Inter-annual variability in krill density at South Georgia: 1997–2012

Antarctic krill (Euphausia superba) are a key species in Southern Ocean ecosystems, maintaining very large numbers of predators, and fluctuations in their abundance can affect the overall structure and functioning of the ecosystems. The inter-annual variability in the abundance and biomass of krill was examined using a 15 year time series of acoustic observations undertaken in the Western Core Box (WCB) survey area to the north of South Georgia, Southern Ocean. Krill targets were identified in acoustic data using a multi-frequency identification window and converted to krill density using the Stochastic Distorted Wave Born Approximation (SDWBA) target strength model. The density of krill within 500 m resets ranged several orders of magnitude (0 to 10 000 gm^-2) and its distribution was highly skewed with many zero observations. Within each survey the mean krill density was significantly correlated with the top 7% of the maximum reset densities observed. Hence, only the densest krill swarms detected in any one year were driving the mean krill density estimates for the WCB in that year. WCB krill density (µ, mean density for the area) showed several years of moderate to high values (µ > 30 gm^-2) interspersed with years (1999-2000, 2004, 2009-2010) of very low density (µ < 30 gm^-2).