title: Can reduced predation offset negative effects of sea louse parasites on chum salmon?
abstract: The impact of parasites on hosts is invariably negative when considered in isolation, but may be complex and unexpected in nature. For example, if parasites make hosts less desirable to predators then gains from reduced predation may offset direct costs of being parasitized. We explore these ideas in the context of sea louse infestations on salmon. In Pacific Canada, sea lice can spread from farmed salmon to migrating juvenile wild salmon. Low numbers of sea lice can cause mortality of juvenile pink and chum salmon. For pink salmon, this has resulted in reduced productivity of river populations exposed to salmon farming. However, for chum salmon, we did not find an effect of sea louse infestations on productivity, despite high statistical power. Motivated by this unexpected result, we used a mathematical model to show how a parasite-induced shift in predation pressure from chum salmon to pink salmon could offset negative direct impacts of sea lice on chum salmon. This shift in predation is proposed to occur because predators show an innate preference for pink salmon prey. This preference may be more easily expressed when sea lice compromise juvenile salmon hosts, making them easier to catch. Our results indicate how the ecological context of host-parasite interactions may dampen, or even reverse, the expected impact of parasites on host populations.
authors.affiliation: Department of Biological Sciences, University of Alberta, , Edmonton, Alberta, Canada , T6G 2E9, School of Resource and Environmental Management, Simon Fraser University, , Burnaby, British Columbia, Canada , V5A 1S6, ESSA Technologies Ltd, , Vancouver, British Columbia, Canada , V6H 3H4, Department of Zoology, University of Otago, , Dunedin 9016, New Zealand, Salmon Coast Field Station, , Simoom Sound, British Columbia, Canada , V0P 1S0, Department of Ecology and Evolutionary Biology, University of Toronto, , Toronto, Ontario, Canada , M5S 3B2, Fisheries and Oceans Canada, Pacific Biological Station, , Nanaimo, British Columbia, Canada , V9T 6N7, Department of Mathematical and Statistical Sciences, University of Alberta, , Edmonton, Alberta, Canada , T6G 2G1.
authors.forename: Stephanie J
authors.initials: SJ
authors.lastname: Peacock
authors.forename: Brendan M
authors.initials: BM
authors.lastname: Connors
authors.forename: Martin
authors.initials: M
authors.lastname: Krkosek
authors.forename: James R
authors.initials: JR
authors.lastname: Irvine
authors.forename: Mark A
authors.initials: MA
authors.lastname: Lewis
mesh_terms: D000818:Animals; D001955:British Columbia; D033342:Copepoda / Q000502:physiology*; D005398:Fisheries / Q000706:statistics & numerical data; D020387:Food Chain; D006790:Host-Parasite Interactions / Q000502:physiology*; D016013:Likelihood Functions; D015233:Models, Statistical; D009026:Mortality; D017683:Oncorhynchus / Q000469:parasitology*; D011235:Predatory Behavior / Q000502:physiology; D013045:Species Specificity
keywords: functional response; model; parasite; predation; salmon; sea lice