The successful implementation of policies that protect the maritime environment by setting limits on the total load of organic matter, nitrogen, and phosphorus in wastewaters has led to improved water quality in the Seto Inland Sea. As a result, red tides (harmful algal blooms) have become less frequent and smaller compared with the 1960s, and fisheries such as the yellowtail (Seriola quinqueradiara) culture fishery have benefited from the reduced risk of red tide damage. The implementation of quality control standards for river water and conversion of paddy fields to urban areas have also reduced the amount of nutrients in the sea derived from surrounding lands and inflowing rivers. These policies, along with a decline in the areas of seagrass beds and tidal flats, have caused oligotrophication of the sea. Recently, the "Nori" (Porphyra spp.) culture fishery has claimed that the oligotrophic seawater constrains Porphyra growth, and that the amount of nutrients entering the sea should be allowed to increase. We propose a new conceptual model for evaluating water quality control levels that uses a risk management method to determine the optimal nutrient load to maximize total fishery production. Because the available data indicate that the relationship between the nutrient load in seawater and fishery production is probabilistic, the damage to fisheries caused by both eutrophication and oligotrophication must be captured as risk. We quantified the cost of the risk as the price of an insurance bond to compensate for fishery damage caused by red tides or reduced growth. The difference between the expected maximum production of a fishery and this cost is an indicator of the impact of policy on the fishery. This indicator will be low both when the nutrient load is too high and when it is too low. Thus, there exists an optimal nutrient load at which the value of the indicator will be maximized. This study evaluated the COD load that maximizes the sum of the expected incomes of these fisheries is 75 ton/day, and the TN load that maximizes the sum is 65 ton/day. Both these amounts are much higher than the current loads. We applied our model to the evaluation of water quality control levels in Sea of Harima-nada, in the eastern Seto Inland Sea, by carrying out statistical analyses of time series data of yellowtail and Porphyra fishery production, and of chemical oxygen demand, total nitrogen, and total phosphorus loads from the surrounding land. Our results suggest that the Porphyra culture fishery would benefit from a change in the environmental pollution control policy.
