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Biol Lett. Jun 23, 2009; 5(3): 328–331.
Published online Mar 18, 2009. doi:  10.1098/rsbl.2009.0033
PMCID: PMC2679930
Hybridization rapidly reduces fitness of a native trout in the wild
Clint C. Muhlfeld,1,2* Steven T. Kalinowski,2 Thomas E. McMahon,2 Mark L. Taper,2 Sally Painter,3 Robb F. Leary,4 and Fred W. Allendorf3
1US Geological Survey, Northern Rocky Mountain Science Center, Glacier National Park, West Glacier, MT 59936, USA
2Department of Ecology, Montana State University, Bozeman, MT 59717, USA
3Conservation Genetics Laboratory, University of Montana, Missoula, MT 59812, USA
4Montana Fish, Wildlife and Parks, University of Montana, Missoula, MT 59812, USA
*Author for correspondence (cmuhlfeld/at/
Received January 19, 2009; Accepted February 20, 2009.
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.
Keywords: fitness, hybridization, introgression, invasive species, reproductive success, cutthroat trout
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