Manila Clam and Pacific Oyster Culture in Isahaya Bay — For the Sustainable Production in Stressful Environment —

Isahaya Bay is a branch of Ariake Sound, Kyushu, where the tidal range is the largest in Japan. Culture of Manila clam, Ruditapes philippinarum, commenced in the 1970s and is currently the most important industry in the Konagaicho Fisheries Cooperative. The Cooperative introduced a demarcated fishery for Manila clam culture, so that the fishermen are in charge of the management of their own culture grounds, such as sand placement on muddy substrate, installation of facilities for accumulating the juvenile clams and preventing predators, and registering landings under the cooperative sales. The annual landing normally reaches 400- 700 metric tons for about 100 individual fishermen. But Manila clam culture has occasionally suffered from mass mortality during summer since 1998 when the barrage in the Isahaya Reclamation Project closed the inner bay area in the previous year. We studied the mass mortality from the aspects of environment and clam physiology and determined that the main cause of mass mortality was anoxia by means of continuous monitoring of water quality factors and analysis of organic acids in the pallial cavity fluid of Manila clams. For sustainable and stable production of Manila clams, prediction and prevention of anoxic water intrusion to the culture grounds are of critical importance. Suspended culture of Pacific oyster, Crassostrea gigas, was introduced in late 1990s in Isahaya Bay, which had naturally produced delicious native oysters near the shoreline. The oyster culture industry has grown up to 175 metric tons a year by 42 fishermen in 2005. However, there are several problems constricting the increase in production; namely, summer mortality after spawning, heavy fouling by barnacles, and lower consumer demand in the local market. Increasing market demand and improvement of the culture technique are necessary.