Two illegally introduced walleye were found in Upper Thompson Lake last week, marking the first documented detection of the non-native predator in Lincoln County.
The female walleye were discovered on Oct. 8 during a routine gill net survey, where Montana Fish, Wildlife and Parks biologists put out nets in waters across the state to learn more about the fisheries’ populations. The walleye, each of which was found in a separate net, were 18 and 21 inches long.
“The walleye are problematic for the fishery; we don’t have walleyes in lakes and rivers in western Montana,” said Dillon Tabish, a spokesperson for FWP’s Region 1. “The closest is in Noxon Reservoir on the Clark Fork River, and we’re working to try to keep them from spreading.”
Adult walleye are considered top predators, meaning humans are the only threat to their existence. Young walleye and their eggs, however, are susceptible to being eaten by other fish.
Tabish noted that the other highly sought after fish in Upper Thompson Lake are rainbow trout, northern pike, yellow perch and large-mouth bass, all of which are non-native species.
“But we don’t need to add more non-native fish to the mix,” he added.
Walleye spawn in the spring, and Tabish said he didn’t know how long the walleye were in Upper Thompson Lake.
Walleye permanently upset the balance of Canyon Ferry Reservoir on the Missouri River after they were illegally planted and discovered in the late 1990s. FWP tried to remove them by netting around known spawning beds, but that effort failed. Eventually, the voracious walleye made significant dents in the perch and trout populations, and FWP was forced to manage Canyon Ferry, Holter and Hauser reservoirs as walleye fisheries now.
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“This is just the latest case of illegal introductions in this region,” Tabish said. “We try every way we can to get the message to anglers that this causes a lot of problems and is narrow sighted. It hurts the fishery and raises costs when biologists have to pursue this instead of doing their regular jobs.”
The state agency plans to investigate the discovery of walleye in Upper Thompson Lake, including additional surveys to gauge the potential distribution and population size.
Tabish said they’ll also collect otoliths — an inner ear bone — fin clips and scales from the two walleye for additional research. The otoliths contain unique identifiers that show where a fish was born, and the scales and fins will help biologists determine the ages of the walleye.
After walleye were discovered in Swan Lake in 2015, testing of the otoliths showed they were brought over from Lake Helena.
FWP hasn’t determined a course of action. Tabish expects his office will present the situation to the Fish and Wildlife Commission at its Thursday meeting to consider an emergency mandatory kill regulation for any walleye caught on Upper Thompson Lake.
“That’s what we did when walleye were discovered in Swan Lake,” Tabish said. “If you caught one you had to kill it and bring it to our office so we could collect samples. We did that on Lake Mary Ronan for pike too, so that’s most likely our plan of action.”
After the mandatory kill regulation on Swan Lake was implemented, no additional walleye were reported, Tabish said.