Survival of Steller sea lions in Alaska: a comparison of increasing and decreasing populations

Abstract

Steller sea lion ( Eumetopias jubatus (Schreber, 1776)) populations have had differing dynamics in different regions of Alaska over the past 30 years. The western population (west of 144°W, near Cape Suckling) declined by approximately 85% between the 1970s and 2000, while the eastern population has increased at a rate of over 3%/year. Past research has indicated that the decline in the western population likely resulted from decreased juvenile survival and smaller declines in adult female survival and reproduction. Based on repeated observations (1987–2003) of sea lions branded as pups at Marmot Island (58.216°N, 151.840°W; western population; branded in 1987–1988) and at the Forrester Island rookery complex (54.859°N, 133.539°W; eastern population; branded in 1994–1995), we used mark–resight analyses to estimate age-specific survival probabilities. Juvenile sea lion survival probability at Marmot Island from 1988 to 1991 was lower than survival estimates at that location in the 1970s (assumed stable population) and lower than juvenile survival at Forrester Island from 1995 to 1998 (increasing population). Adult female survival at Marmot Island from 1992 to 2003 was only slightly reduced compared with that in the 1970s but was substantially lower than that at Forrester Island (1999–2003). In addition, and contrary to the typical pattern (e.g., Forrester Island), adult female survival probabilities at Marmot Island were indistinguishable from adult male survival probabilities. This suggests that regardless of which factors altered the dynamics of the western Steller sea lion population, they differentially affected females.