Target density estimation of mesopelagic fish using a towed, broadband, split-beam, acoustic scattering system

Abstract

The ocean mesopelagic zone (200–1000 m) features a complex temporal and spatial distribution of organisms which may play an important role in ocean biogeochemical cycles. Yet a quantitative account of the abundance, biomass and migration behaviors of mesopelagic inhabitants is still lacking. Shipboard echosounders are limited by acoustic attenuation at higher frequencies constraining the usable frequency band for investigating deep sound scattering layers, as well as large acoustic sampling volumes, a result of the large depths associated with the deep scattering layers, combined with the inherent complexity of mesopelagic communities. Deep-See, a towed platform carrying multiple broadband sonars, focuses on individual targets in the deep scattering layers, to help interpret shipboard data. Target density estimation is the first step towards determining biomass in the mesopelagic zone. Two methods are investigated and applied to a summer 2019 dataset collected using two broadband split-beam echosounders (5.5–18.5 kHz, 18–47 kHz) on Deep-See in the slope waters off the New England Continental Shelf. The echo-counting method utilizes echo magnitude and phase information to determine singe targets while echo-statistics involves comparison of data to a physics-based numerical simulation to predict the statistics of broadband echoes from which target density can be extracted.