Two decades of variability in krill predators at Bird Island, South Georgia and their potential as ecosystem indicators

The breeding performance of seabirds and seals at Bird Island, South Georgia, in the Southern Ocean, has been recorded annually for over two decades as part of a large marine ecosystem monitoring programme. We examined the mechanistic relationships between, and patterns of inter-annual variability in variables for four species to evaluate their potential as ecosystem indicators. These variables included foraging performance, offspring survival, offspring mass, the contribution of Antarctic krill Euphausia superba to the diet, and per capita reproductive performance (PRP: the product of offspring survival and mass). Krill in the diet was correlated across the three bird species (gentoo, Pygoscelis papua; and macaroni, Eudyptes chrysolophus, penguins; and black-browed albatrosses, Thalassarche melanophris) and was a strong predictor of PRP for Antarctic fur seals, Arctocephalus gazella, and gentoo penguins. Offspring survival and hence PRP were largely independent of krill in the diet for macaroni penguins and black-browed albatrosses. The first principal component of all variables summarises the variability associated with krill availability. This was characterised by skewed anomalies with more pronounced but less frequent negatives than positives. Negative anomalies occurred at approximately three year intervals and the most severe occurred in 2009. Above average concurrent sea surface temperature was a necessary but not sufficient condition for extreme (lower 10th percentile) negative anomalies. There were no trends in indicators of krill availability but there were negative trends in some krill-independent variables suggesting changes to other parts of the ecosystem.