Migratory individuals (migrants) should become large before migration because smaller migrants suffer higher mortality during migration. The pattern that smaller migrants start migration later than larger ones may represent a pre-migration tactic to cope with size-selective mortality, since delayed migration may allow smaller migrants to become large. However, processes and consequence of delayed migration have been poorly studied. We should investigate where smaller migrants stay longer before migration and whether it results in significant growth. Since migrants stay in two distinct pre-migration habitats (i.e., natal habitat and migration corridor), we predicted that smaller migrants stay longer in either or both habitats to grow large before migration. Here, we tested this prediction by using masu salmon migrants with a capture-mark-recapture method and individual tracking system using PIT-tags.
Our two-year survey revealed that smaller migrants stayed longer in both natal habitat and migration corridor in one year, but only in migration corridor in the other year. Although smaller migrants showed different behaviors between two years, they achieved larger size increment before migration in both years. These results suggest that masu salmon migrants flexibly change their stay in the structured riverscape to become large before migration.