Interannual variability in Antarctic krill (Euphausia superba) density at South Georgia, Southern Ocean: 1997–2013
Sophie Fielding
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Jonathan L. Watkins
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Philip N. Trathan
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Peter Enderlein
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Claire M. Waluda
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Gabriele Stowasser
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Geraint A. Tarling
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Eugene J. Murphy
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British Antarctic Survey, Natural Environment Research Council
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High Cross, Madingley Road, Cambridge CB3 0ET
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UK
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 m22) 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 (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 Nino/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.
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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 is also thought to be an important influence on ocean
biogeochemistry (Tovar-Sanchez et al., 2007) and thus oceanic carbon
cycles. 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).
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). At the
same time, there have been recent developments in harvesting
technology and products derived from krill that have further focused
interest in exploiting this resource (Nicol et al., 2011). This
combined pressure has raised concern regarding the future sustainability
of krill harvesting under the cumulative pressure of climate change
and increasing fisheries (Schiermeier, 2010; Flores et al., 2012).
The most recent estimate of post-larval krill biomass for the
whole Southern Ocean was 379 MT (Atkinson et al., 2009), with
.50% of this biomass contained within the Atlantic sector.
Atkinson et al. (2004) showed a decrease in krill abundance in the
Southwest Atlantic sector of the Southern Ocean coincident with
a decline in winter sea ice coverage. Trivelpiece et al. (2011) have
more recently identified declines in krill-eating penguin
populations on the Antarctic Peninsula, linking these with declines in
krill abundance. Such changes highlight the importance of
quantifying interannual variability in krill density within the Atlantic
sector of the Southern Ocean (Reiss et al., 2008; Reid et al., 2010),
particularly as these pressures are expected to increase over the
coming decades.
Potentially, the most suitable places to monitor biological
responses to climate change are where many species are highly
thermally sensitive, usually at the margins of their biogeographic ranges
(Hogg et al., 2011). For krill, such a location exists at South Georgia
where sea temperatures are towards the upper limit of the thermal
range of krill (Flores et al., 2012). This archipelago is a large, isolated
land and continental shelf area in the Atlantic sector of the Southern
Ocean (Figure 1). The island is (...truncated)