 Remaining populations survive in isolated populations, mostly in headwater streams above natural downstream barriers.The introduction of rainbow and brown trout into Missouri River tributaries eliminated the westslope cutthroat trout from most of its eastern range in Montana.Introductions of non-native kokanee salmon ("Oncorhynchus nerka"), lake trout ("Salvelinus namaycush") and lake whitefish ("Coregonus clupeaformis") into Flathead Lake and the Flathead River system caused drastic declines in westslope cutthroat trout populations.