The salinity front in the North Equatorial Current: A landmark for the spawning migration of the Japanese eel (Anguilla japonica) related to the stock recruitment

The North Equatorial Current (NEC) region is the only spawning area of the Japanese eel. The 2002 expedition of the R.V. Hakuho Maru indicated that the larval distribution of this species is likely related to a salinity front generated by two distinct water masses in the NEC. The cruise occurred during an El Niño event and the salinity front had moved southward by about 4° of latitude. Smaller larvae (<10 mm TL) were collected just south of the salinity front where these young larvae have never been collected in usual years. Drifters released at stations around the salinity front showed that most of them were transported into the Mindanao Current region. The southward shift of the spawning area associated with a southward movement of the salinity front may have been caused by the 2002 El Niño and would increase larval transport into the Mindanao Current region. Analyses of stable isotope ratios suggested that younger leptocephali have been living in the westward flows south of the salinity front and that their diet may change with development. These findings indicate that differences of the water masses separated by the salinity front probably help to determine the spawning location and the salinity front functions as a landmark for the spawning migration of the Japanese eel.