Family Dominance at the Fry Stage Weakly Influences Mean Family Body Length at Smolting in Hatchery-Reared Steelhead

<p>Steelhead <i>Oncorhynchus mykiss</i> reared in hatcheries rapidly adapt to captivity. However, the traits under selection in captivity are not well understood. Hatchery-reared salmonids are generally more aggressive than wild fish. Therefore, it is possible that selection in hatcheries favors increased aggressiveness because aggressive fish may dominate food resources and grow larger, resulting in a fitness advantage at release. We tested whether family dominance at the fry stage correlates with mean family fork length at smolting of the same families raised in a parallel experiment. Family dominance was assessed when families were first interacting with each other as fry. Juveniles in the parallel experiment were raised at high or low density to determine whether the effect of family dominance on mean family fork length was stronger at low density. More-dominant families were slightly larger at smolting, and the effect changed little between density treatments. Although the effect we observed was modest, these data are consistent with the hypothesis that dominance as fry is a trait under selection in hatcheries.</p> <p>Received April 8, 2016; accepted July 28, 2016 Published online October 7, 2016