Abstract
Brackish Lake Shinji, Japan, has been designated for conservation efforts by the Law Concerning Special Measures for the Conservation of Lake Water Quality. Although the amount of loads flowing into this lake is decreasing, the chemical oxygen demand （COD） is increasing in this lake. To determine the cause of the increase of COD, we applied a multiple regression analysis using the environmental factors that might affect COD of lake water. By multiple regression analysis, only Chl-a concentration was extracted as a factor influencing the COD concentration. And extracted as a factor influencing the Chl-a concentration were taking out by fishing and TP concentration. COD concentration and Chl-a concentration decrease of the catch of Corbicula japonica was suggested to be most effective on the increase of COD and Chl-a in lake water. Reduce of the nutrient loads to the lake has been regarded as the primary measure to decrease COD in the lake. However, COD and Chl-a removal from the system as the fishery of Corbicula japonica has improved the lake water quality. It is, therefore, necessary to consider the influence of the food web and material flow in the ecosystems to effectively manage and conserve the water quality of lakes.
