Partial Migration in Salmonids: Focusing on Asian Endemic Masu Salmon (Oncorhynchus masou) and White-Spotted Charr (Salvelinus leucomaenis)

Partial migration is perhaps one of the most striking features of life-history polymorphism, in which a population displays both migratory and resident behavior. Not surprisingly, both plasticity and genetics are involved in the life-history divergence between residents and migrants. The most plausible mechanism for the persistence and evolution of partial migration is a status-dependent conditional strategy, in which the adoption of alternative migratory tactics is influenced by the status (e.g., body size, condition) of individuals to maximize fitness. To uncover the factors affecting life-history divergence, approaches comparing traits (e.g., growth rate) between residents and migrants are common, but care must be taken in interpreting cause and effect as the onset of life-history divergence precedes the actual timing of outmigration. Individuals planning to migrate will change their behavior and growth rate for a period of 6 months or more to pre-adapt to life at sea, which is very different from life in a river. Early studies of partial migration often dichotomously divided the life histories of residents and migrants, but recent studies have shown there is remarkable diversity within migrants. It is important but challenging to study diverse migration life-history strategies. In this review, we focus on Asian endemic masu salmon and white-spotted charr, which to date have received little attention in studies of partial migration, to seek a comprehensive understanding of the partial migration phenomenon in these salmonids.