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Abstract
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.
Highlights
The mesopelagic zone (200–1000 m depth) is inhabited by numerous macroscopic organisms, such as fish, cephalopods, and crustaceans, and the mesopelagic fish biomass make up a high fraction of the estimated total global fish biomass (Irigoien et al, 2014) and could be harvested on a large scale in the future as a source of marine fat and protein (e.g. Gjøsæter and Kawaguchi, 1980; FAO, 1997)
Vertical distribution and numerical density estimates of fish from nets In total, 50 fish taxa were caught in the trawl at the studied station, with a total averaged numerical density of 2.771 individuals 1000 m–3 integrated over the depth range 0–1200 m
Hullmounted acoustic data collected at the station during daytime when net samples were taken, suggested that the majority of these fishes were caught within the deep scattering layers (DSLs) (∼450–∼650 m depth, Supplementary Figure S1)
Summary
The mesopelagic zone (200–1000 m depth) is inhabited by numerous macroscopic organisms, such as fish, cephalopods, and crustaceans, and the mesopelagic fish biomass make up a high fraction of the estimated total global fish biomass (Irigoien et al, 2014) and could be harvested on a large scale in the future as a source of marine fat and protein (e.g. Gjøsæter and Kawaguchi, 1980; FAO, 1997). There are high uncertainties in biomass estimates, with current global estimates spanning one order of magnitude (Gjøsæter and Kawaguchi, 1980; Irigoien et al, 2014; Davison et al, 2015b; Proud et al, 2019).
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