Abstract
On sandy tidal flats facing the Ariake Sea in Kumamoto Prefecture, annual harvest of clam, Ruditapes philippinarum, markedly decreased from 40,000∼65,000 tons / year to 1,000∼3,000 tons / year. In many trials to recover clam population on the flats, creating a sand cover on the flats with sand collected from sea floor of the offshore area of the Ariake Sea is the only successful measure. On newly created sand covers on the flats, juveniles of the clam are able to grow normally, while they die soon after settlement, forming small shells with several millimeter in length, outside the sand covers. We suspected the negative impact of contaminants in the sediment on the flats, and found manganese in high concentrations (1,400 to 2,900 μg / g)in the sediment where the young juveniles of the clam died. On the flats where the clam harvesting fishery was engaged, manganese concentration of the sediment was less than 500 μg / g. It is very likely that the creation of sand cover brought the effect to the sediment on lowering the concentration of manganese and provided a space free from the negative impact of manganese to juveniles of the clam.
