Repeatability of migratory behaviour suggests trade‐off between size and survival in a wild iteroparous salmonid

Repeatable individual variation in behaviour has been demonstrated in all taxa, though few studies have explored the broader ecological consequences of such consistent behaviour. We tagged and tracked 401 individual sea trout Salmo trutta with passive integrated transponder telemetry over two to four spawning seasons as they migrated between freshwater and marine environments to spawn and feed respectively. We found the timing (time of river entry) and duration of migration (time spent in freshwater and time spent at sea) to be repeatable within individuals across years. In addition, we identified two peaks in return migration timing; a group of summer‐migrating fish and a group of fall‐migrating fish. At the time of first spawning, sea trout differed in size depending on how long the first marine phase was, but summer‐migrating fish were smaller in all subsequent spawnings. Furthermore, summer‐migrating fish had significantly better marine survival than fall‐migrating fish. Our study suggests that the timing of arrival in freshwater is an important fitness‐relevant trait, such that summer‐migrating fish have decreased fitness but remain in somewhat safer conditions, while fall‐migrating fish have increased fitness but are exposed to more risky conditions. Taken together, our findings suggest that migration timing and duration is determined via a trade‐off between size and survival, which resulted in two evolutionary stable strategies. A free Plain Language Summary can be found within the Supporting Information of this article.