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Predicting synergistic effects of resources and predators on foraging decisions by juvenile Steller sea lions

  • Behavioral Ecology - Original Paper
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Abstract

Many theoretical and experimental studies suggest that synergistic interactions between resources and predators influence foraging decisions and their fitness consequences. This framework, however, has been ignored almost completely by hypotheses on causes of the population decline of Steller sea lions (SSLs) (Eumetopias jubatus) in western Alaska. By comparing predictions from a dynamic state variable model to empirical data on the behaviour of individuals instrumented with satellite-linked time-at-depth recorders, we develop and find preliminary support for the hypothesis that, during winter in Prince William Sound, juvenile SSLs (a) underutilise walleye pollock, a predictable resource in deep strata, due to predation risk from Pacific sleeper sharks, and (b) underutilise the potential energy bonanza of inshore aggregations of Pacific herring due to risk from either killer whales, larger conspecifics, or both. Further, under conditions of resource scarcity—induced by overfishing, long-term oceanographic cycles, or their combination—trade-offs between mortality risk and energy gain may influence demographic parameters. Accordingly, computer simulations illustrated the theoretical plausibility that a decline of Pacific herring in shallow strata would greatly increase the number of deep foraging dives, thereby increasing exposure to sleeper sharks and mortality rates. These results suggest that hypotheses on the decline of SSLs should consider synergistic effects of predators and resources on behaviour and mortality rates. Empirical support for our model, however, is limited and we outline tasks for empirical research that emerge from these limitations. More generally, in the context of today’s conservation crises, our work illustrates that the greater the dearth of system-specific data, the greater the need to apply principles of behavioural ecology toward the understanding and management of large-scale marine systems.

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Acknowledgments

Theoretical research and writing were funded by award NA04NMF4390161 from the National Marine Fisheries Service to RE Thorne. NOAA-Fisheries Steller Sea Lion Research Initiative Grant #NA17FX1414 to J. Burns funded the fieldwork, which was authorized by the University of Alaska Anchorage Institutional Animal Care and Use Committee and US Marine Mammal Protection Act permits 782–1532 and 358–1564. Bill Lee assisted with the spatial analysis, and Volker Deecke, Ari Shapiro and Patrick Miller generously allowed the use of their unpublished data on killer whales. We are greatly indebted to Marc Mangel for theoretical guidance and comments on an earlier draft. Special thanks extend to Dave Rosen and Dom Tollit for advice on the parameterisation of physiological variables, and to Volker Deecke for advice on killer whale biology. Nick Wolfe contributed to the development of theoretical ideas and Wendy Palen and Michelle Paddack provided other feedback. Comments by three anonymous referees led to a much-improved revision. AF acknowledges the work of Van Jones for influential subtext when it was most needed.

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Author notes

  1. Alejandro Frid

    Present address: Vancouver Aquarium, PO Box 3232, Vancouver, BC, V6B 3X8, Canada

Authors and Affiliations

  1. Department of Biology, Dalhousie University, Halifax, NS, B3H 4J1, Canada

    Alejandro Frid

  2. Department of Biological Sciences, University of Alaska Anchorage, Anchorage, AK, 99508, USA

    Jennifer Burns

  3. School of Computing Science, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada

    Gregory G. Baker

  4. Prince William Sound Science Center, Cordova, AK, 99574, USA

    Richard E. Thorne

Authors
  1. Alejandro Frid
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  2. Jennifer Burns
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  4. Richard E. Thorne
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Correspondence to Alejandro Frid.

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Communicated by Craig Osenberg.

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Frid, A., Burns, J., Baker, G.G. et al. Predicting synergistic effects of resources and predators on foraging decisions by juvenile Steller sea lions. Oecologia 158, 775–786 (2009). https://doi.org/10.1007/s00442-008-1189-5

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  • DOI: https://doi.org/10.1007/s00442-008-1189-5

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