In the frame of the European project SELFDOTT (From capture based to self-sustained aquaculture and domestication of Bluefin tuna, Thunnus thynnus) an experimental setup to estimate the biomass of caged Bluefin tuna (Thunnus thynnus) in the Spanish Mediterranean coast is presented. The aim is to monitor individual tuna during all stages of growth. 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 a 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 to estimate 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.