Pilot work using a codend video camera for improved acoustic interpretation of backscatter observed during fisheries acoustic surveys.

Abstract

One challenging but crucial element of fisheries acoustic surveys is accurate groundtruthing of the echo return. Assignment and proportioning of areas of backscatter to the correct species or mix of species affects fish biomass calculations. Typically, survey groundtruthing is done by midwater trawls targeting a single scattering layer of interest. However, the depth at which each species is caught is unknown. Therefore, this method is problematic when multiple scattering layers are present. Small scatters are also missed, which may be important contributors to the echo intensity. To address these issues, we conducted pilot work using a video camera mounted in the codend of an open midwater trawl, which enabled us to look at several scattering layers during each tow. For comparison, we conducted tows with a closed net and no camera at the same location. We then examined each acoustic echogram and the corresponding tow in order to compare backscatter calculations attributable to our species of interest. We found that there were some differences in these measurements between the two types of tows. We conclude that this method appears to provide improved knowledge of acoustic backscatter observed during fisheries acoustic surveys and plan to integrate this method into future surveys.