1Freshwater Fisheries Research Division, National Research Institute of Fisheries Science, Fisheries Research Agency, Nikko, Tochigi 321-1661, 2Tochigi Prefectural Fisheries Experiment Station, Yuzukami, Tochigi 324-0404, Japan
Forecasting of the first ascending day and the number of ascending schools of juvenile amphidromous ayu Plecoglossus altivelis altivelis were examined in the Naka River, Tochigi Prefecture, central Japan. We analyzed the relationships among 4 parameters (cumulative sea water temperature from October to March near the estuary of the Naka River, the day of the first observation of an ascending school, the number of ascending schools observed from March to June and the average body weight in late October) over a period of 7-21 years between 1979 and 1999. The first observation day of a school was significantly earlier when the cumulative water temperature from February to March was higher. The number of schools was larger both when the cumulative water temperature in March was higher and the first observation day of a school was earlier. The body weight in late October was smaller when the number of schools was larger. The results show that the first observation day of a school and the number of schools can be forecasted from the cumulative water temperature from February to March, and the cumulative water temperature in March and the first observation day of a school, respectively.