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    Ross Sea Bioregionalisation, Part II: Patterns of co-occurrence of mesopredators in an intact polar ocean ecosystem

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    G. Ballard, D. Jongsomjit and D.G. Ainley (USA)

    We report results of analyses of niche occupation among mesopredators in the Ross Sea region, Antarctica, considering three important components: 1) projected distribution and overlap across the surface of the ocean, 2) capacity to utilize differing amounts of the water column (foraging depth) and 3) diet. Species included were: Antarctic Minke Whale, Ross Sea Killer Whale (ecotype C), Crabeater Seal, Weddell Seal, Emperor Penguin, Adélie Penguin, Light-mantled Sooty Albatross, and Antarctic and Snow petrel. The apex predators, Leopard Seal and Killer Whale ecotype A/B, were not included because of their rarity and, therefore, lack of adequate sighting data on which to generate spatial models. We also did not have adequate data to model Arnoux’s Beaked Whales, Antarctic Toothfish nor Colossal Squid, which likely are also important mesopredators, particularly adult toothfish. We modeled mesopredator species distributions at a 5km/pixel scale, using environmental data and species presence localities from at-sea surveys and other sources. A machine learning, “maximum entropy” modeling algorithm (Maxent) was used to model spatial patterns of species’ probabilities of occurrence, and these data were used to identify areas of importance to species in a conservation prioritization framework (Zonation). Data on depth of diving and diet were taken from the literature. Three patterns of horizontal spatial use of the Ross Sea were apparent: 1) Shelf Break: restricted mostly to the shelf break, which includes outer continental shelf and slope (Light-mantled Sooty Albatross); 2) Shelf and Slope: full use of both the shelf and the slope (Ross Sea Killer Whale, Weddell Seal); and 3) Marginal Ice Zone (MIZ; pack ice surrounding the Ross Sea post-polynya): combinations in which the slope is the main habitat but western and eastern portions of the shelf (where sea ice is persistent) are used as well (Minke whale, Crabeater Seal, penguins, petrels). Diet composition overlapped extensively, but use of foraging space was well partitioned by depth of diving. Horizontally, the entire suite of mesopredators used the entire shelf and slope in a mosaic pattern although, not necessarily during the same season. Spatial modeling of species richness, supported by Zonation analysis, indicated the outer shelf and slope, as well as deeper troughs in the Ross Sea Shelf and Ross Island vicinity to be particularly important to the upper trophic level organisms of the Ross Sea. Our results substantially improve understanding of these species’ niche occupation previously only described using heuristic approaches.