ビンナガマグロ漁況との關係(第 1 報)

Abstract

Based mainly on the latest materia's given by the local fisheries authorities in l'refs. Siduoka, Kanagawa, etc. we have investigated the catch of ‘Binnaga Maguro’ (Germo germo (Lacepede)) in relation to the hydrographical conditions. The seasonal variations of the catch of ‘Binnaga’ in every year are nearly of the same type (Fig. 1); the so-called winter season begins in October, showing its flowering season from Dec. to Feb., and ends in the middle decade of April. And the so-called summer season begins from the late April, showing its flowering season in the interval from middle May to middle June, and ends in July. Inspecting Fig. 2 we notice the remarkable decline of the summer catch (landed mainly at the harbours in Siduoka Pref.) in the adjacent waters of Japan. On the other hand, the winter off-shore fishing grounds were extended year by year to the east with the increase of the landings, and now cover the waters distant about 2, 000 sea-miles from our coast. The former fishing ground moves north-easterly from spring to summer with the advance of Kur03iWO, while the latter (in winter) accumulates in the regions along the line of subtropical convergence. Apparently we can see the fishing grounds during the whole year rotating in the anti-cyclonic sense with the run of Kurosiwo and its back current of salty water-masses. The optimum water temperature of the catch lies at 17°-19°C (max. at about 18°C) in Nov. -Feb., but lying at 18°-21°C (max. at about 20°C) in May and June. The catch represents 2 maxima in winter, one at the salinity of 34•6-14•9% and the other above 35•5%, while in summer only one at 34•8 %. The shoals distributed in the region comparatively near the land lack the large-sized fishes and contain more small-sized ones than off-shore shoals. In conclusion, summarizing the above mentioned facts, the writer (M. UDA) proposes the following hypothetical theory on the migration of Binnaga Maguro, following the instructions kindly given him by Dr. M. TAUTI: There are two partially inter-related stocks, one in the waters near the land (I) and the other in the offing (II). The former consists of the native stock A0 and the incoming stock from (II) B' (of small and partially of middle-sized fish), while the latter (II) consists of the native stock BO and the incoming stock from (I) A' (of medium and partially of small sizes). The recent overfishing of (I), particularly of A, , caused the decline of the yield of `Binnaga Maguro' in summer.