In situ of Cape horse mackerel (Trachurus capensis)

Abstract

The acoustical target strength (TS) of Cape horse mackerel (Trachurus capensis) was measured in situ at 38 kHz using a submersible split beam transducer in combination with a SIMRAD EK 500 echosounder. The transducer head was lowered to scattered aggregations of horse mackerel close to the bottom in order to resolve single echo targets at short range (5-30 m), Echograms were printed for a range intervai of 5-50 meters from the transducer head, Time of reception, compensated and uncompensated TS, range and alongship and athwardship offset angle were recorded from the EK 500 serial port and stored in ASCII- files at an IBM- compatible computer. Single fish targets were tracked using especially developed software and selected for analysis. The measurements suggest a lower TS than presently applied, but the literature is inconclusive on the matter, and the results should therefore be interpreted with caution.