Heterosis and outbreeding depression between strains of young-of-the-year brook trout (Salvelinus fontinalis)

Brook trout (Salvelinus fontinalis (Mitchill, 1814)) supports a lucrative fish aquaculture industry in Quebec and production of this species is mainly oriented to stock enhancement supporting sport fisheries. The aim of this study was to verify the expression of interstrain heterosis, i.e., increased performance of first generation hybrid progeny, during the first stages of development in brook trout. Two wild populations that were recently introduced to fish production in Quebec, the Laval (L) and the Rupert (R) strains, and a domestic strain (D) that is present in most Quebec fish farms were used in this study. The growth of 72 full-sibling families, representing three pure (female D x male D, female L x male L, female R x male R) and five hybrid (female D x male L, female D x male R, female L x male D, female L x male R, female R x male L) cross types, were surveyed from hatching to 2136 degree-days. Both heterosis and outbreeding depression were observed, depending on the hybrid cross type. The occurrence of heterosis was dependent on the developmental stage, and growth advantage at a very early developmental stage did not necessarily translate into bigger size later on. Outbreeding depression in growth (mass or length) was much less common than heterosis, and when occurring, it varied from 9.2% to 33.3% compared with 9.0% to 88.2% improvement in growth traits for heterosis. The results indicate that in early development, there was higher occurrence of heterosis relative to fish mass than to fish length and that, overall, the occurrence of heterosis was unpredictable.