Management of Aquaculture and Marine Environment in an Open-Type Inner Bay Through the Satoumi Approach: The Case of Shizugawa Bay

Abstract

A demarcated fishery right is set on a sea area for aquaculture designated by a governor of a prefecture in Japan. Around Japan, this right area broadly covers open-type inner bays, where oyster culture is one of the most active types of aquaculture. It is necessary to develop a coastal management approach for realizing sustainable aquaculture in a sound marine environment in the open-type inner bay through understanding suspended and benthic material circulation as well as aquaculture impact on the marine environment. We have therefore conducted the monitoring of the marine environment and aquaculture and made studies on material circulation and ecosystem modeling of Shizugawa Bay, a typical rias-type bay as an open-type inner bay since 2014. We have established a council for sharing data obtained from the studies and discussing on marine environment in the bay based on scientific data with stakeholders from Shizugawa Bay since 2015. After the Great East Japan Earthquake in 2011, Shizugawa Branch of Miyagi Prefecture Fishery Cooperative succeeded in reducing oyster culture rafts to 55 and 23% to those before the tsunami event in the sea areas of Shizugawa and Tokura areas, respectively, both located in the head area of Shizugawa Bay. The result of the reduction lead to an increase in the growth rate of oysters, consequently shortening the oysters’ culture period. In situ experiments showed that feces and pseudofeces of young oysters contained less percentage of organic matter than in the case of the old ones, leading to less organic matter on the sea bottom. Nitrogen is a limiting nutrient in Shizugawa Bay. Dissolved iron was insufficient for boosting phytoplankton (diatom) in the surface layer of the center part of the bay in summer. These nutrients were mainly exported from the offshore waters compared to little inflow from the rivers due to their narrow watersheds like as many rias-type bays. The physical–biological coupling ecosystem model of Shizugawa Bay predicted marine environment and aquaculture production based on several scenarios co-produced by scientists and fishermen. Through the Council for the Future Marine Environment of Shizugawa Bay, the stakeholders, including the authors of the study, discussed how many aquaculture rafts and target species were optimal on the basis of the predicted results. The discussion was reflected in the renewal of the demarcated fishing right area in 2018. In Shizugawa Bay, the forestry activity, with a FSC international ecolabel, was linked to oyster culture with an ASC ecolabel through macrophyte beds registered as a Ramsar Site in 2018. These efforts and the reduction in the number of aquaculture rafts are activities called satoumi and satoyama, which enhance biodiversity and productivity through sustainable use of the coastal waters and mountains and will become a new universal coastal area management tool.