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Fishei7 Bulletin 100(1) 



over the same period. For example, California sea lions 

 made up only 20% of the total sea hon count at the Faral- 

 lon Islands in 1938 (Bonnot and Ripley, 1948); but by the 

 mid 1970s, California sea lions were twice as numerous as 

 Steller sea lions during June and July (Fig. 51. 



The role of commercial hai-vest and direct take or ha- 

 rassment of sea lions by humans in this decline is un- 

 certain. Large numbers of sea lions were hunted in Cal- 

 ifornia in the late 1800s for oil, hides, and "trimmings" 

 (which included the whiskers, genitalia, and gall bladder 

 of adult males) that were sold to Chinese markets (Scam- 

 mon, 1874). Hunting sea lions for oil became unprofitable 

 around 1900 because of the reduction in sea lion numbers 

 and the wide-spread availability of petroleum products 

 (Rowley, 1929). A reduced sea lion hai-vest for hides, trim- 

 mings, and (in Mexican waters) pet food, continued until 

 the end of the 1930s when Chinese markets disappeared 

 with the onset of the Japanese-Chinese war and protests 

 were successful in stopping Mexican harvests (Bonnot, 

 1951). During the same period, although fewer sea lions 

 were taken by sportsman, fisherman, and collectors for 

 museums and zoos, rookery abandonments and population 

 declines still persisted in Oregon and southern California 

 (Rowley, 1929; Bonnot, 1931). An additional cause for these 

 population declines may have been the sea lion hunts that 

 were introduced by commercial fisheries around 1900 to 

 reduce competition for fish (Bonnot, 1937). For example, a 

 bounty was offered for Steller sea lions in the early 1900s 

 in areas north of California (Rowley, 1929; Bonnot, 1931; 

 Bonnot, 1951). 



Although numbers hai-vested in California are not well 

 documented and the role of harvest in the decline is not 

 obvious, several arguments can be made that declines in 

 Steller sea lions from the 1940s to 1970s were likely not 

 due to effects of hai-vest alone. During the period of com- 

 mercial harvest, Steller numbers appeared stable (Bonnot 

 and Ripley, 1948), whereas the 75-80% decline was evi- 

 dent after 1947, after commercial hunting and collections 

 had ended, although harassment by fisherman continued. 

 After 1947, the California sea lion population increased 

 exponentially throughout the state from 3050 in 1947 to 

 a minimum of 18,047 in 1970 (Bonnot and Ripley, 1948; 

 Carlisle and Aplin, 1971), whereas numbers of Steller sea 

 lions on the Channel Islands and at the Farallon Islands 

 declined from 80% to 100% during this period. Large in- 

 creases in California sea lions were evident after commer- 

 cial hai-vesting ended, even though many more California 

 than Steller sea lions were likely hunted commercially, 

 poached, or captured because of difficulty hunting in the 

 steep, rocky intertidal areas frequented by Steller sea li- 

 ons (Rowley, 1929; Bonnot, 1951). This reasoning suggests 

 that factors in addition to hai-vest have influenced the 

 population decline. Proposed causes include reduction of 

 the prey base due to overexploitation by commercial fish- 

 eries (Ainley and Lewis, 1974), shifts in prey composition 

 due to ocean warming, and competition for food with grow- 

 ing numbers of California sea lions (Bartholomew, 1967). 



Human disturbance, however, likely played some role in 

 the decline, in respect of which Steller sea lions may be 

 more affected by human disturbance than California sea 



lions. For example, the large Steller sea lion rookeries at 

 San Miguel Island and at Seal Rocks, just off San Fran- 

 cisco, were abandoned permanently because of harassment 

 and shooting by hunters for sea lion trimmings or by fisher- 

 man (Rowley, 1929). Southeast Farallon Island was inhab- 

 ited by fair numbers of lighthouse keepers and their fami- 

 lies (since the mid- 1800s) and egg hunters (men collecting 

 seabird eggs for sale in commercial markets for human 

 consumption) from the mid-1800s to the mid-1900s. High- 

 est human occupancy occurred during World War II, when 

 over 50 military personnel wore added to the island's pop- 

 ulation (Ainley and Lewis, 1974). Families were removed 

 in 1965 and the lighthouse was automated in 1972, after 

 which time only PRBO researchers remained on the island 

 (Ainley and Lewis, 1974). Despite the designation of the 

 North and Middle Farallon Islands in 1909 and the South 

 Farallon Islands in 1969 as a national wildlife refuge, ha- 

 rassment by fisherman and disturbance from low-flying 

 helicopters was common into the 1970s (Ainley and Lewis, 

 1974). Heightened human presence in the mid-1900s likely 

 increased the abandonment of Steller sea lions from the is- 

 lands during the period of dramatic decline. 



Recent population trends Over the last 20 years, the 

 numbers of Steller sea lions on the South Farallon Islands 

 has continued to decline significantly. Numbers of adult 

 females present during the breeding season declined by 

 5.9% per year from 1977 to 1996, although the rate of 

 decline has lessened since the mid to late 1980s (Fig. 3B). 

 This rate of decline is much higher than the 3.6% per year 

 decline reported for adult females by Sydeman and Allen 

 (1999), who used maximum counts and data from all sea- 

 sons, although rates are similar between the two studies 

 when similar data were used (3.2% per year estimated 

 from our study, when data from all seasons were pooled). 

 These findings demonstrate the importance of accounting 

 for seasonal effects when investigating population trends. 

 The rate of decline of 5.9% per year is similar to the rate of 

 decline obsei-ved during the breeding season in the area of 

 greatest decline in Alaska (from Kiska Island to the Kenai 

 Peninsula), where rates of decline varied from approxi- 

 mately 5% (1975-85 and 1990-94) to 16% (1985-90; York 

 etal., 1996). 



Numbers of immature individuals present during the 

 breeding season have also declined by 4.5% per year over 

 the past several decades, but an overall net increase in 

 immature individuals on the islands has been apparent 

 owing to increased numbers in the late fall and early win- 

 ter. Numbers of immature individuals on the Farallon Is- 

 lands in the winter were particularly high from 1984 to 

 1986. Immature individuals have continued to be present 

 in significant numbers during winter in recent years. It 

 is uncertain where these young animals originated from, 

 but overall declines in juvenile counts, coupled with sig- 

 nificant declines in juvenile counts during the breeding 

 season, suggest that increased numbers in winter may 

 represent changes in movement and haulout patterns of 

 juveniles rather than improved juvenile sui-vival in recent 

 years. Increased numbers of subadult males hauled out on 

 the South Farallon Islands during the breeding season in 



