カタクチイワシシラスの主要餌料生物であるＯｉｔｈｏｎａ属かいあし類のシラス漁場内における分布特性

Abstract

Distribution pattern and abundance of cyclopoid copepods belonging to the genus Oithona were studied on the basis of 648 samples collected every 3 or 4 days from July to October, 1984 at 18 stations in Shirasu fishing ground off Hiratsuka in Sagami Bay. Samplings were made in both surface and bottom layers at each station. O. davisae is the most dominant species, comprising 90-100 percent of a total catch of Oithona in number from late July to the middle of September and 70-90 percent from late September to October. The density of Oithona in the bottom layer was about 4 times as high as that at the surface. The bottom densities showed a seasonal change, highest in late July, 425 inds/l ont he average (range 104-1073), at a level of 100-250 inds/l through August, below 100 inds/l in September, and below 50 inds/l in October. O. davisae is abundantly distributed in the bottom waters of the salinity range, 30-33% in the area shallower than 10m deep whose surface layer is usually affected by the water from Sagami River. It is suggested that its habitat overlaps with that of Shirasu and that O. davisae is important as a food of Shirasu.