Effects of water temperature and rearing density in winter on early sexual maturation of ayu Plecoglossus altivelis

In order to investigate the effects of water temperature and rearing density in winter on the sexual maturation of ayu Plecoglossus altivelis, the frequencies of individuals with GSI values exceeding 0.1 for males and 1.0 for females (maturation-initiated individuals) were compared in groups with different rearing conditions. In the group reared at a high water temperature (18°C) from the end of November, the maturation-initiated individuals occurred in about 20% of males and about 10% of females in spring. On the other hand, in the group that was reared at the high water temperature for the first 20 days and then kept at a low water temperature (14°C), the maturation-initiated individuals did not occur in both males and females. In the group reared at the high water temperature and rearing density reduced to 1/10, the frequency of the maturation-initiated individuals was reduced. In the process of seedling production of ayu, it was effective to set the rearing water temperature in winter to 14°C in order to suppress the occurrence of early mature fish. In addition, lowering the rearing density may suppress the frequency of occurrence of early maturing fish in high water temperature.