Factors responsible for the drastic catch decline of the Manila clam Ruditapes phillipinarum in Japan

The annual catch of the Manila clam Ruditapes phillipinarum in Japan increased from 100 thousand tonnes in 1960 to 140 thousand tonnes in 1982. This increase occurred despite a local decrease in coastal fishing grounds due to land reclamation and helped to balance overall production. Since 1984, however, the catch has decreased drastically to only 50 thousand tonnes and has remained at this low level. The authors reviewed published reports relevant to the ecology and resources of Manila clam and concluded that the main factor responsible for the drastic decrease in catch is over-fishing. Over-exploitation of the adults and sub-adults significantly affected and damaged reproduction and source-sink relationships among localities. Disturbance of the clam habitat throughout the year by heavy fishing gear may also be responsible for lowering the survival rate of the juveniles.