The South Fork westslope cutthroat trout project comes to a close later this summer when biologists from Montana Fish, Wildlife and Parks re-stock Sunburst Lake in the Bob Marshall Wilderness with westslope cutthroat trout that are native to the drainage.

The remote lake was the largest lake in the 21-lake project, designed to rid the lakes of hybrid rainbow-cutthroat, rainbow trout and Yellowstone cutthroat trout in the South Fork of the Flathead River drainage.

The lakes, which had been stocked with non-native fish decades ago, which resulted in the hybrids, were in the Bob Marshall Wilderness or upstream from the Hungry Horse Reservoir.

The concern was that the non-native fish from the lakes would eventually dilute the genetic integrity of the South Fork fish. Cutthroat can cross-breed with rainbow and Yellowstone cutthroat trout.

The South Fork of the Flathead River is one of the few remaining pure westslope cutthroat trout drainages in the U.S.

Last fall, crews from FWP used rotenone, a fish poison, to kill the fish in Sunburst, which is an idyllic lake at the head of Gorge Creek at the base of Swan Mountain.

FWP biologist Matt Boyer said the effort went well.

“Sunburst was the biggest lake, in terms of volume,” he said.

He said the stocking this spring would come after the lake thaws out, which would likely be July, as the lake is in a high election basin.

The lake will be restocked with fingerlings raised at the FWP hatchery at Sekokoni Springs near Blankenship. Those fish are from stocks that come from the headwater streams of the South Fork and are genetically pure.

Boyer said it will probably take a few years for the trout to reach a catchable size, though they should grow quickly.

The project started in 2007 and biologist have done one or two lakes a year since then. Six lakes received a “swamping treatment” where large numbers of pure westslope trout are introduced into a drainage. As they breed, they squeeze out of the non-native genes in the population.

Glacier National Park and Waterton Lakes National Park in Canada are also considering using similar techniques to rid lakes of non-native species. Glacier is finalizing its fisheries management plan, which will likely include similar efforts to the South Fork project. Waterton also has lots of opportunities and plenty of cold water, Boyer noted.

FWP could act as a consultant with Canadians if they needed assistance, Boyer noted. Research into non-native fish is continuing. A graduate student will take a close look at lake trout populations in Swan lake this summer, and biologists are also tagging and tracking non-native fish in the mainstem Flathead River and its tributaries to better see where they’re moving and their pervasiveness in the system.