Juvenile Growth Regime and Early Marine Growth Affect Life History Phenotypes of Hatchery-Reared Steelhead, Oncorhynchus mykiss

Husbandry practices affecting emergence timing and growth rate in culture can alter life histories of hatchery salmonine fishes, and may contribute to their lower reproductive success.  Steelhead hatcheries typically produce age-1 smolts (S1), while most wild fish smolt at age 2 (S2) or older.  We examined effects of two growth regimes designed to produce either S1s or S2s on smoltification and age of maturity in individually-tagged Methow River steelhead. We compared growth history in fresh and seawater (SW) in relation to individual life history phenotype.  Results: 1) S1 smolts were significantly larger than parr from the time of tagging (5 months post-ponding) onward, suggesting the growth trajectory promoting smoltification of S1s was established prior to tagging; 2) fish from both groups that matured two years after SW transfer had higher growth during the first 6-12 months in SW; and 3) the S2 regime produced fish with higher SW survival, females with more uniform age of maturity and higher proportion of males maturing one year after SW transfer than the S1 regime.  Conclusions: Both hatchery rearing regime and early marine growth affected age of maturity, and the S2 rearing regime may reduce selection on early growth in the hatchery.