Undi- gested prey remains may be voided via gastric evacu- ation or, possibly, by regurgitation, as proposed for squid beaks (Clarke, 1980; Pitcher, 1980). The validity of in vitro experiments in represent- ing in vivo situations remains a matter of debate, but technical and other considerations make in vivo digestion experiments with dolphins impractical at this stage. Although not engaging strictly in a diges- tion experiment, Kastelein et al. (1993) fed captive Commerson’s dolphins (Cephalorhynchus commer- sonii) on North Atlantic herring (Clupea harengus) and Columbia river smelt ( Thalechthys paci ficus), NOTE Sekiguchi and Best: In vitro digestibility of some prey species of dolphin 391 into which gelatine capsules containing red dye were inserted. They found that only 40 to 155 minutes elapsed before dye appeared in feces, but it is not clear how this relates to the full digestion times of the fish. In vivo experiments with pinnipeds (another marine mammal feeding largely on cephalopods and fish) suggest somewhat faster digestion rates than those in our study. Murie and Lavigne (1985) found no fish hard parts remaining in seal stomachs 18 hours after feeding. However, stomachs could have been voided by regurgitation and gastric evacuation, whereas “hard parts” in our experiments could es- cape from the digestion bags only if they were re- duced to less than mesh size. Thus, their results are not necessarily inconsistent with those of the present study, although mechanical break-down actions of stomachs are likely to produce faster digestion in vivo. The in vitro digestion speeds recorded in the present study differed between species ( Table 3), but there was no consistent correlation with the taxo- nomic position of the prey. Three fish species in the order Clupeiformes (round herring, pilchard, and anchovy) had digestion-rate ratios in the range 2.03- 2.76, although large and small size groups of pilchard had significantly different T.,0 values. However, maasbanker and goby, both in the order Perciformes, showed very different digestion-rate ratios (3.76 and 1.74). While both squid species were digested faster than most fish species, chokka squid was digested more slowly than large round herring, hake, goby, and lanternfish. Bigg and Fawcett (1985) found that the squid Loligo opalescens was digested much faster than herring (Clupea harengus pallasi), both in vitro and in vivo (i.e. in a seal stomach). On the other hand, Jackson et al. (1987) found no difference in the di- gestion rate between fish (hake and anchovy) and squid ( Loligo ) in vitro. LeBrasseur and Stephens ( 1965) reported that fish (salmonids, myctophids, and hexagrammids) were digested faster than squid (gonatids) in their pepsin-hydrochloric acid solution (0.2 g pepsin/1 L, 1.5% HC1, pH 1.8). These in vitro differences quite possibly are the result of variations in the acidity of the solutions used and differences in experimental procedures. It is possible that digestion rates are related to muscle structure. Because pepsin is an enzyme that dissolves protein, the protein composition of a body will have an effect on digestion rate. Greer-Walker and Pull (1975) found that active pelagic fish had higher proportions of red muscle than coastal or deep- sea fish species. They reported that the mean red muscle proportion was 19.8% for Clupeidae, 18.3% for Carangidae, 4.5% for Gobiidae, and 4.5% and 0.6% for the deep-sea fish families Macrouridae and Chimaeridae, respectively. The digestion rates of fish prey found in the present study (Table 3) appear to fit a pattern in which the prey species digested most slowly tend to have the highest proportions of red muscle. Red muscle, containing greater quantities of mitochondria, myoglobin, fats, and glycogen than white muscle, may have stronger resistance to pep- sin in the digestion process. Fish otoliths recovered in the present study were reduced in size, and most hake and maasbanker otoliths completely dissolved within 8-12 h after ex- posure. McMahon and Tash (1979) reported that otoliths in a 0.01 N HC1 solution (pH=2.0-2.5) at 25°C were dissolved completely in 24 h, and a herring otolith in a pH 1.09 to 3.09 solution disappeared in 7 h (Jobling and Breiby, 1986). However, the erosion rate of otoliths of different species in acid varies (Jobling and Breiby, 1986), possibly depending on the ratio of surface area to volume (da Silva and Neilson, 1985). On the other hand, using otoliths recovered from fecal samples of captive harbor seals (Phoca uitulina), Harvey (1989) found no significant rela- tion between the robustness (length/weight) of the otolith and the degree to which the resultant esti- mate of fish length was reduced. In seal stomachs, all otoliths were released from herring skulls within 6 h and no otoliths were found 12 h after feeding (Murie and Lavigne, 1986; Murie, 1987). In the present experiments, only fragile, somewhat eroded otoliths were recovered after about 20 h of digestion in vitro. Consequently, it would be likely that any intact otoliths that are found in dolphin stomachs are from recently ingested fish. Walker et al. (1986) reported the recovery of an- chovy ( Engraulis mordax) otoliths from the stomach of a Pacific white-sided dolphin ( Lagenorhynchus obliquidens ) that had been held in captivity for 8 days without being fed anchovy; this finding suggested the possibility that otoliths can be retained over a pe- riod of one week. In the present experiments, a total of 16 anchovy otoliths (80%) were recovered after 20 h; these otoliths were too eroded, however, to esti- mate original sizes. Because the forestomach of a dolphin contains no glands, gastric juice must be re- fluxed from the main stomach (Harrison et al., 1970), so that the retention of otoliths for as long as 8 days should be viewed as exceptional. The digestion sequences were similar for all ex- perimental species (Table 2). Because otoliths are located inside a fish skull, their size reduction de- pends on when they are initially exposed to stomach acids. In most cases, heads of fish had disintegrated when about 40-60% of the body had been digested (Table 2), usually some 4 to 15 h after digestion be- gan (Figs. 1 and 2), when most otoliths were prob- ably exposed to the acids and began to erode. Harvey 392 Fishery Bulletin 95(2), 1 997 ( 1989) found that lengths of prey estimated from the sizes of otoliths in seal feces were underestimated by an average of 27.5%. Although the erosion rate of otoliths may be different for each species (Jobling and Breiby, 1986), it should be possible to apply cor- rection factors to avoid underestimating fish size. The stage of digestion of fish prey in a stomach, for in- stance, could be used as an index to suggest how much time has passed since feeding. A significant difference in T20 values for different size groups of a particular prey species was only found in pilchard. Smaller anchovy were digested about 1.2 times faster than larger ones. On the other hand, larger round herring were digested about 1.2 times faster than smaller ones (Table 3). These dif- ferences were not significant, however, although there was more variation among samples for anchovy and round herring than for pilchard (Fig. 2). Larger sample sizes may be required to test for differences in digestion rates between different-size individuals of a prey species. Although it has not been possible to calibrate these in vitro experiments with in vivo information, this paper indicates interspecific differences in relative digestion rates for several prey items taken by dol- phins. It should, therefore, be possible to apply “cor- rection factors” to estimate the original amount of par- ticular prey consumed when prey of different digest- ibility occur together in a stomach. However, the wider application of such a method would require the exami- nation of digestion rates for additional prey species. Acknowledgments We acknowledge W. R. Siegfried for allowing us to use the laboratory at the Percy FitzPatrick Institute, University of Cape Town as well as S. Jackson and N. J. Adams who provided us with assistance with laboratory equipment. We also thank K. Findlay for his assistance during experiments. M. Hiroki, Tokyo University of Agricultural Technology, assisted KS with the statistical analysis. Useful comments on this manuscript were received from T. Jefferson, anony- mous reviewers, and the scientific editor. This study was supported by the Benguela Ecology Programme. PBB was supported by the Foundation for Research Development and the South African Marine Corpo- ration through WWF-South Africa. Literature cited Bigg, M. A., and I. Fawcett. 1985. Two biases in diet determination of northern fur seals ( Callorhinus ursinus). In J. R. Beddington, R. J. H. Beverton and D. M. Lavigne (eds.), Marine mammals and fisheries, p. 284-291. George Allen & Unwin, London. Bigg, M. A., and M. A. Perez. 1985. Modified volume: a frequency-volume method to as- sess marine mammal food habits. In J. R. Beddington, R. J.H. Beverton and D. M. Lavigne (eds.), Marine mammals and fisheries, p. 277-283. George Allen & Unwin, London, Clarke, M. R. 1962. The identification of cephalopod “beaks” and the re- lationship between beak size and total body weight. Bull. Br. Mus. Nat. Hist. (Zool.) 8:419-480. 1980. Cephalopoda in the diet of sperm whales of the South- ern Hemisphere and their bearing on sperm whale biology. Discovery Rep. 37:1-324. da Silva, J., and J. D. Neilson. 1985. Limitations of using otoliths recovered in scats to estimate prey consumption in seals. Can. J. Fish. Aquat. Sci. 42:1439-1442. Fitch, J. E., and R. L. Brownell. 1968. Fish otoliths in cetacean stomachs and their impor- tance in interpreting feeding habits. J. Fish. Res. Board Can. 25:2561-2574. Greer- Walker, M., and G. A. Pull. 1975. A survey of red and white muscle in marine fish. J. Fish Biol. 7:295-300. Harrison, R. J., F. R. Johnson, and B. A. Young. 1970. The oesophagus and stomach of dolphins (7 Ytrsiops, Delphinus, Stenella). J. Zool. (Lond.) 160:377-390. Harvey, J. T. 1989. Assessment of errors associated with harbour seal ( Phoca vitulina) faecal sampling. J. Zool. (Lond.) 219:101- 111. Ishihara, Y. 1960. Studies on crystalline whale pepsin. Memo. Fac. Fish., Hokkaido Univ. 8:1-81. Jackson, S., D. C. Duffy, and J. F. G. Jenkins. 1987. Gastric digestion in marine vertebrate predators: in vitro standards. Funct. Ecol. 1:287-291. Jobling, M., and A. Breiby. 1986. The use and abuse of fish otoliths in studies of feed- ing habits of marine piscivores. Sarsia 71:265-274. Kastelein, R. A., J. McBain, and B. Neurohr. 1993. Information on the biology of Commerson’s dolphins (Cephalorhyncus commersonii). Aquat. Mamm. 19:13-19. LeBrasseur, R. J., and K. Stephens. 1965. Relative rates of degradation of some organisms con- sumed by marine salmon. J. Fish. Res. Board Can. 22:1563-1564. McMahon, T. E., and J. C. Tash. 1979. Effects of formalin (buffered and unbuffered) and hy- drochloric acid on fish otoliths. Copeia 1979(1):155-156. Murie, D. J. 1987. Experimental approaches to stomach content analy- ses of piscivorous marine mammals. In A. C. Huntley, D. P. Costa, G. A. J. Worthy, and G. A. Castellini (eds.), Ap- proaches to marine mammal energetics, p. 147-163. Soc. Mar. Mammal., Special Publ. 1. Murie, D. J., and D. M. Lavigne. 1985. Digestion and retention of Atlantic herring otoliths in the stomachs of grey seals. In J. R. Beddington, R. J. H. Beverton, and D. M. Lavigne (eds.), Marine mammals and fisheries, p. 292-299. George Allen & Unwin, London. 1986. Interpretation of otoliths in stomach content analy- ses of phocid seals: quantifying fish consumption. Can. J. Zool. 64:1152-1157. NOTE Sekiguchi and Best: In vitro digestibility of some prey species of dolphin 393 Nordoy, E. S., W. Sermo, and A. S. Blix. 1993. In vitro digestibility of different prey species of minke whales ( Balaenoptera acutorostrata). Br. J. Nutr. 70:485- 489. Pitcher, K. W. 1980. Stomach contents and feces as indicators of harbor seal, Phoca vitulina, foods in the Gulf of Alaska. Fish. Bull. 78:797-798. Sekiguchi, K., N. T. W. Klages, and P. B. Best. 1992. C omparative analysis of the diets of smaller odontocete cetaceans along the coast of southern Africa. S. Afr. J. Mar. Sci. 12:843-861. Smith, G. J. D. 1972. The stomach of the harbor porpoise Phocoena phocoena (L.). Can. J. Zool. 50:1611-1616. Walker, W. A., S. Leatherwood, K. R. Goodrich, W. F. Perrin, and R. K. Stroud. 1986. Geographical variation and biology of the Pacific white- sided dolphin, Lagenorhynchus obliquidens, in the northeast- ern Pacific. In M. M. Bryden and R. J. Harrison (eds.), Re- search on dolphins, p. 441^165. Oxford Univ. Press, Oxford. Zar, J. H. 1974. Biostatistical analysis. Prentice-Hall, Inc., Englewood Cliffs, NJ, 620 p. 394 Development of laboratory-reared sheepshead, Archosargus probatocephalus (Pisces: Sparidae)* John W. Tucker Jr. Sabine R. Alshuth Harbor Branch Oceanographic Institution 5600 North U S. 1, Fort Pierce, Florida 34946 Twenty-two sparid species are known from the western Atlantic and 15 from eastern coastal waters of the United States and Canada, including two in the genus Archo- sargus'. A. probatocephalus , sheep- shead, and A. rhomboidalis, Carib- bean sea bream (Johnson, 1978; Robins et al., 1991). Two previous publications have provided partial descriptions of mid- to late larval and juvenile sheepshead based on wild specimens. Hildebrand and Cable ( 1938) described wild sheeps- head larvae and juveniles, begin- ning with 6-mm-TL larvae. Mook (1977) described osteology of 2-25 mm wild sheepshead, with notes on pigmentation and illustrations of mid- to late larvae and a juvenile. Rathbun (1892) reported a dia- meter of about 800 pm for sheeps- head eggs. Houde and Potthoff (1976) gave a comprehensive de- scription of Caribbean sea bream reared from collected eggs. For other genera in this region, partial descriptions exist for scup ( Stenoto - mus chrysops) eggs and larvae (Kuntz and Radcliffe, 1917; Hilde- brand and Schroeder, 1928; Wheat- land, 1956) and pinfish ( Lagodon rhomboides) eggs, larvae, and juve- niles (Hildebrand and Cable, 1938; Cardeilhac, 1976). This paper de- scribes the size and shape, morph- ometries, pigmentation, feeding, and growth for a series of sheeps- head reared from eggs to 67-day- old juveniles. Materials and methods Specimens Eggs and milt were stripped from a pair of running ripe adults caught in the Indian River just west of Fort Pierce Inlet on 4 April 1984 (398-g female, 367-g male). Larval and juvenile specimens were reared from 14,000 eggs stocked in a 2.4- m diameter cylindrical fiberglass tank holding 3,500 L of water. Fil- tered estuarine water was supplied from the Indian River, and ex- change was increased from zero at day 5 to 300% per day by day 30. During the culture period, water temperature was 22.1-33.2°C (mean 27.0°) and salinity, 27-36 ppt. For the first 7 days, tempera- ture was 23°C and salinity 34-36 ppt. The tank was in a greenhouse that admitted 35% diffuse natural light. The fish were fed cultured rotifers, Brachionus plicatilis (days 3-31); cultured artemia nauplii to adults (days 14-47); cultured and wild copepods, Tigriopus japonicus and Acartia tonsa (days 18-67); bay scallop and penaeid shrimp meal (days 24-77); commercial dry salmon starter feed (days 27-100); wild crab larvae (occasionally dur- ing days 28-45); and commercial soft-moist salmon feed (days 36- 80). Details of spawning, culture conditions, and foods are given in Tucker ( 1987). Specimens were pre- served in 5% formalin buffered with sodium acetate; 145 of the preserved specimens and several live speci- mens were used for the description. Measurements and counts Measurements were made with an ocular micrometer in a stereomicro- scope, except that standard and to- tal length of postflexion larvae longer than 9 mm SL were deter- mined with a millimeter scale. Mean diameters of the yolk and oil globule were determined and vol- umes calculated for ten specimens of each age from 2.5 haf (hours af- ter fertilization) until yolk and oil were exhausted. Notochord length (NL), standard length (SL), total length (TL), snout length, horizon- tal eye diameter, predorsal length (snout to first dorsal spine [snout- DSpl]), snout to first dorsal ray (snout-DRal), snout to pelvic spine (snout-PvSpl), preanus length (snout-anus), and body depth at anus were measured as in Houde and Potthoff (1976). Other mea- surements were upper jaw length — snout tip to posterior margin of maxillary; head length (HL) — hori- zontal distance from tip of snout to anterior margin of cleithrum at body midline; head depth — great- est vertical depth of head; body depth at pelvic fin — vertical dis- tance from dorsal to ventral body margin at base of second pelvic ray (body at Pv); and caudal peduncle depth — least vertical distance from dorsal to ventral body margin. Most specimens were fairly trans- parent, and internal structures such as myomeres were visible dur- ing preflexion without clearing and staining. Vertebrae were not counted. The following counts were taken * Contribution 1142 from Harbor Branch Oceanographic Institution, Fort Pierce, Florida. Manuscript accepted 27 September 1996. Fishery Bulletin 95:394-401 (1997). NOTE Tucker and Alshuth. Development of laboratory-reared Archossrgus probatocephalus 395 from larvae and juveniles with a stereomicroscope: caudal rays, dorsal spines and rays, anal spines and rays, pectoral rays, and pelvic spine and rays. Results and discussion Egg development The planktonic eggs were spherical. The chorion was transparent and smooth; the yolk clear, homoge- neous, and unpigmented; and the single oil globule yellow. The perivitelline space was very narrow ( 12- 39 pm before fertilization, 10-48 pm at 5 haf, and 31-77 pm at 28 haf). Diameter of live eggs at 2.5 haf was in the range of 806-865 pm (mean 824 pm) and was constant until hatching; oil globule diameter range was 187-241 pm (mean 206 pm). At 2.5 haf, mean yolk volume was 254 nL and oil globule vol- ume was 4.58 nL (Fig. 1). Just before hatching (Fig. 2A), the embryo had sparse pigmentation on the snout and behind the eye. Several punctate melano- phores were present on the oil globule. At 23°C, about 90% of the eggs hatched at 28 ±0.5 haf. Larval development Hatchlings had unpigmented eyes, undeveloped mouths, and clear finfolds (ranges 1.58-1.70 mm NL, 1.68-1.78 mm TL, Fig. 2B). The pigmented oil glob- ule was near the posterior margin of the unpigmented yolk sac, close to the anus. Distinct melanophores were visible on the ventral midline, halfway between the anus and the notochord tip. Several small con- tracted melanophores were on the body, but no dis- tinct pattern was seen in the examined material. Sixty-seven percent of the yolk remained and 70% of the oil (Fig. 1). At 15 hah (h after hatching), pigmentation was not visible. At 25 hah, live lar- vae had five distinct vertical bands of yellow pig- ment, one above the yolk sac, three between the anus and the notochord tip, and one at the noto- chord tip (Fig. 2C; yellow pigment not shown here, but see photograph in Tucker, 1986). Six percent of the yolk remained and 33% of the oil (Fig. 1 ). At 45 hah, eyes were only partly pigmented, and the mouth was not yet open. Except for the eyes, no pigmentation was visible in preserved specimens. Two percent of the yolk remained and 10% of the oil (Fig. 1). Between 3 and 4 dah (d after hatch- ing), nearly all larvae developed functioning di- gestive systems and fully pigmented eyes and be- gan to feed on rotifers. Lower jaw length averaged 0.29 mm. Pigmentation was present on the ven- tral surface of the gut and anus. Some melano- phores were visible along the ventral midline. At 73 hah, only 0.2% of the yolk remained and 0.4% of the oil (Fig. 1). At 4.0 dah, larvae were feeding efficiently (Fig. 3A). Rayless pectoral fins were present at 2.37 mm NL. Pigmentation was sparse. Melanophores on the head had disappeared, and those on the gut had contracted. Dendritic melanophores were visible on the surface of the gut and were densest on the dorsal surface. Several distinct melanophores were on the ventral midline. No fin rays were visible. In at least half the specimens, yolk and oil were exhausted; the rest had a trace. At 5.0 dah, shape and pigmentation had not changed appreciably, but by 6 dah, all lar- vae had melanophores on the gut, as well as preanal and postanal pigmentation on the ventral midline. At 9 dah, nine larvae (2.78-3.24 mm NL) were still in preflexion (Fig. 3B), and one (3.50 mm NL) had begun notochord flexion. Pigmentation was as for 6 dah. Number and position of branching melano- phores on the ventral midline were variable. The larva undergoing notchord flexion also had internal melanophores in the center of the auditory vesicle, one branched melanophore on the forehead, and melanophores on the lower jaw angle and throat. At 14 dah, two larvae (4.16-4.66 mm NL) were in preflexion and eight (4.66-5.36 mm NL) in flexion. Four of the flexion specimens were in early flexion and four in midflexion. Rays began forming above the center of the pectoral fin at 4.66 mm NL and just below the center of the caudal fin at 4.91 mm NL (Table 1). At 17 dah, all 10 specimens were in midflexion. Cau- dal rays continued to develop. Rays began forming in the posterior part of the soft dorsal fin at 5.29 mm NL and in the posterior part of the anal fin at 5.36 mm NL. Larvae had two melanophores on the forehead, four Hours after fertilization Figure 1 Yolk and oil globule depletion in sheepshead, Archosargus probatocephalus , eggs and larvae. Triangles represent yolk and dots represent oil. 396 Fishery Bulletin 95(2), 1997 $ Figure 2 Sheepshead, Archosargus probatocephalus , eggs and yolksac larvae: (A) eggs just before hatching (28 h after hatching, 820 mm diameter); (B) newly hatched larva (28 h after fertilization, 1.58 mm NL, 1.68 mm TL); and (C) yolksac larva (21 h after hatching, 2.46 mm NL, 2.59 mm TL). Drawings A and B are from preserved specimens, and C is from a live specimen. melanophores along the ventral mid- line, and one large melanophore on the posterior part of the anus. At 21 dah, all larvae were in late flexion (Fig. 3C ) and were characterized by a more rounded head and increased pigmen- tation. Large dendritic melanophores spread over the gut and along the ven- tral midline. One distinct dendritic melanophore was visible on the fore- head and another behind the eye. Small punctate melanophores were visible on the ventral abdomen. In live larvae, reddish chromatophores were scattered over the body but mainly between the developing dorsal and anal fins (not shown in Fig. 3C, but see photograph in Tucker, 1986). At 28 dah, five larvae had com- pleted flexion. The finfold was gone, and caudal, dorsal, anal, and pecto- ral fin spines and rays were well de- veloped (Table 1). Caudal rays num- bered 21-29 and pelvic rays 0-3. Ten dorsal spines were present in all lar- vae; the last one was yet to form. Rays began forming at the center of the pel- vic fin at 6.24 mm SL. The first and second anal spines were present in all specimens. Punctate melanophores were scattered over the entire body. Prejuvenile development By 28-30 dah, transformation was nearly complete. Adult counts of spines and rays were reached in all fins except for a lack of 0-2 caudal rays, the last dorsal spine, the low- ermost 1-2 pectoral rays, and the last pelvic ray (Table 1). Coloration was similar to that of adults, and 5-6 of the characteristic lateral black bars had formed, but the fish were more slen- der than adults. This could be considered a prejuvenile phase. At 28 dah, two specimens (8.54 and 8.81 mm SL) had become prejuveniles (Fig. 3D). By 38 dah, the adult complement of fin elements was present except for 1-2 caudal rays, one anal spine, and one pectoral ray in some specimens; 1-2 pelvic rays still were missing. Ten of 14 specimens were fully scaled. Juvenile development Between 38 and 53 dah, all specimens reached the juvenile stage and had adult counts for fin elements. By 42 dah, the body had deepened, but the eye was relatively large. By 67 dah, body proportions of the juveniles were similar to those of adults. Proportions Snout length:head length (HL) varied slightly until 53 dah, then increased to 28% at 67 dah (Table 2). Eye diameter:HL was largest at 0.6 dah (69%) and decreased to 36% at 67 dah. Upper jaw length:HL was greatest at 17 dah and least at 67 dah. HL:body length (BL) was least at 0.6 dah and most at 38 dah, then decreased at 53-67 dah. Snout to first dorsal spine:BL, snout to first dorsal ray:BL, and snout to first pelvic spine:BL were greatest at 38 dah. Snout NOTE Tucker and Alshuth: Development of laboratory-reared Archosargus probatocephalus 397 B 1 mm D Figure 3 Sheepshead, Archosargus probatocephalus , preflexion to postflexion larvae (preserved): (A) yolk exhaustion (4 d after hatching, 2.45 mm NL, 2.64 mm TL); (B) preflexion (9 dah, 3.24 mm NL, 3.47 mm TL); (C) flexion (21 dah, 4.97 mm NL, 5.74 mm TL); and (D) prejuvenile (28 dah, 8.81 mm SL, 11.4 mm TL). to anus:BL was minimal at 5-9 dah and greatest at 38 dah. Total length :BL and head depth:BL were least at hatching and most at 38 dah. Body depth at pel- vic fin:BL increased between 28 dah and 38-67 dah. Body depth at anus:BL decreased from 17% at 0.6 dah to 14% at 5—6 dah, then rose to 35—36% for 28- and 38-day-old prejuveniles. Caudal peduncle depth:BL increased between 9 dah and 28-38 dah. At 28 dah, prejuveniles were slightly deeper than postflexion larvae at the pelvic fin and anus. All lengths and depths divided by BL were highest at about 38 dah. Thereafter, postanal length increased relatively faster than the other lengths and depths, leading to a decrease in proportional measurements, except for body depth at pelvic fin:BL, which was constant during 38-67 dah. Growth and survival Growth of sheepshead to 27.5 mm TL at 67 dah (Fig. 4) was similar to that of Caribbean sea bream (Houde and Potthoff, 1976). Mean dry weight rose from 14.5 pg (about 69 (pg wet) at 7 dah to 88 pg (464 pg wet) at 67 dah (Fig. 4); mean wet weight was 7.6 g at 101 dah. Survival was 40% from fertilization to 101 dah, and 100% thereafter to 3 years. Comparison with other sparids Our specimens up to 9 dah (Fig. 3B) were younger than previously de- scribed sheepshead; most larvae were longer than those at the same stage illustrated by other authors (Hilde- brand and Cable, 1938; Mook, 1977), until 6 mm. The 3.9-mm TL larva in Fig. 3B corresponded to the 2-mm specimen illustrated by Mook ( 1977). The 6.1-mm specimen in Fig. 3C fit- ted between Mook’s ( 1977 ) 4- and 4.5- mm specimens and corresponded with Hildebrand and Cable’s (1938) ca. 6-mm specimen. The 8.9-mm specimen in Fig. 3D fitted between Mook’s (1977) 8- and 11-mm speci- mens. As Riley et al. (1995) have dis- cussed, net damage to field-caught larvae can shrink and distort them to different degrees, depending on species and stage. At 23°C, sheepshead hatched at ~28 haf, first fed at ~84 hah ( ~ 112 haf) and exhausted their yolk and oil by ~96 hah (-124 haf). At 26°C, Caribbean sea bream hatched by -22 haf, first fed at about 35 hah (-57 haD, and exhausted yolk by 50 hah (-72 haf) (Houde and Potthoff, 1976). Eggs of gilthead sea bream, Sparus aurata (native to the Mediterranean region), with a mean diameter of 1,020 pm, are larger than those of sheepshead and Caribbean sea bream (Table 3) and contain about twice as much yolk and oil (Ronnestad et al., 1994). At 25 haf, gilthead sea bream eggs had 430 nL yolk and 5.8 nL oil, but sheepshead had only 184 nL yolk and 3.6 nL oil. At 18°C (first 6 h at 15-18°C), gilthead 398 Fishery Bulletin 95(2), 1 997 Table 1 Meristic ranges for 65 reared sheepshead, Archosargus probatocephalus, larvae and juveniles. Days after hatching No. of specimens Caudal rays Dorsal spines Dorsal rays Anal spines Anal rays Pectoral rays Pelvic spines Pelvic rays Preflexion 9 5 0 0 0 0 0 0 0 0 14 2 0-7 0 0 0 0 0 0 0 Flexion 9 1 0 0 0 0 0 0 0 0 14 8 7-11 0 0 0 0 2-3 0 0 17 10 9-17 0 5-10 0 3-9 5-7 0 0 21 2 15-16 0 0-6 0 6-92 7-9 0 0 Postflexion 28 5 21-29 102 11-122 2-3 2 10 11-13 0 0-3 Prejuvenile 28 2 30-327 10 11-12 3 10-11 13-14 l7 4 38 10 30-34 11 11-13 2-3 10-11 14-157 1 3-4 Juvenile 53 10 32-34 11 12-13 3 10-12 15 1 5; 67 10 32-34 11 12-13 3 10-11 15 1 5 1 First reached adult number. 2 First reached adult number, but one more will be added. sea bream hatched at ~55 haf, first fed at -100 hah (-155 haf) and exhausted their yolk at -115 hah (-170 haf) and their oil slightly later. Also at 18°C, pinfish (with similar sized eggs) hatched at -48 haf and were ready to feed sooner, by -76 hah (-124 hafKCardeilhac, 1976). Oil of unfed pinfish was ex- hausted at -150 hah (-198 haf) and yolk at -165 hah (213 haf). Distinguishing characteristics Between hatching and full eye pigmentation, Carib- bean sea bream had little coloration, but sheepshead had five large ventral melanophores and scup three large ventral melanophores. In both sheepshead and scup, one melanophore was over the gut and the sec- ond over the anus. In sheepshead, the remaining three were evenly spaced between the anus and no- tochord tip. In scup, the third melanophore was about halfway between the anus and notochord tip, and each of the three areas of pigment formed more of a lateral band than in sheepshead. From about 2.7 mm to at least 10 mm TL, both sea bream and scup had a ventral row of postanal melanophores associated with myosepta; sheepshead had scattered ventral melano- phores, rather than an evenly-spaced row. By about 9 mm TL, prejuvenile sheepshead had five or six dis- tinct lateral black bars on the body; all juveniles had six. Sheepshead from the Atlantic typically have six complete bars, whereas those from the Gulf of Mexico typically have five (Johnson, 1978). By about 15 mm Days after hatching Figure 4 Growth of sheepshead, Archosargus probatocephalus , lar- vae and early juveniles. The dashed curve represents mean body length and the solid curve represents mean total length. Dots represent mean dry weight. TL, Caribbean sea bream had six indistinct bars (Houde and Potthoff, 1976). By about 25 mm TL, scup had six irregular bars (Johnson, 1978). By about 30 mm TL, pinfish had five or six indistinct bars (Hildebrand and Cable, 1938). Meristics are only slightly helpful for identifica- tion. Western North Atlantic sparids usually have 10 precaudal vertebrae and 14 caudal vertebrae (Jor- dan and Evermann, 1896-1900; Miller and Jorgen- son, 1973; Hoese and Moore, 1977; Johnson, 1978). NOTE Tucker and Alshuth. Development of laboratory-reared Archosargus probatocephalus 399 Table 2 Summary of proportional measurements of 145 reared sheepshead, Archosargus probatocephalus, larvae and juveniles. Except for body length, values are in percentage of head length (HL) or body length (BL). Mean ±standard deviation is on the first line, and range on the second. A zero means <0.5%. For BL, notochord length (NL) was used through flexion and standard length (SL) after flexion. DSpl = first dorsal spine, DRal = first dorsal ray, PvSpl = first pelvic spine, and Pv = pelvic fin. Percent of head length Percent of body length Length Length Depth Events and Body Upper Snout- Snout- Snout- Snout- Body Body Caudal days after length Snout Eye jaw Head DSpl DRal PvSpl anus Total Head at Pv at anus peduncle hatching n (mm) (%) (%) (%) (%) (%) (%) (%) (%» (%) (%) (%) (%) {%) Hatching 0 10 1.65 +0.03 24 ±5 58 ±5 19 ±1 104 ±1 13 ±1 1.58-1.70 14-29 52-67 18-20 103-106 11-15 0.6 10 2.21 ±0.03 22 ±4 69 ±4 14 ±1 47 ±1 105 ±1 14 ±1 17 ±1 2.16-2.25 19-30 64-77 13-14 46-49 103-106 12-14 16-19 1.0 10 2.42 ±0.01 22 ±1 50 ±1 17 ±0 43 ±1 106 ±1 14 ±0 16 ±1 2.34-2.47 20-24 47-52 16-18 41-46 105-107 13-15 15-18 1.9 10 2.59 ±0.02 24 ±1 50 ±1 18 ±0 41 ±1 106 ±0 17 ±0 15 ±0 2.55-2.60 21-26 49-51 17-18 39-43 105-107 16-18 15-17 Eye pigmentation and first feeding 3.0 10 2.55 ±0.06 21 ±2 48 ±2 20 ±0 41 ±1 107 ±0 19 ±1 15 ±0 2.45-2.65 19-24 46-51 19-20 40-44 106-107 18-20 14-15 Yolk and oil exhaustion 4.0 10 2.46+0.16 22 ±2 46 ±2 21 ±1 40 ±2 107 ±1 20 ±2 15 ±2 2.20-2.65 20-26 43-50 20-24 38-44 106-108 17-24 13-17 5.0 10 2.49 ±0.04 22 ±2 45 ±2 20 ±1 38 ±1 107 ±0 19 ±0 14 ±1 2.43-2.56 19-25 42-48 19-22 37-40 107-108 18-20 12-15 6 10 2.43 ±0.09 20 ±1 42 ±1 37 ±4 22 ±0 40 ±1 107 ±1 20 ±1 14 ±1 2.25-2.54 19-22 40-44 29-41 21-22 38-42 103-108 18-20 12-17 9 5 3.10+0.12 22 ±5 43 ±4 36 ±6 22 ±1 39 ±1 107 ±2 21 ±2 16 ±2 3 ±1 2.78-3.24 17-31 39-48 30-43 21-24 38-42 105-111 18-23 14-17 2-4 14 2 4.41 ±0.35 20 ±2 40 ±0 36 ±8 25 ±1 49 ±1 105 ±1 25 ±1 24 ±1 7 ±1 4.16-4.66 18-22 40 28-43 24-25 48-50 104-106 24-27 23-24 6-8 Flexion 9 1 3.50 21 40 31 23 40 106 22 18 4 14 8 5.06+0.21 21 ±3 38 ±2 38 ±6 26 ±1 51 ±1 106 ±2 26 ±1 24 ±1 8 ±1 4.66-5.36 18-26 36-42 34-51 24-27 49-52 103-110 25-27 23-25 8-9 17 10 4.92 ±0.51 23 ±5 37 ±5 41 ±3 28 ±2 54 ±4 114 ±5 27 ±2 26 ±3 11 ±3 3.90-5.41 14-35 26-43 36-44 24-30 49-59 104-124 26-30 21-31 4-13 21 2 4.95 ±0.05 22 ±4 38 ±0 35 ±2 28 ±0 54 ±0 116 ±1 28 ±1 25 ±0 10 ±0 4.92-4.97 19-24 38 33-36 28-29 54 115-117 28-29 25-26 10 Postflexion 28 5 6.60+0.79 22 ±2 41 ±2 32 ±3 34 ±2 40 ±1 65 ±2 37 ±2 59 ±2 124 ±2 34 ±2 33 ±3 31 ±3 14 ±1 5.83-7.93 20-25 38-43 27-36 30-36 39-42 63-67 35-39 57-61 122-126 29-35 29-36 26-35 13-15 Prejuvenile 28 2 8.68 ±0.19 23 ±1 39 ±0 28 ±7 35 ±2 39 ±3 64 ±3 40 ±1 59 ±0 124 ±1 34 ±1 35 ±1 36 ±2 15 ±1 8.54-8.81 22-24 39 23-33 33-36 37-41 62-66 39-40 59 123-124 33-35 34-36 34-37 14-15 38 10 7.22 ±0.64 21 ±1 39 ±1 26 ±1 40 ±1 44 ±2 77 ±2 44 ±1 68 ±2 132 ±3 35 ±1 37 ±2 35 ±2 15 ±1 6.05-10.3 19-23 37-40 24-29 38-42 42-47 74-79 42-46 65-71 126-134 34-38 35-38 30-37 13-16 Transformation 53 10 18.6 ±3.2 22 ±1 38 ±1 26 ±1 33 ±1 34 ±1 63 ±1 36 ±1 61 ±1 129 ±1 28 ±0 38 ±2 33 ±2 13 ±0 13.7-24.7 21-24 35-40 25-27 30-35 33-36 62-66 34-39 60-63 128-130 27-29 36-42 31-36 12-14 67 10 21.5 ±5.2 28 ±3 36 ±2 24 ±2 33 ±1 36 ±2 65 ±2 40 ±2 62 ±2 128 ±0 29 ±1 37 ±1 32 ±2 13 ±0 15.9-33.7 22-34 33-39 20-26 31-35 34-38 62-68 37-42 59-65 126-130 27-31 36-40 31-35 12-14 400 Fishery Bulletin 95(2), 1997 Dorsal ray, anal spine, anal ray, pectoral ray, and caudal ray counts of sheepshead overlap with those of most other sparids (Table 4). For most species of Archosargus, Calamus, Diplodus, Lagodon, Pagrus, and Stenotomus, 11 or 12 dorsal spines are typical, with a range of 10-13. Dorsal rays mostly number 10-12, with a range of 9-16, and anal rays mostly are 10-11, with a range of 8-15. Caudal rays usually are in the range of 32-38, with 9+8 principal rays. For sheepshead, typical counts followed by ranges in parentheses are caudal rays 32-33 (8-9+9+8+7), dorsal spines 12 ( 10-12), dorsal rays 11 (10-13), anal Table 3 Spawning, egg, and hatchling data for sheepshead, Archosargus probatocephalus ; Caribbean sea bream, Archosargus rhomboidalis', pinfish, Lagodon rhomboides; and scup, Stenotomus chrysops. Sheepshead Sea bream Pinfish Scup Location Florida Florida Florida Northeastern U.S. Spawning season Feb-Apr Sep-May Oct-Mar May-Aug Egg diameter (pm) 806-865 800-940 990-1, 0502 800-1,150 Oil globule diameter (pm) 187-241 210-260 ~200; 140-280 Hatching temperature (°C) 23 24 18 22 Incubation time (h) 28 ~<22 48 <40 Hatchling length (mm TL) 1. 7-1.8 2. 1-2.3 ~2 References Present study Houde and Potthoff, 1976 Caldwell, 1957 Cardeilhac, 1976 Kuntz and Radcliffe, 1917 Hildebrand and Schroeder, 1928 Wheatland, 1956 1 Unfertilized. Table 4 Comparison of selected larval and juvenile characters of sheepshead, Archosargus probatocephalus, reared at a mean tempera- ture of 23°C and Caribbean sea bream, Archosargus rhomboidalis, reared at 26°C (Houde and Potthoff, 1976): size and age from first development to completion in all specimens. Less common counts are in parentheses. Because specimens were not taken every day, age ranges are approximate. BL = body length; dah = days after hatching. Archosargus probatocephalus Archosargus rhomboidalis Adult number BL (mm) Age (dah) Adult number BL (mm) Age (dah) Hatchling 1.6-1. 7 2.0-2.22 Flexion 3. 5-5.0 9-21 4. 2-4. 9 9-11 Pectoral rays 15 4.7-10.3 14-38 14(15) 5.0 — 8.0 11-15 Caudal rays (819+9+8+7-8(9) 4.9-13.7 14-53 (10)8-9+9+8+7-8(9) 4.1-10.2 7-16 Dorsal rays 12-13 5.3-13.7 17-53 10-11 5. 0-5. 7 11-13 Anal rays 10-11(12) 5. 4-6.4 17-28 10(11) 5.0-5. 7 11-13 Pelvic rays 5 6.2-13.7 28-53 5 6.6-15.6 26-37 Dorsal spines 11 7. 2-8. 5 28-38 (12)13(14) 5. 7-8.2 13-16 Prejuvenile2 >6.0, <13.7 >28, <53 Pelvic spines 1 8. 5-9. 5 28-38 1 6.6-15.6 26-37 Anal spines 3 8.5-13.7 28-53 3 5.4-7. 1 13-26 Juvenile2 13.7 >38, <53 20.0 ~35 1 Smallest specimens described. 2 Juvenile coloration attained; all fin elements present except last dorsal spine, last pectoral ray, and last one or two pelvic rays. 3 All fin elements present, and fish fully scaled. NOTE Tucker and Alshuth: Development of laboratory-reared Archosargus probatocephalus 401 spines 3, anal rays 10-11 (9-11), pectoral rays 15- 17, pelvic spine 1, pelvic rays 5 (Miller and Jorgenson, 1973; Johnson, 1978). Caribbean sea bream usually have 13 dorsal spines, whereas sheepshead, pinfish, and most other sparids usually have 12. Diplodus spp. have slightly higher counts than most sparids: 11-13 dorsal spines, 13-16 dorsal rays, and 13-15 anal rays. Pagrus species have fewer anal rays, usually 8. Stage of development at a given size could be use- ful for distinguishing larvae. At the same stage, scup tended to be much longer than sheepshead (Fig. 4) and Caribbean sea bream. Kuntz and Radcliffe’s (1917) 25- mm TL scup (their Fig. 36) corresponds with Houde and Potthoff’s (1976) 12.8-mm TL sea bream (their Fig. 5a) and our 8.9-mm TL sheepshead (Fig. 3D). The proportions snout length:BL (3-8%), eye diameter:BL (7-14%), predorsal length:BL, snout to first dorsal ray:BL, snout to pelvic spine:BL, preanus length:BL, and body depth at anus:BL (Table 2) all were similar during development of sheepshead and Caribbean sea bream (Houde and Potthoff, 1976). Eye diameter and body depth are relatively small in scup at 10 mm TL. At that size, eye diameterrHL was about 32% in scup, 37% in sea bream, and 39% in sheeps- head; depth at pelvic fin:SL was 27% in scup, 34% in sea bream, and 37% in sheepshead. Acknowledgments We thank Chuck Sultzman for helping to catch the broodfish, Mary Clark and Bob Eifert for maintain- ing cultures of algae and zooplankton, and Blake Faulkner for assistance in raising the larvae. Literature cited Cardeilhac, P. T. 1976. Induced maturation and development of pinfish eggs. Aquaculture 8:389-393. Hildebrand, S. F., and L.E. Cable. 1938. Further notes on the development and life history of some teleosts at Beaufort, N.C. U.S. Bur. Fish. Bull. 24: 505-642. Hildebrand, S. F., and W. C. Schroeder. 1928. Fishes of Chesapeake Bay. U.S. Bur. Fish. Bull. vol. 53, part 1, p. 1-388. Hoese, H. B., and R. H. Moore. 1977. Fishes of the Gulf of Mexico: Texas, Louisiana, and adjacent waters. Texas A&M Univ. Press, College Station, TX, 327 p. Houde, E. D., and T. Potthoff. 1976. Egg and larval development of the sea bream Archosargus rhomboidalis (Linnaeus): Pisces, Sparidae. Bull. Mar. Sci. 26:506-529. Johnson, G. D. 1978. Development of fishes of the Mid-Atlantic Bight, vol- ume IV. U.S. Fish Wildl. Serv., Biol. Services Progr. FWS/ OBS-78/12, 314 p. Jordan, D. S., and B. W. Evermann. 1896-1900. The fishes of North and Middle America. U.S. Nat. Mus. Bull. 47 (4 parts):l-3313. Kuntz, A., and L. Radcliffe. 1917. Notes on the embryology and larval development of twelve teleostean fishes. U.S. Bur. Fish. Bull. 1915- 1916(35):89-134. Miller, G. L., and S. C. Jorgenson. 1973. Meristic characters of some marine fishes of the west- ern North Atlantic Ocean. Fish. Bull. 71:301-312. Mook, D. 1977. Larval and osteological development of the sheeps- head, Archosargus probatocephalus (Pisces: Sparidae). Copeia 1977:126-133. Rathbun, R. 1892. Report upon the inquiry respecting foodfishes and the fishing grounds. U.S. Comm. Fish. Rep. 16:xli-cvii. Riley, C. M., G. J. Holt, and C. R. Arnold. 1995. Growth and morphology of larval and juvenile cap- tive bred yellowtail snapper, Ocyurus chrysurus. Fish. Bull. 93:179-185. Robins, C. R. (chairman), R. M. Bailey, C. E. Bond, J. R. Brooker, E. A. Lachner, R. N. Lea, and W. B. Scott. 1991. Common and scientific names of fishes from the United States and Canada. Am. Fish. Soc. Spec. Publ. 20, 183 p. Ronnestad, I., W. M. Koven, A. Tandler, M. Harel, and H. J. Fyhn. 1994. Energy metabolism during development of eggs and larvae of gilthead sea bream ( Sparus aurata ). Mar. Biol 120:187-196. Tucker, J. W., Jr. 1986. Sheepshead, the versatile porgy. Tropical Fish Hob- byist 34(5):64-65, 68. 1987. Sheepshead culture and preliminary evaluation for farming. Progr. Fish-Cult. 49:224-228. Wheatland, S. B. 1956. Oceanography of Long Island Sound, 1952-1954. VII: Pelagic fish eggs and larvae. Bull. Bingham Oceanogr. Collect., Yale Univ. 15:234-314. 402 Fishery Bulletin 95(2), 1997 Superintendent of Documents Order Form *5178 □yes, please send me the following publications: subscriptions to Fishery Bulletin (FB) for $32.00 per year ($40.00 foreign). The total cost of my order is $ . Price includes regular shipping and handling and is subject to change. International customers please add 25%. Company or personal name (Please type or print) Additional address/attention line Charge your order. It’s easy! 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