Kyoto Institute of Oceanic and Fishery Science, Kyoto 626-0052, Japan
In order to clarify the stock structure of the Japanese flounder Paralichthys olivaceus in western Wakasa Bay, the first annual ring radius of the otolith was compared between immature and adult fish collected from 1994 to 2000. The average radius of the first ring in adult fish was significantly smaller than that of the same year-class immature fish. This phenomenon was observed in both females and males. Considering the previous reports which showed that there were two distinct populations as early settlers and late settlers, we speculated that the early settlers with larger fin ray number grow fast and migrate westward at one or two years old, whereas the late settlers with smaller fin ray number grow slowly and stay in this area throughout their life.