The teeth on the dredge for catching Japanese surf clams Pseudocardium sybillae are known to be highly selective. In this paper, the various factors affecting the selective action of the teeth of the dredge are analyzed, and the probability of the Japanese surf clams not sifting through the gaps between teeth was calculated for the various shell lengths. Considering that this probability is in proportion to the relative catching efficiency, selectivity curves for the Japanese surf clam dredges

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... ere estimated for various gaps between teeth. The ratio of the shell length of the Japanese surf clam to the distance between teeth indicating the relative catching efficiencies of 0 %, 50 % and 100% respectively, were calculated by using the selective curves. These ratios showed a slight tendency to decrease as the distance between the teeth increased, butfor a distance of 4.0 cm, these values were 1.22, 1.34 and 1.83 respectively.