• archival tag;
  • biological clock;
  • natal stream;
  • swimming depth;
  • swimming speed

The swimming depth of chum salmon Oncorhynchus keta equipped with archival tags was investigated off the Pacific Ocean coast of Hokkaido and North Honshu, Japan. As shown from movements of the fish with disc tags, O. keta swam at shallower depths during the full-moon phase than in the other phases and their swimming speed during this phase was faster compared to other phases. In addition, the circadian rhythm suggests a biological clock. These observations are all consistent with the view that O. keta make use of moonlight in order to navigate at night-time during homeward migration.