Life History Transmitters (LHX) in Steller sea lions: assessing the effects of health status, foraging ability, and environmental variability on juvenile survival and population trends.
Abstract:
The population of the Steller sea lion (Eumetopias jubatus), an apex predator in the North Pacific and Bering Sea ecosystems, has continuously declined over the past two decades along the Aleutian Islands and in the Gulf of Alaska. Steller sea lions are now listed as threatened within their eastern Alaskan range, and as endangered in their western range, under the Endangered Species Act. Despite years of productive research, little is known about the extent to which a hypothesized reduction in juvenile survival--in turn possibly related to reduced prey abundance--contributes to the present population trends.
We will accurately measure survival rates in juvenile Steller sea lions at two locations of a declining population, along the Aleutian Islands and in the Gulf of Alaska. For the first time, our approach will also deliver longitudinal dive effort data spanning several years, as well as detailed dive behavior data from non-surviving sea lions, and thus from the very animals that presumably contribute most to the population decline. Survival rates and long-term dive effort data will be determined through the use of satellite-linked life history transmitters (LHXs), implanted into 60 freeranging young-of-the-year Steller sea lions. As a central part of our project, we will directly test the hypothesized reduction in juvenile Steller sea lion survival.
Our experimental design utilizes a new paradigm for testing additional specific hypotheses pertinent to the decline of Stellers: a previously impossible direct comparison between survivors and non-survivors. We will analyze the relationship between early body mass and condition, health, as well as pollution indicators, near-weaning dive behavior and performance, continued long-term foraging effort and the survival of individual juvenile Steller sea lions. The analysis of seasonal, interannual and ontogenetic changes and variability in dive effort and mortality, as well as of detailed dive effort prior to death, will allow us to assess the relative effects of reduced foraging efficiency, nutritional stress, and possibly predation on survival. Furthermore, we will assess the extent to which juvenile survival can be predicted by parameters that can be measured in young Stellers prior to weaning. Such a predictive analysis might become a powerful tool in the assessment of future population trends before they become obvious at the population level.
Our cooperative project--which integrates interdisciplinary research on population dynamics, foraging ecology, physiology, health, and contaminants--directly addresses key issues and implements specific recommendations of the Bering Sea Ecosystem Research Plan Draft, the Steller Sea Lion Recovery Plan, several workshops on Steller sea lion issues, as well as NPMR. These diverse plans identify needs on Steller sea lion research, highlighting long-term longitudinal monitoring and juvenile survival as two of the most central and pressing issues.
© M. Horning 1999.
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