307 



Abstract— Triennial lidltoni tnnvl sur- 

 \ (•>• data from 1984 to 1996 were used to 

 I'valuati' changes in the summer distri- 

 bution of walleye pollock in the western 

 and central Gulf of Alaska, Ditfercnces 

 between several age groups of pollock 

 were evaluated. Distribution was exam- 

 ined in relation to several physical char- 

 acteristics, including bottom depth and 

 distance from land. Interspecies associa- 

 tions were also analyzed with the Bray- 

 Curtis clustering technique to better 

 iHiderstand community structure. Oiu' 

 ri'sults indicated that although the pop- 

 ulation numbers decreased, high con- 

 centrations of pollock remained in the 

 same areas during 1984-96. However, 

 there was an increase in the number of 

 stations where low-density pollock con- 

 centrations of all ages were observed, 

 which resulted in a decrease in mean 

 population density of pollock within the 

 GOA region. Patterns emerging from our 

 data suggested an alternative to Mac- 

 Call's "basin hypothesis" which states 

 that as population numbers decrease, 

 there should be a contraction of the pop- 

 ulation range to optimal habitats. 



During 1984-96 there was a concur- 

 rent precipitous decline in .Steller sea 

 lions in the Gulf of Alaska. The results 

 of our study suggest that decreases in 

 the mean density of adult pollock, the 

 main food in the Steller sea Hon diet, 

 combined with slight changes in the 

 distribution of pollock (age-1 pollock in 

 particular) in the mid-1980s, may have 

 contributed to decreased foraging effic- 

 iency in Steller sea lions. Our results 

 support the prevailing conceptual model 

 for pollock ontogeny, although there is 

 evidence that substantial spawning may 

 also occur outside of Shelikof Strait. 



Changes over time in the spatial distribution 

 of walleye pollock (Theragro chalcogramma) 

 in the Gulf of Alaska, 1984-1996 



Michiyo Shima 



School of Aquatic and Fishery Sciences 



University of Washington 



1122 Boat St NE 



Seattle, Washington 98105 



E mail address mshimatOu Washington edu 



Anne Babcock Hollowed 



Alaska Fishenes Science Center 



7600 Sand Point Way NE 



BinC15700 



Seattle, Washington 981 15 



Glenn R. VanBlaricom 



Washington Cooperative Fish and Wildlife Research Unit 



School of Aquatic and Fishery Sciences 



University of Washington 



Box 355020 



Seattle, Washington 98195 



Manuscript accepted 21 September 2001. 

 Fish. Bull. 100:307-323 (2002). 



The incorporation of space into ecolog- 

 ical models has increased our knowl- 

 edge of the competition and co-existence 

 among species (Steinberg and Kareiva, 

 1997). It has also shown that the dis- 

 tributions of most organisms are not 

 homogeneous throughout their range, 

 but rather occur as patches. Ajialyses 

 of predator-prey interactions and food- 

 web dynamics are enhanced by consid- 

 ering the habitat association of species. 

 A first step towards this goal is to 

 describe the spatial distributions of the 

 major players in an ecosystem. 



The purpose of our study was to ex- 

 amine changes in walleye pollock (T/ier- 

 agra chalcogramma (Pallas, 1814)) dis- 

 tribution. Walleye pollock (hereafter 

 called "pollock") are a dominant spe- 

 cies in the Gulf of Alaska (GOA). Pol- 

 lock are the major prey of many species 

 of flatfish and marine mammals and 

 have been the predominant species in 

 Steller sea lion [Eumctopias jiibatus 

 (Schreber, 1776)) diets since at least 

 the mid-1970s (Pitcher, 1981; Merrick 

 and Calkins, 1996). Numbers of Stell- 

 er sea lions have declined significantly 

 in recent decades in the western and 

 central GOA. Examination of tempo- 



ral and spatial changes in the abun- 

 dance and population structure of pol- 

 lock is important to understanding the 

 decline in sea lion numbers. Existing 

 and proposed management decisions to 

 protect Steller sea lions include tempo- 

 ral and spatial changes to pollock fish- 

 ing quotas. 



The GOA pollock population peaked 

 in the early 1980s as a result of sever- 

 al strong year classes in the late 1970s 

 (Fig. lA). Since then, the population 

 has had fewer strong year classes. Mod- 

 eled estimates of total biomass of the 

 population declined in the early 1980s 

 and leveled off in the 1990s (Fig. IB). 

 Bottom trawl survey biomass estimates 

 (since 1984) have not reflected the de- 

 cline in the early 1980s (Fig. 10). In- 

 stead, they showed that the demersal 

 fraction of the population changed lit- 

 tle. Compared with other gadoid stocks 

 worldwide, the GOA pollock stock has 

 been lightly exploited; average landings 

 have constituted less than 10% of the 

 stock biomass during the last 20 years 

 (Shima etal,, 2000). 



In this study we document changes 

 in the spatial distribution and popula- 

 tion density of juvenile and adult pol- 



