Family influence on length at release and size-biased survival post release in hatchery-reared steelhead: A mechanism to explain how genetic adaptation to captivity occurs

Genetic adaptation to captivity influences the reproductive success of hatchery-reared steelhead (Oncorhynchus mykiss) spawning in the wild, but the mechanism by which adaptation occurs is not well understood. Because body size-at-release is positively correlated with post-release survival in hatchery-reared smolts, one hypothesis is that hatcheries select for physiological and/or behavioral traits that promote fast growth in captivity. If those juvenile growth traits are maladaptive for spawning or offspring survival in the wild, then that could explain why hatchery fish quickly evolve to have lower reproductive success than natural-origin fish in the wild environment. First-generation hatchery-reared steelhead from the Hood River, (Oregon) have lower reproductive success in the wild than do natural-origin fish, and substantial evidence suggests the fitness difference is genetically based and due to genetic adaptation to captivity. Here we ask whether the 'selection on size-at-release' hypothesis could explain the rapid genetic adaptation observed in this well-studied steelhead population. Using scale analysis, we back-calculated length at ocean entrance to test whether size-selective survival occurred in two cohorts (brood year [BY] 1997 and 2009). In BY 2009, we found evidence of weak size-selective survival (difference of 9 mm between pre-release average length and back-calculated length from surviving adults), but in BY 1997, strong (37 mm difference) size-selective survival was observed. Family identity explained 33% of the variance in fork length before release, and fork length was highly heritable in both BY cohorts. Thus, the requisite genetic variation for response to selection on size-at-release exists in this population. Our results support the hypothesis that size-selective survival does occur after release, and that selection for traits promoting fast growth in the hatchery could be a mechanism by which rapid adaptation to captivity has occurred in the Hood River winter steelhead hatchery program.