Field Survey on Aquatic Environment and "Ayu" Habitat in Urban River

Field measurements were performed to investigate "Ayu" habitat in the Gokase (Ose) river in Miyazaki. "Ayu" is famous and popular, and hence, one of the most important fish in Japan. There has been substantial river engineering for flood control, e.g. dredging of bed, especially near urban areas. This engineering work can damage the fish habitat. In order to protect "Ayu" habitat, it is necessary to predict how the fish respond to habitat damage in the river, i.e. an environmental assessment. The water quality e.g. DO (Dissolved Oxygen), BOD, COD, SS, the temperature, and the discharge are all significant for the fish habitat. The abundance of algae attached to the surface of bed material, e.g. stones, is also significant because it is a food source for the fish. The water quality, the temperature, and the velocity of flow were measured at the several points in one of the artificial spawning beds "Ogura-shita rapid" in the Ose river. Material attached to the river bed was collected, and TR (Total Residue) and Chl.a (Chlorophyll.a) were smaller in summer, and larger in winter, which is different from what is generally expected. The growth rate of algae in the Ose river was found to depend on the velocity of flow, i.e. the value of Chl.a was increased as the velocity was increased, became the largest at a velocity of 0.4 to 0.5m/s, and decreased with increasing velocity above 0.5m/s.