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Long-term decline in krill stock and increase in salps within the Southern Ocean

Abstract

Antarctic krill (Euphausia superba) and salps (mainly Salpa thompsoni) are major grazers in the Southern Ocean1,2,3,4, and krill support commercial fisheries5. Their density distributions1,3,4,6 have been described in the period 1926–51, while recent localized studies7,8,9,10 suggest short-term changes. To examine spatial and temporal changes over larger scales, we have combined all available scientific net sampling data from 1926 to 2003. This database shows that the productive southwest Atlantic sector contains >50% of Southern Ocean krill stocks, but here their density has declined since the 1970s. Spatially, within their habitat, summer krill density correlates positively with chlorophyll concentrations. Temporally, within the southwest Atlantic, summer krill densities correlate positively with sea-ice extent the previous winter. Summer food and the extent of winter sea ice are thus key factors in the high krill densities observed in the southwest Atlantic Ocean. Krill need the summer phytoplankton blooms of this sector, where winters of extensive sea ice mean plentiful winter food from ice algae, promoting larval recruitment7,8,9,10,11 and replenishing the stock. Salps, by contrast, occupy the extensive lower-productivity regions of the Southern Ocean and tolerate warmer water than krill2,3,4,12. As krill densities decreased last century, salps appear to have increased in the southern part of their range. These changes have had profound effects within the Southern Ocean food web10,13.

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Figure 1: Krill, salps and their food.
Figure 2: Temporal change of krill and salps.
Figure 3: Krill–ice relationships.

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Acknowledgements

We thank V. Loeb and R. Hewitt for sharing the extensive data from the AMLR Program; A. Clarke, G. Hosie, S. Chiba, J. Nishikawa, R. Anadón, P. Ward and A. de C. Baker for providing further data and information; P. Fretwell, A. Fleming, S. Grant and S. Thorpe for data handling; and A. Hirst, A. Clarke, P. Ward, K. Schmidt, D. Pond, P. Trathan, G. Tarling and E. Murphy for comments. Satellite data were provided courtesy of NASA GSFC, the SeaWiFS Project and NOAA. The Department of Environmental Affairs & Tourism (Pretoria, South Africa) provided funds and facilities for South African collections, through the South African National Antarctic Program. A.A.'s contribution was funded under the British Antarctic Survey's DYNAMOE Programme.

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Correspondence to Angus Atkinson.

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Supplementary information

Supplementary Information Main Text

Focus on the three key results of the paper, expanding on their robustness to possible artefacts of sampling. Includes Supplementary Information tables 3and 4, and Supplementary Figure captions. (DOC 52 kb)

Supplementary Table 2

Data collection information — metadata for the complete database (follows main text table 1). (XLS 63 kb)

Supplementary Figure 4

Krill distribution (follows main text Figs 1–3). (PPT 472 kb)

Supplementary Figure 5

Fine-scale sampling coverage in the South West Atlantic sector. (PPT 464 kb)

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Atkinson, A., Siegel, V., Pakhomov, E. et al. Long-term decline in krill stock and increase in salps within the Southern Ocean. Nature 432, 100–103 (2004). https://doi.org/10.1038/nature02996

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