Hydraulic jet dredges have been used recently for catching Japanese surf clam in Hokkaido. The catching efficiency of this kind of fishery needs to be clarified.
An attempt was made to estimate the catch efficiency of the Japanese surf clam hydraulic jet dredge which is being used in Tomakomai sea area. The dredge survey was carried out four times using two dredges towed simultaneously in the same fishing ground. One dredge was towed behind the other at an interval of 270cm. The number of shells caught by the dredge was counted and their shell length was measured. Under the condition of the dredging survey, the catching efficiency in a shell length class is considered to be a function of the ratio of clams caught by the front dredge to the number of shells caught by the rear dredge. In this way, the fishing efficiency for hydraulic jet dredge was evaluated. However, the average catch efficiency of hydraulic dredge for Japanese surf clam may be slightly influenced by towing speed. In our survey, at a towing speed ranging between 2.90m/min-4.78m/min, the average catch efficiency obtained was 0.63.
