Analysis of the Fish Finder Records―I Some Informations on Underwater Formation of Tuna Long-line

摘要

Mr. NISHIMURA has pointed out in his report that we have become able to know underwater distribution of tunas. The author on board the Nagasaki-Maru engaged in tuna long-line fishing in the South China Sea and in the waters off the Palau Is., in January, June and December 1961. The author tried to record the underwater formation of the long-line by means of the fish finder obtaining succcessful records as shown in PL.Ⅲ-Fig. 1 in June and analysable dater in December for the first time. Using those records in December, the author calculated the resistance of water against the long-line. From the result of calculation, the author believes that the echo-sounding is the most reliable method to verify the underwater formation of long-line and the distribution of tuna. The available range of the fish-finder, however, is rather limited, and the long-line can not be recorded in many cases by it as the long-line is often moved out of the signal range by currents and other factors, especially on rough sea. Factors affecting the depth of hooks 1. Design of the long-line, the longer the main line, branch lines and float lines are designed, the deeper the hooks sink. 2. Amount of slack allowed during setting-down of line, The larger the distances between each buoy become, the deeper the hooks sink. 3. Effects of current and wind, The stronger the current and wind flow, the shallower the hooks become. Variation of hook-depth after setting-down of line Fig. 1 is the diagrm in which those records show the sinking stages of the deepest part of main line. Half an hour after setting-down of long-line, the main line usually sinks to a maximum catenary depth of 100m, and after one hour, it becomes perfectly balanced. Theoretical diagram of vertical distribution of the gear 1. In case of no effect from current and wind According to reports by YOSHIHARA (1951), MORITA (1955), HAMURO and ISHII (1961) and J. J. GRAHAM (1958), they have all agreeded thet the underwater condition of tuna long-line indicates a catenary curve with any effect from external force. Fig. 2 shows the scale graph of a tuna long-line used by the Nagasaki-Maru. The condition of the main line differs according to the distance between buoys. The actual hook-depth is equal to the sum of the values of the branch line setting-point, length of buoy line and the branch line, i. e., y=a. cosh・x/a, 1/2 of total length of main line=1/2 of buoy distances+(1/2 buoy dist.)3/6a2 2. In case of effect from current and wind a) External force from horizontal right angle direction to the buoy formation is shown in Fig. 3, i. e., y=(buoy line length+a• cosh•-x/a-a)cosθ θ: the heeling angle of long line under effect of external force b) External force from longitudinal direction of the buoy formation is shown in Fig. 4. In any case, the long line always draws a catenary curve and no any other type of curve. Analysis of fish-finder records 1. From the result of analysis conducted in December 3 1961 in the waters off the Palau Is., the author found the amount of external force against long-line in the water. As shown in Fig. 5 and PL IV-Fig. 4, the long-line has been affected by an external force from horizontal right angle direction to the buoy formation. In this case, the speed of external force is 0.2 kuot. 2. As shown in Fig. 6 and PL. in-Fig. 2, the long-line has been affected by an external force from longitudinal direction to the buoy formation. In this case, the speed of external force is 0.26 knot.