Distribution and stable isotope ratio characteristics of Japanese eel leptocephali in relation to hydrographic structure of their Pacific Ocean spawning area

To understand the larval distribution, size variation, and stable isotope ratios of Japanese eel leptocephali in relation to the salinity front and their feeding ecology, larvae from 7 research cruises (2002-2013) in the North Equatorial Current (NEC) spawning area were examined. The smallest early-stage larvae were distributed south of or near the salinity front, confirming that the salinity front is an important oceanic feature to understand spawning locations of the Japanese eel. Larger size larvae tended to distribute into higher latitudes. Transport to northern latitudes with their growth would facilitate transport into the Kuroshio region, but retention in the Subtropical Countercurrent (STCC) might be detrimental. Preleptocephalus isotope ratios reflected maternal ratios, but feeding-stage leptocephali (8-56 mm) tended to have higher partial differential 15N values at lower latitudes typically in areas south of a salinity front. Feeding larvae quickly assimilate isotope ratios from the NEC after spawning and early growth. Large differences of partial differential 13C values of larvae between the NEC and STCC might vary with spatial baselines in the two currents. However, diel vertical migrations should be considered, because the isotope ratios in particulate organic matter distinctly depend on the depth. Comparisons among Japanese eel larvae and other taxa of leptocephali in the NEC illustrate the need for further studies on the trophic ecology of leptocephali.