Temporal genetic analysis of steelhead (Oncorhynchus mykiss) reveals hatchery-induced drift in captivity

The primary evolutionary goal of many hatchery supplementation programs is to minimize genetic change and fitness loss associated with captive breeding. The objective of this study was to assess the genetic stability of a hatchery steelhead (Oncorhynchus mykiss) population from the Mad River (California, USA) over 35 years of captive breeding. The genetic population structure of historical (1974-1975) and contemporary (2000-2010) hatchery and wild stocks were compared using 14 microsatellite loci. Contributions from four non-indigenous populations transplanted to the Mad River were also assessed. The founding steelhead population artificially propagated at Mad River Hatchery was resolved as genetically similar to wild steelhead from natural areas in the Mad River and those from the Eel River. From this starting point, the captive population diverged over 35 years while the wild population retained the historical condition but was also highly admixed (17-44%) with the newly formed hatchery population. Analyses indicated that contributions from non-local populations were not responsible for divergence of the hatchery population. Instead a lower effective population size in the hatchery (Ne=246-285) in comparison to the wild (Ne=1,935-4,356) and closure of the hatchery population to immigration from the wild suggests drift in isolation is responsible for divergence. The small number of spawners used in many years at the hatchery along with variance in reproductive success likely explains the observed level of drift. The evolutionary potential of both hatchery and wild populations may be constrained unless efforts are implemented to reduce divergence among them and mitigate for the deleterious effects of genetic drift in the hatchery population.