Climate change is predicted to wreak havoc on the West’s trout populations over the next century, and one study has examined some Montana rivers to find out how.

Researchers from the U.S. Geological Survey, U.S. Forest Service and Colorado State University studied how five Western river systems have already been affected as climate change has begun to accelerate. Their recent paper provides a preview of things to come for Montana’s trout.

Predictions sometimes don’t seize the public’s attention unless specific examples can be given. Although forecasting models predict trout populations will shrink by 20 to 90 percent over the next 50 to 100 years, fishermen pay more attention when their success drops at their favorite fishing hole.

The researchers focused on rivers in Colorado, Wyoming, Idaho and two river systems in Montana: the Flathead and Greater Yellowstone basins.

Since 1950, the average air temperature above the Montana steams has increased about 0.27 degrees per decade, according to USGS records. This warms the streams, threatening native bull and cutthroat trout in the Flathead area and cutthroat trout in the Greater Yellowstone area.

Other things cause waters to warm even more. The loss of riparian habitat means less shade. Impoundments such as dams slow water, causing it to warm. Finally, the loss of flow means the remaining water warms quicker.

The study found that the annual ebb and flow of streams have changed over the past 60 years. Compared with the 1950s, spring runoff occurs earlier in the year, resulting in lower summer flows for both Montana rivers.

The Flathead system is in a pristine area where habitat is still good, although it has suffered from increasing wildfires. So the biggest problem there is loss of native trout due to hybridization, according to the study.

Native trout depend on colder water, while nonnative trout -- such as rainbow, brown or brook -- thrive in warmer water. With the whole river system warming, native trout have less optimal habitat, and invasive species are moving higher into the headwaters.

In the Yellowstone area, warmer summers have prompted fish managers to issue two fishing closures in the past decade out of concerns that catch-and-release adds additional stress for fish.

Bass are making their way farther up the Yellowstone River as the water warms.

On the Madison River, the number of days where the water temperature became dangerous for trout -- warmer than 70 degrees -- increased from six days a year in the 1980s to 15 days a year in the past decade.

The study’s authors encourage land managers to do what they can to restore healthy riparian habitat, limit impoundments and keep stream flows as high as possible.

They also encourage more long-term fish studies to monitor population declines or responses to improved habitat.

Fish, Wildlife & Parks biologists have been trying to improve habitat for several decades, said regional fisheries manager Travis Horton.

Originally they did it because studies showed restoration would be more productive than stocking, Horton said.

But they’ve recently had more incentive as they watch some fish populations flounder, such as those in the Jefferson River.

“The Jefferson has always been boom and bust depending on whether there was a drought,” Horton said. “So we’ve done a lot of work to keep more water in the system.”

Some of that work involves partnerships with landowners to keep more irrigation water in the streams, among other projects.

“The good news is that a lot of the projects that people do will help,” said Trout Unlimited scientist Seth Wegner. “Nothing exacerbates temperatures like low flow, so adding water back in can make a huge difference.”

Laura Lundquist can be reached at 582-2638 or llundquist@dailychronicle.com. Follow her on Twitter at @llundquist.

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