Fish habitat evaluation based on width-to-depth ratio and eco-environmental diversity index in small rivers

We estimated the performance of river fish habitat evaluation using width-to-depth ratio (WDR) in comparison with eco-environmental diversity (EED) to propose an inexpensive and easy-to-use habitat evaluation procedure, which is applicable to small river construction works. WDR calculation costs less than that of EED. For verification, 25 stations in eight rivers were selected and fish were captured using electrofishing. pH, electrical conductivity, turbidity, dissolved oxygen (DO), water temperature, fraction of forest, farmland, and residential area in each basin were measured to examine possible influence of water quality. Results show that there is no major water quality issue in the target rivers. Although fish habitat is classified as good when WDR is higher than 6, it cannot be evaluated by WDR when it is lower than 6. EED has positive relationship with fish habitat for any WDR value. Thus, if a river geometry design in a river work results in WDR higher than 6, no measures need to be taken regarding fish habitat condition; however, if it is less than 6, it is necessary to examine whether the construction work lowers the EED or not.