Lowry and Forney: Abundance and distribution of Zalophus califormanus 



341 



1998 when waters were warm because of the strong 

 1997-98 El Nino. Increased abundance of juveniles 

 and adult females were observed in this region during 

 previous El Nifios (Huber. 1991; Sydeman and Allen, 

 19991 and during our May-June, July, and September 

 1998 surveys. The increase in adult females in central 

 California in 1998 resulted in an increase in the num- 

 ber of pups counted at Ano Nuevo and South Farallon 

 Islands (106 pups in 1998 vs. 23 in 1997), and below 

 normal births at rookeries in southern California (Low- 

 ry, unpubl. data, Forney et al. 2 ). In contrast to 1998, 

 during the summer of 1999 fewer sea lions were found 

 in central and northern California, especially north of 

 San Francisco (zones A, B, and C), and greater num- 

 bers were found at rookeries in southern California (M. 

 Lowry, unpubl. data) when waters were cold as a result 

 of the La Nina oceanographic condition that began in 

 October 1998 (Hay ward et al., 1999). 



The abundance and distribution of California sea 

 lions were distinctly different between El Nino and 

 La Nina periods. During El Nino, sea lions were very 

 abundant in central and northern California, and were 

 distributed throughout the region. In contrast, during 

 summer 1999 (our only survey that year [La Nina|), sea 

 lions were less abundant than during summer 1998, 

 and they were distributed only south of the San Fran- 

 cisco Bay area. The abundance and distribution pattern 

 of summer 1999 is similar to the observed abundance 

 and distribution pattern described by earlier studies 

 (Chambers, 1979; Griswold, 1985; Weise, 2000; Bonnell 

 et al. 1 ). During periods of elevated sea lion abundance 

 in central and northern California, such as those ob- 

 served during the 1998 El Nino, we would expect 1) 

 increased consumption of prey species because of more 

 sea lions feeding in the area, 2) increased pressure on 

 coastal fisheries resources because sea lions feed on 

 commercially valuable species (see Lowry et al., 1990, 

 1991; Lowry and Carretta, 1999; Weise, 2000), and 3) 

 increased interactions with commercial and sport fisher- 

 ies. The opposite would occur during periods of low sea 

 lion abundance during non-El Nino years. Greater abun- 

 dance of California sea lions in central and northern 

 California during the 1997-98 El Nino event, therefore, 

 would be expected to have a greater effect on salmonids 

 and other sea lion prey species, and on fisheries than 

 would occur during non-El Nino years. 



Abundance of sea lions in central and northern Cali- 

 fornia during 1998 was greater in May- June (spring) 

 and September (fall) and less in July (summer) and 

 December (winter). This bimodal phenomenon, also ob- 

 served in the past (Sullivan, 1980; Bonnell et al. 1 ), is 

 due to migrating subadult and adult male sea lions on 

 their way to (in fall) and from (in spring) Oregon (Mate, 

 1975), Washington, and British Columbia (Bigg, 1988). 

 However, these seasonal differences were not signifi- 

 cantly different, likely because of low power (only one 

 year of data), or because the animals behaved differ- 

 ently from other years. In fact, fewer subadult and adult 

 males were present at southern California rookeries 

 during the 1998 July census (near the end of breeding 



season) than were present during 1997 and 1999 (M. 

 Lowry, unpubl. data). The large number of sea lions in 

 central and northern California during 1998 was the 

 result of a more numerous population (U.S. population 

 estimated at 204,000 to 214.000 in 1999) than existed 

 when previous surveys were conducted in 1980-82 and 

 1995-96 (U.S. population estimated at 76,000 in 1982 

 and at 167,000 to 188,000 in 1995) (Barlow et al. 7 ; For- 

 ney 2 ; Bonnell et al. 1 , and Beeson and Hanan s >. 



In central and northern California, California sea 

 lions have been sighted during aerial surveys (Carretta 

 and Forney"; present study) and tracked with satellite 

 tags (Melin and DeLong, 2000; Melin, 2002) up to 100 

 nautical miles from shore. However, our surveys indi- 

 cated that they forage predominantly within 20 nautical 

 miles from shore. 



The strip transect method assumes that all animals 

 within a strip are sighted by the observer. Although we 

 found no difference in sighting rate between Beaufort 

 sea state scales 0-1, 2, 3, and 4, Carretta et al. 12 found 

 during their 1998-99 line transect survey in waters 

 off San Clemente Island, California, that the effective 

 strip width of pinniped sightings at 213 m altitude 

 was slightly less in Beaufort sea states 3-4 (184 m on 

 each side) than in Beaufort sea states 0-2 (256 m on 

 each side). Their results suggest that if our analysis 

 suffered from reduced detection probability at high 

 sea states, then we may have underestimated at-sea 

 abundance of sea lions or increased the variance of at- 

 sea sea lion abundance. This potential negative effect 

 was minimized in our surveys by surveying at a lower 

 altitude (183 m) than the 213 m altitude surveyed by 

 Carretta et al. 12 



The g{0) correction derived from dive and foraging 

 studies of lactating adult-female California sea lions 

 during late breeding season (July-August) may be an 

 additional source of error in our at-sea abundance es- 

 timates. It may not be representative of nonlactating 

 adult females and other age- and sex-class sea lions, 

 and it may not be representative for all seasons or 

 different oceanographic cycles (e.g., El Nino and non- 

 El Nino). Dive data from various ages and sexes are 

 needed to test these assumptions, but existing dive data 

 from a single age+sex group provided a rough correc- 

 tion to account for animals underwater during at-sea 



11 Carretta, J. V. and K. A. Forney. 1993. Report of two aerial 

 surveys for marine mammals in California coastal waters 

 utilizing a NOAA DeHavilland twin otter aircraft March 

 9-April 7, 1991 and February 8-April 6. 1992. NOAA 

 Tech. Memo. NMFS, NOAA-f M-NMFS-SWFSC-185, 77 

 p. National Marine Fisheries Service, Southwest Fisheries 

 Science Center, 8604 La Jolla Shores Drive, La Jolla, CA 

 92037. 



12 Carretta. J. V., M. S. Lowry, C. E. Stinchcomb, M. S. Lynn, 

 and R. E. Cosgrove. 2000. Distribution and abundance of 

 marine mammals at San Clemente Island and surrounding 

 offshore waters: results from aerial and ground surveys in 

 1998 and 1999. National Oceanographic and Atmospheric 

 Administration admin, report LJ-00-02, 51 p. Southwest 

 Fisheries Science Center, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92037. 



