Improved precision of acoustic surveys of benthopelagic fish by means of a deep-towed transducer

Acoustic measurements of deepwater fish from a deep-towed transducer deployed at depths from 500 to 650 m over a seamount (rising from 1000 to 600 m depth) are compared with those from a vessel-mounted transducer. Algorithms were developed to correct for noise and to analyse signals near the bottom. The analysis showed that the vessel-mounted transducer can significantly underestimate the volume reverberation of deepwater fish by an average factor of 1.86, equating to 2.7 dB. This bias was largely attributed to beam thresholding and calibration, acoustic attenuation from near-surface bubbles, ship motion, and uncertainties in the sound absorption constant. On the steepest slope of the seamount the near-bottom shadow zone was reduced by 50% and the extrapolated biomass in this zone reduced by 47% using the deep-towed transducer. These results confirm that a deep-towed transducer will improve absolute abundance measurements of deepwater resources. (C) 1996 International Council for the Exploration of the Sea