The authors have studied on the ecology of the engraulid fish, Coilia nasus Temminck et Schlegel, which is distributed in Ariake Sound and rivers flowing into the Sound situated in the west coast of Kyushu. In the lower Chikugo River, Coilia nasus, which ascends to the river from the sea for spawning has been caught in large quantities with drift nets, and is one of the important fishes for the fisheries in this river. Drift net samplings, environmental observations and analysis of the daily catch by a fisherman were made at the lower Chikugo River in 1977 and 1978 in order to clarify the ecology of the ascending population of this species. The results obtained are as follows: 1) The ascending activity of Coilia nasus is influenced by increases of the water level from the neap tide to the spring tide and the amount of flowing water in the tidal compartment. 2) The amount of the ascending fishes increases in the afternoon and in ebb tide of daytime. 3) This fish in the lower reaches swims mainly in the surface layer. Ascending to the upper reaches, the swimming layer of the fish changes to the bottom layer. 4) Coilia nasus ascended for the spawning to the upper reaches in a May-August period is 160-410 mm in total length. 5) According to the age determination based on the otolith reading, this species attains 182 mm TL in a year, 260 mm TL in two years and 324 mm TL in three years. The spawning population consists of two and three year old males and over two year old female. 6) Gonadosomatic index (GSI) of the female collected in the spawning area is of a high value from May to mid-June. Main spawning period is deduced to be in mid-June. 7) The female in the spawning area occupies 10-35% in sex ratio. It seems that one female spawns with 2-20 males in a group.
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