Scientific analysis determined that two walleyes discovered in Upper Thompson Lake in October were illegally introduced on separate occasions from the Lower Clark Fork reservoir system.

Montana Fish, Wildlife and Parks said the non-native walleyes, described as “predacious,” probably originated from the Noxon Reservoir near Trout Creek and had been illegally introduced during the summers of 2015 and 2018.

Biologists discovered the fish during a routine fisheries survey. Both were female. One measured 18 inches and the other was 21 inches. FWP officials said it was the first discovery of walleyes in Lincoln County.

The agency said walleyes prey vigorously on smaller fish and pose a risk to other fish species in Upper Thompson Lake, including brook and cutthroat trout, perch and bass.

Dillon Tabish, a Region 1 spokesman for FWP, said there’s no indication to date that a population of walleyes has been established in Upper Thompson Lake.

But he said biologists will continue to survey the lake this spring to search for additional fish.

In a news release, Brian Stephens, a FWP fisheries biologist, expressed concerns about the introduction of walleyes in Upper Thompson Lake.

“This is an ongoing problem and threat to a prized fishery,” Stephens said.

Upper Thompson Lake is part of the Thompson Chain of Lakes along U.S. Highway 2 west of Marion.

FWP said any walleye caught on Upper, Middle or Lower Thompson lakes “must be killed immediately, kept and the entire fish turned into FWP.” The agency said anglers must report the catch within 24 hours to 406-752-5501.

FWP said anyone with information about the planting of walleyes in Upper Thompson Lake is encouraged to call 1-800-TIP MONT.

The agency said moving live fish from one body of water to another is a crime. It noted that introducing fish to new waters can create a host of problems, ranging from damage to existing species to hybridization to spreading disease and parasites.