Monitoring for schools of juvenile Pacific bluefin tuna Thunnus orientalis by using portable stereo vision cameras in a sea net cage

Since field surveys of fish school behavior around fishing gear are difficult to conduct, simulation of such behavior is of great potential significance with regard to the capture process of the fishing gear, especially in case of rare and endangered species of fish. To obtain model parameters for the school behavior of juvenile Pacific bluefin tuna (jPBT) in seawater, a portable stereo camera was used to measure in three dimensions the nearest neighbor distances of five jPBTs and distances between the net and the fish school in a fish net cage at sea. The results of the performance test of the stereo camera showed that the measured distances were 100.6% and 101.9% of the actual measured lengths at distances of 36 cm and 100 cm from the camera, respectively. We obtained five continuous video frames in which all jPBT were pictured within the angle of view. The nearest neighbor distance and the distance between the centroid of the fish school and the side net were 17.6 +/- 5.36 cm and 28.0 +/- 0.26 cm, respectively. Although a measurement error of less than 2% was observed, this is not significant as compared with the errors in previous studies; therefore, the portable stereo camera is effective for monitoring fish schools.