Observation of in situ tuna behavior around the fisheries aggregating devises (FADs) by using passive and active acoustic methods

The by-catch of bigeye tuna (Thunnus obesus) has been serious problem for commercial fisheries of skip jack tuna (Katsuwonus pelamis) due to the strong constrain of quarter for each species. Selective catch of skip jack tuna with avoiding by-catch of bigeye tuna is required for sustainable and effective fisheries. Light stimuli were applied in attempts to remove unnecessary tuna species and let them escape through the mesh or underneath of a round net. The movements of fish were observed with coded pingers (passive method) and a broadband quantitative split-beam echo sounder (active method). Newly introduced micro coded pinger (Fusion Inc., Tokyo) for this observation resulted in longer survival or / and retention of tagged fish comparing with previous survey periods. Three hydrophone array systems were deployed on each survey ships (Shoyo-maru, Nippon-maru, and Nippon-maru´s boat). The pinger transmitted 31 bits M-sequence signal every 1 second. Movements of multiple tagged fish were able to be observed at the same time. Not only depth change of each fish, but also three dimensional locations could be calculated. In the mean time, schools of fish were observed by the broadband echo sounder near the FADs. The 3D tracks of fishes which were estimated by active way with high distance resolution showed the fish swam deeper at sun rising that was consistent with passive monitoring result. Consequently large data sets of the movement of in situ bigeye tuna and skipjack tuna around FADs were obtained.