Age at release affects developmental physiology and sex-specific phenotypic diversity of hatchery steelhead trout (Oncorhynchus mykiss)

Most steelhead trout hatcheries increase growth rate during rearing to produce and release yearling smolts for harvest augmentation, but natural steelhead exhibit variable age of smoltification, so this common rearing practice may not be ideal for programs focused on recovering imperiled wild stocks; therefore, it is important to investigate and compare alternative hatchery rearing methods that promote life history diversity. Over six consecutive years, the Winthrop National Fish Hatchery on the Methow River, WA reared and released paired groups of age-1 (S1) and age-2 (S2) steelhead smolts. To understand how the two rearing methods affected developmental ontogeny and life-history, fish were sampled prior to hatchery release for factors associated with smoltification (size, gill Na+/K+ ATPase activity, and a qualitative smolt phenotype) and sexual maturation (sex, pituitary and testis mRNA transcripts, gonadosomatic index, and plasma 11-ketotestosterone). Our objectives were to quantify..., , , # Data from: Age at release affects developmental physiology and sex-specific phenotypic diversity of hatchery steelhead trout (Oncorhynchus mykiss)

[https://doi.org/10.5061/dryad.x3ffbg7vk](https://doi.org/10.5061/dryad.x3ffbg7vk)

## Description of the data and file structure

We collected steelhead trout (*Oncorhynchus mykiss*) from two different rearing treatments at the Winthrop National Fish Hatchery between 2011 and 2015 prior to hatchery release. At the time of sampling, we collected fork length, weight, sex, and a visual index of smolt phenotype. Gill tissue was collected from a subset of each treatment. No further information was collect on female fish. For males, paired testes were weighed and one testis was preserved for histology and the other was preserved for mRNA analysis. Male blood was collected, centrifuged, plasma was removed and stored. Male pituitary glands were dissected and preserved for mRNA analysis.

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