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NOAA Technical Memorandum NMFS-AFSC-183

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Aerial, Ship and Land-Based Surveys of Steller Sea Lions
(Eumetopias jubatus) in Alaska, June and July 2005 - 2007

Abstract

The National Marine Fisheries Service (NMFS) conducted aerial, land, and ship-based surveys of Steller sea lions (Eumetopias jubatus) in Alaska during June and July 2005, 2006, and 2007. Surveys were timed to assess trends in pup production (late June through mid-July) or abundance of adults and juveniles (non-pups; early through late June). The aerial photographic survey conducted in 2005 provided the first estimate of Steller sea lion pup (newborn) production at all rookeries and major haulouts in Alaska in a single year (n = 15,460; 5,510 in southeast Alaska, and 9,950 within the range of the western stock of Steller sea lion in Alaska (west of 144°W)). Pups were also surveyed during ship and land-based surveys at selected terrestrial locations in 2005 and 2007. Aerial surveys to count non-pups on terrestrial sites within the range of the western stock were conducted in both 2006 and 2007, but neither resulted in a range-wide assessment because of delays caused by weather, maintenance, and issuance of scientific permits. The non-pup aerial survey conducted in 2007 was the first to use both vertical digital and medium-format film photography (both with forward-motion compensation), allowing multiple comparisons of counts at the same sites.

Despite incomplete surveys conducted in 2006 and 2007, the available data indicate that the western Steller sea lion population (non-pups) was stable since 2004 (when the last complete assessment was done). However, there was considerable regional variability in non-pup trends, with increases in the central and western Gulf of Alaska and eastern Aleutian Islands being largely offset by decreases in the eastern Gulf of Alaska and central and western Aleutian Islands. Overall, counts of non-pups increased at an annual rate of 2.1% between 2000 and 2007, but this rate was not significantly different from zero (P = 0.08).

Western stock pup counts exhibited the same regional variability in recent trends as counts of non-pups, but overall they were approximately 4% higher in 2005-2007 than in 2001-2002. Pup counts in southeast Alaska (part of the eastern stock) increased 13% between 2002 and 2005.


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