What was the primary cause leading to the dramatic decline of the Paiute Cutthroat Trout population?

The stocking of non-native species during the 20th century.

The dramatic reduction in the Paiute Cutthroat Trout population stems overwhelmingly from human introduction of non-native fish species into their historically isolated habitat during the 20th century. Specifically, brook trout and rainbow trout were introduced into many high-mountain lakes and streams. These invasive species became aggressive competitors, outcompeting the native Paiute trout for essential food resources, occupying prime areas of habitat, or directly hybridizing with the native trout, thereby diluting the unique Paiute genetics. This external pressure led to the extirpation of the native species from most of its historical range.