Temporal Change of Krill and Salps in the Southern Ocean, 1926-2003. a, Krill density in the SW Atlantic sector (4,948 stations in years with >50 stations). Temporal trends include b, post-1976 krill data from scientific trawls; c, 1926–2003 circumpolar salp data south of the SB. Atkinson, et al., 2004

a, Krill density in the SW Atlantic sector (4,948 stations in years with >50 stations). Temporal trends include b, post-1976 krill data from scientific trawls; c, 1926–2003 circumpolar salp data south of the SB. Regressions of log10(mean no. m-2) on year were calculated for cells with ≥3 years of data, weighted by number of stations in that year. One-sample t-tests supported a post-1976 decrease in krill density in the SW Atlantic (scientific trawls: t = -3.4, P = 0.004, 16 cells, smaller nets: t = -2.5, P = 0.04, 8 cells). Salp densities increased south of the SB after 1926 (t = 3.1, P = 0.004, 32 cells) Green spots denote cells usable in the spatio-temporal model. 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.

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