ABSTRACT: It is said that the tidal current in the Ariake Sea has changed since the construction of the Isahaya Dyke. The change may bring on the environmental deterioration in the Ariake Sea. Especially in the Isahaya Bay, the emergence of oxygen-deficient water has been prominent. In this study, firstly, we simulated the emergence of oxygen-deficient water by incorporating an ecosystem model into an ocean current model and elucidated the mechanism of such a phenomenon both from physical and biological viewpoints. Secondly, we proposed a local engineering solution, i.e. a seawater-quality enhancer, which mixes bottom and surface waters and discharges the mixture at the depth of euphotic layer, and numerically simulated its effects on the ecosystems. It was shown that 16% of the oxygen-deficient water in the Isahaya Bay is reduced by the apparatus with the flow rate of 1-million-ton water per day and 52 % in the 10-million-ton case 8 weeks after the start of operation. In the future, it is necessary that the reliability of the present model is improved by conducting more accurate simulations with incorporating a benthic model, which is deeply concerned with oxygen-deficient water, and by comparing the computational results with measurements more precisely.

ABSTRACT：In this study, applicability of the floating vessel of seawater treatment to remediation technique of water in the closed sea area is examined. As a result, it was found that the floating vessel increased transparency and decreased COD of seawater on the inside sea area closed by the silt protector. In addition, the collected materials on the floating vessel contained many nutrient salts and heavy metal ions. Therefore, using the floating vessel of seawater treatment, it is possible to improve the water quality on the closed sea area.

ABSTRACT : The objective of this paper is to conduct an economic analysis on the role of information in decision making of fishery persons on fisheries resources management using the expected utility theory. Through theoretical discussion and a case study of Stock Management of Snow Crab in Kyoto Prefecture, this paper found that information substitute for financial assistances in encouraging resources management. Even if fishery persons are given information on resources management, they do not always implement it. When the economic burden of resources management on fishery persons is too heavy, it is necessary to provide life security to them to implement the resources management. However, the amounts of money needed to ensure the livelihood of fishery person can be cut down if we provide them with information that reduce uncertainty and raise reliability.