The effect of structural enrichment and increased water flow on the opportunity for domestication selection in hatchery-reared steelhead (<i>Oncorhynchus mykiss</i>)

Hatchery-reared salmon often have lower fitness in the wild than wild fish, a difference that appears to result from rapid adaptation to the hatchery environment. Size at release is heritable and positively correlated with survival at sea. Therefore, selection should favor traits that promote fast growth in the hatchery even if those traits are maladaptive in the wild. Modifying hatchery conditions to reduce the variation in size among families should decrease the opportunity for selection. Using a mix of 15 full-sibling families of winter run steelhead ( Oncorhynchus mykiss), we tested two modifications designed to benefit the normally slower-growing fish: (1) the addition of structure to rearing tanks and (2) increased water flow. Neither treatment substantially changed the variance in size at release relative to controls. Furthermore, there was a very high correlation among family mean size across all environments. We conclude that simple changes to the hatchery environment such as those tested here cannot overcome the large family effects and are unlikely to substantially reduce the opportunity for selection on size at release.