AINLEY FT AL.: POPULATION FLUCTUATIONS OF SEA LIONS 



returning north from breeding sites, offshore 

 waters near the Farallones were no longer as 

 attractive to the pinnipeds as during the spring. 

 The sea lions thus remained along the coast to 

 feed on other prey. Then in 1977, when trawlers 

 no longer fished for whiting off central Califor- 

 nia, the sea lions responded in three ways, all 

 possibly due to increased food supply during 

 summer and fall: 1) Young animals moved 

 farther north or farther off the coast than previ- 

 ously, 2) more adults remained during summer 

 instead of migrating south, and 3) adults return- 

 ing from southern breeding sites moved offshore 

 in larger numbers than they had in previous 

 falls. The size of the sea lion population peak 

 during spring was not affected by termination of 

 the fishery, because fishing was only just getting 

 under way each year at that time. 



Adding coincidental support to the hypothesis 

 that the 1966/76 whiting fishery off central Cali- 

 fornia was indirectly depressing the numbers of 

 California sea lions in the vicinity are data from 

 other localities. Populations of California sea 

 lions at breeding sites on southern California 

 islands have been increasing geometrically for 

 the past several decades (Bartholomew 1967; 

 LeBoeuf and Bonnell 1980; LeBoeuf 6 ). At the 

 crease in numbers at the Farallon Islands is 

 likely a reflection of this. Successive counts at 

 coastal Ano Nuevo Island during the early 1960's 

 also reflected this increase, but beginning some- 

 time between 1963 and 1967 numbers began a 

 decline there that lasted through 1975; since 

 then, however, they have begun to increase again 

 (LeBoeuf and Bonnell 1980; LeBoeuf). At the 

 Monterey breakwater, about 80 km farther 

 south, D. J. Miller 7 has noted that numbers of 

 subadult California sea lions since about 1978 

 have been much higher than in previous years. 



Changes in the occurrence of another pinni- 

 ped, the northern fur seal, Callorhinus ursinus, 

 at the Farallones, provide additional support to 

 the hypothesis. Also an important whiting pred- 

 ator (Fiscus 1979), this species breeds at San 

 Miguel Island in southern California and in the 

 Bering Sea, and during the nonbreeding season 

 frequents waters of the California continental 

 slope. From 1970 to 1976 we observed individual 

 fur seals at the farallones on only 3 single days, 



6 B. J. LeBoeuf, Division of Natural Sciences. University of 

 California at Santa Cruz, Santa Cruz, CA 95064, pers. com- 

 mun. June 1981. 



7 D. J. Miller, California Department of Fish and Game. Mon- 

 terey, CA 93940. pers. commun. June 1981. 



each 2 yr apart. Since then, however, their occur- 

 rence has changed dramatically: the species has 

 occurred annually during the summer and fall, 

 and at least 10 different individuals(determined 

 by tags or peculiar scars) have hauled out, some 

 repeatedly, for periods of variable length. Two 

 that hauled out were tagged at San Miguel; 

 another has hauled out for 5 yr in succession. The 

 fur seal breeding population on San Miguel 

 Island has been increasing geometrically from 

 the early 1960's to the present(LeBoeuf and Bon- 

 nell 1980) and the increasing occurrence of this 

 species on the Farallones is likely a reflection of 

 this trend. The dramatic jump in numbers at the 

 Farallones beginning after 1976, however, is out 

 of line with the continuous increase in breeding 

 numbers. Cessation of the whiting fishery off 

 central California in 1976 may account for the 

 change at the Farallones, just as this may be re- 

 sponsible for the change in population dynamics 

 of California sea lions in central California. 



ACKNOWLEDGMENTS 



Field work at the Farallon Islands was funded 

 by the Point Reyes Bird Observatory, U.S. Fish 

 and Wildlife Service, Marine Mammal Commis- 

 sion, and National Marine Fisheries Service 

 (Marine Mammal Laboratory and Southwest 

 Fisheries Center). Logistic support was provided 

 by the U.S. Coast Guard and the Oceanic Society, 

 San Francisco Bay Chapter. The Farallones com- 

 prise a national wildlife refuge, and we thank 

 the personnel of the San Francisco Bay National 

 Wildlife Refuge for their help. Marine mammal 

 food items were collected under NMFS permit 

 No. 146; fish otoliths were identified by J. E. 

 Fitch and cephalopod beaks were identified by 

 D. G. Ainley; G. Galbraith, Division of Enforce- 

 ment and Surveillance, NMFS, provided data on 

 the occurrence of whiting trawlers. C. S. Strong, 

 T. J. Lewis, R. J. Boekelheide, R. P. Henderson, 

 R. R. Le Valley, S. H. Morrell, J. W. Higbee, B. 

 Bainbridge, K. Darling, and W. Clow assisted 

 with counts. O'B. Young helped to prepare the 

 manuscript. Valuable comments were offered by 

 R. L. DeLong, D. P. DeMaster, C. H. Fiscus, B.J. 

 LeBoeuf, and P. F. Major. 



LITERATURE CITED 



Bailey, K. M. 



1980. Recent changes in the distribution of hake larvae: 

 causes and consequences. Calif. Coop. Oceanic Fish. 

 Invest. Rep. 21:167-171. 



257 



