Abstract
Hatching dates of larval Japanese flounder settling at Yanagihama Beach near Nagasaki in 1990 and 1991 were estimated by measuring daily increments of otolith of 1, 629 specimens (8.5-73.9mm SL). The spawning season of flounder was estimated to extend from late February or early January to the middle or end of April. Although spawning seems to have taken place every day, five prominent peaks appeared in the frequency distributions of hatching dates, mostly falling on spring tides. There are two alternative interpretations of this phenomenon; either there is a semilunar periodicity in spawning, or the larvae derived from eggs spawned during a spring tide have more chance of surviving and being carried to Yanagihama by some oceanographical conditions which previail during spring tides.
