The influence of smolt age and physiological condition on survival and behaviour of wild migrating juvenile sockeye salmon (Oncorhynchus nerka) in British Columbia

Sockeye salmon (Oncorhynchus nerka) smolts typically experience high mortality during outmigration through freshwater and into the marine environment, yet factors influencing survival remain poorly understood. Telemetry studies investigating migration survival have largely focused on tracking large hatchery smolts or larger individuals within wild populations, which may not be representative of the majority of migrants. I used recently developed miniaturized acoustic transmitters (VEMCO V4) to track age-1 sockeye smolts (n = 200) for the first time over ~950 km from Chilko Lake, British Columbia, Canada through the coastal marine environment using large-scale receiver arrays. I compared their survival with concomitantly tagged and tracked age-2 smolts (n = 100). I paired acoustic telemetry with non-lethal gill biopsies and used high-throughput quantitative polymerase chain reaction to assess how infectious agents and host gene expression profiles influence smolt migration survival. Cumulative freshwater survival of age-1 smolts was double that of age-2 survival (56% vs 28%), potentially due to higher proportional tag burdens of age-2 smolts. Although survival between the age classes differed, trends in landscape-specific survival were similar, with the poorest survival occurring the in the upper river tributaries (76% and 53% for age-1 and age-2 smolts respectively over the first 14 km from release), and after ocean entry for age-1 smolts (36%). Three infectious agents (Flavobacterium psychrophilum, ‘Candidatus Branchiomonas cysticola’, and infectious hematopoietic necrosis virus) were most prevalent in tagged smolts, and F. psychrophilum was related to migration survival. Gene expression profiles differed between age groups and were strongly associated with migration survival. Smolts that died earlier in the river had a significantly higher expression of inflammatory (IL-11 and IL-17D) and stress (HSC70 and JUN) response genes than smolts that survived migration. These genes were also more highly expressed in age-2 smolts than age-1 smolts. My work highlights the importance of expanding research to include smaller age-1 smolts and provides important survival estimates for an indicator population of Fraser River sockeye salmon. My work also provides important and novel links between infectious agents and gene expression profiles with migration survival of age-1 and co-migrating age-2 smolts for the first time, highlighting mechanisms contributing to sockeye smolt mortality during migration.Forestry, Faculty ofGraduat