331 



Abstract— The abundance and dis- 

 tribution of California sea lions 

 iZalophus californianus) in central 

 and northern California was stud- 

 ied to allow future evaluation of 

 their impact on salmonids, the eco- 

 system, and fisheries. Abundance 

 at-sea was estimated by using the 

 strip transect method from a fixed- 

 wing aircraft with a belly viewing 

 port. Abundance on land was esti- 

 mated from 126-mm-format aerial 

 photographs of animals at haulouts 

 between Point Conception and the 

 California-Oregon border. The sum of 

 these two estimates represented total 

 abundance for central and northern 

 California. Both types of survey were 

 conducted in May-June 1998. Septem- 

 ber 1998, December 1998, and July 

 1999. A haulout survey was conducted 

 in July 1998. The greatest number 

 of sea lions occurred near Monterey 

 Bay and San Francisco Bay for all 

 surveys. Abundance was high in cen- 

 tral and northern California in 1998 

 when warm water from the 1997-98 

 El Nino affected the region and was 

 low in July 1999 when cold water 

 La Nina conditions were prevalent. 

 At-sea abundance estimates in cen- 

 tral and northern California ranged 

 from 12,232 to 40,161 animals, and 

 haulout abundance was 13,559 to 

 36,576 animals. Total abundance of 

 California sea lions in central and 

 northern California was estimated as 

 64,916 in May-June 1998, 75,673 in 

 September 1998, 56,775 in December 

 1998, and 25,791 in July 1999. The 

 proportion of total abundance to ani- 

 mals hauled-out for the four complete 

 surveys ranged from 1.77 to 2.13, and 

 the mean of 1.89 was used to estimate 

 a total abundance of 49,697 for July 

 1998. This multiplier may be appli- 

 cable in the future to estimate total 

 abundance of California sea lions 

 off central and northern California 

 if only the abundance of animals at 

 haulout sites is known. 



Abundance and distribution of California sea lions 



iZalophus californianus) in central and 



northern California during 1998 and summer 1999 



Mark S. Lowry 



National Marine Fisheries Service 



Southwest Fisheries Science Center 



8604 La Jolla Shores Dr. 



La Jolla. California 92037 



E-mail address: mark.lowryia'noaa.gov 



Karin A. Forney 



National Marine Fisheries Service 

 Southwest Fisheries Science Center 

 110 Shaffer Road 

 Sanla Cruz, California 95060 



Manuscript submitted 1 October 2002 

 to the Scientific Editor's Office. 



Manuscript approved for publication 



14 December 2004 by the Scientific Editor. 



Fish. Bull. 103:331-343 (2005). 



The California sea lion {Zalophus cali- 

 fornianus) is distributed from central 

 Mexico to British Columbia, Canada. 

 Four islands off southern California 

 (Santa Barbara, San Clemente, San 

 Nicolas, and San Miguel Islands) form 

 the reproductive center for the U.S. 

 population, although some pupping 

 occurs at various other haulout sites 

 in central California (Pierotti et al., 

 1977; Keith et al., 1984). The number 

 of individuals off California varies 

 throughout the year because sea lions 

 from Mexico enter and leave Cali- 

 fornia waters and individuals from 

 California migrate southward into 

 Mexico or northward as far as Brit- 

 ish Columbia, Canada (Bartholomew, 

 1967; Bigg, 1988; and Huber, 1991). 

 In southern California, the abun- 

 dance of California sea lions peaks 

 during the summer breeding season 

 (Bartholomew, 1967; Odell, 1975). In 

 central and northern California, the 

 number of sea lions typically increases 

 in the autumn during the north- 

 ward migration, declines in winter, 

 increases in spring as sea lions move 

 to rookeries in southern California 

 and Mexico, and declines in summer 

 (Orr and Poulter, 1965; Mate, 1975; 

 Sullivan, 1980; and Griswold, 1985; 

 Bonnell et al. 1 ). 



Since the mid-1970s, the Califor- 

 nia sea lion population in the Unit- 

 ed States has expanded at an aver- 

 age of 5.0% per year and was most 



recently estimated to be between 

 204,000 and 214,000 individuals in 

 1999 (Forney et al. 2 ). This estimate 

 is roughly 2.7 times greater than 

 in 1981-83 (Bonnell et al. 1 ). As the 

 U.S. sea lion population has grown, 

 concerns have arisen about potential 

 impacts on commercially harvested 

 fish stocks. California sea lions feed 

 on a variety of fish and cephalopods, 

 some of which are commercially im- 

 portant species, such as salmonids 

 (Oncorhynchus spp.). Pacific sardines 

 (Sardinops sagax), northern anchovy 

 (Engraulis mordax), Pacific mackerel 

 (Scomber japonicus), Pacific whiting 

 (Merluccius productus), rockfish (Se- 



1 Bonnell, M. L., M. O. Pierson, and G. 

 D. Farrens. 1983. Pinnipeds and sea 

 otters of central and northern Califor- 

 nia, 1980-1983: status, abundance, and 

 distribution. Center for Marine Stud- 

 ies, Univ. California, Santa Cruz. OCS 

 Study MMS 84-0044, 220 p. Prepared 

 for Pacific OCS Region, Minerals Man- 

 agement Service, U.S. Department of 

 Interior, Camarillo, Calif. 93010, con- 

 tract no. 14-12-0001-29090. 



2 Forney, K. A., J. Barlow, M. M. Muto, 

 M. Lowry, J. Baker, G. Cameron, 

 J. Mobley, C. Stinchcomb, and J. V. 

 Carretta. 2000. U.S. Pacific ma- 

 rine mammal stock assessments: 2000. 

 NOAATech. Memo.: NOAA-TM-NMFS- 

 SWFSC-300, 276 p. National Marine 

 Fisheries Service, Southwest Fisheries 

 Science Center, 8604 La Jolla Shores 

 Drive, La Jolla. CA 92037. 



