520 



Fishery Bulletin 88 (3|. 1990 



sea lions. During the El Nino in 1983 and 1984 the com- 

 mercial catch of market squid was 2010 and 622 short 

 tons, respectively; prior to the El Nifio it was 25,915 

 and 17,951 short tons for 1981 and 1982, respectively; 

 and, after the El Nino it was 11,326 and 23,124 short 

 tons for 1985 and 1986, respectively. The presence of 

 market squid in the diet of sea lions during 1983 in- 

 dicates the availability of this prey to sea lions, despite 

 low availability to commercial catches. However, in 

 1984 when the commercial harvest was at its lowest, 

 market squid was not detected in the sea lions' diet. 

 Both the sea lions' consumption of market squid and 

 the commercial catch were apparently affected by 

 availability. 



The effect of El Nino on the diet of SCI sea lions is 

 also demonstrated by the presence of remains from 

 pelagic red crabs in scat samples. Pelagic red crabs are 

 normally found over the continental shelf of southern 

 Baja California, and are sometimes transported into 

 the SCB and further northward by the Davidson and 

 California Countercurrents in January and February 

 (Boyd 1967). These currents intensified as a result of 

 the 1982-83 El Niiio/Southern Oscillation, affected 

 water temperatures in the SCB from late 1982 to 

 October-November 1984, and carried pelagic red crabs 

 from southern Baja California to the SCB where they 

 were eaten by California sea lions. 



The food consumed by the U.S. population of Califor- 

 nia sea lions could have an impact on the ecosystem 

 and be in direct competition with commercial and sport 

 fisheries. Six of the common prey of California sea lions 

 identified at SCI (jack mackerel, northern anchovy, 

 market squid. Pacific mackerel, Pacific whiting, and 

 rockfish) are of value to commercial or sport fisheries. 

 DeMaster (1983) estimated that California sea lions in 

 U.S. waters (estimated population size 69,700) con- 

 sumed roughly 115,000-295,000 mt of biomass per 

 year. The present consumption would be greater than 

 in 1983 because of the estimated 5% annual increase 

 in the U.S. population (DeMaster et al. 1982). The 

 growth in the sea lion population, and its estimated con- 

 sumption, raises questions. Will there be greater- com- 

 petition between sea lions and fishermen for fish and 

 squid as a result of increased numbers of sea lions? 

 What will be the effect of population growth of sea lions 

 on other elements of the SCB ecosystem? 



Observed temporal differences in diet of sea lions 

 from SCI during the California El Nifio of 1982-84 

 clearly indicate the need for long-term studies of food 

 habits of this predator throughout its range. Short- 

 term studies may reflect oceanographic conditions of 

 relatively short duration, such as El Nino, that strongly 

 influence food supply and sea lion diet. 



Acknowledgments 



We would like to thank Jan Larson of Staff Civil En- 

 gineers, Natural Resources Division, Department of 

 the Navy at North Island, California, for his coopera- 

 tion. We thank Lisa Ferm and all the other people who 

 helped us collect sea lion scats at SCI and Roberta Folk 

 for sorting through many of the scats. The Los Angeles 

 County Museum of Natural History allowed us access 

 to the John Fitch otolith reference collection to iden- 

 tify some of the fish otoliths. We thank Eric Hochberg 

 from the Santa Barbara Museum of Natural History 

 and Cliff Fiscus, now retired from the National Marine 

 Mammal Laboratory, Seattle, for their assistance in 

 identifying some of the cephalopod beaks. We thank 

 Jay Walker, former NMFS employee at SWFC, for his 

 programming assistance. We are indebted to Dr. Nancy 

 Lo, Bruce Wahlen, and Dr. D.W. Macky for their ad- 

 vice with the statistical analysis. Special recognition 

 is given to Henry Orr who provided illustrative ser- 

 vices. We are grateful for the comments we received 

 from Jay Barlow, Peter Boveng, and two anonymous 

 reviewers. 



Citations 



Antonelis. G.A., C.H. Fiscus, and R.L. DeLong 



1984 Spring and summer prey nf California sea lions, Zahphus 

 califomianus, at San Miguel Island, California 1978-1979. 

 Fish. Bull., U.S. 82:67-76. 



Bailey, K.M., and D.G. Ainley 



1982 The dynamics of California sea lion predation on Pacific 

 whiting. Fish. Res. (Amst.) 1:163-176. 



Bindman, A.G. 



1986 The 198.5 spawning biomass of the northern anchovy. 

 Calif. Coop. Oceanic. Fish. Invest. Rep. 27:16-24. 

 Boyd, CM. 



1967 The benthic and pelagic habitats of the red crab. Pleuron- 

 ,;,deH planipe^. Pac. Sci. 21(3):.394-403. 

 da Silva, J., and J.D. Neilson 



1985 Limitations of using otoliths recovered in scats to esti- 

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 42:14.39-1442. 



Dellinger, T.. and F. Trillmich 



1988 Estimating diet composition from scat analysis in otariid 

 seals (Otariidae): Is it reliable? Can. J. Zool. 66:1865-1870. 

 DeMaster, D.P. 



1983 Annual consumption of northern elephant seals and 

 California sea lions in the California current. Calif. Coop. 

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 and Abstracts (unpaginated). 



DeMaster, D.P., D.J. Miller, D. Goodman, R.L. DeLong, and 

 B.S. Stewart 



1982 Asse.ssmentofCalifornia sea lion fishery interactions. In 

 Marine mammals: Conflicts with fhsheries, other management 

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