51 



Abstract— We examinod seasonal ami 

 annual variation in numbers of StcUcr 

 (northern! sea lions iEumetopias juba- 

 tiis) at the South Farallon Islands from 

 counts conducted weekly from 197-4 

 to 1996. Numbers of adult and sub- 

 adult males peaked during the breeding 

 season (May-July), whereas numbers 

 of adult females and immature indi- 

 viduals peaked during the breeding 

 season and from late fall through 

 early winter (September-December). 

 The seasonal pattern varied signifi- 

 cantly among years for all sexes and 

 age classes. From 1977 to 1996, num- 

 bers present during the breeding season 

 decreased by 5.99r per year for adult 

 females and increased by 1.9% per year 

 for subadult males. No trend in numbers 

 of adult males was detected. Numbers 

 of immature individuals also declined 

 by 4.5'^r per year during the breeding 

 season but increased by S.O't per year 

 from late fall through early winter Max- 

 imum number of pups counted declined 

 significantly through time, although few 

 pups were produced at the South Faral- 

 lon Islands. The ratio of adult females to 

 adult males averaged 5.2:1 and declined 

 significantly with each year, whereas 

 no trend in the ratio of pups to adult 

 females was discernible. Further stud- 

 ies are needed to determine if reduced 

 numbers of adult females in recent 

 years have resulted from reduced sur- 

 vival of juvenile or adult females or 

 from changes in the geographic distri- 

 bution of females. 



Population status, seasonal variation in 

 abundance, and long-term population trends 

 of Steller sea lions (Eumetopias jubatus) 

 at the South Farallon islands, California* 



Kelly K. Hastings 



William J. Sydeman 



Point Reyes Bird Observatory 



4990 Shoreline Highway 



Stinson Beach, California 94970 



Present address (for K K Hastings): Alaska Department of Fish and Game 



Division of Wildlife Conservation 



333 Raspberry Rd 



Anchorage, Alaska 99518 

 Email address (for K K Hastings) kellyhaslingsiSfishgame slate ak us 



Manuscript accepted 1 August 2001. 

 Fish. Bull. 100(11:51-62(20021. 



Steller sea lions (Eunwtopias jubatus) 

 range from southern California along 

 the West Coast of North America 

 through the Aleutian and Pribilof 

 Islands to the Kuril Islands and Okhotsk 

 Sea, Japan (Kenyon and Rice, 1961). 

 Major haulouts and rookeries have his- 

 torically been centered at the Aleutian 

 Islands and at islands and mainland 

 sites around the Gulf of Alaska, where 

 over 70% of the world population was 

 located in the 1950s and 1960s (Lough- 

 lin et al., 1984). In 1990, the species 

 was listed as threatened throughout 

 its range under the Endangered Spe- 

 cies Act owing to declines of over 50% 

 from an estimated world population of 

 240.000-300.000 in the early 1960s to 

 116,000 individuals in 1989 (Loughlin 

 et al., 1992). Numerically the decline 

 was most severe in the western Gulf of 

 Alaska where 50-80'% declines occurred 

 (Loughlin et al., 1992). Reduced juve- 

 nile sui-vival appears to be the prox- 

 imate cause for the decline (York, 

 1994); ultimate causes, however, are 

 unknown. Effects of long-term environ- 

 mental change and pollutants on Steller 

 sea lions, and interactions or compe- 

 tition of these sea lions with commer- 

 cial fisheries are potential contributing 

 causes of this decline (NMML'). 



In contrast to rookeries in the west- 

 ern Gulf of Alaska, southeast Alaska 

 rookeries have increased by more than 

 60% over the past three decades ( Lough- 

 lin et al., 1992). Based on differences in 

 population trends and genetics (Bick- 

 ham et al., 1996), a distinction has been 



made between two separate stocks: 1) 

 the eastern stock, ranging from south- 

 east Alaska to California, and 2) the 

 western stock, ranging from the Gulf 

 of Alaska, Aleutian Islands, and Prib- 

 ilof Islands to Russia (LIS. Federal 

 Register 62:24345-24355). In 1997, the 

 National Marine Fisheries Service list- 

 ed the western stock as endangered, 

 whereas the eastern stock remained 

 listed as threatened. However, differ- 

 ences in trends between rookeries in 

 southeast Alaska and those in Cana- 

 da, Oregon, and California may indi- 

 cate that these areas deserve separate 

 management considerations. 



For example, rookeries in Canada 

 and California suffered 40% and 80% 

 declines respectively, from the early 

 1900s to 1970 (Bigg, 1988; Ainley et 

 al.-); declines continued over the past 



* Contribution 790 of the Point Reyes Bird 

 Observatory, Stin.son Beach, CA 94970. 



' NMML (National Marine Mammal Labora- 

 tory). 1995. Status review of the United 

 .States Steller sea lion [Eumetopias juba- 

 tuf) population. Report of the National 

 Marine Mammal Laboratory. National 

 Marine Fisheries Service, Seattle, WA, 

 61 p. [Available from National Marine 

 Mammal Laboratory, 7600 Sand Point Way 

 N.E., Seattle, WA 98115-0070.] 



' Ainley, D. G., H. R. Huber, R. R Henderson, 

 and T. J. Lewis. 1977. Studies of marine 

 mammals at the Farallon Islands, Califor- 

 nia. 1970-1975. Final report to the Ma- 

 rine Mammal Commission. Washington D.C. 

 I NTIS publication number PB274046. Avail- 

 able from Point Reyes Bird Observatory, 

 4990 Stinson Beach, CA 94970.1 



