Assessment of historical and contemporary genetic diversity of steelhead (Oncorhynchus mykiss) in the Lake Michigan basin

Steelhead (Oncorhynchus mykiss) were introduced into the Great Lakes in the late 1800's. Subsequently, natural recruitment across the Lake Michigan basin has been regularly supplemented by hatchery production of strains derived from widely dispersed locales within the species' native range along the west coast of the United States. We used microsatellite markers to assess how the populations of steelhead in Lake Michigan are genetically structured (1) spatially within and among tributaries to lake Michigan, (2) temporally based on time of entry into spawning runs (fall versus spring) as well as between historical and contemporary populations, and (3) between naturalized populations and hatchery strains used for supplementation. Hierarchical analysis indicated significant genetic differentiation between the naturalized populations and hatchery strains (Θ_s = 0.060, P < 0.05), among naturalized populations (Θ_p = 0.003, P < 0.05), and among hatchery strains (Θ_p = 0.105, P < 0.05). However, few significant pairwise genetic differences among naturalized populations exist, no significant differences in allele frequencies between different spawning populations within rivers were observed, and no significant differences in allele frequency between sympatric fall and spring spawning runs were observed.; Prior to 1983, hatchery supplementation of Lake Michigan steelhead populations in Michigan utilized primarily on strain and was largely unsuccessful due to low survival estimates (0.01%) of small (<120mm) hatchery yearlings to smolt stage. Accordingly, contributions of hatchery fish to historical adult spawning runs in Michigan tributaries were low (0–30%) across six major drainages. Large (>150mm) yearlings of multiple hatchery strains have been stocked exclusively since 1983, increasing estimates of survival (90%) to smolting. Consequently, the proportion of hatchery adults in spawning runs increased to 13–79%. We examined the effects of changes in stocking practices on straying of hatchery steelhead and to temporal changes in levels of genetic diversity and relationships among populations for steelhead populations sampled for two time periods (1983–1984 and 1998–1999). Measures of inter-population divergence (mean F_ST) were not significant for either time period. However, spatial genetic relationships among historical and contemporary populations were significantly correlated with geographic distance. Increased numbers of alleles in spawning adults from populations can be attributed to alleles specific to recently introduced hatchery strains.; The increased contribution of hatchery origin individuals to spawning runs in Michigan rivers increases the potential for introgressive hybridization between hatchery and river origin individuals. Therefore, maintenance of genetic diversity within hatchery strains is important to management. We empirically compared six mating strategies used by hatcheries in the Great Lakes region to examine the effects of reproductive variance on measures of genetic diversity. Treatments that minimized reproductive variance by not pooling gametes from multiple individuals, and did not use individuals repeatedly for matings resulted in the lowest estimates of coancestry (inbreeding) and highest effective population size estimates.