Study on the inshore migration mechanism of Conger myriaster leptocephali

Conger myriaster is an important commercial fish species in Japan, Korea and China. The larvae have only been caught in shallow coastal areas, and most of them are large individuals (80-130 mm TL) at the late leptocephalus or metamorphic stage. Because of the lack of information on distributional ecology of small leptocephali, recruitment mechanism of C. myriaster has been a mystery in spite of many catches of large leptocephali in coastal areas. Because mature individuals have not been collected from inshore waters to continental slope where commercial fishing takes place, the spawning ground has been presumed to be in deeper offshore waters. The early life history of C. myriaster from spawning in offshore waters to their inshore migration remains unknown. The distributional pattern of leptocephali in the genus Conger was investigated around the Kuroshio, the Kuroshio Extension in the East China Sea and the western North Pacific in order to determine the inshore migration pattern of C. myriaster leptocephali. The body density of the leptochphali is ascertained to be lower in the offshore areas before migration than in the coastal areas because sea water density in upper layer of the offshore Kuroshio in which the leptocephali maintain neutral buoyancy ranging from 1.023 to 1.024. Therefore, the leptocephali are considered to increase the body density through the inshore migration process. The ability of the larvae to regulate body density is important in controlling their migration from warm to cold water areas because of a negative relationship between seawater density and temperature. The findings of the present study suggest a hypothetical pathway of the migration in which the larvae vertically migrate into deeper depths to approach coastal areas. Simulations of larval dispersal or retention by hydrodynamic models should incorporate the information on larval body density fluctuation.