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Marine megafauna in environmental extremes : : distribution and oceanic dispersal of polar and tropical tetrapods

  • Author(s): Gearheart, Geoffrey
  • et al.
Abstract

Early autumn visual surveys carried out in the Ross Sea in February-March 2013 reveal Emperor penguins congregate and feed in migratory "hubs" located in stable pack ice of the eastern Ross Sea, close to the Antarctic Slope Front. From there, based on their annual cycle, they travel to the marginal ice zone, or the eastern and western Ross Sea colonies. We hypothesize that Adélie penguins migrate with the expanding pack ice from breeding colonies in the western Ross Sea to lower latitudes with sufficient light to forage. This movement appears synchronous, as evidenced by the gradual increase in the relative abundance of this species along our survey transect, and the large concentration of birds in the eastern Ross Sea, beyond the shelf break. The Antarctic Slope Front is a hotspot for Weddell and crabeater seals, the former remaining in pack ice on the continental shelf, the latter being more numerous beyond the shelf break, possibly due to higher concentrations of krill. Acoustic tracking and Lagrangian drifters enabled us to build a model for the first month of dispersal of leatherback hatchlings from Papua, Indonesia. This "critical period" corresponds to the duration of the yolk reserves, the limit beyond which turtles need to in productive waters to survive. We show that hatchlings' movements strongly influence their trajectories, especially in flows not opposed to their swim direction. Offshore the Bird's Head Peninsula, the seasonally reversing New Guinea Coastal Current (NGCC) entrains hatchlings born in summer into the oligotrophic North Equatorial Counter Current (NECC). This process starts within the first hours of dispersal. In October- December, when predominant winds shift, causing the reversal of the NGCC, hatchlings deviate from prevailing currents, reaching after 30 days waters similarly unproductive as the NECC's. Winter dispersal is mediated by the southeastward NGCC. River outflow and upwelling are responsible for the productive areas traversed by hatchlings; the turbid waters potentially shielding turtles from predators. We suggest that local oceanography prevailing when turtles disperse mediates survival, with winter hatchlings having the best chances of overcoming the critical dispersal period

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