Field applications of broadband acoustic scattering techniques for quantifying zooplankton distribution, abundance, and size.

Abstract

Discriminating among sources of scattering remains a key problem in ecological applications of active acoustics. This is particularly true in studies of zooplankton, which are often found in communities of heterogeneous species composition, and in the deployment of acoustics on autonomous platforms, where independent sampling with nets to ground-truth acoustic observations is often not feasible. Broadband measurements can provide substantial improvements in species discrimination by characterizing more fully the frequency spectrum of scatterers relative to traditional single- and multi-frequency narrowband techniques. Here, we present findings from a series of three cruises to the margins of Georges Bank examining patchiness in the distribution of krill and other zooplankton, as well as interactions with predators (fish, seabirds, and marine mammals). A heavily modified commercially-available broadband system spanning a frequency band of 30–600 kHz, with some gaps, was deployed in a towed body to depths of 200 m. Concurrent measurements made with a surface-towed multi-frequency system (43, 120, 200, and 420 kHz) along with ground-truthing information from depth-stratified net sampling and a video plankton recorder allow an assessment of the strengths and limitations of broadband methods for remotely discriminating among sources of scattering and for estimating the abundance and size of animals.