襟裳岬近海に現れるDSLの日変化と水中照度およびプランクトン量との関係について
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The deep scattering layer was observed by means of an ultrasonic finder, 200KC and 28KC, at sta. A off Cape Erimo, south coast of Hokkaido, continuously from 8.30a.m. to 8.30p.m. on 20 August 1961 (Fig. 1). D. S. L. descended from about 90m-110m at 8.30a.m. to about 110-bottom (130m) at about 11.00a.m. when the altitude of the sun was highest. Then, D. S. L ascended during afternoon, reaching the surface layer at night. There was no discontinuous layer of temperature and chlorinity around the D. S. L (Fig. 2), so that D. S. L. appearing in the region at this time was not considered to be caused by any discontinuity of the water. D. S. L. was located throughout the day in the range of 0.2-0.8 lux of light intensities (Table 1, Fig. 3). When the sun was covered with cloud at about 12.30p.m., D. S. L. as well as underwater light intensity ascended to a slight extent (Fig. 3). It was observed that the zooplankton were more abundant in the D. S. L. than the layers above the D. S. L. (Table 2, Fig. 3). The major components of zooplankton population were crustacea, among which Euphausia pacifica composed of two size groups (larger group 17.65mm±2.85mm, smaller group 5.81mm±2.18mm) predominated (Fig. 4). It is concluded that the D. S. L. appearing off Cape Erimo at this season was caused by the aggregation of planktonic crustacea, mainly Euphausia pacifica, and that the D. S. L. was shifted vertically from the morning to the night according to the change of underwater light intensities which, in turn, may have induced the vertical migration of Euphausia pacifica.
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