PEREZ and BIGG: DIET OF NORTHERN FUR SEALS 



recovered only recently (Kondo 1980). The northern 

 fur seal appears to have reacted to this recovery by 

 eating more sardines. A similar change in diet may 

 have taken place off California during the past 50 

 years. The Pacific sardine, Sardinops sagax, was 

 once the most abundant small, schooling fish off 

 California, whereas now northern anchovy is (Mur- 

 phy 1966; Smith 1972; Mais 1974). The Pacific sar- 

 dine population was drastically reduced during the 

 1940's mainly because of fishing pressure and has 

 remained at a relatively low level since, while the 

 northern anchovy increased in abundance during the 

 1950's and the 1960's (Vrooman and Smith 1971; 

 Hart 1973; Wolf and Smith 1985). The Pacific sar- 

 dine may undergo long-term periodic fluctuations 

 in population size (Thompson 1921), and it may now 

 once again be increasing in biomass (Wolf and Smith 

 1985). Northern fur seals have not eaten Pacific sar- 

 dine in recent years, but perhaps they fed on this 

 species prior to the 1940's. The seal may have 

 changed its diet from largely Pacific sardine to 

 northern anchovy. Unfortunately, the stomach con- 

 tents of only two northern fur seals were collected 

 from California prior to the 1950's (Scheffer 1950). 

 Clemens and Wilby (1933) gave the only evidence 

 that sardines were once consumed by these seals in 

 the eastern North Pacific Ocean. They found that 

 sardines were commonly eaten during 1931 off 

 southwestern Vancouver Island. 



An interesting speculation regarding the signifi- 

 cance of small schooling fish to northern fur seals 

 is the relationship between diet and the migration 

 route of the seal. Small schooling fish could be im- 

 portant just because they are abundant and lie along 

 the coastal migration path of northern fur seals. Ka- 

 jimura (1985) argued for this possibility. He sug- 

 gested that the migration pattern of northern fur 

 seals is genetically established and that the seal 

 feeds opportunistically upon whatever prey species 

 are most abundant in its path. He believes that, 

 although food is not a major factor in determing the 

 migration route of northern fur seals, the move- 

 ments of prey species can still alter the local distribu- 

 tion of fur seals. An alternative possibility is that 

 the seals learn the location of the main foods and 

 then selects its migration route to include them. 



9 Yoshida, K., N. Okumoto, and N. Baba. 1981. Japanese 

 pelagic investigation on fur seals, 1979-1980. Far Seas Fish. Res. 

 Lab., Shimizu, Jpn., Fur Seal Resour. Sect., Contrib. No. 41-10, 

 150 p. 



10 Yoshida, K., and N. Baba. 1983. Japanese pelagic investiga- 

 tion on fur seals, 1981-1982. Far Seas Fish. Res. Lab., Shimizu, 

 Jpn., Fur Seal Resour. Sect, Contrib. No. 41-11, 118 p. 



"Yoshida, K., and N. Baba. 1984. Japanese pelagic investiga- 

 tion on fur seals, 1983. Far Seas Fish. Res. Lab., Shimizu, Jpn., 

 Fur Seal Resour. Sect., Contrib. No. 41-12, 67 p. 



Baker (1978) argued for this alternative. He pro- 

 posed that, while some inherited factors may be in- 

 volved in migration, northern fur seals could main- 

 ly search the North Pacific Ocean for the most 

 preferred or abundant food, and thereafter estab- 

 lish the migration route. Such being the case, 

 perhaps inexperience explains why 1-2 yr-old seals 

 are rarely seen inshore feeding with older seals. 

 Also, perhaps squid is not a preferred or sufficiently 

 available food for northern fur seals offshore, 

 because most seals older than 1-2 yr feed inshore 

 on fish. However, at this stage, not enough is known 

 about the factors that control migration of the north- 

 ern fur seal to establish which alternative is true. 

 As Kajimura (1985) has pointed out, factors other 

 than diet are no doubt involved as indicated by the 

 fact that males do not migrate as far south as 

 females. 



ACKNOWLEDGMENTS 



The following people assisted us with the prepara- 

 tion of data for analysis, and with the construction 

 of tables and figures presented in unpublished pre- 

 liminary reports of this study: Julia Bosma, Patricia 

 Bouthillette, Laurie Briggs, Carl Brooks, David 

 Crystal, Ian Fawcett, Gary Fidler, Job Groot, Carol 

 Hastings, Marta Hladyschevsky, Kerry Hobbs, 

 Gerald Hornof, Marilyn Marshall, Elizabeth 

 Mooney, R. Perez, Kenneth Pierce, and Marsha 

 Schad. 



LITERATURE CITED 



Akimushkin, I. I. 



1963. Cephalopods of the seas of the U.S.S.R. [In 

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 Baker, R. C, F. Wilke, and C. H. Baltzo. 



1970. The northern fur seal. U.S. Fish Wildl. Serv., Circ. 

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 Baker, R. R. 



1978. The evolutionary ecology of animal migration. Holmes 

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 Bakkala, R., K. King, and W. Hirschberger. 



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 Baxter, J. L., and J. M. Duffy. 



1974. Inshore fishes of California, 4th revision. Calif. Dep. 

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 Bigg, M. A., and I. Fawcett. 



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969 



