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    A balanced trophic model of the ecosystem of the Ross Sea, Antarctica, for investigating effects of the Antarctic toothfish fishery

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    M.H. Pinkerton, S.M. Hanchet and J. Bradford-Grieve (New Zealand)
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    We report on the development of a mass balanced carbon-budget trophic model of the Ross Sea with which to investigate effects of the fishery for Antarctic toothfish (Dissostichus mawsoni). A survey of the available literature provided an initial set of parameters describing the abundance (seasonal and spatial where possible), energetics (growth, reproduction, consumption), and trophic linkages (diets, predators) for major groups of biota. We also estimated the level of uncertainty on these parameters. The Ross Sea is a low primary production system, with high seasonal, spatial and interannual variability in primary production. In the relative absence of krill, Antarctic silverfish (Pleuragramma antarctica) and mesozooplankton (mainly copepods) are probably the major middle-trophic level link between primary production and the larger predators, though the role of cepahlopods in the system is poorly known. A number of demersal fish species (including Macrourus whitsoni, Bathyraja eatonii, Chionobathyscus dewitti, Antimora rostrata Chionodraco hamatus) are present, but their abundances and feeding characteristics are not well known. Toothed and baleen whales visit the Ross Sea in summer in relatively large numbers. Adelie and emperor penguins have breeding colonies along the Victoria Land coast, and petrels, skua and albatross are seasonal visitors. Weddell, crabeater, leopard and Ross seals are also present in summer, and some may stay in the region year-round. The trophic model was balanced by adjusting the initial set of parameters obtained from the literature and available datasets. We present a novel objective method of adjustment that takes into account estimates of parameter uncertainty, and appropriately handles the huge range of magnitude (>5 orders of magnitude) in trophic flows between different groups of organisms. Biomass, production rates, consumption rates and diet fractions are adjusted simultaneously. We set ecotrophic efficiency to unity for all non-primary producers. Changes to the initial set of parameters needed to obtain balance were significant: up to 62% (biomass, production, consumption), and 40% (diet fractions). The balanced model presented here has not yet been validated and should be considered a work in progress. 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. More information on demersal fish abundance is required to validate this result. Information on what the various species of demersal fish consume is needed to estimate the potential for trophic cascades due to the toothfish fishery. The significance of toothfish in the diets of predators (especially Weddell seal, type-C killer whale, sperm whale) are low in the model, but the model does not consider sub-populations of predators, or localized dependencies on toothfish as prey. More complete information on the abundances, diets, and population structures of top predators in the Ross Sea are needed to investigate these potential effects.