Between 2001 and 2013 the number of breeding pairs of Adélie penguins (Pygoscelis adeliae) at breeding colonies in the southern Ross Sea more than doubled from about 235 000 to more than half a million. It has been suggested that predation release of Antarctic silverfish (Pleuragramma antarctica) due to fishing of one of its predators, Antarctic toothfish (Dissostichus mawsoni), could have contributed to the increase in Adélie penguin numbers. This paper brings together information on the biomass, consumption rates and diets of toothfish and Adélie penguins over the Ross Sea shelf as a first test of the predation release hypothesis. In particular, the examination of 422 Antarctic toothfish stomachs showed that they consume only a small proportion of silverfish (1.9–5.1% by mass) over the southern Ross Sea shelf. These Antarctic toothfish were sampled over three years, between 2011/12 and 2013/14. The mass of silverfish estimated as being released from predation by fishing (577 tWW/y) was equivalent to about 2% of the amount of silverfish consumed annually by Adélie penguins in this region. This result was inconsistent with predation release of silverfish due to the toothfish fishery being responsible for recent increases in the number of Adélie penguins breeding in the southern Ross Sea. Mixed trophic impact (MTI) analysis was used to look for alternative indirect pathways through the food-web by which changes to toothfish could affect Adélie penguins in the Ross Sea. The MTI analysis found only a weak link between changes in toothfish biomass and changes to the biomasses of silverfish and Adélie penguins. Essentially, Antarctic toothfish and Adélie penguins were not assessed as having overlapping diets over the Ross Sea shelf: Antarctic toothfish consume mainly small, bottom-dwelling fishes (especially icefish, and Trematomus spp.) while Adélie penguins consume crystal krill and silverfish in the water column. The large-scale trophic connection between toothfish and Adélie penguins over the Ross Sea shelf is hence weak. We encourage the development of further specific hypotheses of mechanisms by which fishing could affect Adélie penguins in the Ross Sea.
Dr Rohan Currey (New Zealand)