Variable-aperture processing of multibeam echosounder data to better resolve fish locations and improve abundance estimates.

Abstract

Acoustic classifications of fish and estimations of their orientation distributions are possible using multi-frequency or broad bandwidth measurements of their acoustic target strengths (TSs) with knowledge of their scattering directivity pattern and size distributions. To measure TS of in situ fish, single-frequency interferometric methods provide information to detect resolvable single targets and estimate their location within the acoustic beam. This technique is compromised by multiple targets that are unresolvable because of their spacing, but this situation can be mitigated with the use of multi-frequency interferometry. The ambiguity caused by coincident echoes can also be substantially reduced using single-frequency, multiple-aperture interferometry. This method uses phase differences from multiple sub-arrays of a single-frequency (200-kHz) multibeam echosounder to estimate robustly when echoes originate from a resolvable single target and its position within the acoustic beams. Results provide accurate measures of beam-compensated TS and, in cases of low-density scatterers, estimates of their volume density. Multiple-aperture interferometry can significantly improve the utility of single-frequency multibeam echosounders for quantitative measures of fish and zooplankton, and seabed-range detections.