Abstract
Fish sometimes enter into an intake gate of weirs and dams, so that a lot of released larval ayu (Plecoglossus altivelis altivelis) are lost. Some techniques with air bubbles, electricity and transmitted light have been developed to avoid fish from the intake gate. However, a lot of fisheries co-operative associations pointed out that the most of such techniques have no effect. It is qualitatively known that fish avoid the area which is included air bubbles. In this study, the air bubbles are injected from the bottom of channel with changing the flow velocity, and trajectories of fish are recorded. It was found that ayu avoid the bubble area with an increase of air quantity. The swimming speed of ayu is decreased in air bubble area. In addition, the increase of the flow velocity is promoted evasion behavior of ayu.
