For anadromous Pacific salmonid (Oncorhynchus spp.) smolts, the physiological state of individuals can influence migration fate. This critical life stage is typically associated with poor survival and influences population productivity, highlighting the need to identify intrinsic factors associated with outmigration fate. To better understand and identify such factors, we combined acoustic telemetry with nonlethal gill biopsies and used high-throughput real-time quantitative polymerase chain reaction to assess how infectious agents and host gene expression profiles influence migration fate for hatchery steelhead smolts (Oncorhynchus mykiss). Redundancy analyses of gene expression, infectious agent loads, and body condition highlighted gene expression profiles indicative of migratory fate. Smolts never detected after release in the river had significantly elevated expression of the immune genes Il-17D and RPL6, and lower expression of the osmoregulatory gene NKA α1b relative to other individuals. Flavobacterium psychrophilum and “Candidatus Branchiomonas cysticola” were detected in gill samples, but neither influenced survival. We demonstrate rare evidence of gene expression profiles relating to migration fate in juvenile salmonids and highlight potential mechanisms influencing fate for hatchery steelhead smolts.
