Tracking Anguilla japonica Silver Eels Along the West Marina Ridge Using Pop-up Archival Transmitting Tags

Takatoshi Higuchi, Shun Watanabe, Ryotaro Manabe, Tsuyoshi Kaku, Akihiro Okamura, Yoshiaki Yamada, Michael J. Miller, and Katsumi Tsukamoto (2018) Japanese eels Anguilla japonica were tagged in order to understand their behavior in their spawning area. Three silver eels (EEL-A, B, C: TL792, 898, 992 mm) were tagged with pop-up satellite archival transmitting tags (PSATs) and released at different locations near/in their spawning area along the southern part of the West Mariana Ridge. EEL-A showed premature tag pop-up with mostly disordered records and the EEL-C tag did not pop up, while EEL-B showed stable diel vertical migrations during 31 of the 43 days it was tracked. EEL-B swam in shallower layers (411-182 m) during nighttime and deeper layers (563-885 m) during daytime. The mean nighttime swimming depth ± SD of EEL-B was significantly deeper during the full moon (342.4 ± 6.8 m) than the new moon (274.8 ± 16.9 m) and was positively correlated with the moon's altitude. EEL-B reached its maximum depths (851.1 ± 22.8 m) and minimum water temperatures (4.9 ± 0.1°C) during the sun culmination (sun at its highest point in the sky) of each day. The daytime water temperature varied between 4.7 and 5.2°C, staying at an almost constant 5°C. The eel started to dive to deeper water around nautical twilight (sun altitude: -11.6 ± 4.6°) and rise shallower around sunset (sun altitude: -0.8 ± 1.4°); sun altitude and swimming depth were correlated during the dives at dawn and ascents up at dusk. These results suggest that the regular diel vertical migrations of Japanese eels are strictly regulated by both light intensity and the lower limit of water temperature.