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Acoustic sampling has long been a standard survey tool for estimating the abundance and distribution of fish, zooplankton, and their seabed habitat (Kimura, 1929; Sund, 1935; Holliday, 1972a; Nielson et al., 1980). Typically, acoustic surveys are conducted using multifrequency echosounders that transmit sound pulses down beneath the ship and receive echoes from animals and the seabed in the path of the sound waves (Simmonds and MacLennan, 2005). Generally, for surveys of animals, the backscatter signal is normalized to the range-dependent observational volume yielding the volume backscattering coefficient, which provides indications of the target type and behaviour. Objects scatter sound if their product of mass density and sound speed is different from that of the surrounding medium. A fish with a swimbladder has a large acoustic-impedance contrast (Foote, 1980), and thus has a large reflecting area, backscattering cross-section. Plankton, e.g. krill and salps, generally have much lower acoustic-impedance contrasts, but can produce large volume backscattering coefficients when they are aggregated in large densities (Hewitt and Demer, 1991, 2000). Under certain conditions, the summed and averaged volume backscattering coefficients are linearly related to the density of the fish or plankton aggregations that contributed to the echoes (Foote, 1983a). The number density can be estimated by dividing the integrated volume backscattering coefficient from an aggregation of target species by the average backscattering cross section from a representative animal (Ehrenberg and Lytle, 1972). An estimate of animal abundance is then …