Two scales of distribution and biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2 (55°E to 80°E)

Summary

This study provides the first comprehensive krill biomass assessment for the eastern sector of CCAMLR Division 58.4.2 since 2006. The research addresses two critical objectives: updating large-scale biomass estimates to inform fisheries management, and evaluating whether smaller-scale surveys can reliably represent broader regional patterns. The findings confirm that day-time acoustic surveys are more reliable due to diel vertical migration patterns, and demonstrate that strategically designed small-scale surveys can serve as cost-effective monitoring tools between expensive large-scale surveys. This is particularly important given renewed commercial interest in East Antarctic krill fishing and the need for regular monitoring in a changing Antarctic environment. The lower biomass density compared to 2006 (though within expected variation) underscores the importance of continued assessment. The study recommends using day-time data only for biomass estimation and suggests that a series of small-scale surveys extrapolated across latitudinal bands could provide a viable approach for "filling gaps" in monitoring between major survey efforts.

Key Findings

Total krill biomass estimate: 6.48 million tonnes (using day-time data only) or 4.8 million tonnes (using combined day-night data)

Areal biomass density: 8.3 g/m² (day-time) with 28.9% coefficient of variation

Survey area: 775,732 km² in eastern sector of CCAMLR Division 58.4.2 (55°E to 80°E)

Significantly lower krill density observed at night compared to day, suggesting diel vertical migration with krill moving closer to surface at night (above minimum observation depth of echosounder)

Small-scale "Mawson box" survey (4,902 km²) was statistically representative of krill distribution within its latitudinal band (KS-test p=0.98; t-test p=0.44), validating potential for cost-effective smaller surveys

Survey conducted February-March 2021 aboard RV Investigator: 1,188 nautical miles of transects, 34 net trawls

Krill length frequency: mean total length ranged across trawls with no clear spatial pattern

Ice krill (Euphausia crystallorophias) present in samples at ratio of 7.4:1 (Antarctic krill to ice krill)

Krill biomass density considerably lower than 2006 BROKE-West survey estimate (20.5 g/m²) but comparable to 1996 survey of adjacent Division 58.4.1 (5.5 g/m²)

Abstract

Regular monitoring is an important component of the successful management of pelagic animals of interest to commercial fisheries. Here we provide a biomass estimate for Antarctic krill (Euphausia superba) in the eastern sector of the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR) Division 58.4.2 (55°E to 80°E; area = 775,732 km²) using data collected during an acoustic-trawl survey carried out in February and March 2021. Using acoustic data collected in day-time and trawl data, areal biomass density was estimated as 8.3 gm⁻² giving a total areal krill biomass of 6.48 million tonnes, with a 28.9% coefficient of variation (CV). The inaccessibility of the East Antarctic makes fisheries-independent surveys of Antarctic krill expensive and time consuming, so we also assessed the efficacy of extrapolating smaller surveys to a wider area. During the large-scale survey a smaller scale survey (centre coordinates -66.28°S 63.35°E, area = 4,902 km²) was conducted. We examine how representative krill densities from the small-scale (Mawson box) survey were over a latitudinal range by comparing krill densities from the large-scale survey split into latitudinal bands. We found the small scale survey provided a good representation of the statistical distribution of krill densities within its latitudinal band (KS-test, D = 0.048, p-value = 0.98), as well as mean density (t-test p-value = 0.44), but not outside of the band. We recommend further in situ testing of this approach.

Published in

PLOS ONE 17(8): e0271078. https://doi.org/10.1371/journal.pone.0271078

2022

Authors

Martin J. Cox, Gavin Macaulay, Madeleine J. Brasier, Alicia Burns, Olivia J. Johnson, Rob King, Dale Maschette, Jessica Melvin, Abigail J. R. Smith, Christine K. Weldrick, Simon Wotherspoon, So Kawaguchi

Institutions

Australian Antarctic Division, Kingston, Tasmania, AustraliaAustralian Antarctic Program Partnership, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, Tasmania, AustraliaAqualyd Limited, Wakefield, New Zealand

Methods

AcousticBiological sampling DataField

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