The spatiotemporal distribution of seawater intrusion in the Chikugogawa estuary was analyzed, and its relationship with the temporal variation of phytoplankton and engraulid fish, Coilia nasus, was discussed. The Chikugogawa estuary is vertically well mixed for most of the year, and a salt wedge is observed only when the tidal range decreases to 2 m or less. We found that the ratio of chlorophyll-a to pheophytin-a during the semilunar cycle varied according to the change in the mixing conditions in the estuary. Further, the number of sampled estuarine fish was related to the salinity and tidal range within the estuary; the fish was caught when the salinity was low and the tidal range was large. It is necessary to regulate the fresh water discharge in the estuary in order to maintain a low-salinity region that is suitable for the migration of fish and their spawning areas to the downstream of the river mouth barrage.
