Larval Transport of the Japanese Eel
The spawning ground of the Japanese eel (Anguilla japonica) is located in the North Equatorial Current (NEC), to the west of the Mariana Islands near a salinity front at 15°N, 140°E (Tsukamoto 1992; Kimura et al. 1994). The absence of large genetic differences among Japanese eels caught in Japan, China, Taiwan, and Korea (Taniguchi and Numachi 1978; Sang et al. 1994; Ishikawa et al. 2000) suggests that the region west of the Mariana Islands is the only spawning ground for the Japanese eel. Their larvae, termed leptocephali, are transported in the surface water of the NEC from the spawning ground and then the larvae must transfer from the NEC to the strong northward-flowing Kuroshio at the terminus of the NEC east of the Philippine Islands after entirely passive westward transport by the NEC. Figure 1 shows the ocean circulation around the spawning and nursery grounds of the Japanese eel in East Asia. Individuals transported into the Mindanao Current would be lost to recruitment because the Japanese eel is not found in the areas downstream of this current. Therefore, an efficient physical mechanism of larval transport is necessary to successfully complete the migration of the Japanese eel from the spawning ground to their growth habitats. What must be determined to clarify the mechanism of the larval migration is how the adult eels find the spawning ground and how the tiny leaf-like eel larvae migrate 3000 km to East Asia. In this chapter, spatial and temporal fluctuations of the water circulation in the NEC and their effects on the larval migration of the Japanese eel are described, and the possible role of these factors in the observed decline of both the Japanese eel and the Atlantic eels is also discussed.
KeywordsNorth Atlantic Oscillation Index North Equatorial Current Thermal Front Larval Transport Salinity Front
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