抄録
Fishermen are embarrassed upon the population reduction of larval fish by the sluice gate which is constructed with dams and weirs. The terror by the falling object from the upward may be valid to make fish avoid from the sluice gate. In this study, the small object was dropped from the upward toward the water surface of a pool, and the fish trajectories were recorded by a digital video camera. It was found that the ayu avoid from the object very quickly during about 1s, and after that ayu make a school of fish gradually. The swimming speed and also the swimming distance during avoiding decreases with an increase of the fish population due to the crowding.
