Broadband acoustic quantification of mesopelagic zooplankton

Abstract

In addition to its critical role in biogeochemical cycling, the mesopelagic zone has the potential to become an important global fisheries resource. A growing number of studies have focused on estimating the global biomass of mesopelagic fishes, but that of mesopelagic zooplankton is far more uncertain. Yet, quantification of zooplankton biomass and distribution is crucial to both understanding the ecology of the mesopelagic zone and informing policy makers on sustainable exploitation. Shipboard, narrowband, volume backscattering measurements—typically performed at 18 and/or 38 kHz—are likely dominated by gas-bearing organisms, hindering accurate assessment of zooplankton biomass. A towed vehicle, Deep-See, was developed and equipped with broadband acoustics from 1–450 kHz, optical sensors, and environmental sensors to address the challenges associated with systematically acquiring holistic, quantitative data to infer mesopelagic biomass. Broadband backscattering spectra (50–160 kHz) collected by Deep-See in Summer 2018 and 2019 off the New England shelf break are used to classify scattering layers into fluid-like zooplankton (e.g., jellies, copepods, krill) or gas-bearing organisms (e.g., fishes, siphonophores), then physics-based scattering models are used to estimate abundance and biomass. Consistent with the acoustic measurements, digital holographic images collected by Deep-See reveal multiple deep scattering layers, some consisting mainly of weakly scattering krill.