Temporal Stability of Genetic Variation within Natural Populations of Summer Steelhead Receiving Mitigation Hatchery Fish

Stochastic fluctuation in allele frequencies may reflect microevolutionary processes responsible for genetic change, such as small effective population size (N-e), which is a legitimate concern for imperiled populations affected by environmental or anthropogenic factors. In salmonids, recent empirical studies have provided conflicting results regarding the consistency of with in-population genetic variation over time. In the present study. we surveyed the genetic variation at 14 microsatellite loci in two endemic populations of summer steelhead Oncorhynchus mykiss (Tucannon and Touchet) for seven consecutive years and the Lyons Ferry Hatchery stock (LFH) for 4 years. The LFH (mitigation) stock is used to enhance fishing opportunities in southeastern Washington State and not to aid in the recovery of natural populations. We observed statistically significant differences in allele frequencies between replicated collections from the same location. One of twenty-one genic tests for the Tucannon River collections. 9 of 2 1 genic tests for the Touchet River collections, and all genic tests regarding LFH collections were statistically significant. Temporal variation was larger than spatial variation (0.86% and 0.44%, respectively). We also used genetic data to infer effective population sizes for natural steelhead populations and the LFH mitigation hatchery stock, with point estimates of 729.7, 599.4, and 266.7 for the Tucannon, Touchet, and LFH populations. respectively. Despite the temporal variation observed, relative genetic differentiation was stable over time. replicated collections tending to cluster together in factorial correspondence analyses of allele frequency data. However, the genetic data were consistent with the potential for gene flow between the Tucannon River and LFH populations. The potential benefits of stocking genetically differentiated hatchery fish to enhance fishing opportunities in the Tucannon River may be offset by the negative effects of hatchery introgression and the small N-e in hatchery fish.