Wilson et al.: Geographic variation among age-0 Theragra chakogramma 



209 



Kodiak Island, but we do not yet understand how this 

 actually affects walleye pollock in nearshore nurseries. 

 In this article, we present information from a pilot 

 study to better understand the environmental basis for 

 the apparent richness of the Kodiak Island vicinity as 

 a pollock nursery. Our objectives were 1) to examine 

 age-0 pollock size, body condition, growth, and diet for 

 evidence of geographic effect (nearshore versus shelf), 

 and 2) to determine if their potential prey field (i.e., 

 zooplanktonl was associated with this effect. 



Materials and methods 



This study was conducted as an ancillary project during 

 a research cruise off east Kodiak Island, 5-18 Septem- 

 ber 1993 (Fig. 1). In this area, the shelf is about 50 nmi 

 wide and has an offshore bank (Albatross Bank) crossed 

 by deep gullies (Barnabas and Chiniak gullies) extend- 

 ing from the slope to the coast. Bays form the upper 

 reaches of these troughs and receive seasonal influxes of 

 freshwater (Rogers et al. 1 ). Over the shelf, net transport 

 is southwestward (ca. 5 em's) (Stabeno et al., 1995). A 

 boundary current, the Alaska Stream, exists farther 

 offshore and flows rapidly to the southwest (Reed and 

 Schumacher, 1986). 



Sampling was conducted from the NOAA ship Miller 

 Freeman (Fig. 1). Sampling occurred only at night to 

 avoid complications of diel fish movement (Brodeur and 

 Wilson, 1996b) and feeding patterns (Merati and Bro- 

 deur, 1996). A 38-kHz, Simrad-EK500 echo-sounder 

 system was used to help guide our sampling to locations 

 where age-0 pollock were likely present. The targeting 

 of echo signs resulted in an irregular sample-location 

 pattern and biased estimation of fish abundance; how- 

 ever, it focused our sampling at locations where age-0 

 pollock were likely present and thereby contributed to 

 successful fish collections. Sampling was accomplished 

 in four areas: Chiniak Bay, Ugak Bay, Kiliuda Bay, and 

 over the adjacent shelf. All data analyses included these 

 four areas as geographic strata; finer divisions (e.g., in- 

 ner and outer Kiliuda Bay, and NE and Albatross Bank) 

 were not possible given the available data and chosen 

 analytical methods. 



Age-0 pollock were obtained from the four areas with 

 a bottom trawl and a midwater trawl (Wilson et al., 

 1996). The codend of each trawl was lined with a 3-mm 

 mesh net. Towing speed averaged 4.5 k/h. Previous 

 comparisons between these trawls indicated no sig- 

 nificant difference with regard to estimation of age-0 

 pollock size or abundance (Brodeur and Wilson, 1996a; 

 Wilson et al., 1996). Differences in the sampling effort 



Rogers, D. E., D. J. Rabin, B. J. Rogers, K. J. Garrison, and 

 M. E. Wangerin. 1979. Seasonal composition and food web 

 relationships of marine organisms in the nearshore zone of 

 Kodiak Island — including ichthyoplankton, meroplankton 

 (shellfish), zooplankton, and fish. Annual rep. OCSEAP 

 RU553, FRI-UW-7925. 291 p. Fish. Res. Inst., Univ. Wash- 

 ington, Seattle, WA. 



used to collect each sample were corrected by dividing 

 the age-0 catch by the volume filtered. Volume filtered 

 was estimated by multiplying the distance fished (me- 

 ters traveled while at depth) by the mouth opening of 

 the trawl (m 2 ) (Wilson, 2000). Thus, age-0 catches are 

 reported as number of fish per m 3 . 



Size composition of walleye pollock for each area was 

 estimated by measuring the standard length (SL) of 

 fresh age-0 pollock to the nearest millimeter. For large 

 catches, a random subsample of about 300 individuals 

 was used to represent the entire catch; otherwise, SL 

 on every individual was measured. Length frequencies 

 were expanded to the standardized catch estimates. 

 Age-0 juveniles were clearly distinguishable from older 

 pollock (<130 mm versus >150 mm SL) as indicated by 

 Brodeur and Wilson (1996a). Random subsamples of 

 age-0 pollock were also frozen at sea for subsequent de- 

 termination of body condition, age, growth, and diet. 



In the laboratory, length-specific weights of 776 age-0 

 pollock were used to examine area differences in body 

 condition (Table 1). The fish were thawed within four 

 months of collection. Excess water was blotted from each 

 individual, and each specimen was measured to the 

 nearest millimeter SL and weighed whole to the nearest 

 0.01 gram. Afterwards, each carcass was stored in 95% 

 ethanol for eventual gut content analysis. Lengths and 

 somatic weights, obtained from the subset of fish used 

 in the gut analysis, were also analyzed to verify that 

 geographic differences in condition were not dependent 

 on whole versus somatic weight. 



Growth rate was estimated for 128 individuals by 

 using fish length and age data. Age, in days, was esti- 

 mated as the number of daily increments visible in the 

 microstructure of sagittal otoliths following Brown and 

 Bailey (1992). Length-age relationships were examined 

 for evidence of an area effect on growth rates integrated 

 over the period from hatching to capture. We used these 

 relationships to convert the length composition for each 

 sample to a hatching-date distribution, and by summing 

 across samples we then obtained area-specific hatching- 

 date distributions. 



To estimate growth rate realized near the point of 

 capture we measured the width of recent daily otolith 

 increments. Following Bailey (1989), we measured the 

 width of the two outermost, nonoverlapping 5-increment 

 bands on each of 97 sagittal otoliths. These widths 

 were assumed to relate directly to body growth during 

 the first (1-5 days) and second (6-10 days) 5-d periods 

 before capture, and that the increments were deposited 

 while individuals were near the point of capture. Thus, 

 growth rate indices were obtained for three different 

 periods: 1) hatching date to capture date, 2) 1-5 days 

 before capture, and 3) 6-10 days before capture. 



Gut content analysis was conducted on 300 individu- 

 als according to the method of Merati and Brodeur 

 (1996) to determine feeding intensity and taxonomic 

 composition of age-0 prey. No more than 15 fish per 

 sample were examined. Each fish was measured (SL), 

 blotted dry, and weighed immediately prior to dissec- 

 tion. Stomachs were excised between the esophagus and 



