Genetic population structure of bull trout in the East Fork bitterroot river drainage, Montana

Investigation of the genetic population structure of Bull Trout Salvelinus confluentus is useful for developing biologically sound conservation and management strategies. We focused on the East Fork Bitterroot River (hereafter, East Fork), Montana, because it is a relatively undisturbed, connected watershed that contains a migratory life history form of Bull Trout. Fin clips were collected from 17 sites: nine East Fork tributaries, the main-stem East Fork, and seven other tributaries across the Bitterroot River drainage. Considering all the population samples, principal components analysis of allele frequencies at 15 microsatellite loci indicated that the East Fork samples formed a cluster that was distinct from the other Bitterroot River tributary samples. Within the East Fork, there was significant divergence among population samples, with pairwise F_ST ranging from 0.016 to 0.188 and a global F_ST of 0.063. Relative to other Bull Trout studies, levels of genetic variation within our samples were typically higher, while the overall level of differentiation among samples was lower. Based on ONCOR analyses of multiple-locus genotypes, most individuals in the East Fork basin were assigned to their water body of capture, with an average probability of 88%. Within the East Fork, 16 fish that were collected in the main stem were assigned to tributary populations; 26 individuals sampled from seven tributaries were assigned to the main-stem population. In addition, there were five tributaries in which sampled individuals were assigned to tributary populations other than the water body of capture. Based on GeneClass2 analysis, 76 individuals were identified as first-generation migrants. These observations suggest movement and potential gene flow between the main-stem East Fork and its tributaries and between tributaries via the main stem. The main-stem East Fork therefore appears to be an integral component for maintaining the migratory form of Bull Trout in the drainage and may serve as an important vehicle for genetic exchange among tributary populations.