Northern Sea Lion Population Decline in the Eastern Aleutian Islands

Abstract

From June 1975 through June 1977, 6 aerial surveys were conducted along the eastern Aleutian Islands and north coast of the Alaska Peninsula to assess the distribution and abundance of northern sea lions (Eumetopiasjubatus). Site-specific counts of sea lions on rookeries and haulout places were compared to counts made in 1957, 1960, 1965, and 1968. Analysis of total counts from sites replicated among survey years since 1957 indicated that a significant population decline occurred. The 1977 population estimate was below 25,000, whereas in the late 1950's and early 1960's the population exceeded 50,000. The 1977 population level was 44.9-54.7% of the 1957 estimate, and it appeared that the population decline has been greatest since 1968. No cause for the decline was apparent, but several contributing factors are discussed. J. WILDL. MANAGE. 44(1):25-33 The northern or Steller sea lion is the most abundant sea lion in North America and ranges from California north into the Bering Sea, Alaska (Rice 1977). The recent population level in Alaska is estimated at 200,000 animals (Interagency Task Group 1978) with the greatest numbers occurring from the Gulf of Alaska to the western limits of the Aleutian Islands. However, reliable estimates of abundance are not available for much of Alaska. Population studies along the south side of the Alaska Peninsula into the Gulf of Alaska have been conducted by Alaska Department of Fish and Game biologists and others (Calkins et al. 1975, Fiscus et al. 1976), but no surveys have been conducted throughout the Aleutian Islands since the one by Kenyon and Rice (1961). This paper reports on a 1975-77 study of the distribution and abundance of northern sea lions in the eastern Aleutian Islands (Braham et al. 1977). Previous surveys in this area, specifically of the Fox Island group, were generally incomplete and conducted at different seasons (Kenyon and Rice 1961, Mathisen and Lopp 1963, Kenyon and King 1965, Fiscus and Johnson 1968). Resultant population estimates ranged from 45,000 to 50,000 in the late 1950's and early 1960's to 20,000 to 25,000 in the mid-1970's. Studies conducted by Braham et al. (1977) were designed to refine the identification of breeding and haulout sites and to estimate population size for comparison with previous surveys. O. Siebert, King Salmon, Alaska, flew the survey aircraft and provided guidance and knowledge of the area under study. C. Fiscus was instrumental in developing the surveys and provided field expertise during the 1975-76 surveys. Officers and crew of the NOAA research ship Surveyor and personnel at the U.S. Coast Guard Station at Cape Sarichef, Alaska, provided logistical support. Advice and assistance from D. Calkins, J. Faro, K. Pitcher, K. Schneider, and N. Steen, Alaska Department of Fish and Game, are appreciated. Assisting with research were T. Bray, B. Krogman, P. McGuire, R. Mercer, M. Nerini, C. Peterson, N. Severinghaus, and D. Withrow. Graphic arts and manuscript review were providJ. Wildl. Manage. 44(1):1980 25 This content downloaded from 207.46.13.28 on Tue, 30 Aug 2016 05:18:02 UTC All use subject to http://about.jstor.org/terms 26 SEA LION POPULATION DECLINE* Braham et al. ed by the Northwest and Alaska Fisheries Center, NMFS, and typing by L. Hietala. The study was supported by the Bureau of Land Management through interagency agreement with the National Oceanic and Atmospheric Administration, under a multi-year program managed by the Alaska Outer Continental Shelf Environmental Assessment Program (OCSEAP) office. STUDY AREA AND METHODS The study area was the central portion of the range for northern sea lions in Alaska, and extended from the north side of the Alaska Peninsula at Amak Island and Sea Lion Rock west to Samalga Island (Fig. 1). Seven breeding locations were identified: Sea Lion Rock (#45, Fig. 1), Ugamak (#40) and Round (#41) islands, Cape Morgan (#24) on Akutan Island, Bogoslof Island (#10), Ogchul Island (#4), and Adugak Island (#2). Also, 23 haulout sites (non-pupping locations) with more than 45 animals per site during any 1 survey were identified; 19 were on the Fox Islands. The Aleutian Islands are typically of columnar basalt, having coastal substrate types including sandy beaches, cobblestone beaches, and rocky outcrops along vertical cliffs. The adjacent waters are used extensively by cetaceans and fur seals during migration and feeding, and the area is important for Soviet, Japanese, and U.S. commercial fishing. The islands are part of the Aleutian Islands National Wildlife Refuge system. Oil and gas exploration has been proposed for the continental shelf north, south, and east of the Fox Islands. Methods.-Aerial surveys were conducted 17-20 June 1975, 9-13 August 1975, 15-20 June 1976, 19-21 August 1976, 21-25 October 1976, and 28-30 June 1977. The peak of the breeding pe46