ELECTROPHORETIC COMPARISON OF THE REPRODUCTIVE SUCCESS OF NATURALLY SPAWNING TRANSPLANTED AND WILD STEELHEAD TROUT THROUGH THE RETURNING ADULT STAGE

A previous electrophoretic assessment of the natural reproductive success of sympatric transplanted hatchery and wild summer-run steelhead trout Oncorhynchus mykiss (formerly Salmo gairdneri) populations was extended to include returns to the adult life history stage. The mean percentage of offspring from naturally spawning hatchery steelhead decreased at successive life history stages from a potential of 85-87% at the egg stage to 42% at the adult stage. In addition, reproductive success of naturally spawning hatchery steelhead compared to wild steelhead decreased from 0.750-0.788 at the subyearling stage to 0.108-0.129 at the adult stage. In freshwater, the period of greatest differential mortality for offspring of hatchery and wild steelhead occurred from the subyearling to smolt stage, suggesting that influences such as predation and competition affected survival of hatchery offspring to a greater extent than did environmental and ecological effects directly associated with differences in parental spawning time. Differential mortality of hatchery offspring also occurred during the smolt to adult phase, and was of a magnitude similar to that for the egg to subyearling phase. Poorer survival for naturally produced offspring of hatchery fish could have been due to long-term artificial and domestication selection in the hatchery population, as well as maladaptation of the transplanted hatchery stock in the recipient stream. © 1990.