Abstract
The dredge for catching the Japanese surf clam Pseudocardium Sybillae is known to be highly selective.In this paper, the various factors affecting the selective action of the dredge are analyzed, and the probabilities of the Japanese surf clam not shifting through the gaps between the teeth, or through the mesh of the netting bag, are calculated for various shell lengths, using the relation between the posture and the length when the shell passes through these parts. Considering that the probability of making a catch is the product of the two probabilities described above, and that this probability is proportional to the relative catching efficiency, the selectivity curves for the Japanese surf clam dredge were estimated for various gaps between the teeth and for various mesh sizes of the netting bag.The present dredge for the Japanese surf clam is used in Hokkaido, and the smallest Japanese surf clam that can be caught with it has a length of 9.0 cm. The combination of the mesh size of the netting bag and the distance between teeth was examined by using selectivity curves.5.0cm was found to be desirable as the distance between teeth, and 13.0cm as the mesh size of the netting bag.
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