POPULATION FLUCTUATIONS OF CALIFORNIA SEA LIONS AND 

 THE PACIFIC WHITING FISHERY OFF CENTRAL CALIFORNIA 1 



David G. Ainley, Harriet R. Huber, 2 and Kevin M. Bailey 3 



ABSTRACT 



Seasonal fluctuations in the number, age ratios, and diet of California sea lions, Zalopkus califor- 

 nianus, were studied at the Farallon Islands, central California, from 1971 to 1980. During these 

 years, average monthly numbers increased geometrically, except for April and May. Before 1977, 

 the annual peak in population occurred during April and May, almost no animals were present late 

 June to early July, and a slight peak occurred during fall; adult males predominated. Beginning in 

 1977, fall numbers equaled or exceeded those in spring, large numbers remained throughout 

 summer, and subadults predominated. We hypothesize that seasonal fluctuations in sea lion num- 

 bers were related to the availability of their principal prey, Pacific whiting, Merluccius productus, 

 and that the changes that began in 1977 were related to termination of the whiting fishery off central 

 California beginning that year. 



The California sea lion, Zalophus californianus, 

 ranges along the North American west coast 

 from the Gulf of California to British Columbia. 

 Bartholomew (1967) hypothesized that most 

 adult males migrate to the north from breeding 

 sites in Baja California and southern California 

 beginning in midsummer and remain there until 

 the early spring when they return south, and that 

 females and young animals remain in the vicin- 

 ity of breeding areas or move somewhat south- 

 ward during the nonbreeding season. This has 

 become the accepted explanation to account for 

 the seasonal movements in the population (e.g., 

 Mate 1975). Preliminary analysis of census and 

 diet information collected at the Farallon 

 Islands during 1971-80 led to a related hypoth- 

 esis that the movements of male sea lions toward 

 the north could be a response to the seasonal oc- 

 currence and availability of an important prey 

 species, the Pacific whiting, Merluccius produc- 

 tus (Huber et al. 4 ). This information was later 

 quoted by Fiscus (1979). Additional analysis, 

 presented here, provides more insight into the 

 ecological relationship between the two species. 

 The Pacific whiting is an abundant midwater 

 fish of the continental slope and shelf off Cali- 



'Contribution No. 232 of the Point Reyes Bird Observatory. 



2 Point Reyes Bird Observatory, Stinson Beach, CA 94970. 



3 College of Fisheries, University of Washington, Seattle, WA 

 98195 



<Huber, H. R, D. G. Ainley, S. Morrell, R. R Le Valley, and 

 C. S. Strong. 1979. Studies of marine mammals at the Far- 

 allon Islands, California, 1977-78. Final rep., 50 p. Marine 

 Mammal Commission, Wash., D.C.; available Natl. Tech. Inf. 

 Serv., Springfield, VA 22151, as PB80-111602. 



Manuscript accepted November 1981. 

 FISHERY BULLETIN: VOL. 80, NO. 2. 1982. 



fornia. A summary of its biology and of the whit- 

 ing fishery is provided by Dark (1975), Fiscus 

 (1979), and Dark et al. (1980). Pertinent to the 

 present study are the fish's migrational patterns. 

 Pacific whiting migrate vertically from near 

 bottom in daytime towards the surface at night, 

 except in the winter spawning season when they 

 remain at depth. They spawn off the coast from 

 Baja California to central California and mi- 

 grate northwards in spring and summer to feed 

 off the Oregon, Washington, and British Colum- 

 bia coasts. Small adults and juveniles migrate a 

 lesser distance — 1 to 3-yr-olds are mainly located 

 off central and southern California from spring 

 through fall. Also pertinent is the major harvest 

 of whiting conducted by eastern European 

 trawlers off the Pacific coast states and Canada 

 from 1966 to 1976. Much of the fishing was con- 

 centrated in the Farallon area. Under the Fish- 

 ery Conservation and Management Act of 1976 

 (FCMA), the fishery was prohibited off central 

 California (south of lat. 39°N); we hypothesize 

 that termination of the fishery affected the oc- 

 currence of California sea lions and possibly 

 other pinnipeds. 



METHODS 



California sea lions were counted at the south 

 Farallon Islands (lat. 37°42'N, long. 123°00'W), 

 which are situated at the continental shelf break 

 32 km offshore from Point Reyes and Bolinas 

 Point, Calif. Counts were irregular but fairly 

 frequent from 1971 through 1973, but were regu- 



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