Fundamental study to estimate fish biomass around coral reef using 3-dimensional underwater video system

Estimate of fish biomass around coral reef is one of the important information as a component of coral ecosystem. To obtain the biomass, several methods such as echo sounding, experimental fishing, catch by net, analyzing from commercial fishing, diving survey are used. However, coral fish consist of too many species and most of them are relatively small. Also, the complex shape of coral reef interferes with the estimate of number and size of fishes. The authors developed three dimensional (3D) underwater video system to measure the size of coral fishes. The system consists of a camera body (housing two TV cameras), cable, onboard camera controller, digital video recorder and a computer. Two video signals are recorded by the recorder synchronized at every 0.01 second. By playback of the video, selected synchronized two still pictures are stored in analyzing computer. Two pictures are used for analysis by applying triangulation technique. By choosing identical points on two pictures, three dimensional coordinates of the point are calculated, and distance between two points is easily obtained by choosing both points. Data analysis procedure of obtaining the fish biomass was examined. The camera was settled at a typical patch reef, 13 m in height and 30 m in diameter, located in Sekisei Lagoon, and a continuous video record for 45 minutes was obtained at 12 sites. Analysis was carried out as follows; 1) Decision of measurement area at each site. 2) Analysis of fish species and number in the area every five seconds for 30 minutes. 3) Measurement of body length for each fish species by selecting several images sets. 4) Classification of fish shape to six kinds and obtained body length versus weight curves. 5) Calculated total biomass at one moment in a cubic meter. By the analysis, fish biomass at each measurement site was obtained, and l:he data seemed fairly correct compared with diving survey.