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

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.
Methods
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.