title: 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 Ocean, and krill support commercial fisheries. Their density distributions have been described in the period 1926-51, while recent localized studies 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 recruitment and replenishing the stock. Salps, by contrast, occupy the extensive lower-productivity regions of the Southern Ocean and tolerate warmer water than krill. 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 web.
authors.affiliation: British Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road, Cambridge CB3 OET, UK. aat@ bas.ac.uk
authors.forename: Angus
authors.initials: A
authors.lastname: Atkinson
authors.forename: Volker
authors.initials: V
authors.lastname: Siegel
authors.forename: Evgeny
authors.initials: E
authors.lastname: Pakhomov
authors.forename: Peter
authors.initials: P
authors.lastname: Rothery
mesh_terms: D000818:Animals; D000864:Antarctic Regions; D016477:Artifacts; D002734:Chlorophyll / Q000032:analysis; D000077194:Chlorophyll A; D016208:Databases, Factual; D056890:Eukaryota / Q000502:physiology; D033201:Euphausiacea / Q000502:physiology*; D020387:Food Chain; D046448:Ice Cover / Q000382:microbiology; D009792:Oceans and Seas; D011156:Population Density; D012621:Seasons; D012623:Seawater / Q000032:analysis / Q000469:parasitology*; D013696:Temperature; D013997:Time Factors
chemical_list: D002734:Chlorophyll; D000077194:Chlorophyll A