DECLINE IN ABUNDANCE OF THE NORTHERN SEA LION, 

 EUMETOPIAS JUBATUS, IN ALASKA, 1956-86 



Richard L. Merrick, i Thomas R. Loughlin.i and Donald G. Calkins^ 



ABSTRACT 



Aerial, ship, and onshore surveys were conducted to assess the abundance of northern sea lions, 

 Eumetopias jubatus, in southwestern Alaska, from the central Gulf of Alaska through the central Aleu- 

 tian Islands, during June-July of 1984-86. Counts of northern sea lions from these surveys were com- 

 pared with counts made in 1956-62 and 1975-79. These data indicated that the number of adults and 

 juveniles onshore declined 52% from 140,000 animals in 1956-60 to 68,000 in 1985— an annual rate of 

 decline of at least 2.7%. Numbers have declined throughout the region, with the greatest declines in 

 the eastern Aleutian Islands (79%) and the least in the central Aleutian Islands (8%). This was not due 

 to emigration because significant increases have not been noted elsewhere. Between the 1960s and 

 mid-1970s, there were large decreases in the eastern Aleutian Islands and western Gulf of Alaska, and 

 a major increase in the central Aleutian Islands. Beginning in the late 1970s declines occurred in all 

 areas. The causes of the declines are unknown, but they may be associated with disease, prey availability 

 or quality, or a combined effect of these and other factors. Factors which may contribute to the declines 

 include the pre-1973 commercial harvests, entanglement of juveniles in marine debris, incidental takes 

 in fisheries, and killing by fishermen. 



The northern or Steller sea Hon, Eumetopias juba- 

 tus, breeds from the Kuril Islands and Okhotsk Sea 

 through the Aleutian Islands and Gulf of Alaska, and 

 south to California. Loughlin et al. (1984) estimated 

 the maximal population in 1974-80 at 290,000 (in- 

 cluding some pups), of which more than 196,000 

 were in Alaska. The number of northern sea lions 

 counted in Alaska was unchanged since the surveys 

 of Kenyon and Rice (1961) and Mathisen and Lopp 

 (1963) in 1956-60, even though significant declines 

 had occurred in the eastern Aleutian and Pribilof 

 Islands (Kenyon 1962; Braham et al. 1980). These 

 declines were offset by increases in northern sea lion 

 numbers in the central and western Aleutian Islands 

 (Fiscus et al. 1981). 



Concern over the decline in northern sea lion 

 numbers in the eastern Aleutian Islands prompted 

 the National Marine Mammal Laboratory (NMML) 

 and the Alaska Department of Fish and Game to 

 conduct surveys in 1984, 1985, and 1986 at sites 

 throughout southwestern Alaska. These included 

 aerial, ship, and onshore surveys of rookeries and 

 major haul-out sites from Kiska Island in the cen- 

 tral Aleutian Islands to the Barren Islands in the 

 central Gulf of Alaska, as well as observations dur- 



ing two breeding seasons at Ugamak Island, a major 

 rookery in the eastern Aleutian Islands. Together 

 with earlier data for the Aleutian Islands (Kenyon 

 and Rice 1961; Kenyon 1962^; Kenyon and King 

 1965*; Braham et al. 1980; Fiscus et al. 1981) and 

 for the Gulf of Alaska (Mathisen and Lopp 1963; 

 Calkins and Pitcher 1982^), these data present a 

 30-yr record of counting northern sea lions, albeit 

 sporadically, in Alaska waters. The objectives of this 

 paper are 1) to report the results of surveys con- 

 ducted between 1984 and 1986, thus describing the 

 current distribution and numbers of northern sea 

 lions in much of Alaska, 2) to compare those counts 

 with the historical data, and 3) to discuss the state 

 of knowledge on causes of the decline in sea lion 

 numbers. 



'National Marine Mammal Laboratory, Northwest and Alaska 

 Fisheries Center, National Marine Fisheries Service, NOAA, 7600 

 Sand Point Way N.E., Seattle, WA 98115. 



^Alaska Department of Fish and Game, 333 Raspberry Road, 

 Anchorage, AK 99502. 



'Kenyon, K. W. 1962. Sea otter studies, population and distri- 

 bution (with notes on Steller sea lion and emperor goose). Un- 

 publ. Rep., U.S. Fish Wildl. Serv., Branch Wildl. Res., Seattle, 47 

 p. Available from Northwest and Alaska Fish. Cent. Natl. Mar. 

 Mammal Lab., NMFS, NOAA, 7600 Sand Point Way N.E., Seattle, 

 WA 98115. 



^Kenyon, K. W., and J. G. King, Jr. 1965. Aerial survey of 

 sea otters and other marine mammals, Alaska Peninsula and Aleu- 

 tian Islands, 19 April to 9 May 1965. Processed Rep., U.S. Fish 

 Wildl. Serv., Bur. Sport Fish. Wildl., Seattle, 52 p. Available from 

 Northwest and Alaska Fish. Cent. Natl. Mar. Mammal Lab., 

 NMFS, NOAA, 7600 Sand Point Way, N.E., Seattle, WA 98115. 



^Calkins, D. G., and K. W. Pitcher. 1982. Population assess- 

 ment, ecology and trophic relationships of Steller sea lions in the 

 Gulf of Alaska. Alaska Dep. Fish and Game, Final Rep. RU243, 

 128 p. Alaska Department of Fish and Game, 333 Raspberry 

 Road, Anchorage, AK 99502. 



Manuscript accepted December 1986. 

 FISHERY BULLETIN: VOL. 85, NO. 2, 1987. 



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