Montana Fish, Wildlife and Parks is proposing a “mandatory kill” regulation for walleyes in Swan Lake after two of the invasive fish were found to have been illegally introduced into the popular fishery.

The state Fish and Wildlife Commission will decide whether to add the new rule to the 2016 fishing regulations at its Dec. 10 meeting in Helena.

“This is a very aggressive approach that we’re taking,” Mark Deleray, the agency’s regional fisheries manager, said Monday. “The only more aggressive regulation would be to close the lake to fishing entirely, and we decided not to pursue that at this time. We’re throwing everything at it.”

Two walleyes were discovered in the lake in October as Fish, Wildlife and Parks wrapped up its annual gill netting of invasive lake trout at Swan Lake.

While the walleyes do not represent the first such introduction in the region, state biologists are concerned because the large predatory fish have a reputation for taking over and displacing other species once they get into a body of water.

Deleray said it’s the first time a mandatory kill has been proposed in Northwest Montana. Under the regulation, anglers who catch a walleye would be required to kill it immediately, report the catch within 24 hours and turn the whole fish in to the state agency within 10 days.

“We didn’t want to provide a benefit to a person who introduces fish illegally by allowing them to take fish home or even producing a catch-and-release fishery,” Deleray said. “So this proposal does neither of those.”

The new regulation would take effect March 1, 2016. Fish, Wildlife and Parks is investigating the walleye introduction. Samples from the two captured walleye are being analyzed to determine whether they reproduced in the lake, but the agency doesn’t expect to get the results back for at least another month.

A reward of $25,000 is offered to anyone with information that results in a conviction of those responsible for the walleye introduction.