Climate Change and Invasive Species are Driving Native Trout Declines in Montana

Scientists from the University of Montana, the U.S. Geological Survey, and Montana Fish, Wildlife & Parks have concluded a study on climate change and its affect on five trout species in the northern Rocky Mountains. The group has deduced that the declining distribution of native trout is due to warming stream temperatures and competition from non-native trout species.

The new study, published in Science Advances, a multidisciplinary journal of the American Association for the Advancement of Science, looked at native Westslope cutthroats and bull trout in addition to invasive brookies, browns, and rainbows.

Researchers assessed how native and invasive trout populations in Montana shifted in terms of distribution over the last 30 years and offered predictions about the future. The work involved analyzing more than 30 years worth of data collected and maintained by Montana Fish, Wildlife & Parks—primarily some 22,000 data points created during electrofishing surveys in streams and rivers. The study area included 79,352 miles of stream within two major river drainages, the Columbia River and Missouri River basins.

Bull and Cutthroat Trout Are in Trouble

According to a University of Montana report: “Researchers found native bull trout and Westslope cutthroat trout occupancy—defined as the amount of stream where a species is present—declined by 18 and 6 percent, respectively, between 1993 and 2018 and are predicted to decrease by an additional 39 and 16 percent by 2080. Although invasive brook trout also were expected to decline, brown and rainbows have expanded their range due to rising water temperatures—and appear poised to prosper during future climate change.”

While climate change was identified as the likely culprit overall for the declining native trout, different drivers in the decline were identified for each species.

Bull trout, designated a threatened species under the Endangered Species Act, were called “habitat specialists” by the researchers, “requiring cold, connected, high-quality, and complex riverine habitats.” Warmer waters and lower summer water levels are degrading stream habitat, likely causing the bull trout decline.

Westslope cutthroat trout, though, are most greatly impacted by the invasive trout species. Brook trout can outcompete native cutthroats, and rainbows readily crossbreed with Westslope cutthroat trout, creating hybrids. The threat from invasive rainbow trout was called “particularly concerning” since a warming climate lets these more adaptable fish expand their range.

In the study’s discussion section, the authors noted: “Invasive rainbow trout and brown trout persisted in warmer streams with higher flow, whereas brook trout persisted in streams with cooler temperatures and relatively lower flow. Native bull trout persisted in colder streams with higher flow. In contrast, native cutthroat trout had high persistence probabilities across a wide range of temperature and flow regimes.” This resulted in an unexpected projection that cutthroats, in the absence of invasive species, could occupy more habitat at the end of the century than at present despite rapid climate change.

Donovan Bell, the study’s lead author and a doctoral candidate in UM’s Wildlife Biology Program, said, “Our two native trout species in Montana will decline in the future unless appropriate conservation action is taken. Our results suggest that tailoring conservation strategies to specific species and specific climate-change threats is important for native fish conservation.”

For example, the report states, conservation of bull trout in streams and rivers may be better aimed at protecting, reconnecting, and restoring critical cold-water habitat. On the other hand, suppression of invasive trout species likely is more effective for the conservation of Westslope cutthroat trout.

This latest study doesn’t present quite as dire a situation as earlier studies that projected substantial declines in both native and invasive trout. A broad-scale study from 2011 in the interior western United States projected marked declines in native cutthroat trout (58 percent), invasive brook trout (77 percent), brown trout (48 percent), and rainbow trout (35 percent) by 2080. These projections were made using the A1B emissions scenario, developed by the Intergovernmental Panel on Climate Change and assuming, among other things, high carbon-dioxide levels for first the half of the 21st century, followed by a gradual decrease after 2050.

This newer study predicts smaller declines in cutthroat trout (16 percent) and brook trout (15 percent), and increases in brown trout (2 percent) and rainbow trout (10 percent), with the biggest increases west of the Continental Divide (21 and 19 percent, respectively).

It’s Not Too Late to Take Action

“Globally, climate-induced changes to aquatic habitats are predicted to threaten at least one-third of freshwater fishes, and some invasive species could take advantage of such changes,” said Clint Muhlfeld, a USGS scientist and study co-author. “These scenarios seem to be playing out in our backyard with native and invasive trout.”

Andrew Whiteley, a study co-author and UM associate professor, said Montana already has lost populations of cold-adapted native fish species, and this likely will continue as climate change progresses over this century. “This is particularly troubling in a state where cold-water fisheries now contribute nearly $650 million a year to our economy,” says Whiteley, who studies fisheries and conservation genetics. “But all is not lost for these economically, ecologically, and culturally important species as long as appropriate conservation action is taken.”