Automatic passing fish detection system using RFID (Radio Frequency IDentification) technology was developed to provide information on movement and behavior of Ayu fish (plecoglossus altivelis) within a fishway. In this study, movements of Ayu fish were deduced from detection signals of sensing antennas placed at each remote location to be monitored. To accomplish this, a newly application software which help controlling multiple RFID readers, determining fish locations and logging the detection status was developed. Using developed system, the effect of decreasing light levels on the behavior of Ayu fish within the fish tank was examined. It was found from the result that most of Ayu fish ascended or descended the channel response to the change of light levels and their location was associated with water velocity.
