832 



Fishery Bulletin 98(4) 



comparison with their study is complicated by differences 

 in gear and survey strategy. One possible explanation 

 for the effect of region on fish size is growth rate vari- 

 ability. Growth rates, estimated from larval and juvenile 

 length-at-age data, were found to increase from southwest 

 to northeast, at least during 1987 (Yoklavich and Bailey, 

 1990; Brown and Bailey 1992). Prey quality (Merati and 

 Brodeur, 1996), and physical conditions (Strickland and 

 Sibley, 1989) have also been observed to vary, the suppo- 

 sition being that the Kodiak Island area is a richer nurs- 

 ery habitat than are more southwestern areas. Variation 

 in time of spawning is another possibility; particularly in 

 1986 and 1988, when regional differences in growth were 

 not evident (Bailey et al., 1996b). It appears, however, that 

 time of spawning throughout the western GOA is fairly 

 synchronous (Picquelle and Megrey, 1993; Muter and Nor- 

 cross, 1994). except near the Shumagin Islands were it 

 happens about one month earlier (Wilson et al.^). Size- 

 related migration and mortality are other possible effects 

 on fish size but evidence is lacking. 



Spatial differences in size may translate into spatial dif- 

 ferences in mortality. In a review of size-selective mortality 

 among juvenile fishes, Sogard (1997) identified over-win- 

 ter mortality and predation as major size-selective pro- 

 cesses. These mortality vectors may be particularly strong 

 because the first-winter gi-owth of juvenile pollock in the 

 GOA is negligible (Brodeur and Wilson, 1996a), and pre- 

 dation is prevalent (Brodeur and Bailey, 1996). Thus, geo- 

 graphic differences in age-0 size may contribute a spatial 

 component to year-class strength determination. The poor 

 relationship between age-0 size and year-class strength, 

 however, suggests that age-0 size is a minor factor, at least 

 in terms of determining relative recruitment among year 

 classes. 



The practice of targeting echo layers was a drawback of 

 these data. This practice has been shown to bias estimates 

 of fish density (Wilson et al., 1996), but its effect on 

 distribution patterns is uncertain. It is possible that the 

 area represented by mean fish density varied with year 

 and region, thereby causing density to be a poor indicator 

 of abundance. Estimates of year- and region-specific den- 

 sity were, however, closely related to the absolute abun- 

 dance estimates of Bailey and Spring (1992) (Pearson, 

 r=0.78. P=0.008, /) = 10). On the positive side, the practice 

 of targeting echo sign does reduce the effort needed to col- 

 lect biological samples (Brodeur and Wilson, 1996b), but 

 this must be carefully weighed against possible adverse 

 effects. 



In summary, age-0 pollock appear to have been evenly 

 distributed throughout much of the western GOA during 

 1985-88, except during 1987 when a strong, east-to-west 

 increase in fish density was observed. The Kodiak region 

 was an important pollock nursery despite its position 

 upstream from what is presumed to l)e the most impor- 

 tant spawning area in the GOA. The Kodiak region was 

 particularly well suited as a recruitment predictor owing, 

 perhaps, to a relative abundance of large-size individuals. 

 Questions emerged about annual variability in the origin 

 and dispersal of young pollock, and regarding the possibil- 

 ity that geographic effects on sun'ival or retention could 



affect the relative production of GOA recruits among dif- 

 ferent spawning areas. 



Acknowledgments 



I thank many people at the Alaska Fisheries Science 

 Center for assistance with this project, in particular Kevin 

 Bailey. Ric Brodeur, Mike Canino. Anne Hollowed, and Art 

 Kendall for commenting on earlier drafts of this manu- 

 script. A great deal of statistical assistance was provided 

 by Kathy Mier and Susan Picquelle. Additional comments 

 by Olav Rune Godo (Institute of Marine Research, Bergen, 

 Norway), the AFSC Publications Unit, Sarah Fisher, and 

 three anonymous reviewers are gratefully acknowledged. 

 Finally, I wish to thank Herb Shippen for overseeing 

 the age-0 surveys, which occurred before my time at the 

 AFSC. 



Literature cited 



Ander.';on. J. T., E. L. Dalley, and J. E. Carscadden. 



1995. Abundance and distribution of pelagic 0-group cod 

 iGadus morhua) in Newfoundland waters: inshore versus 

 offshore. Can. J. Fish.Aquat. Sci. 52:115-125. 



Bailey, K. M.. R. C. Francis, and K. F Mais. 



1986. Evaluating incidental catches of 0-age Pacific hake to 

 forecast recruitment. CalCOFI Rep, 27:109-112. 

 Bailey, K. M.. R. D. Brodeur. and A. B. Hollowed. 



1996a. Cohort sui-vival patterns of walleye pollock. Titer- 

 agra chalcogramma. in Shelikof Strait, Alaska: a critical 

 factor analysis. Fish. Oceanogr 5(suppl. 1):179-188. 

 Bailey. K, M.. A. L. Brown. M. M. Yoklavich. and K. L. Mier 



1996b. Interannual variability in growth of larval and juve- 

 nile walleye pollock Theragra chalcogramma in the west- 

 ern Gulf of Alaska, 1983-91. Fish. Oceanogi-. 5(suppl. 

 1):137-147. 

 Bailey. K. M.. ,S. J. Picquelle, and S. M. Spring. 



1996c. Mortality of lai-val walleye pollock Theragra chalca- 

 gramma in the western Gulf of Alaska, 1988-91. I'lsh. 

 Oceanogi-. 5(suppl. 1):124-136. 

 Bailey K. M., and S. M Spring. 



1992. Comparison of larval, age-0 juvenile and age-2 recruit 

 abundance indices of walleye pollock. Theragra chalco- 

 gramma. in the western Gulf of Alaska. ICES J. Mar Sci. 

 49:297-304. 

 Brodeur. R. D.. and K. M. Bailey 



1996. Predation on the early life stages of marine fish: a case 

 study on walleye pollock in the Gulf of Alaska. In Sur- 

 vival strategies in early life stages of marine resources (Y. 

 Watanabe. Y. Yamashita. and Y. Oozeki. eds. I. p. 245-260. 

 A. A. Balkema. Rotterdam, Netherlands, 367 p. 



Brodeur, R. D., M. S. Busby and M. T. Wilson. 



1995. Summer distribution of early life stages of walleye 

 pollock, Theragra chalcogramma, and associated species in 

 the western Gulf of Alaska. Fish, Bull. 93:603-618. 

 Brodeur, R. D.. and M. T Wilson. 



1996a. A review of the distribution. ecolog>'. and population 

 dynamics of age-0 walleye pollock in the Gulf of Alaska. 

 Fish. Oceanogr. Sisuppl. 11:148-166, 



1996b. Mesoscale acoustic patterns of juvenile walleye 

 pollock {Theragra chalcogramma) in the western Gulf 

 of Ala.ska. Can. J. Fish. Aquat. Sci. ,53(91:1951-1963. 



