Climate change has produced “moderate” to “significant” habitat fragmentation for trout in the Madison River and Green River watersheds, a study has found.

The study predicts cutthroat trout will lose ground to nonnative brook and rainbow trout in northwest Wyoming. It also forecasts the closure of more streams due to heat-stressed trout.

“Climate change is often thought of as a future abstraction, but our case histories illustrate that this is not the case,” the study says. “Stream environments across the Rocky Mountains have been changing in ways that have important implications for trout populations.”

The paper’s chief author said the study was a “retrospective analysis” or a measure of climate changes to date.

“Basically, we were pulling together data sets that already existed,” U.S. Forest Service scientist Dan Isaak said in a phone interview from Boise, Idaho.

“The Past as Prelude to the Future for Understanding 21st-Century Climate Effects on Rocky Mountain Trout” was published in Fisheries, a scientific journal. Two of the five watersheds analyzed are in Wyoming.

Even with seven authors, collecting data was difficult because monitoring equipment that was in place for decades is lacking, the biologist said.

Better monitoring is vital

“One of the hopes is that people will start to establish those monitoring programs,” Isaak said. “We haven’t typically been monitoring smaller headwater streams. We haven’t really set up good systems where people are dedicated to monitoring temperature at the same site over a long period of time.”

One exception to that is the Madison River in Yellowstone National Park. Temperatures on the Madison have been recorded throughout the year since 1977, a rarity for a stream in the West.

Isaak’s paper shows that hotter waters in the Madison have had an effect on trout. There were an average 4.6 thermally stressful days a year for trout during the 1980s, but that tripled to 15 days in the most recent decade.

In July, Yellowstone officials closed sections of the Gibbon, Firehole and Madison rivers due to high water temperatures.

“It’s probably more likely than not that they’re going to do that more and more in the future,” he said, “unless they can find a way to cool the rivers down.”

Mean annual air temperatures warmed by 0.8 degrees Celsius across the West in the 20th century. The effects of that heating on rivers and trout are far from uniform, Isaak said.

Cutthroat take it hard

“Streams around Jackson, for example, are high-elevation,” he said. “There are places where there are streams too cold, and climate change will make those streams more suitable to support a trout population.”

That trend holds true to the Upper Green River watershed, which winds its way out of the high-elevation Wind River Range.

“The upper extents of many streams across the Green River Basin are currently too cold to support recruitment of juvenile fish, and these areas could become more suitable with temperature increases,” the study says.

That aside, climate change and other human-caused disturbances have hit Green River Basin cutthroat hard, the study shows.

Colorado River cutthroat trout, the resident species there, currently occupies just 14 percent of its native range.

“Remaining populations of Colorado River cutthroat trout are highly fragmented and often inhabit only isolated headwater stream sections above natural and anthropogenic barriers that prevent upstream invasions from non-native brook trout, brown trout and rainbow trout,” the study says.

One goal of his research is to organize a broader system that can measure climate change impacts on trout, Isaak said.

“Probably the most important insight in the paper is that we need a way to think strategically across the landscape,” he said. “We need to develop a game plan that points toward the smartest places to take action.

“We’re kind of slowly moving in that direction, but we’re not that good at it yet,” Isaak said. “We don’t have good monitoring data, don’t have good hydrologic models.

“Until you have a base set of maps, it’s really hard for people to work together because we don’t have a common database we can all look at.”