A model for the larval migration of the Japanese eel: roles of the trade winds and salinity front

A model that helps explain the mysterious long-distance migration of the Japanese eel (Anguilla japonica) is presented, based on oceanographic observations, satellite buoy drift experiments, and samplings of eel larvae taken in 1991. The trajectory of a 150 m depth buoy relased in the spawning area strongly suggests that A. japonica larvae spawned just south of the salinity front are transported westward by the North Equatorial Current (NEC). The larvae are then thought to be entrained into the Mindanao Current flowing southward along the Philippine Islands where A. japonica juveniles are scarcely distributed. These controversial results lead to the assumption that eel larvae are transferred from the NEC to the northward flowing Kuroshio, which distributes the eel larvae to the growth habitats of eastern Asia. In this eel larvae transfer model, a northward Ekman transport caused by trade winds plays an important role in explaining the wind-induced northward shift of the larvae together with the onset of diel vertical migration. Assuming that leptocephali greater than 20 mm initiate the vertical migration, a westward wind velocity greater than 5 to 10 m s^-1 should be high enough to diminish the southward current velocity. When the physical and geophysical conditions — such as the salinity front for spawning activity, the water tunnel for westward larval transport, the Ekman transport by the trade wind for transfer of the larvae from the NEC to the Kuroshio, and the strong velocity of the Kuroshio for rapid transport to growth habitats — are well matched with the timing of the onset of vertical migration, large-scale eel migration could result.