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Offshore fish cage culture is being popularized in China. However, it is difficult to observe fish activity and the status of the net cage by naked eye because of the deep and turbid sea water. Aquaculture farmers, therefore, need a way to remotely observe fish activity and the status of the net cage on a regular basis without having to visit the cages themselves and give an alarm signal in the time when the bred fish in net cages escape. This paper describes the utility of using acoustic technology to monitor fish activity and distribution in offshore aquaculture fish cages, and remotely telemeter data back to shore. The monitor consists of three functional parts: a scanning sonar system located in the center of the net cage on offshore, a controlling and data processing display system located on the shore base station, a wireless data communication system. The prototype instrument was designed and the tests in pool and in the field of Xiamen harbor were carried out. Results show that the system has good performance in monitoring the fish activity, distribution and the status of the net cages under different environment conditions. The sea states effects on the performance of the monitoring system. Further testing and development is required.