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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 1b 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. Pour les saumoneaux de salmonides anadromes du Pacifique (Oncorhynchus spp.), l'etat physiologique des individus peut influencer le resultat de la migration. Cette etape cle du cycle de vie est typiquement associee a une faible survie et elle influence la productivite des populations, d'ou l'importance de cerner les facteurs intrinseques associes au resultat des migrations de sortie. Pour mieux comprendre et cerner ces facteurs, nous avons combine la telemetrie acoustique a des biopsies de branchies non letales et utilise l'amplification en chaine par polymerase quantitative en temps reel et a debit eleve pour evaluer l'influence d'agents infectieux et des profils d'expression genique des hotes sur le resultat de la migration pour des saumoneaux de truite arc-en-ciel (Oncorhynchus mykiss) issus d'ecloseries. Des analyses de redondance de l'expression genique, des charges d'agents infectieux et de l'embonpoint font ressortir des profils d'expression genique qui predisent le resultat de la migration. Les saumoneaux jamais detectes apres leur lacher dans la riviere presentaient une expression significativement elevee des genes de l'immunite Il-17D et RPL6 et une expression plus faible du gene de l'osmoregulation NKA 1b par rapport aux autres individus. Flavobacterium psychrophilum et 'Candidatus Branchiomonas cysticola' ont ete detectes dans les echantillons de branchies, mais ni l'un ni l'autre n'influencait la survie. Nous presentons de rares preuves d'un lien entre les profils d'expression genique et le resultat de la migration chez des salmonides juveniles et decrivons des mecanismes qui pourraient influencer le destin des saumoneaux de truite arc-en-ciel issus d'ecloseries. [Traduit par la Redaction], Introduction Large-scale migrations are an important life history component for Pacific anadromous salmonids (Oncorhynchus spp.). Towards the end of their freshwater residence, individuals undergo dramatic physiological changes prior to migrating [...]

Annual migrations by juvenile Pacific salmon Oncorhynchus spp. smolts are predictable, presenting opportunities for predators to exploit these seasonal prey pulses. Directly observing predator–prey interactions to understand factors affecting predation may be possible via dual‐frequency identification sonar (DIDSON) acoustic imaging. Within Chilko Lake, British Columbia, prior telemetry and stomach content analyses suggested that the out‐migration of Sockeye Salmon Oncorhynchus nerka smolts influences the movements and aggregations of Bull Trout Salvelinus confluentus that feed extensively on smolts during their out‐migration. Bull Trout captured at a government‐installed counting fence exhibited high consumption of smolts, but it is only assumed that feeding occurred directly at the fence. We used DIDSON to assess fine‐scale predator–prey interactions between Sockeye Salmon smolts and Bull Trout over 10 d during the 2016 smolt out‐migration. We found that smolt–Bull Trout interactions were correlated with smolt densities at the counting fence, consistent with the prior diet studies in the system. Predator–prey interactions were also coupled with nocturnal migratory behaviors of Sockeye Salmon smolts, presumably to minimize predation risk. These results demonstrate that DIDSON technology can record interactions between predators and migrating prey at a resolution that can identify variability in space and time and provide insight on the role of anthropogenic structures (e.g., counting fences) in mediating such interactions. [ABSTRACT FROM AUTHOR]