EMPIRICAL DETERMINATION OF IN-SITU TARGET STRENGTHS OF 3 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 hors e mackerel in the course of routine surveys. These species have differ ent numerical packing densities, which necessitates treating the TS da ta differently in each case, as the performance of single-target detec tors is strongly dependent on target densities. Evidence for the prese nce of multiple targets in their TS distributions is presented, and em pirical methods of extracting mean target strengths from them describe d. 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. (C) 1996 Internationa l Council for the Exploration of the Sea