Seasonally recurring resource subsidies can be a potential driver of intraspecific life-history variation in consumers in coupled ecosystems. Red-spotted masu salmon (Oncorhynchus masou ishikawae) is known as the southernmost salmon that expresses the partially migratory strategy, with both residents and migrants coexisting within a population. Because individuals that exceed a threshold body size in first autumn become migrants in this species, we predicted that terrestrial invertebrate subsidies occurring from late summer through autumn would effectively increase the migrants and thus could contribute to maintaining migratory polymorphism. To test this, we conducted a large-scale field experiment, in which the input rate of the terrestrial invertebrate subsidies was manipulated. As a result, individuals in subsidized stream reaches grew, on average, 53% faster than those in control reaches. This allowed a greater proportion of individuals to reach the threshold body size in the autumn, and consequently, the proportion of migrants was greater in the subsidized reaches compared to the control reaches. That is, both migrants and residents were observed in the subsidized reaches, while the control reaches consisted mostly of residents. Our findings highlight for the first time that seasonal ecosystem linkages play a key role in maintaining migratory polymorphism in partially migratory animals.
Migratory polymorphism is influenced by both environmental and genetic factors, and identifying whether variation in life histories is driven by phenotypic plasticity or strongly regulated by genetic factors is important for understanding the maintenance process of life-history variation and the ability of individuals to adapt to the future environmental change. Recent advances in molecular biology techniques have allowed us to elucidate the genetic basis of various life-history traits in wild organisms, this enables us to investigate the genotype-environment interaction for maintaining life-history variation by integrating genomic analysis and field experiments. Therefore, as a first step, we explored genomic regions that are associated with partial migration in red-spotted masu salmon by using genome-wide single nucleotide polymorphisms (SNPs). We will present the latest results and discuss future perspectives for a better understanding of the causes and consequences of migratory polymorphism in forest–stream–ocean meta-ecosystems.