Can Alternative Hatchery Rearing Practices Lessen Fitness Loss in Steelhead Trout?

Hatchery rearing environments may affect behavioral, physiological or morphological development that can influence breeding success or induce selection that results in genetically based fitness loss.  The relative fitness of hatchery-origin salmon and steelhead is generally less than that of natural-origin fish.  Differential breeding success, offspring fitness, or both may contribute to lower fitness of hatchery fish, and these effects vary depending on the species, type of hatchery, and broodstock type.  Studies of steelhead trout indicate greater reductions in fitness than in other species after a single generation of hatchery culture. Studies of one steelhead population steelhead have demonstrated a genetic basis for a rapid decline in fitness after one generation.  Two subsequent published studies implementing similar ‘common garden’ study designs for coho salmon and Chinook salmon have not detected a genetic basis for fitness loss, although fish reared in the hatcheries suffer significant fitness loss when they return as adults, implicating developmental effects of the hatchery environment.  Fitness loss in steelhead is hypothesized to result from accelerated growth regimes and associated selection for fish that grow rapidly under hatchery conditions.  Evidence of selection on body size after release comes from studies in the Columbia River and Snake River Basins.  Experimental hatchery growth regimes that more closely mimic natural age-at-smoltification (e.g., age-2) appears to reduce viability selection on growth rate by providing the time for a greater percentage of the hatchery population to achieve thresholds for successful migration.  Research is underway to determine whether two-year steelhead rearing programs can lessen the degree of hatchery-induced selection and fitness loss.