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.

人们经常研究鳗鱼的运动方式，以了解它们在产卵迁移之前在同一条河的淡水和微咸水中的运动。尽管otolith的微化学分析估计鳗鱼使用了淡水，咸淡水和海水栖息地，但尚未观察到日本鳗鱼在海上的河道间移动。在先前的研究中，银鳗似乎会在10月至12月发生产卵迁移，因此，在此期间迁移到海中的鳗鱼被认为是产卵迁移。在关于鳗鱼运动模式的声学遥测研究的进展中，我们发现了第一个证据，表明日本鳗鱼通过海上在遥远的河流之间移动。在日本和歌山县阿久川川进行的研究中，我们于2019年4月8日发布了15个带有声学标签的黄相鳗鱼。在2020年7月中旬，释放到Akugawa河中的15只带标签的鳗鱼中的3只在后来约15公里的Hidari-Aizu河中被当地钓鱼者捕获。这表明3只带标签的鳗鱼中有2只在10月和12月向海迁移。直到2019年6月30日才在释放点检测到另一只带标签的鳗鱼，由于未知原因，在被捕到Hidari-Aizu河之前再也没有发现过。在研究期间，其余12个带标签的鳗鱼留在了Akugawa河中。这些发现表明，日本鳗鱼可能会向海迁移，而不仅仅是产卵。这些信息对于了解鳗鱼的生活史多样性和未来的保护计划非常有用。