Optimal risk-taking theory applied to marine conservation: harbour seals in Prince William Sound

I sought theoretical insight on synergistic effects of resources and predators that are potentially relevant to the decline of harbour seals in Prince William Sound and to indirect effects of fisheries. Simulations predicted that compensatory foraging effort by seals will mitigate potential loss of energy reserves when resources decline, but only at the cost of higher predation rates, even if predator densities remain constant. A second study predicted net energy gain and predation risk per foraging dive, parameterising an analytical model with field data on seal behaviour, resource distributions, and use of depth by Pacific sleeper sharks and killer whales. Analyses suggested that risk of mortality from sharks and net energetic gain were greatest when seals foraged in deep strata, and empirical data showed individual variation in use of these strata. Plots of the individuals’ predicted energy gain against predicted predation risk fit best when relative danger from sharks was assumed to be much greater than that from killer whales. The first two studies combined suggest that, theoretically, overfishing of near-surface fatty fishes might increase shark predation rates on seals. A third model predicted an asymmetric trophic cascade in which indirect effects of sleeper sharks on resources were mediated by seal avoidance of riskier strata. Risk management by seals is predicted to reduce mortality on the dangerous resource (deep pollock) while increasing mortality on the safer resource (shallow herring), and the bycatch of sharks altered this dynamic. Although empirical data are lacking to test most predictions and various assumptions, the three models derive from first principles of behavioural ecology and provide a rigorous basis for predicting indirect effects of fisheries. Further, overfishing of sharks and of resources used by marine mammals are pressing global problems which cannot be addressed by empirical studies alone; indirect interactions between species are too complex to be elucidated without theoretical guidance and rapid exploitation often outpaces the acquisition of data relevant to conservation. Thus, theory presented here is important for assessing the potential damage wrought by different fishery scenarios, informing decisions that attempt to optimise exploitation and conservation, and guiding empirical research.

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