Remote sizing of fish-like targets using broadband acoustics

Rokas Kubilius,Gavin J Macaulay,Egil Ona

doi:10.1016/j.fishres.2020.105568

Rokas Kubilius, Gavin J Macaulay + Show 1 more

Open Access

https://doi.org/10.1016/j.fishres.2020.105568

Copy DOI

Journal: Fisheries Research	Publication Date: Apr 27, 2020
Citations: 12	License type: cc-by

Affiliation: Norwegian Institute of Marine Research

Abstract

Broadband echosounder systems provide very high range resolution that could resolve different parts of fish and hence provide estimates of fish size. The potential for, and accuracy of, direct acoustic sizing of fish was tested on fish-like targets of known dimensions and orientations. Prolate spheroids, 91–477 mm long, with and without an air-filled inclusion (simulating a fish swim-bladder), made from polyvinyl-alcohol-cryogel, were suspended in an acoustic beam and rotated while the backscattered signal was recorded. The echoes from linear frequency modulated pulses of 45−90 and 160−260 kHz with a fast and a slow amplitude modulation were pulse-compressed to provide data with a range resolution of 6.6 mm at its highest. Echoes from the target boundaries were manually identified, tracked and used to directly estimate the thickness and length of the targets. An accuracy of ±11−17 mm was achieved for targets longer than about 200 mm and thicker than about 20 mm when using the 160−260 kHz pulse. The length of the air-filled inclusion could also be estimated for the larger targets. Echoes from the slow amplitude modulation were easier to interpret than the fast amplitude modulation despite the lower range resolution, due to the lower pulse compression temporal sidelobes. The results show that remote acoustic sizing of fish is feasible and of sufficient accuracy for practical use.

Highlights

Oceanic pelagic trawling and purse seine fishing accounts for about 20 % and 30 % respectively of the global landings of pelagic fishes (Watson et al, 2006)
Pre-catch knowledge of fish size is important during pelagic trawling operations as undersized and/or low-quality fish can lead to financially unsustainable operations
In this paper we demonstrate that, i) pulse-compressed broadband signals can provide a spatial range resolution that is high enough to resolve the dimensions of medium-sized fishes, ii) the boundaries of fish-flesh-like objects can be separated and identified within the total scattering from the target, and iii) the length and thickness of a fishflesh-like target can be inferred and measured remotely and directly

Summary

Introduction

Oceanic pelagic trawling and purse seine fishing accounts for about 20 % and 30 % respectively of the global landings of pelagic fishes (Watson et al, 2006). For example, omni-directional sonar is the primary tool to locate fish schools of interest. Pre-catch knowledge of fish size is important during pelagic trawling operations as undersized and/or low-quality fish can lead to financially unsustainable operations. There are few effective methods to conveniently obtain this size and quality information, apart from acoustic techniques, which have a sufficiently large operating range. Optical instruments are a potential sizing tool, but due to their limited operating range require deployment from a vessel with the additional complexities that this entails, and we focus here on the development of acoustic techniques

Methods

Results

Conclusion