Abstract. Gastrointestinal 



tract contents were evaluated from 

 73 female and juvenile male north- 

 ern fur seals (Callorhinus ursinus) 

 for analysis of their diet in the 

 Bering Sea. Fur seals were col- 

 lected from August to October of 

 1981, 1982, and 1985. Juvenile 

 walleye pollock (Theragra chalco- 

 gramma) and gonatid squid were 

 the primary prey. Pacific herring 

 (Clupea pallasi) and capelin 

 (Mallotus villosus), considered im- 

 portant fur seal prey in previous 

 reports, were absent from the diet. 

 Prey species and size varied 

 among years and between near- 

 shore and pelagic sample loca- 

 tions. Interannual variation in the 

 importance of pollock in the diet of 

 fur seals was positively related to 

 year-class strength of pollock. 

 Midwater (n=23) and bottom 

 (rc=116) trawls were conducted at 

 the location of fur seal collections 

 to determine availability of fish 

 and squid relative to prey species 

 eaten by fur seals. The species and 

 size composition of prey taken by 

 fur seals was similar to midwater 

 trawl collections, but differed from 

 bottom trawl catches. Contrary to 

 earlier conclusions that northern 

 fur seals are opportunistic in their 

 feeding habits, we conclude that 

 fur seals are size-selective mid- 

 water feeders during the summer 

 and fall in the eastern Bering Sea. 



Prey selection by northern fur seals 

 (Callorhinus ursinus) in the eastern 

 Bering Sea 



Elizabeth Sinclair 



Thomas Loughlin 



National Marine Mammal Laboratory, Alaska Fisheries Science Center, 



National Marine Fisheries Service, NOAA 



7600 Sand Point Way, N.E., Seattle, Washington 98 11 5 



William Pearcy 



College of Oceanography, Oregon State University 

 Oceanography Administration Building 1 04 

 Corvallis, Oregon 97331 



Manuscript accepted 19 August 1993 

 Fishery Bulletin: 92:144-156 (1994) 



The Pribilof Island population (St. 

 George and St. Paul Islands) of 

 northern fur seals (Callorhinus 

 ursinus) represents approximately 

 75% of the total species breeding 

 population. Between 1975 and 

 1981, the Pribilof Island population 

 declined from 1.2 million to an es- 

 timated 800,000 animals (York and 

 Hartley, 1981; Fowler, 1985). Abun- 

 dance levels on St. Paul Island ap- 

 pear to have stabilized (York and 

 Kozloff, 1987) at a level 60-70% 

 below estimates of the 1940s and 

 1950's, and at one-half the esti- 

 mated carrying capacity (Fowler 

 and Siniff, 1992). The number of 

 animals continues to decline on St. 

 George Island (York, 1990). 



The objectives of this study were 

 to determine the species and size of 

 prey eaten by northern fur seals in 

 the eastern Bering Sea, to compare 

 the seals' present diet with that 

 prior to the population decline, and 

 to examine the seals' consumption 

 of prey relative to prey availability. 

 Previous studies on the feeding 

 habits of northern fur seals in the 

 eastern Bering Sea (Scheffer, 

 1950a; Wilke and Kenyon, 1952; 

 Wilke and Kenyon, 1957; North 

 Pacific Fur Seal Commission Re- 

 ports 1962, ' 1975, 2 and 1980 3 ; 

 Fiscus et al., 1964; Fiscus et al., 

 1965; Fiscus and Kajimura, 1965) 



were conducted prior to the 1975- 

 81 population decline and prior to 

 the 1970s development of a com- 

 mercial walleye pollock (Theragra 

 chalcogramma) fishery in the 

 Bering Sea. Neither the size of fur 

 seal prey, nor fur seal selection of 

 prey relative to real-time availabil- 

 ity have been previously examined 

 in detail. 



Methods 



Northern fur seals were collected 

 from 17 to 28 October 1981; from 

 24 September through 6 October 



1 North Pacific Fur Seal Commission Re- 

 port on Investigations from 1958 to 1961: 

 Presented to the North Pacific Fur Seal 

 Commission by the Standing Scientific 

 Committee on 26 November 1962, 183 p. 

 Available: Alaska Fish. Sci. Cent., NOAA, 

 NMFS, 7600 Sand Point Way NE., 

 BinC15700, Seattle, WA 98115-0070. 



- North Pacific Fur Seal Commission Re- 

 port on Investigations from 1967 through 

 1972: Issued from the headquarters of the 

 Commission, Washington, D.C., June 

 1975, 212 p. Available: Alaska Fish. Sci. 

 Cent., NOAA, NMFS, 7600 Sand Point 

 Way NE., BinC15700, Seattle, WA 

 98115-0070. 



3 North Pacific Fur Seal Commission Re- 

 port on Investigations during 1973-76: 

 Issued from the headquarters of the Com- 

 mission, Washington, D.C., February 

 1980, 197 p. Available: Alaska Fish. Sci. 

 Cent., NOAA, NMFS, 7600 Sand Point 

 Way NE., BinC 15700, Seattle, WA 

 98115-0070. 



144 



