52 



Fishery Bulletin 100(1) 



37''42'4.'S'-N  



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_17"41'I5"- 



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SOUTH FARALLON ISLANDS 



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 Isl,.n,i4 



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. Indian 

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Figure 1 



Map of the South Farallon Islands, including Southeast F'arallon Island and West End Island. Steller sea 

 lions were counted weekly from 1974 to 1997 from Lighthouse Hill, and several gi'ound areas: North Land- 

 ing. Cormorant Blind Hill, Sewer Gulch, and Garbage Gulch. 



several decades at several California rookeries (Westlake 

 et al., 1997; Sydeman and Allen. 1999; Le Boeuf et al.'). 

 Wliereas over 2000 Steller sea lions used the Channel Is- 

 lands in the late 1930s, only 50 animals were obsei^ed 

 there in 1959 (Bartholomew and Boolootian, 1960). Pup- 

 ping at San Miguel Island, an historical rookery, has not 

 been observed since 1981 (NMMLM. Therefore to better 

 understand patterns and causes of the population decline, 

 trends and status of the eastern stock at southern rooker- 

 ies deserve further investigation. 



The Farallon Islands (:37°42'N. 123°00'W). 40 km off the 

 coast of San Francisco, California, are currently one of the 

 most southerly haulout and breeding areas for Steller sea 

 lions; Aiio Nuevo Island is the only rookery farther south. 

 The Farallon Islands consist of three groups of islands: 

 South Farallones (two islands. Southeast Farallon and 

 West End, separated by a small surge channel). Middle 

 Farallon (an intertidal rock), and North Farallones (four 

 large sea stacks; Fig. 1). Although the status of Steller sea 

 lions in California prior to 1800 was poorly documented. 

 Steller sea lions bred at the Farallon Islands in the 1800s 



3 Le Boeuf. B. J., K. A. Ono. and J. Reiter. 1991. History of the 

 Steller sea lion population at Ano Nuevo Island. 1961-1991. 

 Final report to National Marine Fisheries Service, Southwest 

 Fisheries Science Center. La Jolla.CA. Admin, report LJ9145C, 

 9 p. [Available from National Marine Fisheries Service. South- 

 west Fisheries Science Center. P.O. Box 271. La Jolla. CA 

 92038.) 



and early 1900s (Allen. 1880; Rowley. 1929) and were the 

 most abundant sea lion in California and at the Farallon 

 Islands from the early to mid 1900s (Rowley, 1929; Bon- 

 not and Ripley, 1948). A large amount of data is now avail- 

 able to examine seasonal variation and long-term trends 

 at the Farallon Islands from historical surveys conduct- 

 ed from 1927 to 1970 by the California Department of 

 Fish and Game (CDFG; Bonnot and Ripley, 1948; Ripley 

 et al., 1962; Carlisle and Aplin, 1971) and from surveys 

 conducted weekly by Point Reyes Bird Obsei-vatory (PR- 

 BO) from 1971 to 1996. Although maximum numbers de- 

 clined significantly from 1974 to 1997 for the total popula- 

 tion ( 1.6% per year) and for adult females (3.6% per year; 

 Sydeman and Allen, 1999), it is unknown whether num- 

 bers of other age classes also declined and in which sea- 

 sons declines occurred. To understand proximate causes 

 and consequences of the decline, several questions have 

 yet to be addressed: have reduced pup production and re- 

 duced reproductive rates also occurred in recent years?; 

 and what effect has the decline had on the adult sex-ra- 

 tio? Finally, seasonal variation in counts for different sex- 

 es and age classes and variation in the seasonal pattern 

 among years also have not been examined in detail at the 

 Farallon Islands. The objectives of our study were to ex- 

 amine 1) seasonal variation in numbers among sexes and 

 age classes; 2) trends in numbers from 1974 to 1996 by 

 age class, sex, and season; and 3) averages and trends in 

 pup production, reproductive rate, and adult sex-ratio. 



