Oncorhynchus mykiss
Rainbow trout ("Oncorhynchus mykiss") and brown trout ("Salmo trutta") are popular gamefish but are not native to the Bitterroot River watershed and pose significant threats to native trout. In Montana, rainbow trout are only native to the upper Kootenai River in the state's extreme northwest corner. Non-native rainbow trout pose one of the greatest threats to cutthroat trout by hybridization, producing "cutbows". In addition, non-native brook trout ("Salvelinus fontinalis") often displace native cutthroat trout and bull trout in small streams.