State wildlife officials plan to dose a pond in Thompson Falls with a common piscicide in a bid to rid the waterbody of nonnative goldfish.

The recent invasion of an unnamed pond near the Riverfront Motel is part of a larger trend of illegal fish introductions, said Dillon Tabish, the communication and education manager for Montana Fish, Wildlife and Parks Region 1. The state wildlife agency has documented more than 435 instances of illegally introduced fish in Northwest Montana alone. Commonly introduced species include popular game fish like northern pike and smallmouth bass, as well as aquarium fish like koi.  

In the past 20 years, state fisheries biologists have identified seven instances in which goldfish were illegally introduced to waterways in Northwest Montana. 

“In general, in these situations, there are lots of possibilities,” said Tabish. “Folks don’t want to maintain them anymore in an aquarium, but don’t want to kill them. Someone likes goldfish. Some may think they want to sell them. None of these introductions are legal.” 

Goldfish are quick breeders and skilled survivalists, making them a formidable threat to many North American waterways. The East Asian natives gobble up aquatic plants, fish eggs and fish larvae, challenging native fish species through competition and predation. Their feeding activity can also make water cloudier and reduce the amount of sunlight that reaches underwater plants. 

The unnamed pond in Thompson Falls does not host a native fishery, but biologists expressed concern that the goldfish infestation could spill over to nearby waterways. While the pond is not connected to the Clark Fork River, a long culvert under the highway could provide a seasonal connection between the pond and a nearby wetland complex. 

After the goldfish infestation was discovered in August 2025, biologists scoured the pond with a drift boat and electrofishing backpacks. While they succeeded in killing 2,000 goldfish, the officials ultimately determined that a more comprehensive approach was necessary to eradicate the gilled invaders.      

An environmental assessment released on Dec. 30, 2025, proposes dosing the pond with rotenone, a commonly utilized piscicide derived from the roots of certain tropical plants. The chemical seeps into the bloodstream of fish through their gills. 

“Mammals, birds, and other non-gill breathing organisms do not have this rapid absorption route into the bloodstream and are not affected by consuming treated water or dead fish at concentrations used in fisheries management,” states the environmental assessment. 

Fisheries biologists anticipate using a solution with 5% rotenone at a dose of 3 to 4 parts per million. Preliminary tests will determine the exact dosage needed to kill the invasive goldfish. 

The chemical will remain in the pond for about a week before naturally degrading. Wildlife managers may use a secondary agent, potassium permanganate, to neutralize the rotenone if follow-up testing shows the piscicide is not degrading on its own.  

Most fish killed in the process are expected to sink to the bottom of the pond, where they will typically decay within the span of 1-2 weeks. Any dead fish that float to the top of the pond will be collected and disposed of by state fisheries officials. 

Officials may complete follow-up spot treatments if the initial dose of rotenone does not prove effective. 

A price tag is not included in the environmental assessment, but a 2021 eradication near Big Sandy cost the agency $24,618. Officials spent $3,800 that same year removing koi and goldfish from a pond near the Bitterroot River. 

Montana Fish, Wildlife and Parks anticipates the public will be barred from accessing the pond for up to 30 days during the rotenone treatment. 

Comments on the project will be accepted through Jan. 13, 2026 and may be sent to [email protected]. 

The project is one of three priority removal projects that the agency plans to present to the Montana Fish and Wildlife Commission for a vote on Feb. 12. 

Reporter Hailey Smalley can be reached at 758-4433 or [email protected].