Biology Letters
Restricted accessConservation biology

Hybridization rapidly reduces fitness of a native trout in the wild

Clint C. Muhlfeld

Clint C. Muhlfeld

US Geological Survey, Northern Rocky Mountain Science CenterGlacier National Park, West Glacier, MT 59936, USA

Department of Ecology, Montana State UniversityBozeman, MT 59717, USA

[email protected]

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,
Steven T. Kalinowski

Steven T. Kalinowski

Department of Ecology, Montana State UniversityBozeman, MT 59717, USA

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,
Thomas E. McMahon

Thomas E. McMahon

Department of Ecology, Montana State UniversityBozeman, MT 59717, USA

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Mark L. Taper

Mark L. Taper

Department of Ecology, Montana State UniversityBozeman, MT 59717, USA

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Sally Painter

Sally Painter

Conservation Genetics Laboratory, University of MontanaMissoula, MT 59812, USA

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Robb F. Leary

Robb F. Leary

Montana Fish, Wildlife and Parks, University of MontanaMissoula, MT 59812, USA

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and
Fred W. Allendorf

Fred W. Allendorf

Conservation Genetics Laboratory, University of MontanaMissoula, MT 59812, USA

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    Human-mediated hybridization is a leading cause of biodiversity loss worldwide. How hybridization affects fitness and what level of hybridization is permissible pose difficult conservation questions with little empirical information to guide policy and management decisions. This is particularly true for salmonids, where widespread introgression among non-native and native taxa has often created hybrid swarms over extensive geographical areas resulting in genomic extinction. Here, we used parentage analysis with multilocus microsatellite markers to measure how varying levels of genetic introgression with non-native rainbow trout (Oncorhynchus mykiss) affect reproductive success (number of offspring per adult) of native westslope cutthroat trout (Oncorhynchus clarkii lewisi) in the wild. Small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 per cent, with only 20 per cent admixture. Despite apparent fitness costs, our data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and high reproductive success of a few males with high levels of admixture. This outbreeding depression suggests that even low levels of admixture may have negative effects on fitness in the wild and that policies protecting hybridized populations may need reconsideration.

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