豆南海域におけるサンマColoabis saira (BREVOORT) 稚魚の分布-I

Abstract

During the months of November and December large schools of the saury, Cololabis saira, would migrate southward along the Pacific coast of Japan, traversing an area extending from Oshima to Miyakejima (islands). However, only small schools of the fish would usually be observed in the south of the latter island except in December 1948 when nearly two metric tons of the fish were hauled off Hachijo Island some 60 miles south of Miyakejima. In an attempt at throwing light upon the exceptional catch on that occasion oceanographic data in the region in December 1948 has been compared with those of the same month of the previous year. In consequence, the Oyashio, a cold current, has been found prevailing in the area in 1948 with a surface isotherm of 20°C, receding as far south as beyond Hachijo island. Whereas, in 1947 that temperature line expanded to the north of the island likely under the in-fluence of the Kuroshio, 2 warm current which was dominating then and there (Fig. 1). The fact is suggestive that numbers of the adult saury on their way of southern migration seem to come over the sea around Hachijo Island only when the isothermal zone of 20°C. has withdrawn south of the island. The length and weight of a few specimens fished in the area are shown in Table 1 together with some relevant data. In the results of spawning surveys of the saury carried out by authors in 1953 and 1954 in the region under study, 276 youngs measuring 65 to 75 mm in total length became available (Table 2, Figs. 2 and 3). On the basis of knowledge thus acquired in regard to the maturity of the adult as well as occurrences of the young it may be asserted that the spawning in the region takes place once a year during November to the following May with February and March as its height: water temperatures suitable for the youngs approximately range from 17 to 23°C. in the spawning area covering from the vicinity of Oshima to Torishima or perhaps more southerly to the Bonin Islands. Particularly the youngs were abundant in the counter Kuroshio current. An assumption derived from these findings, but yet to be proved, is that considerable amounts of the spawners would travel southward to Ki'nansho (bank), while the hatched young running up north along the counter Kuroshio current. A. question as to whether or not the present species would spawn to any extent in summer when then pass the region on their way back to the north is left to be solved.