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

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Abstract:

Steller sea lion (Eumetopias jubatus (Schreber, 1776)) populations have had differing dynamics in different regions of Alaska over the past 30years. 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.

Les dynamiques des populations de lions de mer de Steller (Eumetopias jubatus (Schreber, 1776)) ont divergé dans les diverses régions de l’Alaska au cours des 30 dernières années. La population occidentale (à l’ouest de 144°W, près du cap Suckling) a décliné d’environ 85 % entre les années 1970 et 2000, alors que la population orientale a augmenté de plus de 3 %/an. Des recherches antérieures ont indiqué que le déclin de la population occidentale est vraisemblablement dû à une diminution de la survie des jeunes et à une réduction moindre de la survie et de la reproduction des femelles adultes. D’après des observations répétées (1987–2003) de lions de mer marqués au fer dans leur jeunesse à l’île Marmot (58.216°N, 151.840°W; population occidentale; marquée en 1987–1988) et au complexe de roqueries de l’île Forrester (54.859°N, 133.539°W; population orientale; marquée en 1994–1995), nous avons pu estimer les probabilités de survie en fonction de l’âge à l’aide de techniques de marquage et de re-signalisation. La probabilité de survie des jeunes lions de mer à l’île Marmot de 1988–1991 est plus faible que les estimations de survie au même endroit dans les années 1970 (population stable présumée) et elle est inférieure à la survie des jeunes à l’île Forrester de 1995–1998 (population croissante). La survie des femelles adultes à l’île Marmot (1992–2003) est un peu plus faible que dans les années 1970, mais elle est nettement inférieure à celle des femelles adultes de l’île Forrester (1999–2003). De plus, les probabilités de survie des femelles adultes ne peuvent être distinguées de celles des mâles adultes, ce qui est différent du patron habituel (comme, par exemple, à l’île Forrester). Ces données indiquent que, quels que soient les facteurs qui aient modifié la dynamique démographique de la population occidentale des lions de mer de Steller, ils ont affecté les femelles de façon particulière.

Document Type: Research Article

Publication date: August 1, 2006

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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