A balanced ecosystem model is used to explore the system-level characteristics of the food-web of the Ross Sea shelf and slope before the advent of commercial fishing for Antarctic toothfish (Dissostichus mawsoni) in the region. We evaluate: (1) biomass and flow of organic matter by trophic level; (2) mixed trophic impact; and (3) ecological importance. Note that the analysis considers food-web structure and function at the spatial, temporal and ecological scale of the ecosystem model, i.e. the whole Ross Sea shelf and slope area, averaged over a typical year, and in 35 trophic groups. Effects at smaller spatial and temporal scales, and effects concerning only parts of trophic groups, are not resolved by this analysis. The Ross Sea food-web is a partially inverted biomass pyramid with a pronounced peak in biomass in the lower-middle part of the food-web, a result of high biomass of mesozooplankton and benthic invertebrates. The biomass of top predators (trophic levels>4.5) is only 0.5% of the total living biomass in the Ross Sea (bacteria excluded). The six groups with the highest ‘indices of ecological importance’ in the food-web of the Ross Sea are phytoplankton, mesozooplankton, Antarctic silverfish (Pleuragramma antarcticum), small demersal fishes, Antarctic krill (Euphausia superba) and cephalopods. Pelagic fishes and crystal krill (E. crystallorophias) are also likely to have high importance in the Ross Sea food-web. These eight groups should be the priorities for monitoring for large-scale ecosystem change in the region, for example due to climate change, trophic cascades or regime shift. The analysis presented here does not support the hypothesis that changes to the abundance of toothfish in the Ross Sea will propagate through the food-web; it appears that Antarctic toothfish only have a moderate index of ecological importance in the food-web of the Ross Sea shelf and slope. Changing the biomass of Antarctic toothfish on the Ross Sea shelf and slope is likely to have the greatest effect on the demersal fish community there.
Sarah Mackey (CCAMLR Secretariat)