Sinclair et al.: Prey selection by Callorhmus ursinus 



53 



B * 



C l/i 



IB C 



£: O 



ra E 

 a> = 



>• c 



SI 



o « 



°- g. 



o 



CL 



90 - 

 80 - 

 70- 

 60 - 

 50 - 

 40- 

 30 - 

 20 - 

 10- 

 - 



100 



1978 1979 1980 1981 1982 1983 1984 1985 



Pollock year class 



80 



70 - 



2 50 



 1981 

 □ 1985 



1978 1979 1980 1981 1982 1983 1984 1985 



Pollock year class 



Figure 7 



Estimates of walleye pollock iTheragra 

 chalcogramma) year-class strength 1978- 

 85 (Bakkala et al., 1987), and the relative 

 abundance of specific year classes in 

 northern fur seal gastrointestinal tracts. 



is size and the tendency to form dense schools. In 

 this sense, a "juvenation" of walleye pollock in the 

 Bering Sea (Swartzman and Haar, 1983) may have 

 provided fur seals with a newly abundant but un- 

 stable resource, due to large fluctuations in the 

 annual year-class strength of walleye pollock and 

 due to potential displacement of other prey species 

 (Pacific herring and capelin). During years of low 

 pollock recruitment, fur seals may switch to other 

 prey such as capelin and Pacific herring, and expe- 

 rience food limitation only if these alternate prey 

 resources have been displaced or depleted. Histori- 



cal records of northern fur seal diet are inadequate 

 to either support or refute an "alternate prey" ar- 

 gument. However, we suggest that when juvenile 

 walleye pollock are unavailable, such as in our 1981 

 sampling season, female and juvenile fur seals se- 

 lect other specific prey of the same size and eat adult 

 walleye pollock only if these other preferred prey are 

 not available. 



During their summer breeding season, northern 

 fur seals consume the most abundant and available 

 fish and squid in the eastern Bering Sea. Walleye 

 pollock make up an estimated 50% of the ground- 

 fish biomass in the eastern Bering Sea and Aleutian 

 Islands area (Walters et al., 1988) and dense aggre- 

 gations of 0-age pollock occur off the Pribilof Islands 

 June through mid-August (Smith, 1981). Kubodera 

 and Jefferts (1984) suggested gonatids are the ma- 

 jor pelagic cephalopod group in the Bering Sea, 

 where large increases in abundances of larval and 

 postlarval gonatid squid occur in early June. Among 

 Bering Sea gonatids, Gonatopsis borealis and 

 Berryteuthis magister are considered to be among 

 the most numerically dominant (Jefferts, 1983; 

 Kubodera and Jefferts, 1984). 



Selection by northern fur seals of a wide variety 

 of numerically dominant prey species throughout 

 their migratory range has led to the general conclu- 

 sion that they are non-specific, opportunistic feed- 

 ers (Kajimura, 1985). Northern fur seals are flexible 

 in their feeding habits, as indicated by the variation 

 in GI contents of seals collected between California 

 and Alaska. Nonetheless, fur seals concentrate on 

 an average of three primary species within each 

 oceanographic subregion (Perez and Bigg, 1986). In 

 addition, fur seal consumption of walleye pollock, 

 gonatid squid, and bathylagid smelt in the eastern 

 Bering Sea is consistent throughout historical 

 records, despite the wide variety of prey available 

 to fur seals within their diving range. Based on this 

 study, we conclude that female and young male fur 

 seals select juvenile and small-sized fish and squid, 

 despite the availability of larger prey types within 

 their diving range. This study demonstrates that 

 female and young male fur seals are size-selective 

 midwater shelf and mesopelagic feeders, at least 

 during the breeding and haul-out season in the east- 

 ern Bering Sea. 



Acknowledgments 



Otolith identifications for the 1981 samples were 

 made by the late J. Fitch. Otolith identifications for 

 1982 and 1985 were based on the otolith reference 

 collections at the National Marine Mammal Labo- 



