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Trout swim in an irrigation diversion ditch along Rattlesnake Creek.

Photo courtesy Rob Roberts

Unlike dogs, trout don’t make healthy mutts.

Don’t expect hybrid vigor when rainbow trout interbreed with cutthroats in Montana’s high mountain streams. Despite the rainbow’s success as the most widely distributed game fish in the world, and the cutthroat’s remarkable ability to thrive through wildfires and landslides, their co-mingled offspring tend to be too dumb to live long.

That fact leaps out of analysis on one of the largest genetic data sets anywhere of Rocky Mountain cutthroat trout at the University of Montana’s Conservation Genetics Lab. In a recently published paper, the researchers looked at what happened to native trout after decades of artificial stocking in lakes and rivers.

“Climate change isn’t going to get these populations in the long run,” said lead study author Clint Muhlfeld of the U.S. Geological Survey.

“It’s immediate things that humans have done. We thought that if we provided cold water, the cutthroats were protected – we could prevent invasive trout from hybridizing. But our data clearly show cold water won’t prevent rainbow trout invasion," he said. "Over 60 percent of our hybridized sites were in cold waters. And those are just little time bombs waiting for the right environmental conditions.”

Here’s the problem: Cutthroat trout spent millennia evolving traits that help them live in Rocky Mountain waterways. Like landlocked salmon, they swim from lakes, up rivers to the same little creeks where they were born to lay the next generation of eggs. They time that spawning to happen right after spring runoff, when the creek bottoms have been freshly scoured by floods of new gravel and nutrients from the melting snowpack.

When catastrophes like wildfire-triggered landslides dump tons of silt into those streams, cutthroat respond with a breeding frenzy to compensate.

Rainbows, in contrast, don’t care where they spawn. And they lay their eggs just before runoff, making them vulnerable to obliteration in the floods. They benefit from the fact humans stocked them by the millions in same systems cutthroats depend on. And they’re genetically capable of interbreeding with the native trout.

“Cutthroats have been here since the last glacial period,” Muhlfeld said. “They’ve proven themselves. Rainbows have only been here since the early 1900s, and they came from hatcheries. They’re not facing flooding or wildfires or droughts. You put them out in the wild, and they don’t survive as well.”

Put them in the gene pool, and the problem spreads. The study found that adding 20 percent rainbow genetic material to a cutthroat’s DNA cut its reproductive success in half. The resulting “cut-bow” mutt fish dwindle over several generations and disappear.

Thanks to 20th century efforts to plant about 2 million rainbow trout in Montana lakes and rivers, many places have reached such a level of hybridization that little can reverse the trend. Montana Fish, Wildlife and Park fish biologist Ladd Knotek said some places get managed for sport fishing conditions, while drainages with genetically pure cutthroat trout get special attention.

“Around here, we have about 30 pure cutthroat populations isolated that can’t be hybridized,” Knotek said. “We strategically leave barriers in place to protect the purity of those fish.”

The problem isn’t rivers like the Blackfoot or Flathead, where cut-bows mingle with brown, brook, rainbow and brook trout. It’s in the isolated mountain creeks where purebred cutthroats have their genetic strongholds. Too many rainbows find their way up there, dilute the cutthroats’ survival traits, and provide nothing in return.

We know this thanks to new DNA analysis techniques pioneered in Missoula and applied to a massive database of fish genetics compiled over decades by local biologists.

The research brought together a generation of fish experts, including the University of Montana’s conservation biology leader Fred Allendorf, David Schmetterling of Montana Fish, Wildlife and Parks, Ryan Kovach of the Northern Rocky Mountain Science Center in West Glacier, Diane Whited at the Flathead Lake Biological Station and 10 others.

“A lot of hybridization studies focus on small-scale, single-drainage projects,” Muhlfeld said. “This one looked at 17 river basins all across Montana, in all three continental drainages – the Missouri, the Upper Columbia and the Saskatchewan. And because it’s a long-term genetic monitoring program that the state’s been doing for decades, we can look at patterns across not only a broad geographic area, but through time as well. And that’s where you see consistent patterns emerge.”

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Natural Resources & Environment Reporter

Natural Resources Reporter for The Missoulian.