Increases in Adélie penguins in the Ross Sea: Could the fishery for Antarctic toothfish be responsible?

Abstract

The Ross Sea is home to about a third of the world population of Adélie penguins (Pygoscelis adeliae). Between 2001 and 2013 the number of breeding pairs of Adélie penguins at colonies in the southwestern Ross Sea more than doubled. It has been suggested that this increase was caused by the fishery for Antarctic toothfish (Dissostichus mawsoni) leading to mesopredator release of Antarctic silverfish (Pleuragramma antarctica), a shared prey of toothfish and Adélie penguins. Cases of mesopredator release in marine systems have been widely reported, but limited information on marine predators often hinders quantitative investigation of these changes. The present study has brought together information from multiple models to estimate the biomass of silverfish that could be released from predation through the effects of the toothfish fishery. New diet data for toothfish are presented which show that Antarctic toothfish and Adélie penguins do not have significantly overlapping diets in the southwestern Ross Sea. The mass of silverfish released from predation due to the effects of fishing was estimated to be 128tonnes wet-weight per year (tWW/y; 5th–95th estimation interval of 46–358tWW/y) in 2013, equivalent to less than 2% of the biomass of silverfish estimated to be consumed annually by Adélie penguins. Even if toothfish consumed only silverfish, the predicted predation release effect would still not be sufficient to explain the observed increase in the number of Adélie penguins in the southern Ross Sea. The results of the modelling are hence 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 southwestern Ross Sea.