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, new data from the examination of the contents of 615 Antarctic toothfish stomachs showed that toothfish consume only a small proportion of silverfish (1.9–5.6%W i.e. by weight of prey) over the southern Ross Sea shelf, including in McMurdo Sound (0.4–6.4%W) and Terra Nova Bay (1.4–6.0%W). These Antarctic toothfish were sampled over four years, between 2011/12 and 2014/15 as part of the sub-adult Ross Sea survey.
The mass of silverfish estimated as being released from predation by fishing (643 tWW/y) was equivalent to 2.4% of the amount of silverfish estimated to be 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.
As a sensitivity test, we also estimated the amount of predation release of silverfish if toothfish consumed 100%W of silverfish i.e. ate only silverfish. In this case, the predation release effect was larger but still not sufficient to explain the observed increase in the number of Adélie penguins in the southern Ross Sea.
We encourage the development of further specific hypotheses of mechanisms by which fishing could affect the wider Ross Sea ecosystem. However, we consider that understanding the ecosystem effects of the toothfish fishery on the demersal fish community of the Ross Sea slope and on Weddell seals and type-C killer whales are higher priorities.