Fig 1.
Major bathymetric (color) features of the study area in the western North Pacific Ocean.
Solid white curves with arrows indicate major ocean currents in top 300 m average. The magenta box represents the spawning area. The black ellipse marks the v-eel release area. The two red curves are the hypothesized seaward migration routes from (solid) Tsukamoto[15] and (dash) Yokose [16]. The star is located next to Mikawa Bay. The yellow curves identify the main ocean currents.
Table 1.
Parameters used in the numerical experiments [note that in all experiments, v-eels are programmed to move vertically (DVM), with swimming at depths of 200 m at night and 600 m during the day].
Fig 2.
(a)(b)Top 600 m mean flow field and (c)(d) vertical profile of ocean velocity at 134.2E [magenta lines in (a)(b)] show the cases of (a)(c) a strong Kuroshio and (b)(d) a weak Kuroshio. White lines show the reference v-eel migration depths.
Fig 3.
An example of the 3D trajectories of v-eel in Exp. 3A (true navigation, 0.15 m s−1). Colored dots are the ocean current speed.
The black curve is the horizontal projection of the migration path. Red arrows are the along-path ocean velocities.
Fig 4.
Time evolution (days) of the depth-integrated trajectories of v-eels for each tested scenario at swimming speed of 0.15 m s−1.
Left panels are for cases A in which the Kuroshio is strong, and right panels are for cases B in which Kuroshio is weak. (from top to bottom) Exp. 1 (random), Exp. 2 (compass), Exp. 3 (true navigation), and Exp. 4 (against Kuroshio). Gray dots mark the locations where the number of v-eels is less than 1%. The magenta box shows the spawning area.
Fig 5.
Percentage of v-eels arriving at the spawning area over time in Exp. 2A (compass orientation 0.15 m s−1), Exp. 3A (true navigation, 0.15 m s−1), Exp. 6A (0.1 m s−1), Exp. 7A (0.65 m s−1), and Exp. 8A (0.65 m s−1 against the Kuroshio).
The yellow area marks the spawning period.
Table 2.
Mean migration duration for successful v-eel arrival (days) from the south coast of Japan to the spawning area located 2300 km further south.
Cases A and B are the strong and weak Kuroshio cases, respectively. Mean migration duration is significantly different between case A and case B for each experiment (Wilcoxon test, p < 0.001).
Table 3.
Mean arrival rate (%) for successful v-eel arrival from the south coast of Japan to the spawning area.
Mean arrival rates are significantly different among experiments (chi-square test, p < 0.001).
Fig 6.
Same as Fig 2 but for v-eels swimming with true navigation at different swimming speeds: (from top to bottom) Exp. 5 (0.05 m s−1), Exp. 6 (0.1 m s−1), Exp. 7 (0.65 m s−1), and Exp. 8 (0.65 m s−1 against the Kuroshio).
Fig 7.
Mean along-path ocean speeds during (dash) daytime and (solid) nighttime for (red) Exp. 3A and (blue) Exp. 3B.
Fig 8.
Schematic diagram showing how a v-eel swims across the Kuroshio.
Black arrows represent the Kuroshio condition. During nighttime, the v-eel stays near the surface. The eastward strong Kuroshio together with the southward swimming direction of the v-eel leads to an east-south-east movement. The v-eel dives to deeper water during the daytime, where the weaker (than at the surface) Kuroshio and southward swimming results in a southeast migration. A few days later, the v-eel is able to cross the Kuroshio because of the accumulation of southward swimming.
Fig 9.
Along-path meridional velocity (m s−1) in the Kuroshio, recirculation region, subtropical gyre, and mean velocity for (a) nighttime and (b) daytime in Exp. 3.