インド洋の海面熱赤道とキハダおよびキハダと混獲されるメバチ漁場の移動
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The authors analysed the data concerning the oceanographic and fishing conditions of the Indian Ocean, which were obtained from the tuna fishing vessels operating there during 1957 to 1962 and made a series of charts which show the highest water temperature zones (29.0-31.0°C) so-called thermal equator and the areas where the catches of tuna were more than 3.1 tons per boat-day. From these charts, the authors obtained the following results: 1) The pattern of the thermal equator seems to durate for 30-50 days, and in waters south of the equator it has a tendency to last about 10 days longer than that of north of the equator. 2) The thermal equator moves northward and reaches its northern extremity from mid April to mid June in the area north of equator though there is a small difference among the years. The season of the northern extremity in the Bay of Bengal is about 10 days earlier than that in the Arabian Sea. 3) In the area south of the equator, a part of the thermal equator begins to move southward in early June to mid September. Then it further moves southward and reaches its southern extremity in early February to early March. Here, the location and the direction of the thermal equator at the east and west ends vary considerably year by year. 4) The main fishing grounds for yellowfin tuna, Thunnus albacares, and bigeye tuna, T. obesus, which are caught together with the yellowfin tuna seem to move seasonally in correspondence with the seasonal movement of the thermal equator. The fishing ground is usually formed along the thermal equator. Especially, in the southern Indian Ocean, there is a tendency that fishing grounds are found in the lower-temperature water masses which are located just south of the thermal equator. 5) Therefore, the merit using the surface isotherms including the location of the thermal equator will increase as a factor to forecast where the main fishing ground for yellewfin tuna will be formed.
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