Progress towards a trophic model of the ecosystem of the Ross Sea, Antarctica, for investigating effects of the Antarctic toothfish fishery

We report on the further development of a carbon-budget trophic-model of the Ross Sea with which to investigate effects of the fishery for Antarctic toothfish (Dissostichus mawsoni). The Ross Sea is a low primary production system, with production being localised in space and time. In the relative absence of krill, Antarctic silverfish (Pleuragramma antarctica) are probably the major middle-trophic level link between primary production and the larger predators. Mesozooplankton (mainly copepods), and demersal fish (especially, Macrourus whitsoni, Bathyraja eatonii, Chionodraco hamatus, C. antarcticus) are other key linking species. The trophic model presented here is not complete and should be considered a work in progress. Overall, the model is close to balance, with total exports of organic carbon (mainly respiration) exceeding primary production by 7%. However, individual groups are generally not balanced, due in part to limited information on diet fractions of Ross Sea organisms. Methods to adjust diet fractions to take into account the relative abundances of prey items are suggested but not applied. The current version of the trophic model suggests that Antarctic toothfish have the potential to exert considerable predation pressure on some species of demersal fish. The significance of toothfish in the diets of predators (especially Weddell seal, type-C killer whale, sperm whale) cannot be tested reliably by the current version of the model, which is not spatially or seasonally resolved, and does not consider sub-populations of predators. More complete information on the abundances and diets of top predators in the Ross Sea are needed, especially with regard to the spatial, seasonal and interannual variability of these properties, and the population structures. Other recommendations for fieldwork, and work currently underway or planned by New Zealand scientists in the Ross Sea, are given.