Behavioral, Physiological, and Morphological Changes in a Steelhead Hatchery Supplementation Program Associated with Genetic Divergence

Hatchery supplementation programs often seek to minimize genetic divergence between interbreeding hatchery and wild fish in an attempt to thwart domestication.  Morphological, physiological, and behavioral differences between hatchery and wild reared fish have been described; however, few have quantified how these traits change in relation to genetic divergence.  Therefore, we examined differences in morphological, physiological, and behavioral traits between hatchery and naturally reared steelhead in years when rearing types were genetically similar and when there was genetic divergence.  Genetic divergence between hatchery and naturally reared steelhead was estimated by quantifying allelic diversity at up to 19 selectively-neutral microsatellite loci from 2004 to 2010.  Pairwise comparisons of bootstrapped F_ST values between hatchery and naturally reared groups showed significant genetic divergence between hatchery and naturally reared steelhead smolts from 2006 to 2010.  We found that naturally reared steelhead smolts were significantly shorter and more salinity tolerant (Gill Na⁺, K⁺ -ATPase activity, plasma Na⁺, and plasma osmolality) than hatchery steelhead smolts; however, the degree of these relationships did not change significantly after genetic divergence.  The median migration date (May 4^th), of hatchery steelhead were the same before (2003-2005) and after (2006-2009) genetic divergence, but the median migration date of wild reared fish  averaged eight days later for years when genetic divergence was observed.  Phenotypic differences between hatchery and wild reared steelhead that initially arose due to rearing effects did not show noticeable change after the divergence of neutral genotypes.  Trends in phenotypic diversity for supplemented populations should be characterized using a variety of factors.