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) farms in the Spanish Mediterranean coast is presented. The aim is to monitor individuals 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 values of target strength of the specimens and information on the orientation of tuna in the acoustic beam and the fish size. To achieve a complete information, an experiment has been designed that allows us to get the acoustic measurements ventral and dorsal individuals accommodated in the cage (using in a first attempt a multiplexed echosounder with two 200KHz split-beam transducers), while stereoscopic systems provides visual information and allows estimate the size and weight of the specimens captured by the system. Preliminary results indicate that combined use of both techniques provides more precise values of the parameters needed to determine the TS as a function of fish orientation and for proper monitoring of tuna in floating cages