Montana Fish, Wildlife & Parks is beginning a project to protect and expand a recently discovered population of native westslope cutthroat trout in Elk Creek, a tributary to the Madison River in southwestern Montana.

Westslope cutthroat trout in Elk Creek are isolated to a small headwater section, and most of the stream is occupied by nonnative brook trout, which outcompete native fish for resources. Elk Creek and the Madison River are part of the upper Missouri River basin, where populations of nonhybridized westslope cutthroat trout are estimated to remain in less than 6 percent of their historically occupied habitat.

The project aims to expand the distribution of westslope cutthroat trout by about 8 miles and serve as an additional source of aboriginal trout for future introductions.

“This project will help ensure the survival and continued genetic diversity of native westslope cutthroat trout in this Madison River tributary,” said Jenna Sirmon, FWP’s lead fisheries staff member on the Elk Creek efforts.

“We also recognize the value of nonnative species to anglers, and FWP will continue to manage most tributaries in the Missouri headwaters for nonnative fish for recreation opportunities,” said Mike Duncan, FWP’s fisheries program manager in southwestern Montana.

FWP fisheries staff are scheduled to begin removing brook trout from the project area in Elk Creek the week of Aug. 25. This year’s phase of the project is expected to last about a week. An additional treatment phase of the project will happen in 2026, and subsequent stream sampling will help ensure the project’s success.

Brook trout will be removed from the project area using a plant-derived fish toxicant called rotenone. Rotenone is routinely used in fisheries management to remove fish species that are incompatible with management goals, and it breaks down quickly in the aquatic environment. Crews will collect westslope cutthroat trout from the project area before rotenone application and return them to the streams when the project is complete. To ensure there are no negative effects to downstream organisms outside of the project area, treated waters will be detoxified through duration of the project.

FWP published an environmental assessment for this project in early 2025 and solicited public comments. Comments received were supportive of the project.