A nursery aquaculture pond (1200m2 × 1.2m) for Japanese flounders, Paralichtys olivaceus, was constructed on a sandy beach. It utilizes wave energy for exchanging water in itself. To estimate how much inflow rate is needed to keep water qualities proper for flounders, a numerical model to forecast water quality was made. It consists of momentum equation, continuity equation and diffusion equations (water temperature, salinity, dissolved oxygen (DO)). The diffusion equation of water temperature includes the tem of heat flux depending on time through water surface・ The diffusion equation of DO includes the tem of consumption rates by suspended matter, sediment and respiration of flounders. However DO production is neglected. This model also can calculate the effect of coastal underground water upon the water quality in the pond. Heat flux was calculated by 'bulk' fomulas. DO consumption rate by suspended matter was measured by jn situ light and dark bottle oxygen method. That rate by sediment was measured in the similar method. That rate by flounders was calculated from the equation of expressing the relation between body weight and respiration rate. The quality of coastal underground water was measured on the spot and the quantity of that was estimated from the pumping tests. This model made it clear that the inflow rate of 227m3/hr is required to maintain the DO concentration over 3.0 ml/l all over the pond, and 313m3/hr is required to maintain it over 3.5ml/l. We could propose the useful method in deciding inflow rate in comection with water quality change.
