Fish farms, parasites, and predators: implications for salmon population dynamics

Authors

  • Martin Krkošek,

    1. Centre for Mathematical Biology, Department of Biological Sciences and Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, Alberta T6G 2G1 Canada
    2. School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98105 USA
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    • Present address: Department of Zoology, University of Otago, 340 Great King Street, P.O. Box 56, Dunedin 9054 New Zealand. E-mail: martin.krkosek@otago.ac.nz

  • Brendan M. Connors,

    1. Earth to Ocean and Evolutionary and Behavioural Ecology Research Groups, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
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  • Helen Ford,

    1. School of Environmental Studies, University of Victoria, Victoria, British Columbia V8W 3R4 Canada
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  • Stephanie Peacock,

    1. School of Environmental Studies, University of Victoria, Victoria, British Columbia V8W 3R4 Canada
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  • Paul Mages,

    1. Earth to Ocean and Evolutionary and Behavioural Ecology Research Groups, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
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  • Jennifer S. Ford,

    1. Biology Department, Dalhousie University, Halifax, Nova Scotia B3H 4J1 Canada
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  • Alexandra Morton,

    1. Salmon Coast Field Station, Simoom Sound, British Columbia V0P 1S0 Canada
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  • John P. Volpe,

    1. School of Environmental Studies, University of Victoria, Victoria, British Columbia V8W 3R4 Canada
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  • Ray Hilborn,

    1. School of Aquatic and Fishery Sciences, University of Washington, Seattle, Washington 98105 USA
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  • Lawrence M. Dill,

    1. Earth to Ocean and Evolutionary and Behavioural Ecology Research Groups, Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6 Canada
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  • Mark A. Lewis

    1. Centre for Mathematical Biology, Department of Biological Sciences and Department of Mathematical and Statistical Sciences, University of Alberta, Edmonton, Alberta T6G 2G1 Canada
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  • Corresponding Editor: K. Stokesbury.

Abstract

For some salmon populations, the individual and population effects of sea lice (Lepeophtheirus salmonis) transmission from sea cage salmon farms is probably mediated by predation, which is a primary natural source of mortality of juvenile salmon. We examined how sea lice infestation affects predation risk and mortality of juvenile pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon, and developed a mathematical model to assess the implications for population dynamics and conservation. A risk-taking experiment indicated that infected juvenile pink salmon accept a higher predation risk in order to obtain foraging opportunities. In a schooling experiment with juvenile chum salmon, infected individuals had increased nearest-neighbor distances and occupied peripheral positions in the school. Prey selection experiments with cutthroat trout (O. clarkii) predators indicated that infection reduces the ability of juvenile pink salmon to evade a predatory strike. Group predation experiments with coho salmon (O. kisutch) feeding on juvenile pink or chum salmon indicated that predators selectively consume infected prey. The experimental results indicate that lice may increase the rate of prey capture but not the handling time of a predator. Based on this result, we developed a mathematical model of sea lice and salmon population dynamics in which parasitism affects the attack rate in a type II functional response. Analysis of the model indicates that: (1) the estimated mortality of wild juvenile salmon due to sea lice infestation is probably higher than previously thought; (2) predation can cause a simultaneous decline in sea louse abundance on wild fish and salmon productivity that could mislead managers and regulators; and (3) compensatory mortality occurs in the saturation region of the type II functional response where prey are abundant because predators increase mortality of parasites but not overall predation rates. These findings indicate that predation is an important component of salmon–louse dynamics and has implications for estimating mortality, reducing infection, and developing conservation policy.

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