There are four primary reasons for the decline of this species. Habitat loss is considered to be a widespread problem. Cutthroat trout have declined due to poor grazing practices, historic logging practices, mining, agriculture, residential development and the lingering impact of forest roads. Fish have been unable to use countless miles of spawning habitat due to dewatering of streams for irrigation and because of barriers created by dams and road culverts. Non-native species have also taken a huge toll on westslope cutthroat trout.
Experience shows that once brook trout dominate a stream, cutthroat trout never regain it.
Other non-native species like lake trout, brown trout and northern pike prey on cutthroat trout. East of the Continental Divide, westslope cutthroat trout were apparently displaced very rapidly after the introduction of brown trout, rainbow trout and brook trout. Thus, ironically, barriers that disrupted historical migration routes for westslope cutthroat trout have sometimes served to protect them from non-native species. A third reason for decline is the more insidious role of hybridization with other species. Westslope cutthroat trout do hybridize with rainbow trout and even other non-native cutthroat trout subspecies.