Electronic Supplementary Material: Can reduced predation offset negative effects of sea louse parasites on chum salmon?

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  • The .zip file contains R code and data that accompanies the paper \"Can reduced predation offset negative effects of sea louse parasites on chum salmon?\". The R code includes three main files: (1) code to compile chum salmon spawner-recruit data from escapement, catch and age -at-return, (2) code to fit a Ricker population model testing for an effect of sea louse abundance on farmed or wild salmon on chum salmon productivity, and (3) code to solve a host-macroparasite model that includes the effect of predation in a multi-host system. Supporting data include escapement, catch and age-at-return data made publicly available by Fisheries and Oceans Canada. Details of the files within are given in the README.txt file. Abstract for the paper: 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 could 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.

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    Attribution-NonCommercial 3.0 International
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    • Peacock, S.J., B. Connors, M. Krkosek, J. Irvine and M.A. Lewis. Can reduced predation offset negative effects of sea louse parasites on chum salmon? Proceedings of the Royal Society B: Biological Sciences (In press).