No abstract

We examined the reproductive performance of female Steller sea lions (Eumetopias jubatus) in order to evaluate the hypothesis that low pup production was associated with nutritional stress and to assess whether reduced birth rates could have been a factor in a recent large-scale decline in numbers. Nearly all (97%) sexually mature females were pregnant during early gestation. However, by late gestation, pregnancy rates had declined to 67 and 55% in the 1970s and 1980s, respectively, owing to reproductive failures. We found that body condition, as depicted by mass index and blubber index, had a positive effect on the probability that a female would be pregnant during late gestation. Age, age2, and lactation were also associated with pregnancy status during late gestation. These findings support the hypotheses that reproductive failures were associated with lowered nutritional status and that the resulting low birth rates were a proximate factor in the decline. We speculate that abortion is a part of the reproductive strategy of the female Steller sea lion that enhances overall reproductive performance during times of suboptimal nutrition. A major shift in oceanic regime occurred in the Gulf of Alaska during the late 1970s that resulted in a reduction of about 50% in the overall biomass of fishes and a shift in species composition. Prey resources may not have been adequate to successfully support the Steller sea lions' "energetically expensive" reproductive/foraging strategy during the period of our study.

Over the past 24 yr, 8,596 Steller sea lion (Eumetopias jubatus) pups were branded on their natal rookeries throughout Alaska with the objectives of determining survival rates, recruitment, movements, and site fidelity. Our objectives here were to examine the extent of dispersal of Steller sea lions away from their natal rookeries, movements between stocks, and degree of natal rookery fidelity. Pups (<1 yr old) usually remained within 500 km of their natal rookery. Branded juveniles dispersed widely and were resighted at distances up to 1,785 km from their natal rookeries. Adults generally remained within 500 km of their natal rookeries. No interchange of breeding animals between the ES (eastern stock) and WS (western stock) was observed. Although natal rookery fidelity was prevalent, 33% of the 12 observations of females branded in the WS during 1987–1988 and 19% of the 29 observations of females branded in the ES during 1994–1995 were observed with newly born pups at sites other than their natal rookeries. Steller sea lions generally conformed to the metapopulation concept as depicted by Hanski and Simberloff (1997), with local breeding populations (rookeries) and movements among these local populations having the potential of affecting local dynamics.

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.