A state biologist has pinpointed Lake Helena as the source of two walleye illegally introduced to Swan Lake in 2015.

The fish’s discovery that year prompted an aggressive response from state regulators, who view the non-native, highly predaceous fish as a serious threat to already-strained local fish, like bull trout and kokanee salmon.

In addition to imposing a “mandatory kill” order for all walleye caught in the lake, Montana Fish, Wildlife and Parks is also searching for the individual or group who introduced them. By identifying their origin, fisheries biologist Sam Bourret has moved that investigation along.

“Identifying the source of where these fish came from ... brings us one step closer to finding the person or persons who did this,” the agency’s Regional Information and Education Program Officer, Dillon Tabish, told the Daily Inter Lake.

So far, the agency’s work has focused on the fish’s inner ear bones, the otoliths. Like tree rings, these bones add new layers as the fish grow.

Analysis of these layers has already revealed that the fish were introduced in spring 2015. By examining them for traces of water chemistry, Bourret determined that they had originated from Lake Helena, about 10 miles from the state capital.

“That helps us whittle it down from all the fisheries in the state of Montana,” Tabish explained.

But while cutting-edge science has narrowed the investigation’s focus, officials are still calling on Montanans to come forward with information.

“We need help from local citizens to catch those responsible for the illegal introduction,” FWP Region One Fisheries Manager Mark Deleray said in a press release.

The state offers rewards up to $15,250 for information about illegal fish introductions. In addition, Montana Trout Unlimited has pledged to reward $20,000 for a tip that results in the conviction of the individual or individuals responsible for illegally introducing the walleye into Swan Lake.

The latter group has no patience for anglers who risk major ecological damage by introducing a few non-native fish.

“These self-styled, Johnnie Appleseed fish-spreaders are out there, and they’re wreaking havoc,” Bruce Farling said last year, when he was the group’s executive director. “It’s damaging to fishermen in the state, and it’s damaging to the economy that’s based on fishing.”

Anyone with possible information on who introduced walleye into Swan Lake is encouraged to call 1-800-TIP MONT. Callers do not need to identify themselves and may be eligible for the cash reward.

Reporter Patrick Reilly can be reached at preilly@dailyinterlake.com, or at 758-4407.