Evaluating the success of a conservation reintroduction: The case of bull trout in the Wallowa River

Abstract

Bull trout in the Wallowa River watershed were considered extirpated in the 1950s. In 1997, bull trout from the adjacent Imnaha River watershed were reintroduced into the Wallowa River watershed. We evaluated whether bull trout are currently present in the Wallowa River watershed and, if so, whether they appear to be the result of the 1997 reintroduction. From 2010 to 2018, we captured 181 Salvelinus spp. The majority (64.5%) of these individuals were bull trout. Bull trout in the Wallowa River watershed were more genetically similar to those from the Imnaha River watershed (pairwise FST = 0.102) than to the other populations we examined. They also exhibited genetic evidence of a recent bottleneck (observed heterozygosity was 0.598, significantly greater than expected). Modeled estimates of size (541–581 mm), survival (<0.0001–0.0015 probability) and age (14–22 years) for the reintroduced fish indicated the bull trout captured in the Wallowa River watershed were unlikely to be remnants from 1997. These results suggest the 1997 reintroduction has resulted in natural reproduction and recolonization of bull trout in the Wallowa River watershed. Whether the abundance and genetic diversity of these bull trout is sufficient to allow the population to persist over an ecological time period is unclear.