Genetic Characterization of Bull Trout from the Walla Walla River Basin

A total of 25 collections of bull trout were analyzed from eight different collection sites within the Walla Walla River Basin and two collection sites in the Yakima River Basin. Adult bull trout were analyzed from five sites and juvenile bull trout were analyzed from five sites. Sixteen nuclear microsatellite DNA loci that are included in the standardized suite of loci were used to examine the levels and patterns of genetic variation. The multi-locus genotypes generated for these bull trout were analyzed to determine population structure of the adult and juvenile bull trout collections. Tests of population subdivision, factorial correspondence analysis, and the neighbor-joining tree suggested the five adult collections were significantly different from one another. The five juvenile collections were also all significantly different from one another. The Touchet River adult collection clustered with the juvenile collections, but was significantly different from them in the genotypic tests of differentiation. The collections of juvenile bull trout from the North Fork Touchet, Wolf Fork and Burnt Fork were significantly different from one another with most of the statistical comparisons we employed, while bull trout from the Lewis and Spangler Creek collections could not be differentiated from the other groups and tended to overlap or group with bull trout from the North Fork Touchet or Wolf Fork. Assignment tests were used to determine stock-of-origin percentage of migratory adult bull trout that were collected at Dayton Dam Trap. The highest percentage of migratory bull trout at the Dayton Dam came from Wolf Fork (50.6%) and the N.F. Touchet River (39.0%), while Lewis Creek and Spangler Creeks accounted for 5.2% each, and no adult bull trout assigned to Burnt Fork.