Spawning Success of Bull Trout Transported above Main-Stem Clark Fork River Dams in Idaho and Montana

Three main-stem dams have fragmented the Clark Fork River in Montana and Idaho for nearly 100years. Since 2001, Bull Trout Salvelinus confluentus collected below the farthest downstream dam, Cabinet Gorge Dam, have been manually transported upstream so that they can return to natal spawning tributaries and contribute gametes to numerically depressed populations upstream from the dams. Although redd counts and radio telemetry data suggest that these adults attempted to spawn after upstream transport, spawning success has not been documented or quantified. We used genetic parentage assignments to determine whether adult Bull Trout were successfully spawning after being transported upstream. Parentage data were also used to examine reproductive success and spawning behavior of fish transported upstream (transports) as well as Bull Trout that spawned in the same tributaries but were not transported upstream (nontransports). We genotyped 1,362 juvenile Bull Trout collected in two Clark Fork River tributaries, East Fork Bull River and Graves Creek, from 2008 to 2010 as well as 184 adult Bull Trout collected below Cabinet Gorge Dam and in Clark Fork River tributaries from 2004 to 2010. Parentage data indicated that Bull Trout collected below Cabinet Gorge Dam did spawn after upstream transport; nearly 27% of the juveniles sampled in this study had at least one upstream transport parent. Reproductive success varied widely among the adults we assigned as parents. We observed a variety of spawning patterns including adults that spawned only once, annual repeat spawners, and alternate-year spawners. Our data provide evidence that the upstream transport program is meeting its goal of increasing the number of spawning adults in Bull Trout populations upstream from Clark Fork River dams and highlights the fact that nontransported fish also make an important contribution to the local tributary populations.