隠岐島周辺におけるスルメイカの漁況学的研究-III

表層流が漁場形成におよぼす作用について

抄録

Tremendous numbers of the squid, Ommastrephes sloani pacificus STEESTRUPS, occurring during October to following March, have made the water around the Oki Islands one of the most important fishing grounds for them in the Japan Sea. With a view to elucidating mechanisms through which their shoals emerge into particular areas, the landing statistics by set nets for the 1950-'51 to 1958-'59 seasons (Table 1) have been examined in association with the data of tagging experiments and oceanographical surveys.
1) The data relevant to drift bottles recovered and drift buoys traced in 1955 indicate that all the surface currents around the southern mouth of the Oki Straits flow westward heading to Nakanoshima and Urago Bay (Fig. 4). Points of the bottle recovery coincided with those localities where tagged squids were recaptured. While some of those places have provided favorable sites for set nets particularly with a great concentration of the animal, the others are valuable grounds for angling boats aiming at the squid (Table 1, Figs. 1 and 3).
2) However, a check on Figs. 1 and 4 will reveal that some of the points to which drift bottles were often driven do not always produce successful catches even when they are located close to a favorable fishing ground. Because a water boundary or a counter circulation existing in the areas seems to separate those waters from the surface currents which bring the squid into the fishing grounds (Fig. 5).
3) From previous studies and commercial fishing practices it has been inferred that the squid diurnally repeats vertical migration, emerging from the bottom layer in the early evening and sinking down there at dawn. A postulation proposed here in regard to their horizontal migration is that shoals of the squid, after floating up to the surface, are transported from area to area along with the movement of water masses more possibly than by their own locomotion.
4) During autumn to winter thousands of the squid would often concentrate in Urago Bay. As the concentration occurs always at midnight in the bay shallower than 50m., they seem to take the rest in a deeper water outside the bay during the daytime.
5) Comparing the catches which accrued to Urago Bay and Nakanoshima during the 1955-'56 and 1956-'57 seasons, one may see that the fishing had success or failure alternatively at one of the places and in one of the seasons (Fig. 2). Those phenomena are possibly attributed to the surface current which, changing its direction from season to season, brings a greater number of squid to either one of the places.
6) Numbers of drift bottles released off the Oki Islands were recovered at points scattering over the coastal regions of Japan. According to Fig. 5, which combines these points with an annual average catch of squid and depth contours of these regions, an essential condition commonly applicable to the coastal regions presenting a favorable fishing ground for the squid is that a region should be provided with the 200m. depth contoure drawing close to the coast and with the surface current heading ashore.
7) Much remains to be solved in regard to detailed mechanisms inducing the squid to migrate vertically as well as horizontally. It may be a first approach to the questions to determine factors responsible for diurnal movements of the squid.