The purpose of this study is to investigate the water level manipulation in Lake Biwa paying attention to the spawning environment of the indigenous and endemic cyprinid fishes. We constructed the model analyzing the relationship between environmental condition (such as water level and precipitation) and spawning of Cyprinus carpio, Carassius sp., and two endemic species, C. buergeri grandoculis and C. cuvieri. Using this model, the number of eggs spawn on the reed beds is calculated in 3 monitoring points (Enshoji, Hariye, and Shin-hama) from 2004 to 2008, and this model calculates fairly well the variation in seasons and years. Predicting the effect of measures such as the change of water level manipulation on spawning, it is recommended to maintain water level for long-term higher in June and July, supposed the peak period of their spawning, and reproduce the stock laying eggs in higher water temperature.
