Captive-bred populations of a partially migratory salmonid fish are unlikely to maintain migratory polymorphism in natural habitats.

Variation in life history is fundamental to the long-term persistence of populations and species. Partial migration, in which both migratory and resident individuals are maintained in a population, is commonly found across animal taxa. However, human-induced habitat fragmentation continues to cause a rapid decline in the migratory phenotype in many natural populations. Using field and hatchery experiments, we demonstrated that despite both migrants and residents being maintained in captive environments, few individuals of the red-spotted masu salmon, Oncorhynchus masou ishikawae, became migrants in natural streams when released prior to the migration decision. Released fish rarely reached the threshold body size necessary to become migrants in natural streams, presumably owing to lower growth rates in natural than in captive environments. The decision to migrate is often considered a threshold trait in salmonids and other animal taxa. Our findings highlight the need for management programmes that acknowledge the effects of the environment on the determination of the migratory phenotypes of partially migratory species when releasing captive-bred individuals prior to their migratory decisions.