Exploring the links between swimming performance, glucocorticoid profiles, behavioral types and cardiac morphology in migrating Atlantic salmon (Salmo salar) smolts.

It has been become increasingly clear that swimming performance is crucial for the migratory success of wild Atlantic salmon (Salmo salar) smolts, yet the associated traits remain largely unexplored. In this study, smolts were caught during their downstream migration to the sea and were categorized as strong or poor swimmers based on their time to fatigue in a swimming test. Behavioral profiles were characterized through a confinement and novelty test, and relationships to cardiac morphology and glucocorticoid profiles were investigated. Strong swimmers increased activity over time in the confinement test, while poor swimmers showed the opposite trend, indicating differences in boldness. There were no differences in plasma cortisol in groups of fish with contrasting swimming performance. Still, strong swimmers had higher cortisone levels 24 h after the swim test, suggesting a protective mechanism against elevated cortisol in this group. No differences in heart morphology were found between the groups. In addition, acoustic telemetry revealed a trend toward lower survival rates for strong swimmers in predator-rich environments. Overall, our results highlight a link between glucocorticoid profiles, boldness and swimming capacity in Atlantic salmon smolts. Further studies are needed to confirm the observed survival patterns and to better understand how predator pressure may shape the fitness consequences of these traits in migrating smolts.