Studies on the fishways of Nagaragawa Estuary Barrage and migratory fish species that passed the barrage were conducted when the estuary barrage project was planned, before and during the construction of the barrage, and before the barrage went into operation. After the barrage went into operation, its monitoring began, and it is still being carried on. The investigations and studies conducted at the planning stage and before the commencement of barrage construction yielded information on the habitats and helped the development of seed production techniques of ayu (sweetfish) and satsukimasu salmon, which were likely to be affected by barrage operation. Through subsequent studies conducted during the construction of the barrage, knowledge was gained about the habitat of migratory bottom fish species, and the type of fishway structure to be adopted was determined according to the study results. Before the gates went into operation, findings from the studies and deliberations conducted thus far were verified through test operations. In the monitoring of the estuary barrage after it went into service, the migratory habits of fish species likely to be affected by barrage operation were studied, and effectiveness of the fishways was confirmed.