The relationship between target strength frequency response and vertical swim velocity: a new approach for fish discrimination

Hikaru Homma,Ilia Ostrovsky

doi:10.1051/alr/2021011

Abstract

In-situ identification of fish species using acoustic methods is a key issue for fisheries research and ecological applications. We propose a novel approach to fish discrimination based on the relationship between target strength frequency response (TS(f)) and vertical swim velocity (VSV), as a proxy of fish body orientation. The measurements were carried out with a wideband echosounder on live fish of five species confined in a net cage. The data show a large dependence of TS(f) on VSV. To compare the variability of frequency responses of different fishes, we calculated ΔTS(f, VSV) as the difference between the TS(f) at given VSV and the TS(f) at VSV = 0, i.e. when the fish was swimming horizontally. We demonstrated that the relationships between ΔTS and VSV were similar for fish of the same species but dissimilar for different species. This implies that the acoustic fish discrimination in nature might be performed when the variations of the VSV can be measured from acoustically tracked fish. This can be a promising method for remote fish discrimination, for instance, for fish with diurnal vertical migrations. Further validation of this approach for fish recognition is required.

Highlights

Remote acoustic identification of fish species has been a major issue for fishery management and stock assessment in different marine and freshwater ecosystems
Recent studies suggest implementing wideband echosounders for species identification, which allow obtaining frequency response of backscattering strength, i.e. the backscattering strength can be presented as a function of acoustic frequency (e.g., Demer et al, 2017; Berges and van Helmond, 2018)
The main aim of this work is to study the dependence of frequency responses of various fish species on vertical swim velocity (VSV), as a proxy of body orientation, and reveal the species-specific patterns that can be used for remote discrimination between five studied species

Summary

Introduction

Remote acoustic identification of fish species has been a major issue for fishery management and stock assessment in different marine and freshwater ecosystems. One of the promising methods is to use multiple echosounders to detect species-specific variations of backscattering strengths at different acoustic frequencies (e.g., Gorska et al, 2004; Korneliussen, 2018; Boyra et al, 2019). Recent studies suggest implementing wideband echosounders for species identification, which allow obtaining frequency response of backscattering strength, i.e. the backscattering strength can be presented as a function of acoustic frequency (e.g., Demer et al, 2017; Berges and van Helmond, 2018). Many fish have gas-bearing swimbladders, which are responsible for the strong backscatter of acoustic waves. The characteristics of the backscattered signal largely depend on the morphology of the swimbladder.

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