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

Abstract

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 interannual variability in the abundance and biomass of krill was examined using a 17-year time-series of acoustic observations undertaken in the Western 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 m−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 krill densities observed. Hence, only the densest krill swarms detected in any one year drove the mean krill density estimates for the WCB in that year. WCB krill density (µ, mean density for the area) showed several years (1997/1998, 2001–2003, 2005–2007) of high values (µ > 30 g m−2) interspersed with years (1999/2000, 2004, 2009/2010) of low density (µ < 30 g m−2). 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 the highest correlation between krill density and winter SST (August SST) from the preceding year. A quadratic regression (r2 = 0.42, p < 0.05) provides a potentially valuable index for forecasting change in this ecosystem.

Drivers of intrapopulation variation in resource use in a generalist predator, the macaroni penguin

“…data). Consequently, we infer that although krill occurred in greater density during 2002 (Fielding et al 2014), they were distributed lower in the water column such that accessibility to foraging macaroni penguins was reduced.…”

“…Acoustic surveys carried out in a standard area to the northwest of South Georgia documented a 3-fold change in local krill density during the 5 years with available feather data (Fielding et al 2014). The density of krill was very low in 2001 (< 30 g m ; Fielding et al 2014).…”

“…The density of krill was very low in 2001 (< 30 g m ; Fielding et al 2014). The recruitment of krill at South Georgia is thought to reflect seasonal advection from spawning grounds farther south (Atkinson et al 2004), and therefore the life stage and oceanic origin of these krill may vary annually (Frazer et al 1997, Trull & Armand 2001, Schmidt et al 2003, Stowasser et al 2012.…”

“…These conditions provide, to some extent, a natural experiment for examining how intrapopulation variation in resource use changes over different temporal and spatial scales. For example, the local biomass of Antarctic krill Euphausia superba at South Georgia can fluctuate several-fold between years (Hewitt et al 2003, Fielding et al 2014, and substantially impact the diet and activity budgets of predators that depend on krill as a primary prey resource (Croxall et al 1999). Macaroni penguins Eudyptes chrysolophus are one of the most important avian consumers of krill in this region (Brooke 2004), and also exhibits limited sexual dimorphism in size (Williams 1995).…”

Images as proximity sensors: the incidence of conspecific foraging in Antarctic fur seals

“…Average frequency of other seal encounters was higher in austral summer 2000, than in 2001 and 2002 (Tukeys Honest Significant differences, p < 0.01), but differences between 2001 and 2002 were not significant (p = 0.30). Such interannual differences in prey biomass are well reported and likely drive this difference in detection of conspecifics [29][30][31]. The frequency of other seal encounters appeared to increase with the total krill encounter rate.…”