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South Fork Westslope Cutthroat Trout Project

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Welcome to the South Fork Flathead Westslope Cutthroat Trout Conservation Program.The South Fork Flathead River drainage contains 355 lakes and approximately 1,898 miles of stream habitat. This drainage was isolated in 1952 by the construction of Hungry Horse Dam nearly five miles upstream of its mouth. The South Fork Flathead River, above Hungry Horse Dam, contains one of the largest genetically pure populations of native westslope cutthroat trout in the nation, and represents 50 percent of the Montana range for genetically pure, large, interconnected populations.

However, historic stocking introduced non-native trout species (primarily rainbow trout and Yellowstone cutthroat trout) into several headwater lakes. By the late 1950’s, fish managers became aware of the negative impacts that past stocking could have on the native westslope cutthroat. By the late 1980’s, genetic analysis showed that the nonnative trout were breeding with the native trout and producing hybridized offspring. These hybrid fish, originating from headwater lakes, were dispersing downstream and threatening the genetic integrity of the entire South Fork Flathead River drainage.

Of the roughly 355 lakes in the South Fork drainage, 21 were identified as being the sources of hybridized fish. It was determined that if the source populations of these hybridized fish were removed from the headwater lakes, the threat of hybridization to the genetically pure cutthroat in the river would be greatly reduced. During 2006, a proposal was adopted that would use fish toxins (piscicides) to remove the current fishery in 21 headwater lakes and replace it with genetically pure westslope cutthroat trout. During the fall of 2007, the first two lakes associated with this project were treated with the piscicide rotenone. The remaining lakes will be treated at a rate of one or two lakes per year. All of the lakes will be restocked with pure westslope cutthroat trout. See Background Information for in-depth details about this project and related documents.

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