Hastings and Sydeman Population status of Eumetopias jubatus at the South Farallon Islands, California 



61 



recent years may have resulted from increased emigration 

 or movement of subadult males from Ano Nuevo Island 

 due to increased competition for the declining number of 

 females there. A stable number of adult males, couplfxl 

 with declines in numbers of adult females, has resulted in 

 a significant reduction in the adult male-to-female ratio 

 on the South Farallon Islands during the breeding season 

 in recent years. 



These results demonstrate that reduced numbers of 

 Steller sea lions on the Farallon Islands in recent years 

 have been driven by reduced numbers of adult females 

 during the breeding season, although reproductive rate 

 and pup mortality rate were stable at this peripheral rook- 

 ery. Patterns were similar at Ano Nuevo, where there were 

 sharp declines in numbers of females and pups during the 

 breeding season but where no trend in reproductive rate 

 w-as apparent from 1962 to 1990 (Le Boeuf et al.'l. How- 

 ever, unlike the Farallon Islands, number of males at Ano 

 Nuevo during the breeding season also declined sharply 

 during the same time period (Le Boeuf et al.'). Although 

 the rate of decline at the Farallon Islands has lessened in 

 recent years, large declines of 9.9*^^ per year for pups and 

 31.5'~r per year for older animals may have occurred at 

 Ano Nuevo from 1990 to 1993. when negative effects of the 

 1992 El Nino may have affected estimates from this short 

 time series (Westlake et al., 1997). 



It is unknown whether reduced numbers of adult fe- 

 males and immature individuals present during the breed- 

 ing season have resulted from reduced survival or chang- 

 es in geographic distribution. Because significant declines 

 in Steller sea lions from historical numbers and over the 

 past several decades have occurred at San Miguel Island. 

 Alio Nuevo Island, and the South Farallon Islands, gi-eater 

 monitoring and protection by state or federal agencies of 

 the southern populations are warranted. Estimates of age- 

 class specific sui-\'ival rates of females are needed to deter- 

 mine if reduced numbers of females are due to increased 

 juvenile or adult mortality. More intensive studies track- 

 ing individual Steller sea lions in California are required 

 to determine if declining numbers indicate a northward 

 shift in the breeding range and to document migratory 

 movements of males and females. Population dynamics 

 and movements of prey of Steller sea lions, dietary overlap 

 with California sea lions, and interactions of sea lions with 

 commercial fisheries in California must be examined to 

 determine natural and anthropogenic causes for changes 

 in sea lion numbers or distribution. 



Acknowledgments 



H. R. Huber deserves special recognition for her contri- 

 butions during the early years of our study. Financial 

 support for manuscript preparation was provided by 

 the National Oceanic and Atmospheric Administration. 

 National Marine Fisheries Ser\ice. Southwest Fisheries 

 Science Center under contract 40JGNF600336 to W. J. 

 Sydeman. The Friends of the Farallones. Homeland Foun- 

 dation. Roberts Foundation, Bradford Foundation, and 

 Exxon Corporation also provided funds for data prepara- 



tion and fieldwork. We are particularly grateful to D. G. 

 Ainley for initiating pinniped studies on the Farallon 

 Islands in 1971. We also sincerely thank Nadav Nur and 

 (irey Pendleton for statistical advice and reviews of the 

 manuscript. We also thank the many obsei-\-ers who have 

 conducted surveys over the past three decades: D. Ainley, 

 G. Ballard, B. Boekelheide, H. Carter, S. Emslie, P. Hen- 

 derson, M. Hester, H. Huber, S. Johnston, J. Lewis, E. 

 McLaren, S. Morrel, J. Nusbaum, J. and T. Penniman, P. 

 Pyle, T. Schuster, J. Walsh, and others. General studies 

 of marine mammals at the South Farallon Islands have 

 been graciously supported over the years by the Marine 

 Mammal Commission. LI.S. Fish and Wildlife Service 

 (USFWS), and the Gulf of the Farallones National Marine 

 Sanctuary. In particular. USFWS and the San Francisco 

 Bay National Wildlife Refuge have provided 28 years of 

 financial, logistical, and moral support; to those involved, 

 we offer sincere gratitude. We also thank the Farallon 

 Patrol for transport to and from Southeast Farallon 

 Island. Michael Rehburg assisted with creating the Far- 

 allon Island map. This manuscript benefited greatly by 

 suggestions from Andrew Trites and several anonymous 

 reviewers. 



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