Empirical determination of in situ target strengths of three loosely aggregated pelagic fish species

In situ measurements of target strength (TS) at 38 kHz have been taken from night-time scattering layers of anchovy, pilchard, and Cape horse mackerel in the course of routine surveys. These species have different numerical packing densities, which necessitates treating the TS data differently in each case, as the performance of single-target detectors is strongly dependent on target densities. Evidence for the presence of multiple targets in their TS distributions is presented, and empirical methods of extracting mean target strengths from them described. After careful assessment of the quality of each data point, and the removal of unwanted influences where possible, the data were used to fit TS/length expressions for the three species.