340 



Fishery Bulletin 103(2) 



In 1999, the majority of sea lions were found between 

 the San Francisco Bay area and Point Conception (zones 

 D through G). Zone E had the greatest number of sea 

 lions (Table 2); the majority of these animals hauled out 

 at Ano Nuevo Island. As in 1998, juveniles and adult- 

 females or young-males were the most prevalent age 

 and sex classes (Table 2). Only seven pups were counted 

 in the study area during July 1999. The number of sea 

 lions counted in 1999 was 52% of that counted in July 

 1998. 



Total abundance 



There was a significant correlation (r=0.468, P=0.024) 

 between at-sea abundance and haulout abundance within 

 zones. Total abundance of California sea lions in central 

 and northern California during 1998 was estimated 

 to be 64,916 in May-June, 75,673 in September, and 

 56,775 in December. Total abundance in July 1999 was 

 estimated at 25,791 individuals. The proportion of total 

 abundance to animals hauled-out was 1.77, 2.13, 1.77, 

 and 1.90, respectively, with a mean of 1.89 and a CV for 

 small samples (Sokal and Rohlf, 1995) of 0.09. Using the 

 mean multiplier of 1.89 on haulout counts obtained in 

 July 1998 (Table 2), when at-sea abundance could not 

 be estimated, we estimated total abundance as 49,697 

 (CV=0.09) animals for that period. 



Discussion 



This abundance study of California sea lions in central 

 and northern California successfully integrated two 

 methods: 1) strip transect surveys to estimate abun- 

 dance at sea; and 2) aerial photographic surveys to esti- 

 mate haulout abundance. TheglO) detection probability 

 derived from previously published dive data allowed esti- 

 mation of total abundance, including animals expected 

 to be underwater during at-sea strip transect surveys. 

 Previous surveys where transect methods similar to ours 

 were used in the Southern California Bight in 1975-78 

 and in central and northern California in 1980-83 

 (Bonnell and Ford, 1987; Bonnell et al. 1 - 10 ) did not have 

 information for deriving ^(0), and, therefore, densities of 

 sea lions at sea were underestimated in these studies. 

 California sea lions were abundant in central and 

 northern California during May through September 



Bonnell, M. L., B. J. Le Boeuf, M. O. Pierson, D. H. Dett- 

 man, G. D. Farrens, C. B. Heath, R. F. Gantt, and D. J. 

 Larsen. 1980. Summary of marine mammal and seabird 

 surveys of the Southern California Bight area 1975-1978. 

 Vol. 3: Investigators reports, part 1 — pinnipeds of the South- 

 ern California Bight, 535 p. Univ. Calif, Santa Cruz, Calif. 

 95064. Final Report to the Bureau of Land Management, 

 under Contract AA550-CT7-367. [NTIS PB81-248-71.1 



