Improving the Accuracy of Krill Target Strength Using a Shape Catalog

Fiona Bairstow,Sven Gastauer,C Tom A Brown,Martin J Cox,Tom Edwards,So Kawaguchi,Luke Finley

doi:10.3389/fmars.2021.658384

Abstract

Antarctic krill are subject to precautionary catch limits, based on biomass estimates, to ensure human activities do not adversely impact their important ecological role. Accurate target strength models of individual krill underpin biomass estimates. These models are scaled using measured and estimated distributions of length and orientation. However, while the length distribution of a krill swarm is accessible from net samples, there is currently limited consensus on the method for estimating krill orientation distribution. This leads to a limiting factor in biomass calculations. In this work, we consider geometric shape as a variable in target strength calculations and describe a practical method for generating a catalog of krill shapes. A catalog of shapes produces a more variable target strength response than an equivalent population of a scaled generic shape. Furthermore, using a shape catalog has the greatest impact on backscattering cross-section (linearized target strength) where the dominant scattering mechanism is mie scattering, irrespective of orientation distribution weighting. We suggest that shape distributions should be used in addition to length and orientation distributions to improve the accuracy of krill biomass estimates.

Highlights

Antarctic krill are a vital component of the complex ecology of the Southern Ocean (Mauchline and Fisher, 1969; Punchihewa and Krishnarajah, 2013)
We suggest that shape distributions should be used in addition to length and orientation distributions to improve the accuracy of krill biomass estimates
The work presented here is a step toward improving the accuracy of Antarctic krill biomass estimates through the use of a shape catalog

Summary

Introduction

Antarctic krill (hereafter “krill”) are a vital component of the complex ecology of the Southern Ocean (Mauchline and Fisher, 1969; Punchihewa and Krishnarajah, 2013). Commercial fisheries target krill to supply aquaculture and a growing demand for nutraceuticals (Kawaguchi and Nicol, 2020). To balance the potentially competing interests in krill, and to ensure long-term sustainability, fishing activities are controlled by setting precautionary catch limits through the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR). Key to setting precautionary catch limits is the estimator of krill biomass which is used as an input in the Generalized Yield Model (Constable and De la Mare, 1996). Krill biomass estimates are obtained from acoustic-trawl surveys, a fisheries independent method, typically carried out using ship-based active acoustic instruments and nets (Hewitt and Demer, 2000). Echosounders, are used to produce acoustic waves modulated at discrete frequencies which are scattered by targets such as krill, while the sound

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