No evidence for a decline in the density of Antarctic krill Euphausia superba Dana, 1850, in the Southwest Atlantic sector between 1976 and 2016

Summary

This study challenges previous reports of major Antarctic krill population declines. By reanalyzing the KRILLBASE database with improved methods, researchers found no evidence for significant long-term decline in krill numbers in the Southwest Atlantic.The findings contradict earlier research suggesting 38-81% declines per decade. Instead, krill populations appear stable over four decades with considerable year-to-year variation. The researchers argue that previously reported declines resulted from uneven sampling and failure to account for different types of nets used to collect data.The authors note that massive population declines would have caused obvious effects on animals that depend on krill for food, which haven't been observed. These findings suggest that current thinking about climate-driven changes in Antarctic ecosystems needs revision, with important implications for managing krill fisheries.

This map shows how krill populations have changed over time in different areas of the Southwest Atlantic Ocean. Each purple dot represents krill density measured at a specific place and time. The straight lines show the general trend in each area - green areas (+) show increasing krill populations, while red areas (–) show decreasing populations. The number in each area shows how many samples were taken there. The grey shading around the lines shows the uncertainty in the measurements.

Key Findings

No evidence that krill populations have significantly declined in the Southwest Atlantic

Krill numbers appear stable over time but vary considerably from year to year

Previous reports of major declines likely resulted from not properly accounting for different sampling methods

The lack of widespread effects on krill predators supports the finding that krill populations are stable

Abstract

The Antarctic marine environment is changing, and changes in the Southwest Atlantic sector have included decreases in sea ice and increases in water temperature. Associated with these changes is a reported 38% and 81% per decade decline in the numerical density (hereafter density) of Antarctic krill Euphausia superba Dana, 1850, between 1976 and 2003. Few changes in other components of the ecosystem that could be attributed to such a change, such as a mass decline in krill-dependent predators, have been detected. In an ecosystem so dependent on this keystone species, a massive population decline in krill ought to have had an obvious effect. In the absence of such an effect, it is timely to revisit the issue of the purported decline in krill density. The original analysis that indicated a decline in krill density was based on the 2004 version of KRILLBASE, a database of net samples. We analysed the publicly available and updated version (version 1, accessed 30 November 2017) and our analyses did not suggest a significant decline in krill density. Rather, after accounting for sampling heterogeneity and habitat variables, average krill density appears to have been stable but with considerable inter-annual variability. Since our results were unable to find any evidence for a decline in krill density we recommend a re-appraisal of many of the paradigms that underlie much of the recent thinking about ecosystem change Antarctic waters. Such a revision is necessary to provide a firmer foundation for predictions of the effects of climate change and resource extraction on the Southern Ocean ecosystem.

Published in

Journal of Crustacean Biology

2018

Authors

Cox, M.J., Candy, S., de la Mare, W.K., Nicol, S., Kawaguchi, S., Gales, N.

Institutions

Australian Antarctic Division S Candy Statistical Modelling Pty Ltd Institute for Marine and Antarctic Studies, University of Tasmania

Methods

Biological sampling Data

Read the full paper

Go to doi.org