Migratory bull trout (
Salvelinus confluentus
) historically spawned in tributaries 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 tributaries have been in decline. Each year adult bull trout return to
the base of Cabinet Gorge Dam when most migratory bull trout begin their spawning
migration. However, the origin of these fish is uncertain. We used eight microsatellite loci
to compare bull trout collected at the base of Cabinet Gorge Dam to fish sampled from both
above and further downstream from the dam. Our data indicate that Cabinet Gorge bull
trout are most likely individuals that hatched in above-dam tributaries, reared in Lake Pend
Oreille, and could not return to their natal tributaries to spawn. This suggests that the risk
of outbreeding depression associated with passing adults over dams in the Clark Fork system
is minimal compared to the potential genetic and demographic benefits to populations
located above the dams.