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Environmental factors found to control trout hybridization

Posted date: November 10, 2016

Continuing to build the case of hope for westslope cutthroat trout

MISSOULA, Mont., November 09, 2016 - Anglers and conservationists can rest easier knowing that scientists have just identified ways to locate and preserve genetically pure westslope cutthroat trout populations into the future. A new study released today provides quantitative estimates of which specific environmental factors, such as stream size and temperature, affect the distribution of cutthroat trout and their hybrids with rainbow trout.

Scientists at the U.S. Forest Service Rocky Mountain Research Station developed a model that accurately predicts where hybrids could occur throughout the range of westslope cutthroat trout in Idaho and Montana under current and future scenarios. Those predictions are embedded in a series of downloadable user-friendly digital maps that provide managers with stream-specific information about where conservation investments may be most beneficial this century. The study used 13,315 genotyped fish from 558 sites across 55,234 km of stream to show where and how pure westslope cutthroat will survive.

Previous Forest Service studies found the long-predicted hybrid swarms of rainbow and westslope cutthroat trout are actually rare in headwater streams and that many genetically pure populations of westslope cutthroat trout continue to persist. This new study tells us why: westslope cutthroat trout and rainbow trout are ecologically segregated based on the habitats in which they can be successful. Westslope cutthroat trout do better in smaller, colder streams; and rainbow trout do better in warmer, larger streams. In intermediate habitats, hybrids are common. Proximity to a strong population of rainbow trout also affects the degree of hybridization likely to occur. The study found that hybridization is more common where both species are native (west of the continental divide) and have occurred together for thousands of years. Given that rainbow trout do better in warmer streams, it is likely their populations will expand to some degree with climate change, but many streams in the Rocky Mountains will remain very cold and continue to provide habitat for pure westslope cutthroat trout populations this century.

“The most rewarding thing about this study is that we have identified the defining factors for hybridization and therefore can point to management options for these streams that can actually influence the ability of pure westslope cutthroat to exist into the future,” said lead author Michael Young, Ph.D. Research Fisheries Biologist with the U.S. Forest Service. “For example, continued stocking of rainbow trout ensures ongoing propagule pressure by that species and can sustain hybridization at levels above those that environmental characteristics would permit. Thus, if stocking stops, hybridization should decline. Similarly, if westslope cutthroat trout can be favored, say by riparian management to increase shading and promote cooler water temperatures, or by connecting a headwater population to colder habitats, hybridization is expected to decline. In addition, the predictions of this model can be used to triage conservation efforts, focusing on areas where managers are most likely to have conservation success. This is great news for conservationists and the angling community.”

Another unique element to this study is that the scientists developed a framework for doing these sorts of studies consistently in the western United States, and it’s a framework that could apply to other species if useful and necessary. Click here for additional information on the project.

Citation: Young MK, Isaak DJ, McKelvey KS, Wilcox TM, Bingham DM, Pilgrim KL, et al. (2016) Climate, Demography, and Zoogeography Predict Introgression Thresholds in Salmonid Hybrid Zones in Rocky Mountain Streams. PLoS ONE 11(11): e0163563. doi:10.1371/journal.pone.0163563



The Rocky Mountain Research Station (RMRS) is one of seven units within the U.S. Forest Service Research and Development. RMRS maintains 14 field laboratories throughout a 12-state territory encompassing parts of the Great Basin, Southwest, Rocky Mountains, and the Great Plains. RMRS also administers and conducts research on 14 experimental forests, ranges and watersheds and maintains long-term research databases for these areas. While anchored in the geography of the West our research is global in scale. To find out more about the RMRS go to You can also follow us on Twitter at




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Jennifer Hayes
Public Affairs Specialist