Abstract
There are many tidal flats in coastal areas of Ariake Sound, where a fishery for the clam Ruditapes philippinarum is well developed. However, a serious reduction in the clam catch has occurred in recent years, especially, in the coastal areas of Kumamoto, where the annual catch of 65, 732t in 1977 has decreased to 2, 000 t or less. Various causes for this decrease have been suggested, including environmental deterioration, regional topography, food habit of local people and predation. The clam populations in Kumamoto have two reproductive peaks, from spring to early summer and in autumn. However, in recent years, the populations have been maintained by recruits from the earlier of these seasonal peaks. Recently, the clam has come to be distributed higher in the tidal flats. In Kumamoto, this species of clam can reach commercial size within 1 year after settlement, and almost all individuals are fished during the period from spring to autumn of the next year. Predation, especially by the eagle rays, is another serious problem for maintenance of the clam population.
