The purpose of this project was to document the extent of introgression in the North Fork Clearwater drainage. This information is pertinent to guiding fish management strategies, including Dworshak resident fish mitigation stocking, to ensure the persistence of native westslope cutthroat trout in this extensive watershed.
Introgression by exotic trout has been documented as the greatest threat to the conservation of native westslope cutthroat trout Onchorynchus clarki lewisi in northern Idaho and western Montana. Genetically pure populations of westslope cutthroat trout are thought to occur in less than 4% of the historic range. Although the North Fork Clearwater drainage is considered a stronghold for populations of westslope cutthroat trout in Idaho, rainbow trout O. mykiss and Yellowstone cutthroat trout 0. c. bouvieri have been widely introduced in the drainage. Rainbow trout are present throughout the North Fork Clearwater River and into the major tributaries, potentially threatening the genetic integrity of native westslope cutthroat trout in most of the drainage Dworshak resident fish mitigation released rainbow trout into Dworshak Reservoir, encompassing the lower 86.2 km of the North Fork Clearwater River. Once released, unobstructed upstream access is available far into the watershed tributaries.
This study detected a broad zone of westslope cutthroat trout and rainbow trout hybridization at low to intermediate elevations in the Clearwater River basin. Genetically pure populations occur in about 25% of the steam habitat in the North Fork Clearwater and Lochsa basins, many of which are restricted to smaller, higher elevation streams. The study suggested that managers should focus on protecting these high elevation habitats, maintaining connectivity among habitats and expanding the pure populations.
*This project is currently closed and no staff are assigned to it.