Abstract
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.
Highlights
Antarctic krill (Euphausia superba), hereafter krill, is a key species in the Antarctic marine foodweb as a result of its large biomass (Atkinson et al, 2009) and important role in the Antarctic foodweb as prey to fish, squid, penguins, other seabirds, and marine mammals including seals and whales (Croxall et al, 1999)
Since the late 1970s, krill has been the target of an international fishery (Nicol et al, 2011), managed within an ecosystembased framework that is regulated in accordance with rules agreed by the Commission for the Conservation of Antarctic Marine Living Resources (CCAMLR)
We examine relationships between krill density at South Georgia and indices of climate variability, and compare with past interpretations of variability and change
Summary
Antarctic krill (Euphausia superba), hereafter krill, is a key species in the Antarctic marine foodweb as a result of its large biomass (Atkinson et al, 2009) and important role in the Antarctic foodweb as prey to fish, squid, penguins, other seabirds, and marine mammals including seals and whales (Croxall et al, 1999). Krill have a circumpolar habitat, constrained to the north by the Antarctic Polar Front (APF) and to the south by the continent (Atkinson et al, 2009). This habitat is undergoing rapid environmental change such as loss of sea ice (Stammerjohn et al, 2008), increasing sea temperatures (Turner et al, 2005; Whitehouse et al, 2008) and ocean acidification (Orr et al, 2005); with recent changes and impacts summarized by Flores et al (2012). There have been recent developments in harvesting technology and products derived from krill that have further focused
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