Project aims to expand range of recently discovered Westslope cutthroat trout population in Elk Creek
By Micah Drew DAILY MONTANAN
Montana Fish, Wildlife and Parks is launching a conservation project next week to protect and expand a recently discovered population of native westslope cutthroat trout in the headwaters of a Madison River tributary.
Elk Creek stretches more than a dozen miles into the foothills southwest of Gallatin Valley. In upper reaches near the creek’s headwaters, a small population of native westslope cutthroat trout population lives in a stretch of river of approximately 0.8 miles.
The project aims to restore cutthroats to roughly eight miles of stream by removing nonnative brook trout during the next year.
“This project will help ensure the survival and continued genetic diversity of native westslope cutthroat trout in this Madison River tributary,” Jenna Sirmon, FWP’s lead fisheries staff member on the Elk Creek efforts, said in a press release.
The species of trout is listed as a “species of special concern” by the state of Montana. Westslope cutthroat historically occupied more than 1,461 miles of tributary streams and rivers in the Madison River basin, according to FWP, but today there are only 22 populations in small headwater tributaries — covering just 14% of their historic range.
Westslope cutthroat have declined due to competition and hybridization with non-native fish, such as brook trout. The small population in Elk Creek has been threatened from the introduction of brook trout downstream from an FWP fishery.
According to FWP’s Westslope Cutthroat Trout Conservation Strategy for the Missouri River Headwaters of Southwest Montana, the long-term goal for WCT conservation is to restore protected conservation populations of WCT to 20% of their historic tributary distribution east of the Continental Divide.
Under the restoration project’s decision notice, released this spring, FWP personnel will remove brook trout located upstream of a constructed fish barrier using rotenone, a plant-derived fish toxin. The treatment, lasting 3-5 days, will be repeated within a year, after which environmental DNA sampling will verify whether all nonnative fish in the stream were successfully removed. If so, the isolated population of westslope cutthroat, which will be collected by fisheries staff prior to treatment, would be returned to the stream and able to occupy roughly eight miles of stream.
According to the environmental assessment of the project, rotenone inhibits oxygen transfer at the cellular level and is effective on fish due to its rapid absorption into the blood stream through the gills. The treatment is not known to pose a threat to terrestrial and avian animals that consume treated waters or dead fish.
After treatment, cutthroat in Elk Creek would be monitored during the next five to 10 years to document their expansion in the drainage. Experience from other projects indicates that salvaged WCT released back to the stream following non-native trout removals will fill the available habitat within three to six years, according to FWP.
Downstream of the fish barrier on Elk Creek, FWP will continue to manage nonnative species, such as brook trout.
“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,” Mike Duncan, FWP’s fisheries program manager in southwestern Montana, said in a press release.
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.
