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Analysis Reveals Origin of Illegally Introduced Walleyes in Upper Thompson Lake

Fishing - Region 1

Tuesday, March 24, 2020

Kalispell, MT — An analysis of two walleyes found last year in Upper Thompson Lake confirmed the non-native fish were illegally introduced on separate occasions from the Lower Clark Fork reservoir system.

Montana Fish, Wildlife & Parks biologists received the results of an otolith microchemistry analysis of the walleyes that revealed both fish were first-generation illegal introductions and were not born in Upper Thompson Lake, a popular fishery that is part of the Thompson Chain of Lakes west of Marion along U.S. Highway 2.

The analysis indicates both fish likely originated from Noxon Reservoir near Trout Creek and were illegally introduced on two separate occasions during the summers of 2015 and 2018.

“This is concerning because it was not a one-time attempt to illegally introduce walleye in this lake. This is an ongoing problem and threat to a prized fishery,” Brian Stephens, FWP fisheries biologist, said.

Any walleye caught on Upper, Middle, and Lower Thompson lakes must be killed immediately, kept and the entire fish turned into FWP. Anglers must report the catch within 24 hours to (406) 752-5501.

Anyone with possible information on the walleyes in Upper Thompson Lake are encouraged to call 1-800-TIP MONT. Callers do not need to identify themselves and may be eligible for a cash reward.

FWP biologists discovered the two walleyes in October 2019 during a routine fisheries survey. It was the first documented detection of the predacious non-native fish in Lincoln County. Both fish were female and measured 18 and 21 inches, respectively.

Moving live fish from one body of water to another is a crime. There are important reasons for this law:

  • Introduced fish may compete with native or already established species.
  • Introduced fish may behave differently in a new habitat -- they may not improve and are likely to harm the fishery.
  • Introduced fish may hybridize (interbreed) with established species.
  • Introduced fish may carry and spread new diseases and parasites.
  • Introduced fish may alter the existing habitat.
  • Illegal introductions can 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.

For more information, visit fwp.mt.gov.