Abstract
The endangered western stock of the Steller sea lion (Eumetopias jubatus) – the largest of the eared seals – has declined by 80% from population levels encountered four decades ago. Current overall trends from the Gulf of Alaska to the Aleutian Islands appear neutral with strong regional heterogeneities. A published inferential model has been used to hypothesize a continuous decline in natality and depressed juvenile survival during the height of the decline in the mid-late 1980's, followed by the recent recovery of juvenile survival to pre-decline rates. However, these hypotheses have not been tested by direct means, and causes underlying past and present population trajectories remain unresolved and controversial. We determined post-weaning juvenile survival and causes of mortality using data received post-mortem via satellite from telemetry transmitters implanted into 36 juvenile Steller sea lions from 2005 through 2011. Data show high post-weaning mortality by predation in the eastern Gulf of Alaska region. To evaluate the impact of such high levels of predation, we developed a conceptual framework to integrate density dependent with density independent effects on vital rates and population trajectories. Our data and model do not support the hypothesized recent recovery of juvenile survival rates and reduced natality. Instead, our data demonstrate continued low juvenile survival in the Prince William Sound and Kenai Fjords region of the Gulf of Alaska. Our results on contemporary predation rates combined with the density dependent conceptual framework suggest predation on juvenile sea lions as the largest impediment to recovery of the species in the eastern Gulf of Alaska region. The framework also highlights the necessity for demographic models based on age-structured census data to incorporate the differential impact of predation on multiple vital rates.
Highlights
The endangered, western population segment of the Steller sea lion (Eumetopias jubatus) has declined to about 20 percent of peak levels recorded four decades ago, with locally divergent but overall stable trends in the Gulf of Alaska (GOA) and Bering Sea Aleutian Islands (BSAI) [1]
Several hypotheses describing forcing on these mesopredators in the GOA – BSAI region have been advanced, including the resource-driven junk food [8] and ocean climate [9] hypotheses, and the consumer-driven sequential megafaunal collapse hypothesis
From the combination of previously unavailable empirical data and qualitative conceptual framework we propose an alternative hypothesis to the postulated depressed natality for present day forcing of the Steller sea lion population in the eastern GOA
Summary
The endangered, western population segment of the Steller sea lion (Eumetopias jubatus) has declined to about 20 percent of peak levels recorded four decades ago, with locally divergent but overall stable trends in the Gulf of Alaska (GOA) and Bering Sea Aleutian Islands (BSAI) [1]. Several hypotheses describing forcing on these mesopredators in the GOA – BSAI region have been advanced, including the resource-driven junk food [8] and ocean climate [9] hypotheses, and the consumer-driven sequential megafaunal collapse hypothesis. Resource driven hypotheses primarily postulate changes in abundance, distribution and accessibility, composition or nutritional quality of prey [8,11] These changes may be natural (i.e. driven by episodic changes in ocean climate) or anthropogenic (i.e. through large scale industrial fishing). Poor health and condition, and unbalanced energy budgets may compromise an individual’s ability to evade predation Effects in both top-down and bottom-up categories may exhibit density dependent and density independent characteristics. Resource effects commonly exhibit density dependence at high abundance, whereas consumer effects are apt to be density independent at high abundance with possible density effects at low abundance [11]
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