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The influence of resource subsidies on animal growth, survival and reproduction is well understood, but their ultimate effects on life history have been less explored. Some wild species have a partially migratory life history, wherein migration is dictated based upon threshold traits regulated in part by the seasonal availability of resources. We conducted a large-scale field manipulation experiment where we provided a terrestrial invertebrate subsidy to red-spotted masu salmon. Individuals in stream reaches that received a subsidy had, on average, a 53% increase in growth rate relative to those in control reaches. This increased growth resulted in a greater proportion of individuals reaching the threshold body size and smolting in the autumn. Consequently, 19–55% of females in subsidized reaches became migratory, whereas 0–14% became migratory in the control reaches. Our findings highlight seasonal ecosystem linkage as a key ecosystem property for maintaining migratory polymorphism in partially migratory animals. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]