Abstract. The microdistribution of Oithona davisae Ferrari and Orsi inhabiting shallow waters of Ariake-kai, western Kyushu, Japan, was investigated in 1980-1981 by means of a pumping method. Water samples (10 1) were repeatedly collected from the surface to the bottom at intervals of 0.5 m. This species was dispersed when the tidal current ran fast (more than 20 cm s-~), but aggregated in the upper layer during the period of slow current. Their density was highest when the current stopped. Aggregation of O. davisae presumably induced the continuous flat swarm near the surface. It is to be expected that the shallow waters in Ariake-kai would be favorable as nursery grounds for the fish larvae and juveniles which depend on the presence of such swarms.

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In the investigations at the three stations in Hiuchi-nada, twenty-one species of copepods were identified. The period of occurrence of every species was, in the main, similar to that recorded in my previous work in Bingo-nada situated in the northern outside of Hiuchinada. Among them, Paracalanus parvus, Oithona similis, Oithona nana, Corycaeus affinis, Corycaeus andrewsi and Microsetella norvegica occurred dominantly or subdominantly. From the results of the present survey and my previous work in Bingo-nada, the biological characteristic of the sea area extending across the central part of the Setonaikai from south to north is considered as follows. In the northern part of Bingo-nada area, the water mass is mingled violently with the polluted water caused by the sewage of factories and cities, and so Acartia clausi and Oithona nana are dominant. On the contrary, in the central part of Hiuchi-nada area where Oithona similis occurred dominantly, such effect of the polluted water seems to be the weakest.

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