Another lake in Bob Marshall eyed for use of fish poison

Montana officials are considering adding Koessler Lake as part of an effort to remove non-native species and restore westslope cutthroat trout in the Bob Marshall Wilderness.

If the state Fish, Wildlife and Parks Commission approves the proposal, there would be no limits on catching fish this summer before biologists treat the lake in the South Fork Flathead River drainage with the fish poison rotenone in September. In previous years, the lakes were stocked with Yellowstone cutthroat, rainbow trout and other non-native fish.

Matt Boyer, a fisheries biologist, said 12 mountain lakes have been chemically treated. Another six, he said, have had “genetic swamping,” where large numbers of westslope cutthroat trout have been added to crowd out non-native fish.

“Genetic data drives the strategy,” Boyer said. “In six of the lakes, genetic swamping showed the most promise and was an effective tool. They were three in the Jewel Basin — Upper and Lower Three Eagle lakes and Pilgrim Lake — as well as Woodward, George and Pyramid lakes in the Bob Marshall.”

Anglers at Koessler Lake are limited to three fish per day. That would be lifted if officials approve poisoning the lake in preparation for restocking with westslope cutthroat trout.

Eighteen of 21 lakes in the South Fork drainage have already been treated. Koessler, Sunburst and Handkerchief are the three remaining.

Of the lakes treated since 2007, nearly half a million westslope cutthroat trout have been stocked.

Westslope cutthroat trout are one of two subspecies of trout in Montana. The fish can serve as an indicator of the health of their ecosystem, and biologists have been keeping a close eye on the species because it has been threatened with habitat loss and hybridization with rainbow trout and Yellowstone cutthroat.