Study results show trout hybridization.

In the western US, all 14 subspecies of native cutthroat trout have declined and several are listed as threatened or been petitioned for listing. Along with habitat degradation and fragmentation, nonnative species introductions are a main factor in these declines. Hybridization can be a major consequence of exotic species introductions, and has contributed to the decline and extinction of many native fishes worldwide, including all subspecies of cutthroat trout.

Hybridization with introduced salmonids is probably the greatest threat facing westslope cutthroat trout (Oncorhynchus clarkii lewisi) (WCT). Although theoretical and empirical studies predict that hybrids between WCT and rainbow trout (O. mykiss) (RBT) will experience outbreeding depression, the resulting fitness decline and loss of local adaptations may not be readily apparent. Furthermore, the unidirectional nature of hybridization can allow introgression, genes of one species blending with another, to spread even when hybrids have reduced fitness.

Although the upper Flathead River system in Montana and British Columbia (Canada) is considered a regional and range-wide stronghold for westslope cutthroat trout, the existence of these populations is threatened by hybridization with nonnative rainbow trout. Previous studies indicate that increased straying by hybrids contributes to the spread of hybridization in the upper Flathead River. However, the genetic and ecological factors that determine invasion success and the establishment of future sources of hybridization remain poorly understood in the natural environment.

Scientists at NOROCK and their partners at the University of Montana, Montana State University, and Montana Fish, Wildlife & Parks are examining this phenomenon. In a study to identify the potential impacts of hybridization on fitness of native trout, scientists used paternity analysis with molecular DNA markers to assess the reproductive success of approximately 800 trout in Langford Creek, a stream in the Flathead drainage.

For the first time scientists evaluated how various levels of human-mediated hybridization affect fitness of native trout in the wild and found that small amounts of hybridization markedly reduced fitness of male and female trout, with reproductive success sharply declining by approximately 50 percent. Despite severe fitness penalties, however, data suggest that hybridization may spread due to relatively high reproductive success of first-generation hybrids and a few males with high levels of non-native genes. These results suggest that many inland native cutthroat trout in western North America may be at greater conservation risk than previously thought.

The long term impact of trout hybridization between native and non native trout is unknown, though scientists fear significant loss of native populations may continue to occur, resulting is the loss of unique genetic and ecological adaptations. This study has broader implications beyond cutthroat trout as many other endangered species face similar complex genetic and policy issues surrounding hybridization, such as the Scottish wildcat, red wolves, Florida panther, and Spotted Owl and the approach may be readily adapted to these species.

Currently, there is no official policy for dealing with hybrids under the Endangered Species Act. The most contentious issue has been deciding how much hybridization is permissible to maintain the evolutionary legacy of species, and these issues have been addressed on a case-by-case basis. The study results shows that fitness can quickly decline in nature when previously isolated salmonid species interbreed, and that policies protecting hybridized populations may need reconsideration.

Results from the study are currently featured in the March 2009 edition of the Biology Letters in print and online. Click here Hybridization Rapidly Reduces Fitness of Native Cutthroat Trout in the Wild to view the recently published article.

More information about this project can be found at http://nrmsc.usgs.gov/research/hybrid_trout

For more information contact:
Clint Muhlfeld
406-888-7926
cmuhlfeld@usgs.gov