While invasive mussel discoveries worry the state’s fisheries experts, Swan Lake provided a small ray of sunshine after Montana Fish, Wildlife and Parks’ annual lake-trout netting program was completed last month with no new walleye caught in the popular fishery.

That wasn’t the case a year ago, when the annual netting program — implemented to conserve the lake’s threatened bull trout population by removing non-native lake trout — turned up a pair of walleyes. Subsequent testing determined the potentially destructive species was the result of an illegal introduction rather than an established population.

“It is too early to say that there is not potentially a problem — the glass-half-full, though, is that we have not seen any evidence of reproduction to date,” Mark Deleray, the agency’s regional fisheries manager said last Tuesday.

In other parts of the state, such as Noxon and Canyon Ferry reservoirs, walleye introductions have led to flourishing populations of the large predatory fish and negatively impact established fisheries. Deleray noted that additional years of netting will be needed before he can confidently say whether Swan Lake’s kokanee salmon and bull trout populations are out of the woods.

“Assuming those fish were a year or two old, at least, when they were introduced into Swan Lake, under those assumptions we’d expect to see reproduction in the next year or two,” he said. Walleye begin reproducing at three to four years of age, which can create a lag time between introduction and when a newly population becomes noticeable.

Depending on whether more were introduced, a pair of walleyes capable of breeding could evade the ongoing netting work and recreational fishing until they begin spawning.

A “mandatory kill” regulation put into place last year requires any angler who hooks a walleye to kill it rather than placing the fish back in Swan Lake. Those who catch walleye are also required to turn the fish in to Fish, Wildlife and Parks so state scientists can determine whether it originated in the lake or elsewhere.

Deleray said no walleyes have been reported since the initial pair caught in October 2015.

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