Two-Year Spawning Migration as a Life-History Strategy of Sea Trout (Salmo trutta L.) in Large, High-Latitude River Systems

Migrating animals are found throughout the animal kingdom, as movement between habitats can be vital for survival, growth, and reproduction. Mature sea trout (Salmo trutta L.) undertake seasonal migrations from marine feeding areas to freshwater spawning habitats, but the migration dynamics may vary considerably between rivers of different morphology and location. In this study, we used radio-telemetry with multi-year tags to observe the riverine movements and spawning migration behaviour of wild sea trout tagged in a large, northern Baltic Sea river system, the Tornio River. Both immature and mature trout were observed to have similar overwintering behaviour as they mainly overwintered in the same locations in the lower river. Many immature trout returned to overwinter for multiple consecutive winters without spawning, a behaviour that has been rarely reported for sea trout in rivers of the northern Baltic Sea. The results from a support vector model highlighted the importance of trout age and previous spawning experience for the direction of the migration, as previously spawned and older trout were more likely to migrate upstream for spawning while their younger counterparts returned to sea in summer after spending the winter in freshwater. The riverine movement patterns indicate that the majority of mature sea trout migrating upstream for spawning in the river system have adopted a spawning migration strategy, which extends for almost two calendar years. Mature sea trout enter freshwater in autumn the year prior to spawning, migrate to spawning habitats and spawn in the year following freshwater entry, and overwinter in-river for a second time post-spawning. This migration strategy has not been previously reported for sea trout native to Baltic Sea river systems. The extended freshwater stays and numerous migrations between sea and river habitats in large, high-latitude rivers need to be considered in management to ensure adequate protection of both immature and mature sea trout.