Volume backscattering (Sv in dB or sv in m2•m-3) is the summation of the contribution from all targets within the sampling volume scaled to 1 m3. Total area backscattering (Sa in dB or sa in m2•m-2) is the summation of sv over all depth and therefore scales to 1 m2. Sv is a measure of the density of organisms and the primary measurement for acoustically estimating fish densities and abundance. The volume backscattering coefficient sv is defined in Equation  as sv = ρ < σbs>. This equation is the basis for estimating fish density from acoustics.