Elevated Temperature and Stock Source, but Not Genotypic Sex, Affect Early Growth in Oncorhynchus mykiss

Conditions experienced during early development can have a significant impact on phenotypic variation and subsequent life history options, particularly among species that normally express multiple phenotypes. Oncorhynchus mykiss is known as steelhead, and rainbow trout, in its anadromous and freshwater resident forms, respectively. Using several steelhead pairs as a parental source of embryos, we compared wild and hatchery-sourced O. mykiss to evaluate to what extent source, genotype (chromosomal sex) and differing temperatures influenced growth and sexual development. Among fish of all treatments raised at 5 C^o above ambient from fertilization to swim up, sex ratios were not significantly different from 1:1; subsequent genotyping demonstrated concordance between genotypic and phenotypic sex. Therefore, early exposure to elevated temperature did not affect sexual development. Additionally, genetic sex did not affect growth rates. Temperature regime and stock source, however, did. Wild fish tended to be smaller post-hatch, but exhibited accelerated growth compared with hatchery source fish. Fish exposed to elevated temperatures exhibited faster growth, resulting in larger terminal size. These growth differences associated with stock source and rearing temperature suggest that phenotypic variation in O. mykiss is likely to increase with projected climate change, with unknown consequences to life history trajectories