While crossbreeding with other fish remains the biggest risk to Montana’s westslope cutthroat trout, new DNA testing shows the populations may be tougher than expected.
“If you care about pure westslope cutthroat, there are a lot of them out there and a lot that don’t show any sign of hybridization,” said Kevin McKelvey, lead author of a study from the Rocky Mountain Research Center’s National Genomics Center for Wildlife and Fish Conservation in Missoula. “The majority of the high portions of basins we looked at are pure, unhybridized cutthroat trout. Even areas farther downstream still have pure populations. That’s something that’s good to know.”
During the 20th century, almost 400 million hatchery-raised rainbow trout were stocked in Montana lakes and streams. Those nonnative fish have the capability of interbreeding with native cutthroat trout. The resulting hybrids tend to be less vigorous and may lose some survival traits that natives depend upon. For example, a recent study of trout in the wildfire-scarred drainages of the Bitterroot Mountains found that silt from eroding hillsides crushed populations of rainbow and brook trout, but triggered a reproductive surge in cutthroats.
Over the years, rainbows and brown trout took over the rivers and lakes where cutthroat used to spend part of their development. That has left smaller, higher-altitude streams as the cutthroat stronghold. The loss of habitat has been serious enough to raise discussion about putting cutthroats under federal Endangered Species Act protection.
The paper was published in the journal Ecology and Evolution with co-authors Michael Young, Taylor Wilcox, Daniel Bingham, Kristine Pilgrim and Michael Schwartz. It looked at fin clips from 3,865 fish caught in 188 locations on 129 streams in Montana and northern Idaho. While only a third of those sites harbored only cutthroats, the biologists found little sign of hybridization even in places where rainbows were present.
Unlike past surveys that could look at five to eight DNA species markers, the Genomics Center’s new test uses 86 markers. That gave the researchers a much more fine-grained look at each fish’s parentage and evidence of cross-breeding. Nevertheless, they found more than 70 percent of the cutthroats showed no signs of hybridization.
That’s good news for Montana state fisheries managers, who’ve been trying a variety of ways to support native cutthroat populations.
Montana Fish, Wildlife and Parks Region 2 fisheries manager Pat Saffel said the research could explain why some places are more receptive to genetic mixing.
“We’re trying to understand their life history and find out where hybridization is going,” Saffel said. “These genetic techniques have been progressing rapidly.”
For instance, biologists have built fish barriers near the bottom of Silver Bow Creek between Butte and Anaconda, which was recently restored from a century of toxic mine waste. Then they stocked the new waterway with native cutthroat. The barrier should keep nonnative rainbows and brown trout from moving up from their Upper Clark Fork River habitat, and reveal how well cutthroat can colonize a new place without unnatural competition.
“There are a number of pure cutthroat populations around Missoula and western Montana,” Saffel said. “But they’re a fraction of what we historically had, and hybridization is a major reason. These small populations are isolated, and this is how we’re addressing that. We’re trying to find bigger landscapes where cutthroats can live, so they’re more likely to persist into the future.”
That’s going to take a lot more time and work. The Genomics Center study involved teams of two biologists electrofishing portions of public land streams in remote parts of the Rocky Mountains. On places with lots of roads, they might sample four spots a day. Brushier country might limit that to one a day.
Meanwhile, the habitat keeps changing. Cold stream temperatures appear to give cutthroat an advantage in upper drainages, but they might lose that as mountain snowpacks continue to shrink. Whirling disease radically changed the makeup of many Montana streams, turning Missoula’s Rock Creek from a rainbow to a brown trout fishery in a couple of decades.
“With what we’re starting to know about the status of hybridization now, we can model it on landscape pretty well,” McKelvey said. “But this is just a one-off sample in time. That helps us understand: Is it growing or shrinking? In some places or other places? But we need a research structure to do that.”