2015 Volume 10 Issue 4 Pages 202-214
Long plate-like sediment samples were collected using a Geoslicer technique at five different sites on the sandy tidal flats along the eastern coast of Ariake Bay, Kyushu, western Japan, where there are major harvesting sites of the edible clam Ruditapes philippinarum. The deposition process of the sediment and fossil contents of the shellfish in these Geosliced samples were examined, dating the deposition process of the sediment to at least 400 years ago with a trace of a giant tsunami that occurred in 1792, and radio-active matters (14C contained in the wood pieces, 210Pb and 137Cs), and describing the distribution of the fossil contents of the shellfish in the sediment samples. These results indicate that dense patches of R. philippinarum were established later than 180 to 190 years ago at the sampling sites, and it is very likely that this species was introduced to the sandy tidal flats in Ariake Bay in an anthropogenic way. This conclusion coincides with archaeological evidence collected from shell mounds established in the coastal areas of Ariake Bay during the Jomon and Tumulus Periods (13,000 BC to 600 AD). Ruditapes philippinarum was very rare among fossil shells collected from the shell mounds except one created at the mouth of the bay. This species does not favor the water conditions in the estuary where the salinity tends to be variable due to the inflow of freshwater from rivers. It seems that various kinds of human manipulation are essential for R. philippinarum to be maintained in large numbers on the tidal flats at the mouths of the rivers in the inner parts of Ariake Bay.