緑川河口干潟における盛砂後のアサリ（Ｒｕｄｉｔａｐｅｓ ｐｈｉｌｉｐｐｉｎａｒｕｍ）の個体群動態

Abstract

During 1970s, 40, 000 to 65, 000 tons of a clam, Ruditapes philippinarum, were harvested each year on the tidal flats in Kumamoto Ariake Region. However, in the past two decades, the tonnage harvested drastically declined due to the collapse of the clam-population in the whole area of the tidal flats. Fortunately, one of the measures to recover the clam population on the tidal flats, which is creating a sand cover on the tidal flats with clean sands collected from the sea floor in the offshore areas, succeeded to re-establish the dense patches ofthe clam with a density between 1, 000 to 6, 000ind. /m2. We have monitored the population dynamics of the clam since a new sand cover was set on the tidal flats in May 1997. On the sand cover, the planknotonic larvae of the clam recruited four times in a year. The newly recruited individuals grew to 30 mm in shell length in less than ten months. The production of the clam patches was estimated to 226.7 gDW/m2/year (only dried flesh weight) and 7, 832.0gWW/m2/year (wet weight including shell). To support this scale of secondary production of the clam, we are able to predict that a large amount of the primary production of more than 470 gC/m2/year is required. Although we were not able to confirm the presence of the primary producers on the tidal flats, the results of the population study of the clam indicate that the large potential of primary production was still preserved on the tidal flats in Kumamoto Ariake Region.