Divergence and shift in the migratory life history of a salmonid fish during the transition to a new environment

Migration strategy requires complex life‐history decisions, and how variation in migration strategies can increase or decrease during migration to novel environments is fundamentally important for population persistence and adaptation for migratory animals. An anadromous form of red‐spotted masu salmon exhibited limited sea migratory patterns probably due to high migration costs in its southernmost habitat range. Contrary to this, we here found that the masu salmon have increased and shifted their migratory patterns when they migrate to freshwater lakes (i.e. adfluvial migration). Adfluvial migrants spent 1–3 years in lakes whereas anadromous migrants spend only half a year at sea. In addition, adfluvial migrants returned to the river to reproduce after maturation, whereas all anadromous migrants did so before maturation. Furthermore, males often adopted migratory tactics, shifting the migration propensity from female‐biased to unbiased, in a lake. The diversification of the years of migration and the shifts in return‐migration timing and sex‐ratio of migrants were only found in deep lakes, where migrants can continue migrating in the favour of the cold water of lakes. The present study demonstrated that a migratory salmonid extensively diversified and shifted its migratory life history, often requiring complex adjustments in life history traits with a genetic basis, in novel environments.