The surprise discovery of walleye in Upper Thompson Lake on Oct. 8 has prompted state wildlife officials to place a catch, kill and report order on the invasive, non-native fish.
Montana Fish, Wildlife and Parks (FWP) biologists made found the walleye during a routine gill net survey. Officials set two nets in the upper section of Upper Thompson Lake and caught two female walleye—one in each net— measuring 18 inches and 21 inches.
The species is not native to Montana and had not been previous detected in the lake.
In 2015, the Fish and Wildlife Commission adopted a similar plan of action when walleye were found in Swan Lake, not far from Flathead Lake. Since enacting the mandatory kill order for Swan Lake, FWP said there have been no reports of walleye in that water body.
In an effort to uncover more details about the two unwanted walleye found in Upper Thompson, biologists extracted otoliths (ear stones), fin clips and scales to their age and origins. Walleye spawn in the spring, so officials are unsure how long walleye have taken up residence in Upper Thompson.
Mike Hensler, FWP fisheries program manager, said walleye spawning demands are similar to kokanee salmon.
“I bring that up because the Thompson Lakes, primarily Middle Thompson Lake have enough spawning habitat to sustain a wild kokanee population,” said Hensler.
It remains unclear if walleye can reproduce in the Chain of Lakes, where FWP stocks rainbow trout and kokanee.
Hensler said the number of eggs a female can produce is related to its body weight—around 30,000 eggs per pound. Between the two adult females captured last week, Hensler said they could produce as many as 150,000 eggs.
“As you can see, if a male is present, it may not take long for a population to get established,” Hensler said.
Another concern is the potential distribution of walleye. FWP officials worry that walleye can migrate between Upper, Middle and Lower Thompson lakes through a stream channel that connects the water bodies.
Hensler also highlighted an outlet stream that leaves Lower Thompson Lake, enters the Thompson River and flows into the Clark Fork River upstream of Thompson Falls Dam as another concern.
“If walleye become established in the Thompson Chain of Lakes, they could serve as a source to expand the distribution of walleye upstream of Thompson Falls Dam, including in the Flathead and upper Clark Fork Rivers,” said Hensler.
Though walleye are revered as a great sport fish, and organizations like Walleye Unlimited have argued that the fish is native to Montana, FWP currently manages walleye as an invasive species. Walleye are considered predacious and are a threat to both native and non-native species.
“They are not tied to any particular habitat so will likely use all portions of the lake and feed on whatever species is available to them,” Hensler said.
That includes kokanee salmon, trout, pumpkinseeds, pike minnows, and especially yellow perch and crayfish.
At the top of the fish food chain, adult walleye face no predators besides humans.
In the late 1990s, walleye were discovered in the Canyon Ferry Reservoir outside of Helena. This illegal introduction irrevocably changed the reservoirs existing ecosystem, forcing biologists to intervene. FWP biologists’ attempt to remove the walleye by netting near spawning beds failed, resulting in drastic dips in perch and trout populations.
With more surveys, biologists hope to determine the prevalence and origin of walleye within the Thompson Chain of Lakes.
FWP asks that anyone with possible information about the walleye in Upper Thompson Lake 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.