Size-selective mortality occurs in smolts during a seaward migration, but not in river residents, in masu salmon (Oncorhynchus masou)

Salmonid fish often experience size-selective mortality when descending the river (i.e., seaward migration). However, it is unknown whether size-selective mortality is specific to this life history (i.e., migrants), or is shared by an alternative life history (i.e., residents). In this study, we investigated the size-dependent mortality patterns of masu salmon (Oncorhynchus masou) migrants and residents during the migration period (i.e., April to June) in the Horonai River, northern Japan. By conducting an individual-based study using PIT tags and antennas, we show that larger migrants more likely survived the seaward migration than smaller migrants, but size-dependent survival was not detected in river residents during the same period. These results suggest that size-selective mortality is specific to the river-descending migrants in masu salmon in their seaward migration period. We attribute this finding to the presence of piscivorous fishes (e.g., brown trout Salmo trutta) which occupy the migration corridor and consume masu salmon migrants, whereas such piscivorous fish do not occur in the river section farther upstream inhabited by residents.

Automated summary

This study investigated the size-dependent mortality patterns of masu salmon migrants and residents in the Horonai River in northern Japan. The results showed that size-selective mortality is specific to the river-descending migrants during their seaward migration period, which may be attributed to predation by piscivorous fishes.