Aerial, ship, and land-based surveys of Steller sea lions (Eumetopias jubatus) in the western stock in Alaska, June and July 2004 and 2004

Abstract

The National Marine Fisheries Service (NMFS) conducted aerial, land, and ship-based surveys of the western stock of Steller sea lions (Eumetopias jubatus) in Alaska during June and July 2003 and 2004. Survey effort in 2003 consisted of sea lion pup (newborn) counts from land or skiff/ship at 13 sites in the eastern Aleutian Islands and in the Gulf of Alaska. Efforts in 2004 included: (1) a medium-format (MF) photography, vertically-oriented aerial survey of adult and juvenile sea lions (non-pups) at 262 sites spanning the entire range of the western stock in Alaska, and (2) pup counts from land or skiff/ship at 20 sites in the central and eastern Aleutian Islands and in three sub-areas of the Gulf of Alaska.

Non-pup sea lion population trends in the western stock were analyzed using counts at groups of consistently surveyed sites: (1) ‘1970s trend sites’ (n = 93) - surveyed since the mid-1970s; (2) ‘1990s trend sites’ (n = 161), a group that includes all the 1970s sites and others surveyed since 1991; and (3) Kenai-Kiska trend sites – sites located between 150EW (Kenai Peninsula) and 177EE (Kiska Island). In all previous years, non-pups were counted from projected 35 mm slides. Comparison of non-pup counts from both 35 mm and MF images taken in June 2000 indicated that MF counts tended to be greater than 35 mm counts. To integrate the 2004 MF with the long-term 35 mm data for sea lion population trend analyses, sub-area totals of non-pup counts in 2004 were reduced by 3.64% to account for film format differences in resolution and orientation.

Non-pup counts at trend sites in the western stock increased 11-12% between 2000 and 2004. The trend observed between 2000 and 2002 continued from 2002 to 2004. Between 2000 and 2004, the estimated average annual rate of change at all the 1970s trend sites was +2.9% (P < 0.05), while at all 1990s trend sites, the annual rate of change was not significantly different from zero. These short-term trend data suggest that the overall decline in the western Steller sea lion population may have abated. However, the western population has not significantly increased in size and in 2004 was 28-30% smaller than in 1991, and 69% smaller than in 1985 at the Kenai-Kiska trend sites.

Changes in sea lion non-pup counts between 2000 and 2004 have not been uniform across the range of the western stock and appear to follow the pattern in sub-area trends observed in the 1990s. Between 1991 and 2000, the sea lion population in the core of the western stock’s range in Alaska (eastern and central Aleutian Islands and western Gulf of Alaska) was relatively stable, while the population at the eastern (eastern and central Gulf of Alaska) and western (western Aleutian Islands) edges declined considerably. Between 2000 and 2004, the decline in non-pup counts slowed at the edges, while increases were observed in the core.

While there are no recent (2003 or 2004) data for the western and much of the central Aleutian Islands, recent regional trends in pup counts were generally similar to those observed in non-pup counts. Pup counts in the eastern Aleutian Islands and eastern Gulf of Alaska have increased since 1998 but have been relatively stable since 1994 in the western Gulf of Alaska. Pup counts have been declining unabated in the central Gulf of Alaska since the 1980s and were the lowest on record in 2003-2004.


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