Fragmentation of riverine systems: the genetic effects of dams on bull trout (Salvelinus confluentus) in the Clark Fork River system

Migratory bull trout (Salvelinus confluentus) historically spawned in tribu taries of the Clark Fork River, Montana and inhabited Lake Pend Oreille as subadult and adult fish. However, in 1952 Cabinet Gorge Dam was constructed without fish passage facilities disrupting the connectivity of this system . Since the construction of this dam, bull trout populations in upstream tr ibutaries have been in decline. Each year adult bull trout return to the ba se of Cabinet Gorge Dam when most migratory bull trout begin their spawning migration. However, the origin of these fish is uncertain. We used eight m icrosatellite loci to compare bull trout collected at the base of Cabinet G orge Dam to fish sampled from both above and further downstream from the da m. Our data indicate that Cabinet Gorge bull trout are most likely individu als that hatched in above-dam tributaries, reared in Lake Pend Oreille, and could not return to their natal tributaries to spawn. This suggests that t he risk of outbreeding depression associated with passing adults over dams in the Clark Fork system is minimal compared to the potential genetic and d emographic benefits to populations located above the dams.