State officials plan to eradicate fish in a tributary of the North Fork of the Flathead River this summer in a bid to expand the range of native westslope cutthroat trout.

Once abundant throughout western Montana, populations of westslope cutthroat trout have declined over the past century, due in large part to hybridization with introduced varieties of trout.  

A 2022 genetic study of the trout in McGinnis Creek, which flows into the North Fork of the Flathead River about 12 miles north of Columbia Falls, found that 72% of the population’s genetics could be traced back to nonnative Yellowstone cutthroat trout. The fish were likely introduced as part of generic cutthroat trout stocking efforts in the 1940s. 

Montana Fish, Wildlife and Parks are now proposing a project to dose a 4.5-mile stretch of McGinnis Creek with rotenone. The commonly used piscicide would kill the fish in the stream, clearing the way for state fisheries biologists to reintroduce genetically pure westslope cutthroat trout. 

“Projects that restore westslope cutthroat trout, such as the one proposed for McGinnis Creek, are necessary to ensure the continued survival of this native species, conserve remaining genetic diversity, meet statutory obligations to prevent listing under the Endangered Species Act, and preserve angling opportunities for this native species,” reads a March 4 environmental impact statement for the project. 

While the project is not expected to have any long-term adverse effects on the environment, the environmental analysis noted the possibility of short-term disturbances from the use of rotenone. The chemical is commonly used in fish removal projects as it is readily absorbed into the bloodstream through a fish’s gills.  

A relatively small dose of rotenone can quickly kill fish, but mammals, birds and other animals that breathe air must consume massive amounts of the chemical to see similarly fatal results. One calculation from the California Department of Fish and Game found that a 22-pound dog would have to drink 7,915 gallons of treated lake water or eat 660,000 pounds of poisoned fish within 24 hours to receive a lethal dose of rotenone.  

Other gilled animals aren’t so lucky. The environmental assessment noted that populations of aquatic invertebrates and zooplankton typically take about a year to rebound after the application of rotenone.  

Young amphibians are also susceptible to the poison. Officials stated they would apply the rotenone to McGinnis Creek on Aug. 31, after most tadpoles have metamorphosed into adults, to avoid significantly harming native species of frogs, toads and salamanders. 

The project is expected to continue for two or three days, during which time access to McGinniss Creek will be restricted.  

Rotenone breaks down easily in water, with all traces of the chemical typically disappearing within 24-48 hours of application, but officials are also proposing several failsafe systems to ensure only targeted reaches of McGinnis Creek are impacted. 

Potassium permanganate will be applied in downstream reaches of McGinnis Creek to bind to residual rotenone molecules and neutralize the chemical as it leaves the designated project area. Officials will measure residual levels of potassium permanganate and monitor so-called “sentinel fish” in cages for signs of toxicity to ensure the rotenone is fully neutralized. 

Officials plan to use environmental DNA to ascertain the success of the rotenone treatment. An additional treatment may be completed in early fall of 2027 to target any residual fish. 

Officials plan to stock the stream with non-hybridized westslope cutthroat trout from Hay Creek, another nearby tributary to the North Fork of the Flathead River. The fish will be released into McGinnis Creek in spring 2027 or 2028, depending on the results of initial environmental DNA testing. 

Montana Fish, Wildlife and Parks is collecting public comments on the proposed project through April 2. Comments may be emailed to Sam Bourret at [email protected]. 

Reporter Hailey Smalley can be reached at 406-758-4433 or [email protected]. If you value local journalism, pledge your support at dailyinterlake.com/support.