News & Features

Illegally Introduced Walleyes Discovered in Upper Thompson Lake

Illegal introductions can wreak havoc on fisheries, recreational opportunity

Montana Fish, Wildlife and Parks biologists found two walleyes in Upper Thompson Lake earlier this month during a routine fisheries survey, signaling the first documented detection of the predacious nonnative fish in Lincoln County and marking anther illegal introduction in Northwest Montana.

FWP staff set a pair of nets Oct. 8 in the upper section of Upper Thompson Lake, a popular fishery that is part of the Thompson Chain of Lakes west of Marion along U.S. Highway 2. Each net caught a single walleye. Both fish were female and measured 18 and 21 inches, respectively.

FWP will begin an initial investigation, including follow-up surveys to understand the potential walleye distribution and population size.

Biologists collected otoliths (ear bones), fin clips and scales from the two fish for additional research. Otolith analysis will be performed to determine whether the captured walleyes were born in the lake.

Anglers cannot move any live fish from the water in which the fish are caught. Illegal introductions can have significantly negative impacts on lakes and rivers. They can often lead to fewer recreational fishing opportunities for native fish, as well as collapsing fisheries and altered food webs.

Anyone with possible information on the walleyes in Upper Thompson Lake is encouraged to call 1-800-TIP MONT. Callers do not need to identify themselves and may be eligible for a cash reward. Anglers should report additional sightings of walleye to FWP at (406) 752-5501.

Moving live fish from one body of water to another is a crime. There are important reasons for this law, FWP officials say, including: the proclivity of introduced fish to compete with native or established species; introduced fish may behave differently in a new habitat; introduced fish may hybridize with established species; introduced fish may carry and spread new diseases and parasites; and introduced fish may alter the existing habitat.

Illegal introductions can also raise management costs by requiring planting more or larger fish or even chemical rehabilitation to maintain or restore the fishery. The result is less fishing opportunity and higher costs for anglers.