Original Article Interannual variability in Antarctic krill (Euphausia superba) density at South Georgia, Southern Ocean: 1997–2013

• Main Authors: Sophie Fielding, Jonathan L. Watkins, Philip N. Trathan, Peter Enderlein, Claire M. Waluda, Gabriele Stowasser, Geraint A. Tarling, Eugene J. Murphy
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Antarctic; Southern Ocean; Antarc*; Antarctic Krill; Euphausia superba
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.1026.4812; http://icesjms.oxfordjournals.org/content/71/9/2578.full.pdf

Antarctic krill (Euphausia superba) are a key species in SouthernOcean ecosystems,maintaining very large numbers of predators, and fluctuations in their abundance can affect the overall structure and functioning of the ecosystems. The interannual variability in the abundance and biomass of krill was examined using a 17-year time-series of acoustic observations undertaken in theWestern Core Box (WCB) survey area to the northwest of South Georgia, Southern Ocean. Krill targets were identified in acoustic data using a multifrequency identification window and converted to krill density using the Stochastic Distorted-Wave Born Approximation target strength model. Krill density ranged over several orders of magnitude (0–10 000 g m22) and its distribution was highly skewedwithmany zero observations.Within each survey, themean krill density was significantly correlated with the top 7 % of themaximum krill densities observed. Hence, only the densest krill swarms detected in any one year drove themean krill density estimates for the WCB in that year. WCB krill density (m, mean density for the area) showed several years (1997/1998, 2001–2003, 2005–2007) of high values (m. 30 g m22) interspersed with years (1999/2000, 2004, 2009/2010) of low density (m, 30 g m22). This pattern showed three different periods, with fluctuations every 4–5 years. Cross correlation analyses of variability in krill density with current and lagged indices of ocean (sea surface temperature, SST and El Niño/Southern Oscillation) and atmospheric variability (Southern Annular Mode) found