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Rainbow and Cutthroat Trout Hybridization

Posted date: January 13, 2016

Long-held beliefs that pure trout species would suffer extinction from hybridization are being re-evaluated


Missoula, Mont., Jan. 12, 2016 – Scientists at the U.S. Forest Service Rocky Mountain Research Station and National Genomics Center for Wildlife and Fish Conservation announced today in a new paper in the journal Ecology and Evolution that pure cutthroat trout populations are widespread and in fact, there are more pure fish that originally thought.

Speculation about the decline of pure trout due to hybridization has been around since the 1950s. Anglers and scientists both thought their days of seeing pure westslope cutthroat trout in the Northwest were limited. The thought was that eventually, the trout would mate with introduced rainbow trout and create hybrid swarms, or groups of hybridized fish that continue to breed with each other until all fish are hybrids. In Montana alone, nearly 400 million rainbow trout have been stocked over the last century, thus the prospects for finding pure westslope cutthroat trout seemed poor. Westslope cutthroat trout are a prized native sportfish in the Northern Rockies and the state fish of Montana and Idaho and have previously been petitioned for listing under the Endangered Species Act.

Westslope cutthroat trout. Photo: National Forest Foundation
Westslope cutthroat trout. Photo: National Forest Foundation

Using genetic analyses of markers known as “single nucleotide polymorphisms,” the scientists were able to study hybridization in much greater detail than previous analyses.  Analyzing 3,865 fish from 129 streams in Montana and Idaho the scientists found that greater than 70 percent of fish sampled showed no signs of hybridization. “This is a better situation than anyone concerned or interested in these fish could have hoped for,” said lead author, Kevin McKelvey, a research scientist. “We found that most of the fish in the streams we sampled were not hybrids.” The scientists have shown systematically where pure native populations exist and where hybridization is a problem.

Why are hybrid trout not as widespread as so many people had predicted for decades? “That is not currently resolved, but most likely it is linked to environmental conditions and we will continue research to help address this question” said McKelvey.

Related Publications

McKelvey, Kevin S. ; Young, Michael K. ; Wilcox, Taylor M. ; Bingham, Daniel M. ; Pilgrim, Kristine L. ; Schwartz, Michael K. , 2016

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