Study on Reason Why Ayu Migrates From Sea to River

The ayu, Plecoglossus altivelis altivelis, migrates from the sea to the river in spring. Hotta(1953) pointed out that the reason why the ayu migrates is coincidence of water temperatures in the river and sea. Wada(1990) pointed out that the water level at the sea greatly affects on the migration number. However, a collective view has not been obtained. In this study, the water level, water temperature, pH, DO, COD, electric conductively, turbidity when the ayu migrates and those when the ayu does not migrate were compared. The ayu wants to migrate when the water temperature is high, the sea water level is high. On the other hand, the ayu does not want to migrate when the discharge is high, due to the water temperature decreases. In such a situation, sometimes DO becomes high.