Migration strategy requires complex life-history decisions, and how its variation 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 almost only one sea migratory pattern probably due to high migration costs in its southernmost habitat range of anadromous salmonids. Contrary to this, we here found that the masu salmon have increased their migratory patterns when they migrate to freshwater lakes (i.e., adfluvial migration). Specifically, adfluvial migrants spent 1–3 years in lakes whereas anadromous migrants are known to spend only half a year at sea. In addition, adfluvial migrants returned to the river to reproduce after maturation (called, mature run), whereas all anadromous migrants did so before maturation (premature run). The diversification of the years of migration and the shift in return-migration timing 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.

Data for individual fish captured in lakes or rivers. All information, including ages and strontium isotopic ratio, are included.