A computer-implemented method for determining the biomass of tuna in a region of water, wherein measurement means comprising an acoustic sensor and image capture means are arranged submerged under said region of water, said measurement means being in known positions and aimed in an upward direction, wherein the method comprises the following steps: - during a measurement interval in which tuna pass through said region of water: - said acoustic sensor takes acoustic measurements by emitting acoustic signals and receiving echoes, thus generating an echogram; and - said image capture means capture images, generating a sequence of images containing images of said tuna; further comprising the steps of: - determining a value of the number of tuna that have passed through said region of water during said measurement interval using said echogram; - determining a tuna weight value using said sequence of images; - determining a biomass value of tuna that have passed through said region of water during said measurement interval from said value of the number of tuna and from said tuna weight value; wherein determining said tuna weight value comprises: - selecting tuna from said sequence of images; - determining a calculation for the size value for each of said selected tuna; - determining a calculation for the individual weight value for each of said selected tuna from said size value; and - determining said tuna weight value as the mean value of said individual weight value for each of said selected tuna; wherein said biomass value is determined as the multiplication of said value of the number of tuna by said tuna weight value; wherein said sequence of images comprises a plurality of frames, and wherein selecting tuna from said sequence of images comprises the following steps: - segmenting said frames to obtain first candidate objects; - filtering said first candidate objects by their geometric characteristics, obtaining second objects which are a subset of said first objects, wherein said second objects are selected as tuna. - during a measurement interval in which tuna pass through said region of water: - said acoustic sensor takes acoustic measurements by emitting acoustic signals and receiving echoes, thus generating an echogram; and - said image capture means capture images, generating a sequence of images containing images of said tuna; further comprising the steps of: - determining a value of the number of tuna that have passed through said region of water during said measurement interval using said echogram; - determining a tuna weight value using said sequence of images; - determining a biomass value of tuna that have passed through said region of water during said measurement interval from said value of the number of tuna and from said tuna weight value; wherein determining said tuna weight value comprises: - selecting tuna from said sequence of images; - determining a calculation for the size value for each of said selected tuna; - determining a calculation for the individual weight value for each of said selected tuna from said size value; and - determining said tuna weight value as the mean value of said individual weight value for each of said selected tuna; wherein said biomass value is determined as the multiplication of said value of the number of tuna by said tuna weight value; wherein said sequence of images comprises a plurality of frames, and wherein selecting tuna from said sequence of images comprises the following steps: - segmenting said frames to obtain first candidate objects; - filtering said first candidate objects by their geometric characteristics, obtaining second objects which are a subset of said first objects, wherein said second objects are selected as tuna. - said acoustic sensor takes acoustic measurements by emitting acoustic signals and receiving echoes, thus generating an echogram; and - said image capture means capture images, generating a sequence of images containing images of said tuna; - determining a value of the number of tuna that have passed through said region of water during said measurement interval using said echogram; - determining a tuna weight value using said sequence of images; - determining a biomass value of tuna that have passed through said region of water during said measurement interval from said value of the number of tuna and from said tuna weight value; - selecting tuna from said sequence of images; - determining a calculation for the size value for each of said selected tuna; - determining a calculation for the individual weight value for each of said selected tuna from said size value; and - determining said tuna weight value as the mean value of said individual weight value for each of said selected tuna; - segmenting said frames to obtain first candidate objects; - filtering said first candidate objects by their geometric characteristics, obtaining