Overview of Masu Salmon in Japan: With Special Reference to Alternative Migratory Tactics

Masu salmon (Oncorhynchus masou) are an Asian endemic Pacific salmon, and anadromous masu salmon support one of the most commercially valuable fisheries in northern Japan. As with some other salmonids, masu salmon exhibit partial migration. Subsequent to the parr stage, fish descend to the sea in spring after smolting, spend a fixed period of 1 year in the sea for feeding and then return to the natal river to breed in autumn. Most migrants descend to the sea at age 1+ years, but some migrants descend to the sea at age ≥2+ years. In contrast, some fish do reproduce precociously without migration to the sea. Resident male salmon can mature at age 0+ years, whereas resident female salmon can mature at age 1+ years, though resident female salmon are rare. Migrants are semelparous, whereas residents are iteroparous. An experimental study showed that both resident and migratory female salmon can produce both migratory and resident offspring; therefore, different migratory tactics are found within the same gene pool. Fitness calculation indicated that individuals exhibited a status-dependent conditional strategy, that is, the adoption of alternative migratory tactics is influenced by the status of individuals to make the best of a situation.