Echo-counting in the mesopelagic zone using a broadband split-beam acoustic scattering system

Abstract

The ocean mesopelagic zone (200–1000 m depths) is a largely unexplored and remote ecosystem. Recent studies have suggested that the biomass of mesopelagic fish may be up to ten times larger than previously reported, more that the rest of the ocean combined. Interpretation of shipboard measurements of mesopelagic scattering layers in terms of abundance and biomass is complicated by large sampling volumes, complex assemblages of organisms, and highly uncertain target strength models. To address some of these challenges, Deep-See, a towed sensor platform carrying multiple broadband acoustic systems, has been developed, allowing real-time measurements of target strength and abundance. The focus here is on a new broadband split-beam echosounder (5.5 kHz–18.5 kHz) deployed in summer 2018 and 2019 off the New England shelf. This system is used to estimate the density of mesopelagic organisms using echo-counting and echo-statisticsapproaches. The parameters selected for echo-counting are discussed in order to achieve accurate estimates of abundance and also to identify and track individual targets. The calibrated broadband spectra of individual targets are used to determine the number of organisms with resonance frequencies in the band. [Funding provided by the WHOI Ocean Twilight Zone Audacious Project and an NSF MRI grant.]