DIET OF NORTHERN FUR SEALS, CALLORHINUS URSINUS, 

 OFF WESTERN NORTH AMERICA 



Michael A. Perez 1 and Michael A. Bigg 2 



ABSTRACT 



Data recorded from the stomach contents of 18,404 northern fur seals, Callorhinus ursinus, mostly 

 females aged >3 years collected off western North America during 1958-74, were analyzed to determine 

 the relative importance of each prey species by region, subregion, and month. When weighted for energy 

 content, the primary food species were small schooling fishes. Between western Alaska and California 

 from December to August the most significant prey species were northern anchovy, Engraulis mordax 

 (20%); Pacific herring, Clupea harengus pallasi (19%); capelin, Mallotus villosus (8%); Pacific sand lance, 

 Ammodytes hexapterus (8%); Pacific whiting, Merluccius productus (7%); salmon, Oncorhynchus spp. 

 (6%); Pacific saury, Cololabis saira (4%); and rockfishes, Sebastes spp. (4%). Other food species eaten 

 in this area consisted of a wide variety of squids (17%) and other fishes (7%). In the eastern Bering Sea 

 the main prey species from June to October were juvenile walleye pollock, Theragra chalcogramma (35%); 

 capelin (16%); Pacific herring (11%); and squids, Berryteuthis magister and Gonatopsis borealis, which 

 comprise most (30%) of the remaining diet of northern fur seals in this region. In all areas off western 

 North America, fishes were the main food species of these pinnipeds in neritic waters, while squids were 

 the most important prey in oceanic waters. Typically three prey species comprised 80% of their diet 

 in any one area, although the composition of the diet varied in type and importance by region and month. 



The northern fur seal, Callorhinus ursinus, is found 

 in the Bering Sea, Sea of Okhotsk, and throughout 

 the North Pacific Ocean, north of approximately lat. 

 32 °N off western North America and lat. 36 °N off 

 Asia (Baker et al. 1970; Fiscus 1978). Although its 

 pelagic distribution is extensive, the main concen- 

 trations lie over the continental shelf. There are 

 three main stocks of this species. The largest stock 

 breeds on the Pribilof Islands in the eastern Bering 

 Sea and migrates primarily to coastal waters be- 

 tween the Gulf of Alaska and California. The other 

 two stocks breed on the Commander Islands in the 

 western Bering Sea and on Robben Island off north- 

 ern Japan. Both stocks migrate primarily along the 

 Asian coast. To determine the diet of the Pribilof 

 Islands population, the United States and Canada, 

 under the auspices of the North Pacific Fur Seal 

 Commission, conducted annual pelagic studies dur- 

 ing 1958-74 to collect stomach contents and other 

 biological information. 



The results of research on the diet of northern fur 

 seals by the United States and Canada during 

 1958-74 have been presented in many annual and 

 2-6 yr summaries submitted by each country to the 



North Pacific Fur Seal Commission. Kajimura (1984) 

 cited most of these reports. Spalding (1964), Stroud 

 et al. (1981), and Kajimura (1985) also published 

 reports on diet collected since 1958. Studies on the 

 food habits of the northern fur seal prior to 1958 

 include Lucas (1899), Clemens and Wilby (1933), 

 Clemens et al. (1936), Schultz and Rafn (1936), May 

 (1937), Wilke and Kenyon (1952, 1954, 1957), Taylor 

 et al. (1955), and Kenyon (1956). 



Investigations to date have reported that north- 

 ern fur seals eat a wide variety of fishes and squids. 

 However, the relative importance of each prey 

 species has remained uncertain because substantial 

 differences often existed between values of relative 

 importance derived by volumetric measure and 

 those derived by frequency of occurrence. For ex- 

 ample, squids were important (averaging 39%) in 

 the diet using frequency of occurrence but not 

 significant (15%) using volume (Bigg and Fawcett 

 1985; Perez and Bigg 3 ). The long-suspected reason 

 for this difference was that squid beaks accumulated 



Northwest and Alaska Fisheries Center, National Marine Mam- 

 mal Laboratory, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way N.E., Seattle, WA 98115. 



department of Fisheries and Oceans, Pacific Biological Station, 

 Nanaimo, British Columbia V9R 5K6, Canada. 



3 Perez, M. A., and M. A. Bigg. 1980. Interim report on the 

 feeding habits of the northern fur seal in the eastern North Pacific 

 Ocean and eastern Bering Sea. In H. Kajimura, R. H. Lander, 

 M. A. Perez, A. E. York, and M. A. Bigg, Further analysis of 

 pelagic fur seal data collected by the United States and Canada 

 during 1958-74, Part 2, p. 4-172. Unpubl. rep. Northwest and 

 Alaska Fisheries Center, National Marine Mammal Laboratory, 

 National Marine Fisheries Service, NOAA, 7600 Sand Point Way 

 N.E., Seattle, WA 98115. 



Manuscript accepted July 1986. 



FISHERY BULLETIN: VOL. 84, NO. 4, 1986. 



957 



