Mapping the scatterscape of pelagic side scan sonar targets relative to oceanographic features

Abstract

Sonar reconnaissance of fishes for stock assessment and research has been an effective and minimally invasive method of gathering abundance and distribution data on scales of 10s to 100s of km since the 1950s. Yet, classification of fishes remains one of the greatest challenges of active sonar surveys. Many variables affect sonar reflection, including size, shape, orientation to the sonar source, the spatial relationship of individuals in a school to each other, and the number and distribution of individuals within a school. The long wavelengths of low frequency (typically <60 kHz) that allow depth penetration provide poor small scale resolution for identifying objects. High frequency side scan sonar (600—900 kHz), while imaging only over short ranges, can resolve individual fish and thus orientation and behavior relevant to understanding low frequency sonar returns and ecology. We demonstrate here that autonomous underwater vehicles (AUVs) offer a mechanism for putting side scan sonar transducers near potential targets together with telemetry, imaging, and oceanographic sensors, and can thus work together with low frequency sonar to develop holistic scatterscapes of oceanographic features, inclusive of information on species identity, orientation, behavior, abundance, individual size, and feature stability.