Effects of Changing Gradient Angle of Notch and Discharge on Migration of Fish

摘要

Fishways have been constructed to facilitate migration of fish past dams, waterfalls and rapids. The most of fishway in Japan is pool-and-weir fishway. One of the factors that influences on the migration rate in pool-and-weir fishway is the gradient angle of notch. Wada pointed out that the recommended value of the gradient angle of notch in pool-and-weir fishway is 60 degree. However, there is no measurement at the angle more than 60 degree. In this study, gradient angle of notch in a pool-andweir fishway was changed in the range from 46 to 82 degree. Moreover discharge was systematically changed from 1 to 13 liter per sec. The migration rates of ayu, Plecoglossus altivelis altivelis, were obtained with the aid of two sets of digital video cameras. The number of used fish is 100. Velocity measurements were conducted with a 3-D electromagnetic current meter. It was found that the migration rates of ayu take the highest value when the gradient angle of notch is from 55 to 64 degree, irrespective of the flow velocity. These results correspond to Wada's point. The migration rates are controlled by the falling velocity from the notch, the distance between the falling flow and orientation area and the angle of fish and falling flow. It was found that there are three factors to cause the migration. The three factors are (1) ayu want to rest close to the falling flow, (2) the angle of fish is close to the angle of falling flow and (3) the falling flow velocity is less than the burst speed and larger than the cruising speed. When one or more of these factors is not filled, the migration becomes difficult.