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Aerial and ship-based surveys of Steller sea lions (Eumetopias jubatus) in Southeast Alaska, the Gulf of Alaska, and Aleutian Islands during June and July 1992

Abstract

Aerial and ship-based surveys of Steller sea lions (Eumetopias jubatus) were conducted during June and July 1992 from Forrester Island in Southeast Alaska to Attu Island in the western Aleutian Islands. A total of 34,844 adult and juvenile sea lions were counted at 95 "trend sites" (sites surveyed consistently since the 1970s and are thus appropriate for monitoring trends). The 1992 count represents a decline of 70.2% from 1979 (116,804) and a decline of 4.4% (P = 0.043) from 1991 (36,459). The annual rate of decline from 1979 to 1992, based on linear regression, was 9.6% (P = 0.0026). Of the 95 trend sites, 32 are rookeries. Estimated annual rates of decline for these rookeries were 10.2% (P < 0.001) for 1979-92 and 5.4% (P = 0.06) for 1989-92. Sixty-nine of the trend sites are located between the Kenai Peninsula and Kiska Island. At these sites, we counted 20,679 adult and juvenile sea lions, which represents declines of 76.9% from the 1975-79 counts (89,364) and 4.9% (P = 0.034) from 1991 (21,737). Estimated annual rates of decline, based on linear regression, were 10.0% (P = 0.002) for 1975-79 to 1992 and 3.7% (P = 0.026) for 1989 to 1992. For 26 trend rookeries in the Kenai to Kiska area, estimated annual rates of decline were 9.8% (P < 0.001) for 1975-79 and 4.4% (P = 0.067) for 1989-92. During the past year, numbers of sea lions counted at trend sites increased only in the eastern Aleutian Islands (from 4,231 in 1991 to 4,839 in 1992) and the western Aleutian Islands (from 2,411 to 2,869). Counts at trend sites declined in Southeast Alaska (from 7,715 in 1991 to 7,558 in 1992), in the eastern Gulf of Alaska (from 4,596 to 3,738), in the central Gulf of Alaska (from 6,273 to 5,721), and in the central Aleutian Islands (from 7,499 to 6,399). Counts of sea lions in the western Gulf of Alaska in 1991 and 1992 remained essentially unchanged (3,734 and 3,720, respectively). We counted 2,951 live pups at six rookeries in 1992. The number of pups increased at three of the four rookeries last surveyed in 1990: Akutan-Cape Morgan (+25.8%), Chernabura (+8.8%), and Chowiet (+9.1%) and declined sharply at Sugarloaf Island (-46.6%). Regionally, numbers of pups increased in Southeast Alaska (+24.2%), in the eastern (+16.9%) and western Gulf of Alaska (+13.9%), and in the eastern Aleutian Islands (+9.3%), but declined in the central Gulf of Alaska (-29.3%) from 1989-90 to 1991-92. Overall the number of pups declined by 4.4%, the same rate that the number of adult and juvenile sea lions declined from 1991 to 1992.


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