Mass estimates of individual gas-bearing mesopelagic fish from in situ wideband acoustic measurements ground-truthed by biological net sampling

Abstract

A new acoustic approach to estimate the mass of individual gas-bearing fish at their resident depth at more than 400 m was tested on Cyclothone spp.. Cyclothone are small and slender, and possibly numerically underestimated globally as individuals can pass through trawl meshes. A towed instrumented platform was used at one sampling station in the Northeast Atlantic, where Cyclothone spp. dominated numerically in net catches, to measure in situ acoustic wideband target strength (TS) spectra, i.e. acoustic scattering response of a given organism (”target”) over a frequency range (here, 38 + 50–260 kHz). Fitting a viscous–elastic scattering model to TS spectra of single targets resulted in swimbladder volume estimates from where individual mass was estimated by assuming neutral buoyancy for a given flesh density, such that fish average density equals that of surrounding water. A density contrast (between fish flesh and seawater) of 1.020 resulted in similar mass–frequency distribution of fish estimated from acoustics/model and Cyclothone spp. caught in nets. The presented proof of concept has the potential to obtain relationships between TS and mass of individual gas-bearing mesopelagic fish in general.