Influence of trawling on in situ estimates of cape horse mackerel (Trachums trachums capensis) target strength

Abstract

The influence of trawling on in situ estimates of Cape horse mackerel (Trachurus trachurus capensis) largel strength was studied by means of a 38kHz split-beam transducer. TS frequency distributions were found to shift downwards during trawling, resulting in average individual largel strengths up to 12 dB lower than prior to and after the trawl. It is suggested that this effect was due to increased tilt angles us a consequence of escape responses to the trawl. In order to obtain accurate absolute estimates of biomass, target strength should clearly be determined at survey speeds, to keep conditions comparable to those applying during echo integration. Using only target strength data collected at survey speeds, preliminary regressions between TS and fish length were obtained. © 1994 International Council for the Exploration of the Sea.