The system consists of one master station and eight secondary stations and ultrasonic pingers. Each secondary station is composed of an ultrasonic transducer, an ultrasonic transceiver and a radio transceiver. The master station's unit consists of eight channel radio receivers, a transmitter and a 32 bit personal computer. The master unit fixes the fish 3D positions by measuring the time difference between sound reception at the secondary units. At the time the master unit's computer displays the fish position, while recording measured time differences into the hard disk. Using this system, Emerald Rock Cod (Trematomus bernacchii) was tracked successfully nine times.