Aerial and land-based surveys of Steller sea lions (Eumetopias jubatus) from the western stock in Alaska, June and July 2001 and 2002

Abstract

The National Marine Fisheries Service (NMFS) conducted aerial and land-based surveys of the western stock of Steller sea lions (Eumetopias jubatus) in Alaska during June and July of 2001 and 2002 at all sites from 144° W to 172° E, the westernmost point in the Aleutian Islands.

Survey effort in 2001 was restricted to pup counts at selected sites in the Aleutian Islands and the Gulf of Alaska. The 2002 survey included the entire western stock: aerial surveys of non-pups at all sites and pup counts at all rookeries except those counted in 2001. In June 2002, a total of 26,602 non-pups were counted on all surveyed sites (n = 259). Of these non-pups, 14,320 were on 30 trend rookeries and 5,020 were on 54 trend haul-out sites for a total of 19,340 on all 84 trend sites. This was an increase of 5.5% for all trend sites and 6.8% for trend rookeries from June 2000, the first region-wide increase observed since standardized aerial surveys began in the 1970s. Counts were still down compared to 1998 (-5.4% for all trend sites and -1.2% for trend rookeries) and down more than 30% since 1990. From 1991 to 2002, the population declined by an average of 4.1% per year (P < 0.001: 95% C.I.= -2.8% to -5.5%) for all trend sites and 3.9% per year (P = 0.002: 95% C.I.= -2.3% to -5.6%) at the trend rookeries.

The Kenai Peninsula to Kiska Island index area, a core subarea within the Alaska portion of the western stock, included 22,221 non-pup sea lions at 220 of the 259 surveyed sites. Of these, 12,893 were on 26 trend rookeries and 3,130 were on 44 trend haul-out sites (16,023 non-pups on all 70 trend sites). These counts represented increases from June 2000 of 4.9% and 9.8%, respectively, for all trend sites and for trend rookeries. Trend rookeries in the Kenai-to-Kiska region also increased by 6.4% relative to 1998, although all trend sites were down 2.4%, and both were down about 25% from 1991. From 1991 to 2002, Kenai-to-Kiska counts declined by an average of 3.1% per year (P < 0.001: 95% C.I.= -2.0% to -4.2%) for all 70 trend sites and 3.2% per year (P = 0.006: 95% C.I.= -1.4% to -5.1%) for the 26 trend rookeries from 1991 to 1998. The 2002 aerial survey included replication of 27 sites in the Gulf of Alaska and 30 sites in the eastern and east-central Aleutian Islands. Coefficients of variation calculated from the replicate counts were 0.029 for all sites, 0.044 for the Gulf of Alaska sites, and 0.038 for sites in the Aleutian Islands.

Region-wide pup counts are on a 4-year schedule (including 2002), with counts at selected sites during intervening years. During June and July 2001, NMFS personnel counted 3,837 live pups at 11 rookeries and 90 live pups at five haul-out sites in Alaska. During June and July 2002, NMFS counted 5,472 live pups at 27 rookeries and 178 live pups at six haul-out sites. Pups were counted in both years at Marmot, Ugamak, and Semisopochnoi Island rookeries and at the Seguam Island, Turf Point haul-out site. Using a composite of 2001 and 2002 pup counts, the total pup count for the western stock was 8,345 live pups at 35 rookeries and 244 live pups at 10 haul-out sites. This represented a decline of 11.2% since 1998 for the western stock in Alaska.


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