Annual variation in riverscape habitat use by a diadromous fish before oceanic migration

Many animals reap significant benefits from migration, yet they face high mortality rates during the journey. As the mortality during migration typically operates more strongly on smaller migratory individuals (migrants), small pre-migrants among migratory animals tend to grow better than larger ones prior to migration, enabling them to attain a larger body size. A common growth strategy for smaller pre-migrants is extending their growth period compared to larger ones. Although this tactic has been documented in previous studies, the landscape-scale habitat utilization that allowed smaller pre-migrants to stay longer prior to migration remains largely unknown. Here, we focused on two distinct riverscape habitats (nursery habitat and transitional habitat) and investigated where smaller pre-migrants of masu salmon (Oncorhynchus masou) stayed longer than larger ones prior to oceanic migration. We conducted an individual monitoring survey on their habitat utilization in two distinct migration years. We found that in 1 year, smaller pre-migrants stayed longer in the transitional habitat compared to the nursery habitat, whereas in the other year, they stayed longer in the nursery habitat compared to the transitional habitat. This suggests that smaller pre-migrants can adjust their duration of stay in each riverscape habitat. Our findings highlight the importance of conserving the entire pre-migration habitat to enhance the resilience of migratory animals in the face of rapid environmental changes.