Small-scale genetic structure and variation in steelhead of the Grande Ronde River, Oregon, USA

Patterns of genetic variation in summer-run steelhead Oncorhynchus mykiss from the Grande Ronde River, Oregon, were evaluated with 20 microsatellite loci to determine the level of fine-scale genetic structure and influences from hatchery-reared stocks. Included were temporal collections of three wild populations (Lookingglass Creek, Catherine Creek, and the upper Grande Ronde River) and one hatchery population (Wallowa Hatchery). Average gene diversity was moderate to high in all collections (range= 0.794-0.815). Genetic variance among sample locations was greater than that of temporal collections within sites, as pairwise exact tests within populations were not significant except for Lookingglass Creek (P = 0.00005). Structure among the four populations was evident from pairwise tests' which yielded 23 significant results from 28 comparisons (adjusted critical value = 0.0018), but Catherine Creek collections were not significantly different from Wallowa Hatchery collections. None of the eight collections had a significant probability of differing from mutation drift equilibrium (e.g., recent bottleneck) under the two-phased mutation model (P = 0.060-0.985) when analyzed with BOTTLENECK software. Additionally, the ratios of allele number to allele range (M values) for the eight collections (0.773-0.819) were not within the range that suggests a recent decrease in effective population size (M < 0.68 implies a probable bottleneck). Results from this study suggest the existence of gene flow between steelhead from Catherine Creek and Wallowa Hatchery, but no evidence for low diversity or recent bottleneck events was detected in any of the four populations. This study provides several pieces of evidence to suggest that steelhead in the Grande Ronde River should be a conservation priority as a source of genetic diversity for the Snake River basin.