Application of an unsupervised clustering algorithm on in situ broadband acoustic data to identify different mesopelagic target types

Mette Dalgaard Agersted,Babak Khodabandeloo,Thor A Klevjer,Yi Liu,Webjørn Melle,Roland Proud

doi:10.1093/icesjms/fsab167

Mette Dalgaard Agersted, Babak Khodabandeloo + Show 4 more

Open Access

https://doi.org/10.1093/icesjms/fsab167

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Abstract

Abstract The mesopelagic zone (200–1000 m depth) contains high fish species diversity but biomass and abundances are uncertain yet essential to understand ecosystem functioning. Hull-mounted acoustic systems (usually 38 kHz) often make assumptions on average target strength (TS) of mesopelagic fish assemblages when estimating biomass/abundance. Here, an unsupervised clustering algorithm was applied on broadband acoustic data (54–78 kHz), collected by a towed instrumented platform in the central Northeast Atlantic, to identify different mesopelagic target types based on similarity of individual TS spectra. Numerical density estimates from echo-counting showed spatial differences in vertical distribution patterns of the different target types and TS spectra data suggested that &gt;30% of the gas-bearing targets had high resonance frequencies (&gt;60 kHz) with low scattering strength at 38 kHz. This conceptual study highlights the importance of separating targets into different target groups to obtain correct backscatter information and to account for all relevant scatterers when estimating average TS at 38 kHz, in order to achieve more accurate biomass/abundance estimates. It furthermore demonstrates the use of a towed broadband acoustic platform for fine-scale numerical density estimates as a complementary method to hull-mounted acoustic data to increase knowledge on mesopelagic ecosystem structure.

Highlights

The mesopelagic zone ranging from 200 to 1000 m depth potentially possesses high abundance and biomass of fish (Gjøsæter and Kawaguchi, 1980; Irigoien et al, 2014; Davison et al, 2015; Proud et al, 2019) and crustaceans, squids, and jellyfish are present
Our findings of target type categories indicate that likely more than 30% of the acoustically identified targets had resonance frequencies >38 kHz and low target strength (TS) at this normally applied frequency for density estimates of mesopelagic fish
Our results further suggest that towed acoustic platforms are a great supplement to hull-mounted acoustics, both for direct density estimates via echo-counting and for obtaining TSs for different target groups at different frequencies, at depths where higher acoustic frequencies cannot reach if transmitted from the surface

Summary

Introduction

The mesopelagic zone ranging from 200 to 1000 m depth potentially possesses high abundance and biomass of fish (Gjøsæter and Kawaguchi, 1980; Irigoien et al, 2014; Davison et al, 2015; Proud et al, 2019) and crustaceans, squids, and jellyfish are present. A proportion of the mesopelagic fauna performs diel vertical migration (DVM) (Gjøsæter and Kawaguchi, 1980; Klevjer et al, 2016), feeding in the euphotic zone during night while descending to deeper depths at dawn to hide from visual predators (Hays, 2003). This behaviour leads to active transport of carbon from upper waters to the deep ocean The vertical position of DSLs has been linked to various environmental parameters such as oxygen concentration (Bianchi et al, 2013; Klevjer et al, 2016) and light intensity

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