Studies on the Ark Shell in Omura Bay-I

1) In order to estimate the total number of ark shells Scapharca broughtoni (SCHRENCK) living in a limited area and to clarify catching efficiency of this species caught by ark shell dredge, towing experiments were made in a rectangular test area, 600m×10m, in southern Omura Bay, Nagasaki Pref., Japan (Fig. 1). The test area was 10m in depth with a muddy bottom. In addition to the native ark shell population, a total of 4, 950 individuals of marked shell obtained from the same bay one day before were released. From June 16 to 17, 1965, one week after releasing, two dredges were towed simultaneously from a boat (1.1 tons, 6 p. s.) at an average speed of 80cm/sec. Thirty tows were made altogether. Specifications of the iron dredge used in the present study are as follows: i) The entrance flame, 125cm in width and 27cm in height, furnishes with 22 hooks. The hook is 32cm in length and the aperture between hooks is 4.5cm. ii) The mesh size of nylon collecting bag attached to the flame is 6cm (Fig. 2). 2) The results of towing experiments are summarized in Table 1. At first, the total number of native ark shells in the test area was estimated at 2, 388 (Fig. 3). Then the accuracy of this method was tested by estimating the number of marked shells released. The figure obtained (4, 969) coincided fairly well with that actually released (4, 950), indicating the reliability of the method. 3) Applying the estimated total number of native ark shell in the area, the catching efficiency for each towing was calculated both for native and marked shells (Table 2). The catching efficiency of the present dredge was estimated as 0.138±0.055 at a confidence level of 95 per cent.