Supplementation of wild populations with captive-bred individuals is often ineffective for boosting long-term productivity of wild populations. On the other hand, it remains unknown whether supplementation can act to maintain life-history variation in natural habitats, which is also important for 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 rapid decline in a migratory phenotype among many natural populations. By using field and hatchery experiments, we here demonstrated that while migrants and residents could be maintained in captive environments, few fish became migrants in natural streams in red-spotted masu salmon Oncorhynchus masou ishikawae. Released captive-bred fish rarely reached the threshold body size necessary to become migrants in natural streams, presumably due to lower growth condition 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 supplementation programs to acknowledge environmentally induced changes in life-history decisions for partially migratory species.

Experimental stocking was conducted in 10 sections of seven streams in an upper drainage of the Arida River system in southwest Honshu, Japan. Data S1 and S2 sheets in the excel file were obtained in the experiment.

Captive-bred fish from the two hatcheries were reared in outdoor mesocosms. Data S3 sheet in the excel file was obtained in the mecosocm experiment.