Invasive species are one of the greatest threats to global biodiversity (Vitousek et al. 1997
). One of the most pernicious threats to native species is hybridization and genetic introgression with exotic taxa (Rhymer & Simberloff 1996
). These irreversible changes have contributed to the demise and extinction of many populations and species of plants and animals. Furthermore, hybridization is exacerbated by habitat modification (Allendorf et al. 2001
), so this problem will probably become more serious with increasing land use and global climate change. Consequently, effective conservation of native biota can be enhanced by an understanding of the ecological and evolutionary consequences of human-mediated hybridization. Such data for endangered and rare species in nature, however, are limited.
Hybridization is more common in fishes than in any other vertebrate taxa (Leary et al. 1995
). Many fish species have external fertilization and similar mating behaviours, facilitating interbreeding. In undisturbed ecosystems, reproductive isolation is maintained primarily by spatial and temporal reproductive isolation rather than well-developed pre- and post-mating isolating mechanisms (Mayr 1963
). However, such barriers to interbreeding may be lost because of habitat disturbance and introduction of non-native species (Allendorf et al. 2001
). This is particularly true for salmonids, where widespread introgression among native and non-native taxa has replaced native species over large areas throughout their native ranges (Leary et al. 1995
). Rainbow trout (Oncorhynchus mykiss
), the most widely introduced salmonid in the world (Lever 1996
), produces fertile offspring when crossed with cutthroat trout (Oncorhynchus clarkii
), and introgression often continues until a hybrid swarm is formed and the native cutthroat genomes are lost (Allendorf & Leary 1988
). A major consequence of such interspecific hybridization may be outbreeding depression due to the break-up of co-adapted gene complexes and disruption of local adaptations (Barton & Hewitt 1989
). Introgression poses a serious threat to all 14 subspecies of cutthroat trout in western North America due to widespread stocking and invasion of non-native trout into historical cutthroat trout habitats (Trotter 2008
); two subspecies are now extinct, five are listed as threatened under the Endangered Species Act (ESA) and seven have been petitioned for listing.
Interbreeding between westslope cutthroat trout (Oncorhynchus clarkii lewisi
) with non-native rainbow trout exemplifies the conservation challenges of interspecific hybridization. Westslope cutthroat trout historically occupied aquatic habitats throughout the Columbia, Fraser, Missouri and Hudson Bay drainages of the United States and Canada. However, non-hybridized populations now persist in less than 10 per
cent of their historic range, with many restricted to small, fragmented headwater habitats, where their long-term sustainability is uncertain (Trotter 2008
). Distinguishing between hybrid and non-hybridized cutthroat trout based on morphology is difficult especially when individual fishes contain low levels of rainbow trout admixture. Owing to this morphological similarity, westslope cutthroat trout populations with less than 20 per cent non-native admixture have been included as part of the native taxon in a recent status review (Department of the Interior (DOI) 2003
). The decision to use the 20 per cent threshold continues to be controversial because the potential fitness effects of this amount of admixture are unknown, and the threshold used will influence the decision to list by determining the number of populations classified as part of the native taxon (Haig & Allendorf 2006
). Consequently, there is a need to better understand how introgression affects fitness. Here, we describe how a wide range of levels of genetic admixture affect reproductive success of a native trout in the wild.