Abstract
The mechanisms of oceanic animal migration remain enigmatic. Adult Japanese eels start their long-distance oceanic migration from coastal areas to breed near the West Mariana Ridge. We tracked acoustically tagged eels released in the Kuroshio Current (KC) area near Japan (five silver-phase eels, three of which had impaired swim bladders) and a tropical/subtropical (TS) area near/in the spawning area (two yellow-phase and three silver-phase eels). We analyzed their active swimming and transport by water currents. The strong flow of the KC dominated the eels’ movements in the north, and TS area; their swimming influenced their movements. In the KC area, greater distances were covered at night than during the day, because eels swam in shallower layers with strong currents at night. Three and one eel in the TS and KC area in the upper 400 m showed counterclockwise and clockwise movements around the time of solar culmination, respectively. The meta-analysis showed that eels released at middle latitudes (20°–34° N) generally swam southward through currents, whereas those released at low latitudes (12°–13° N) generally swam northward through currents. Our study suggests the influence of the surrounding current and a potential effect of solar cues on the movements of Japanese eels.
Highlights
Temperate catadromous eels of the genus Anguilla are among the most enigmatic animals exhibiting a longdistance-oriented migration[1, 2]
The first Pop-up satellite archival tags (PSATs) study of Japanese eels demonstrated that they moved along the K C14; the drag caused by the external tag could affect their swimming p erformance[38–40]
The mean vectors of 10-min-interval swimming vectors during day and night over the tracking periods were calculated to measure the average velocity by active swimming over each period, i.e., the net displacement divided by the time traveled, in which the magnitude of the mean vector was defined as effective swimming speed
Summary
Temperate catadromous eels of the genus Anguilla are among the most enigmatic animals exhibiting a longdistance-oriented migration[1, 2]. Anguillid eels are born in the open ocean, their larvae grow drifting with oceanic currents[1, 3, 4] They migrate to freshwater and estuarine growth habitats. Satellite-based tracking relies on ocean circulation models[17, 18], but the daily locations reconstructed from the PSAT data have high uncertainty with more than 100 km on average[17] This uncertainty must cause difficulty in differentiating passive transport and active swimming. The first PSAT study of Japanese eels demonstrated that they moved along the K C14; the drag caused by the external tag could affect their swimming p erformance[38–40] It remains in question whether the hypothesized migration route has been traced without the effects of drag by the external tag
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