River to river: First evidence of eel movement between distant rivers via the sea

Eel movement patterns have been frequently studied to learn about their movements within the fresh- and brackish waters of the same river before their spawning migrations. Although otolith microchemistry analyses estimated that the eels used fresh-, brackish, and marine water habitats, inter-river movements of Japanese eels via the sea have not been observed. In previous studies silver eels appear to migrate for spawning in October-December, and so eels that migrate to the sea during this period have been considered as migrating for spawning. In the progress of our acoustic telemetry study on eel movement patterns, we found the first evidence that Japanese eels move between distant rivers via the sea. During the study in the Akugawa River, Wakayama Prefecture, Japan, we released 15 acoustically tagged yellow-phase eels on 8 April 2019. Three of the 15 tagged eels released into the Akugawa River were later caught in the ~ 15 km distant Hidari-Aizu River by a local angler during early mid-July 2020. This showed that 2 of 3 tagged eels migrated seaward in October and December. The other tagged eel had been detected only at a releasing point until 30 June 2019 and due to an unknown reason was never detected again before capture in the Hidari-Aizu River. The remaining 12 tagged eels stayed in the Akugawa River during the study period. These findings indicate that Japanese eels may migrate seaward for purposes other than just spawning. This information is valuable for understanding the eels’ life-history diversity and for future conservation plans.