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Fishery Bulletin 103(2) 



bastes spp.), and market squid (Loligo opalescens) (Low- 

 ry et al., 1990. 1991; Lowry and Carretta, 1999; Weise, 

 2000). Effects on these resources have been estimated 

 for Monterey Bay only, where during the 1997-98 El 

 Nino sea lions consumed an estimated 269.1 to 804.7 

 metric tons (t) of salmon, 988.4 to 2206.8 t of sardine, 

 and 533.4 to 1827.4 t of rockfishes annually (Weise, 

 2000). Recently, salmon in central and northern Cali- 

 fornia have experienced population declines and some 

 stocks have been listed as threatened or endangered 

 under the U.S. Endangered Species Act. Although a 

 variety of factors are responsible for the decline (e.g., 

 logging, dams, agriculture, fishing), some salmonid 

 populations are at such reduced levels that predation by 

 sea lions may negatively affect their recovery (NMFS 3 ). 

 Sea lions also have been documented as interfering 

 with recreational fisheries by consuming bait and chum 

 and depredating hooked fish (Fluharty 4 ). 



Existing methods of population assessment have been 

 based on pup counts obtained at California sea lion 

 rookeries near the end of the breeding season and total 

 population has been estimated by extrapolating data 

 from a life history model (Barlow and Boveng, 1991; 

 Boveng 5 ; Barlow et al. 6 "; Forney et al. 2 ). However, this 

 approach cannot be used outside of the breeding season 

 or in nonbreeding areas. Previous studies of California 

 sea lion abundance and distribution in central and 

 northern California during 1980-82 (Bonnell et al. 1 ) 



'NMFS (National Marine Fisheries Service). 1997. In- 

 vestigation of scientific information on the impacts of 

 California sea lions and Pacific harbor seals on salmonids 

 and on the coastal ecosystems of Washington, Oregon, and 

 California. NOAA Tech. Memo. NMFS-NWFSC-28, 172 

 p. Northwest Fisheries Science Center, 2527 Montlake Blvd. 

 E., Seattle, WA 98112-2097 and National Marine Fisher- 

 ies Service, Northwest Region, 7600 Sand Point Way N.E., 

 Seattle, WA 98115-0070. 



4 Fluharty, M. J. 1999. California sea lion interactions with 

 commercial passenger fishing vessel fisheries: a review of log 

 book data from 1994, 1995, and 1996. California Department 

 of Fish and Game Admin, report 99-2, 21 p. [Available from 

 California Department of Fish and Game, Marine Region, 

 San Diego Field Office, 4949 Viewridge Avenue, San Diego, 

 CA 92123.] 



5 Boveng, P. 1988. Status of the California sea lion popula- 

 tion on the U. S. west coast. National Oceanographic and 

 Atmospheric Administration admin, report LJ-88-07, 26 

 p. Southwest Fisheries Science Center, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037. 



6 Barlow, J., R. L. Brownell Jr., D. P. DeMaster, K. A. Forney, 

 M. S. Lowry, S. Osmek, T. J. Ragen, R. R. Reeves, and 

 R. J. Small. 1995. U.S. Pacific marine mammal stock 

 assessments. NOAA Tech. Memo. NMFS, NOAA-TM-NMFS- 

 SWFSC-219, 162 p. National Marine Fisheries Service, 

 Southwest Fisheries Science Center, 8604 La Jolla Shores 

 Drive, La Jolla, CA 92037. 



7 Barlow, J., K. A. Forney, P. Scott Hill, R. L. Brownell Jr., J. 

 V. Carretta, D. P. DeMaster, F Julian, M. S. Lowry, T. Ragen, 

 R. and R. Reeves. 1997. U.S. Pacific marine mammal stock 

 assessments: 1996. NOAA Tech. Memo. NMFS, NOAA- 

 TM-NMFS-SWFSC-248, 223 p. National Marine Fisheries 

 Service, Southwest Fisheries Science Center, 8604 La Jolla 

 Shores Drive, La Jolla, CA 92037. 



and 1995-96 (Beeson and Hanan 8 ) included only ani- 

 mals on land; animals at sea were either not considered 

 or were included as a rough estimate. An assessment 

 approach was, therefore, needed to provide quantitative 

 estimates of California sea lion abundance in central 

 and northern California that included both animals at 

 sea and on land. 



This study uses a combination of the strip-transect 

 method (to estimate at-sea abundance) and aerial pho- 

 tographic counts (to estimate abundance of sea lions 

 on land) in order to estimate the total abundance of 

 California sea lions in central and northern California. 

 Abundances were estimated separately for seven lati- 

 tudinal zones within central and northern California. 

 This study also describes distribution of sea lions by 

 age and sex class in central and northern California, 

 describes offshore distribution of sea lions, and intro- 

 duces a new multiplier that can be used to estimate the 

 total abundance of California sea lions at sea and on 

 land, when only an estimate of the number of animals 

 on land is available. 



Methods 



Survey dates and areas 



Surveys were conducted during May-June, July, Septem- 

 ber, and December 1998, and July 1999. The May-June 

 survey occurred when salmonid smolts were migrating 

 out of rivers (NMFS 3 ), the July survey when the United 

 States stock of California sea lions was expected to be 

 distributed mostly in California coastal waters, and the 

 September and December surveys when adult salmon 

 were migrating into rivers (NMFS 3 ). The study area 

 encompassed the waters and shoreline of central and 

 northern California from Point Conception (34°26.8'N, 

 120°28.0'W) to the California-Oregon border (42°00.0'N, 

 124°12'W) within approximately sixty nautical miles of 

 the coast (Fig. 1). 



Strip-transect surveys 



A twin-engine, high-wing Partenavia PN68C- or PN68- 

 observer model aircraft was flown at an airspeed of 185 

 km/h during strip-transect and coastal haulout surveys. 

 Abundance of sea lions at sea was determined by using 

 the strip-transect method because previous aerial sur- 

 veys in central California indicated that densities of sea 

 lions would be too great in some areas to obtain reliable 

 measures of perpendicular distances for line-transect 

 density estimation. Previous aerial surveys using line 

 transect methods, conducted at 213 m altitude, indicated 

 a relatively flat detection function for sea lions between 



Beeson, M. J., and D. A. Hanan. 1996. An evaluation of 

 pinniped-fishery interactions in California. Report to the 

 Pacific States Marine Fisheries Commission, 47 p. [Available 

 from Pacific States Marine Fisheries Commission, 205 SE 

 Spokane St., Suite 100, Portland, OR, 97202-6413.] 



