Who, When, Where: Migration Timing and Spatial Variation in Juvenile Sea Trout

Abstract

Salmo trutta, is a remarkably flexible species with a range of life history strategies, including resident and migratory. Spawning usually occurs in smaller streams in the autumn; eggs are buried in the gravel bed until spring when the newly hatched fry emerge and develop into juveniles during the next 1-4 years. For the migratory form, sea trout, the juvenile stage represents a key transitionary phase when individuals move from freshwater to the marine environment. This thesis investigates variation in juvenile sea trout during this critical period with a focus on migration timing and spatial variation in traits within sea trout streams. Across five studies this work provides evidence of spatial sorting of juvenile phenotypes within streams and relates migration timing to both individual characteristics and geographic location. Timing is a crucial aspect of migration and has profound impacts on survival. Here, migration timing was investigated at both the population and individual level. Results showed that timing of spring migration differed significantly between sea trout streams in four countries across Europe. This suggests that large-scale environmental gradients play a role in determining migration timing and therefore timing may be particularly sensitive to climate change. Further investigation also revealed differences in timing related to individual characteristics including body size and boldness. In a Swedish stream it was shown that the size of migrants decreased over the migration period and that more fish originating from a downstream site were detected migrating than from an upstream site. Additionally, early migrants were found to be more bold than late migrants. This thesis also explored seasonal differences in migration in a Danish stream and showed that autumn migrants were generally larger than spring migrants and that individuals from further upstream were more likely to migrate in the autumn than in the spring. These findings suggest that size-dependent energetic thresholds and spatial growth differences may play a role in shaping seasonal migration strategies. Previous studies have shown that juveniles do not move very far in the stream prior to migration, which allows for a comparison of the characteristics of offspring of long and short distance migrants. Within stream variation was investigated with juveniles originating from upstream and downstream sections of several streams. Results showed differences in behaviour, physiology, morphology, and migration timing along river gradients, although system-specific patterns indicated locally adapted responses. Several mechanisms could be driving these patterns such as density-dependent processes, differences in habitat quality and parental migration distance variation. Together, these results underscore the importance of maintaining connectivity within stream systems to preserve juvenile phenotypic diversity, which is critical for population resilience in the face of ongoing environmental change.