Shima et a\ Spatial distribution of Themgra chalcogmmmci in the Gulf of Alaska 



321 



to a reliance on shore.side processing. The shift moved fish- 

 inj; ()[)orati()ns closer to shore after the mid-19S0s (Fritz^). 

 Studies in other regions have found that groundfish spe- 

 cies (Pacific whiting and haddock) disperse as a result of 

 trawling. However, within minutes of a vessel's passage, 

 the fish usually revert back to their original distribution 

 (Ona and Godoo, 1990; Nunnallee"^). It should be noted 

 that this behavior is the result of disturbance by a single 

 vessel. It is possible that the activity of multiple vessels 

 that make up commercial fisheries could cause long-term 

 redistribution of pollock. Studies are currently underway 

 to examine the potential effect of commercial fisheries on 

 the distribution of pollock (Hollowed et al.''). 



Predator-prey interactions also influence the distribu- 

 tion of marine fish. Adult pollock are capable of exhibit- 

 ing a strong top-down influence on juvenile abundance 

 through cannibalism (Livingston, 1991). However, in the 

 GOA tlie incidence of cannibalism is minimal (Yang, 1993), 

 perhaps because, as shown in our study, the ranges of 

 adult and juvenile pollock do not overlap. Spatial distribu- 

 tions showed that the bulk of both adult and juvenile pol- 

 lock occurred in the Kodiak and Chirikof regions in the 

 GOA. Mean density for both juveniles and adults was high 

 within 20 nmi from land. However, age-0 (and often age-1) 

 pollock were in gi'eatest density in shallow waters <20 nmi 

 from land, whereas adult pollock tended to occupy deeper 

 waters. Age-2 pollock, which are beyond the size range 

 that adult pollock target as food in the Bering Sea, tend 

 to have a distribution similar to that of adults. The sepa- 

 ration between juveniles and adults could be a seasonal 

 effect due to the summer sampling season of the surveys 

 because juvenile pollock are known to migrate ontogeneti- 

 cally (Brodeur and Wilson, 1996). However, the pattern of 

 spatial separation between age-0 and age-1 pollock and 

 adult pollock may indicate avoidance of adults by juvenile 

 pollock. 



An examination of changes in density of juvenile pol- 

 lock in relation to geographic location allowed evaluation 

 of the prevailing conceptual model for pollock ontogeny in 

 the Gulf of Alaska (Kendall et al, 1996). The majority of 

 spawning for the GOA stock occurs in the Shelikof Strait 

 region (Kendall and Picquelle, 1989; Hinckley et al., 1991; 

 Kendall et al., 1996). Survey and ocean modeling studies 

 show that lai-vae and early juveniles are advected by the 

 Alaska Current towards the Shumagin Islands (Hinckley 



"Fritz, L.W. 1993. Travvllocations of walleye pollock and Atka 

 mackerel fisheries in the Bering Sea, Aleutian Islands and Gulf 

 of Alaska from 1977-92. U.S. Dep. Commer., AFSC Processed 

 Report 93-08. 162 p. 



* Nunnallee, E. P. 1991. An investigation of the avoidance reac- 

 tions of Pacific whiting 'Merluccius productus) to demersal and 

 midwater trawl gear ICES CM. 1991/B:.5, 16 p. lAvailable 

 from Alaska Fisheries Science Center, NMF.S/NOAA. 7600 Sand 

 Point Way NE, Seattle. WA 981 1.5.) 



3 Hollowed, A. B., C. Wilson, M. Shima, and P Walline. 2001. 

 Study to determine the efTect of commercial fishing on walleye 

 pollock distribution and abundance. In Steller sea lion investi- 

 gations. 2000 ( B. S. Fadelv, ed. ), p. 89-104. U.S. Dep. of Commer., 

 NOAA, NMFS, AFSC Processed Report 2001-0,5. 



et al., 1991; Hermann et al., 1996; Hinckley, 1999). Our re- 

 sults are partially consistent with the conceptual model for 

 pollock ontogeny: the area encompassing Shelikof Strait 

 and west to the Shumagin Islands (i.e. the Kodiak and 

 Chirikof regions) was important for all juvenile age groups. 

 However, the disproportionate increase of pollock in the 

 Kodiak region in the 1990s, with two strong year classes 

 (1988 and 1994), supports the suggestion that substantial 

 spawning may also occur outside of Shelikof Strait (Smith 

 et al., 1984; Brodeur and Wilson, 1996). 



The species composition of the cluster in which pollock 

 was included changed little over the course of the survey 

 years. Adult pollock were usually located together with 

 commercially important flatfish, arrowtooth flounder, and 

 Pacific cod. This finding is consistent with other findings 

 from the eastern Bering Sea where pollock often were as- 

 sociated with snow and Tanner crabs iChionoecetes spp.). 

 Pacific cod, flathead sole, Greenland turbot (Reinhardtius 

 hippoglossoides (Gill, 1861)), and yellowfin sole iLimanda 

 aspera (Pallas, 1814)) from 1978 to 1981 (Walters and 

 McPhail, 1982). 



Depth contours sei-ved as demarcations of station clus- 

 ters in our study as well as in others conducted in the 

 eastern Bering Sea and the eastern Pacific Ocean along 

 the western coast of the United States (Gabriel and Tyler, 

 1980; Walters and McPhail, 1982; Jay, 1996). The consis- 

 tency in how the stations were grouped together is com- 

 mon throughout all these studies. Walleye pollock in the 

 eastern Bering Sea also dominated clusters of stations in 

 the central shelf or the outer shelf Other species common- 

 ly found together with pollock in these gi"oups were yel- 

 lowfin sole and Pacific cod. 



A hypothesis relating pollock distribution to sea lion 

 foraging behavior 



Because pollock predominate the Steller sea lion diet (Mer- 

 rick and Calkins, 1996), the temporal and spatial changes 

 in pollock abundance and distribution could affect Steller 

 sea lions. The Steller sea lion population underwent pre- 

 cipitous declines in the 1980s and declines continued in 

 the 1990s but not as steep (Sease et al., 1999; NMFSi"). 

 The rate of decline continues to be high towards the center 

 of the Gulf of Alaska (1.50-158°W), whereas the western 

 Gulf region (158-162°W) has stabilized (i.e. the rate of 

 decline has decreased). 



Survey results suggest that the "forage density hypoth- 

 esis" may apply to Steller sea lions in the GOA. There is 

 evidence indicating that Steller sea lions target dense ag- 

 gregations of prey (Sinclair and Zeppelin, in press), so that 

 the lower mean density of pollock may reduce the number 

 of successful Steller sea lion foraging trips in the central 

 GOA. This effect may have prevented the sea lion popula- 

 tion from recovering from the precipitous decline that oc- 



'" NMFS (National Marine Fisheries Service). 1995. Status 

 review of the United States Steller sea lion, Eumetopiasjuba- 

 tiis, population. National Marine Mammal Laboratory, Alaska 

 Fisheries Science Center. 7600 Sand Point Way NE, .Seattle. 

 WA 98115, 61 p. 



