The change of dissolved oxygen in oyster culture grounds and the decline of growth of oysters

摘要

The oyster culture grounds in Hiroshima bay maintain approximately 24,000 t production level of the shucked meat, but the growth of oyster has been declined since 1960's. The reason, is though to be the decrease of dissolved oxygen in sorrounding waters caused by deterioration of bottom conditions due to the aging of culture ground. This article addresses the dissolved oxygen concentration in Hiroshima bay and its relation to the amount of chlorophyll a and the transparency level, which are deemed to affect seasonal and yearly changes of the oxgen concentration. The fluctuation of dissolved oxygen concentration in Hiroshima bay are mainly affected by the assimilation and the respiration of phytoplankton which is the major suspended matter in this area. These are related to the fluctuation of transparency level and the compensation depth. In addition, it is presumed that the consumption of dissolved oxygen by the bottom materials is smaller in comparison with that by the suspended matter. The main reasons of the decline of oyster growth in Hiroshima bay are estimated as follows; I) The long-term chronic water pollution caused by the dumping of sewage and industrial at the inner area. II) The inflow of clean low nutrients off-shore water into the growing grounds at the outer area.