Abstract: In fisheries acoustics the bias introduced by a system threshold in the integration of echoes from fish and other marine organisms has long been recognized. Some simple methods for evaluating such a bias uses Rayleigh assumption applied to fish backscattering length distribution. More adequate methods require that the target strength distribution of the fish and directivity pattern of the transducer be known. The method proposed here uses statistical approach for correction of target strength probability density function (PDF). The theory of conditional target strength PDF and conditional beam pattern PDF is derived. It provides correction not only for mean value of target strength but for whole distribution. The data obtained from simulations are presented and compared with those obtained from single-beam inverse processing.