The Clark Fork River runs from Thornton Lake near Anaconda, Montana to Lake Pend Oreille in Idaho, and is home to native Bull Trout populations that spawn in tributaries throughout the watershed. Juvenile Bull Trout throughout the watershed will choose to remain in the tributary they emerged from or venture out to mature in larger bodies of water, including the main channel of the Clark Fork River and Lake Pend Oreille, the largest lake in Idaho. These fish return to their natal streams as maturing adults to spawn in upstream tributaries within the Clark Fork River watershed.
The completion of Cabinet Gorge Dam (1952) and other mainstem dams prevented Bull Trout from returning to their natal streams. In the autumn after completion of Cabinet Gorge Dam, biologists surveying spawning of Kokanee Salmon observed Bull Trout pooling below the dam. These were believed to be Bull Trout attempting a return to their upstream natal stream.
In the late 1990s genetic work indicated that yes, the Bull Trout pooling below Cabinet Gorge Dam most likely represented fish from upstream tributaries. In 2003, Abernathy Fish Technology Center, in conjunction with our partner, Avista Corporation, began developing a protocol using genetic markers to distinguish Bull Trout from the various spawning populations throughout the Clark Fork River watershed. This work allowed Avista Corporation and partners, for the first time in nearly 50 years, to begin regular passage of Bull Trout above Cabinet Gorge Dam.
Beginning In the spring of each year Avista Corporation conducts night electrofishing trips below Cabinet Gorge Dam. Adult Bull Trout captured during these surveys are measured and retained for genetic analysis at a holding facility near the base of Cabinet Gorge Dam. Fin clips are taken from captured Bull Trout and shipped for next day delivery to Abernathy. Upon arrival, fin clips are extracted, genotyped, and analyzed typically within six hours. Originally Bull Trout would be assigned to one of nineteen possible populations within four regions found both above and below the three mainstem dams on the Clark Fork River. With time, we have steadily increased our knowledge of the diversity in the watershed and can now assign the fish to one of 50 populations within the four regions that are currently in the project.
After Bull Trout are assigned to a population, the information is passed to Avista Corporation in order to facilitate the transport of that individual. Avista biologists will use our data to inform their decisions about where to release fish. They will move fish either upstream above one of three mainstem dams, or release them back where they found them, downstream of Cabinet Gorge Dam. Since this project began in 2004, over 600 individual fish have been analyzed as part of this program and nearly 500 of these have been assigned to tributaries upstream of Cabinet Gorge Dam. Our data have been instrumental in restoring the life-cycle of the fish in the Clark Fork River, and helping to maintain genetic diversity and the persistence of the species in this part of its native range.