Abundance and trends in the breeding population of Adélie penguins (Pygoscelis adeliae) in the western Ross Sea

The Adélie penguin (Pygoscelis adeliae) is an indicator species used by the CCAMLR Ecosystem Monitoring Program (CEMP) to detect effects of anthropogenic activities (e.g. commercial fishing) on the Antarctic marine ecosystem.  The goal of this study was to assess annual variation and trends in the number of Adélie penguin breeding pairs at colonies in the western Ross Sea.  High angle oblique aerial photographic surveys of colonies were acquired for the breeding seasons between 1981 and 2012, and counted.  On average 870,465 pairs of Adélie penguins breed in the western Ross Sea each summer, with just over a quarter (27.5%) at colonies on Ross and Beaufort Islands (southern Ross Sea meta-population).  The aggregated colonies of Cape Bird and Cape Crozier had a negative per capita growth rate of -1.8% for the years, 1981-2000 followed by a positive per capita growth rate of 5.5% for the years, 2001-2010.  In contrast, a single declining trend line best represented the number of breeding pairs at Cape Royds for the years, 1981 to 2012.  Colony growth rates for the southern Ross Sea meta-population showed a striking level of synchrony through time.  The partial rate correlation functions for the rates of change versus loge-transformed colony size all showed a significant negative correlation at lag 1 indicative of direct density dependence in the number of pairs returning to the colony each year.  In recent years a number of the Adélie penguin colonies have reached their highest levels since New Zealand began its aerial counts in 1981.  However, the reason for the positive growth rate at Cape Bird and Cape Crozier, but not Cape Royds, remains unknown.  It is likely that the adverse oceanographic and sea ice conditions caused by the grounding of the giant iceberg events (2000-2005) off the southern Ross Sea colonies resulted in adults choosing to abandon or skip breeding in the worst affected years, especially 2001.  The increases in the southern colonies are in contrast to what we observed at Cape Hallett and other northern colonies which exhibited an overall decline or no trend respectively up until our last survey in 2006.  It is unlikely the penguins along the northern Victoria Land coast were affected by the giant icebergs during their breeding since they did not ground for any period adjacent to those colonies suggesting an alternate factor(s) are affecting the birds in that region quite differently.  Two hypotheses: (i) increases in the length of season and spatial extent of sea ice in the north-western Ross Sea region and/or (ii) an increase in a common prey species of the Adélie penguin, Antarctic silverfish (Pleuragramma antarcticum), brought on by changes in the abundance of a silverfish predator, the Antarctic toothfish (Dissostichus mawsoni), by commercial fishing or other reasons, have been postulated as explanations for the increases observed in the southern Ross Sea meta-population, especially over the last decade.