Since a lot of estuarine fish inhabit the Chikugo River and fishermen carry out drift net fishing, this river is unique among the rivers in Japan.The relationship between the catch number of the estuarine fish, i.e., Coilia nasus, and the salinity, temperature, suspended sediment concentration (SSC), and tidal current was investigated. The salinity, temperature, and SSC were measured at a station 14.6 km from the river mouth. The tidal current was calculated by conducting a one-dimensional dispersion simulation. A drift net was floated over the cross-section of the river between 14.6 km and 16 km from the river mouth, and the fish catch number was recorded for every cast. The catch density of Coilia nasus was higher in June when the water temperature was 23oC, salinity was 0.07 psu, SSC was 20 mg/l, and freshwater discharge was 90 m3/s. It was suggested that Coilia nasus was swimming in a low-salinity and low-SSC region near fresh water. Data on the catch per unit effor during the semi-diurnal tidal cycle showed that higher catch rates were obtained 30 min after slack high tide, when the current velocity was 0.4 m/s.
