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    Adélie penguin colony size predicts south polar skua abundance on Ross Island, Antarctica

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    D.J. Wilson, P.O’B. Lyver (New Zealand), A.L. Whitehead (Australia), T.C. Greene (New Zealand), K. Dugger (USA), B.J. Karl, J.R.F. Barringer, R. McGarry (New Zealand), A.M. Pollard and D.G. Ainley (USA)
    Submitted By:
    Mr Doug Cooper (CCAMLR Secretariat)

    Top predators are useful ecological indicators of changes in marine prey stocks and other ecosystem drivers. In the Ross Sea region, most south polar skuas (Catharacta maccormicki) nest near Adélie penguin (Pygoscelis adeliae) colonies, feeding on fish, penguin eggs and chicks, and carrion. We estimated skua abundance at Adélie penguin colonies on Ross Island in two consecutive years and tested for a relationship between these estimates and penguin colony sizes. We used distance sampling to estimate numbers of skuas and skua breeding pairs (based on nests), at five penguin nesting areas of varying size within the three main Ross Island colonies. We considered skua populations in ice-free areas within 500-m buffers around penguin sub-colonies at Cape Bird and within 1000 m at Cape Crozier and Cape Royds, where skuas were more widely dispersed. Estimated skua numbers were highest at Cape Crozier (4054–4892 individuals) and lowest at Cape Royds (141–152). Comparison with our skua breeding population estimates suggested that most skuas at these locations were breeding or attempting to breed. We found a strong log-log linear relationship between numbers of skuas and penguin breeding pairs. Many historical paired estimates of skua numbers and penguin colony sizes in the Ross Sea fell below this regression line. We probably overestimated skua numbers at some colonies by overestimating the areas that skuas occupied. Applying our regression model to published mean Adélie penguin colony sizes at 23 locations predicted 18 000 skuas (9 000 breeding pairs) total in the western Ross Sea, which is also likely an overestimate. We plan to revise our estimates and regression model by redefining the areas surveyed and recalculating abundances. We propose to validate the model by surveying skuas at a subset of Adélie penguin colonies. The revised, validated skua population estimate can then inform ecosystem models applied to fisheries management in the Ross Sea.