Environmental DNA detects a possible Japanese eel spawning event near a video-recorded anguillid eel in the open ocean

Facultative catadromous eels migrate back to the sea to reproduce, but their spawning behavior and locations have remained elusive. Using environmental DNA (eDNA), we identified a likely spawning site location and time of spawning of the Japanese eel. We detected Japanese eel eDNA at 400 and 600 m and recorded a likely sighting of this species at about 220 m using a deep-tow camera system 6 d before the new moon. A strong eDNA signal was obtained at 400 m from the apparent spawning event the morning after the estimated peak of eel spawning, 3 d before the new moon. These findings indicate that Japanese eels were already within the area where they were going to spawn at least 6 d before the new moon and then may have spawned near the strong eDNA station 3 d before the new moon. We concluded that the eDNA analysis is useful in searching for spawning sites and determining the timing of spawning of aquatic organisms with external fertilization that causes a temporary surge in eDNA, although prior knowledge of likely spawning sites is needed.