1National Research Institute of Fisheries Engineering, Fisheries Research Agency, Hasaki, Ibaraki 314-0421,
2National Research Institute of Aquaculture, Fisheries Research Agency, Nansei, Mie, 516-0193, Japan
Mechanisms of foot damage occurrence in the clam Meretrix lamarckii were studied to improve the quality of clams caught by clam dredging. The damaged clams were classified into two groups: clam with severed foot (cut clams) and clam with partly severed foot. The rate of foot damage increased with increasing water temperature and the rate of cut clams increased with increasing towing speed. Closing of valves by adductor muscle in response to tactile stimuli did not cause the foot damage. It is suggested that the foot was cut when the shell of the clam was pressed against the teeth of the dredge and sand, with its foot outside the shell. At the temperature of 25°C, clams dug faster and deeper into sand than at 10°C. Therefore, the vigorous burrowing activity of clams and burrowing deeper at higher water temperature may cause the higher rate of cut clams. The towing speed should be kept as slow as possible to reduce the rate of cut clams.