Walleyes have been illegally introduced into Swan Lake, prompting fears by state officials that a population of the invasive species could establish itself in the popular fishing spot.

Mark Deleray, the regional fisheries manager, said Tuesday that during the agency’s fall netting program to remove lake trout from Swan Lake, two 17-inch immature male walleye were caught during the last two weeks.

“The next thing will be to understand what they’re doing, where they’re spawning, how successful they may be and how many are out there,” Deleray said. “If they are successful in reproducing, it would be really hard to eradicate them. We don’t have any tools specific to walleye where we could just remove them from the lake, and that’s very unfortunate.”

State fisheries biologists will study the earbones of the captured fish to try to determine how long they were in the lake, he added.

The walleye, a popular sport fish native to the Missouri and Mississippi drainages east of Montana, has been relatively uncommon in Western Montana, with most introductions happening east of the Divide.

Deleray said the only reproducing walleye population in this part of the state has lived in Noxon Reservoir for about 20 years.

Hopefully, Deleray said, if there are more walleyes in Swan Lake, they won’t successfully establish themselves.

Walleyes previously had been found in Lake Five, but that population ultimately died off before getting established.

If more walleyes are detected in Swan Lake, Deleray said removal actions likely will include limitless walleye fishing in the lake.

“In the smaller lakes, where we have tools like toxicants, you could potentially remove them from the water,” he said. “In a larger lake, a larger system like Swan Lake, that’s not really an option.”

In the short term, the agency is offering a $15,000 reward for any information that leads to conviction of the person or people responsible for the introduction.

“These illegal actions can be very long-lasting, and it can be very expensive for Fish, Wildlife and Parks and the public to deal with,” he said. “The worst-case scenario is they do well and expand. The way that works is the new fish become part of the system, and they take away from other species.”

Swan Lake already has a problem with lake trout pushing out kokanee salmon and bull trout, and the state agency has been using a netting program for the past eight years to try to reverse some of the gains that introduced population has made.

Introducing non-native species is punishable under Montana law by up to a $10,000 fine and the loss of fishing and hunting privileges in the state.

Fish, Wildlife and Parks is asking anyone with information to contact the state using the anonymous hotline, 1-800-TIPMONT.

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com.