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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 from the mainstem Flathead River by the construction of Hungry Horse Dam in 1952. The newly created reservoir and the remaining south fork river maintain a unique assemblage of native fish such as bull trout, mountain whitefish, pygmy whitefish, westslope cutthroat trout and suckers. Within Montana, the South Fork watershed comprises more than half of the remaining interconnected populations of genetically pure westslope cutthroat trout, a species that has declined to less than 10% of its historic range due to habitat degradation, and hybridization and competition with introduced fishes. Yet, even within this cutthroat trout stronghold, historic stocking of headwater lakes and the downstream movement of nonnative rainbow and Yellowstone cutthroat trout has led to the spread of hybridization and the gradual loss of locally adapted gene pools in native westslope cutthroat populations.

To protect the legacy of this native trout and the fishery it supports, biologists with Montana Fish, Wildlife, and Parks and the US Forest Service are implementing a landscape scale conservation strategy to preserve westslope cutthroat in the South Fork Flathead watershed. The goal of this effort is to remove the sources of nonnative trout from 21 headwater lakes where hybridization is occurring and reestablish native westslope cutthroat trout populations. To achieve this goal, biologists use the piscicide rotenone to eradicate the current fishery in order to restock it with native cutthroat. The first two lakes associated with this project were treated with rotenone during the fall of 2007. The remaining lakes will be treated at a rate of one or two per year ending with Sunburst Lake in 2017. View Schedule of Lakes to be Treated 9 KB. All of the treated lakes will be restocked with pure westslope cutthroat trout. See Background Information for in-depth details about this project and related documents.

Determining the appropriate source of westslope cutthroat to use for this conservation effort is complex. The population genetic structure of this species is such that substantial genetic divergence exists between populations, even at small spatial scales. For example, large genetic differences exist between westslope cutthroat from tributaries such as Youngs, Danaher, Gordon, and Little Salmon. These genetic differences among populations result from many generations of adaptation to local environments and are important for long term preservation of the species. As a result, biologists are using local stocks of westslope cutthroat from within the Bob Marshall Wilderness for restoration efforts in the South Fork Flathead.

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