Biomass estimation of bluefin tuna in sea cages by the combined use of acoustic and optical techniques

,SUMMARY In this paper, an experimental setup to estimate the biomass of caged bluefin tuna (Thunnus thynnus) in the Spanish Mediterranean coast is presented. Aims are to monitor individual tuna during all stages of growth, to control the transfer of fish caught in purse seines to towing cages and to be able to estimate the total number of specimens and the biomass transferred. To do this, we propose a combined system of acoustic and optical techniques (both non-intrusive), in order to obtain direct values of acoustical target strength (TS) and information on the orientation of tuna inside the acoustic beam and their size. To achieve complete information, ventral and dorsal acoustic measurements are done using in a first attempt a multiplexed scientific echosounder with two 200 KHz split-beam transducers, while a stereoscopic video system provides optical information and allows estimating the size and the weight of the specimens. Preliminary results indicate that this combined use of techniques provides precise information to determine the TS as a function of fish orientation and for proper monitoring of tuna in floating cages. RESUME