Occurrence of seabirds and marine mammals in the pelagic zone of the Patagonian Sea and north of the South Orkney Islands

The Patagonian Sea is one of the most productive ecosystems in the Southern Hemisphere. Unlike other coastal regions, however, few studies exist on the top predators in its pelagic zone. In March 2017, a survey of seabirds and marine mammals was carried out on board the R/V Puerto Deseado in the Patagonian Sea, which extends from the South Atlantic Ocean to the north of the South Orkney Islands, Antarctica. Four of the five oceanographic regimes described in this region were studied, and 23 seabird species and five marine mammal species were recorded. Great shearwater Puffinus gravis, Antarctic prion Pachyptila desolata, and fin whale Balaenoptera physalus were the most abundant species. In the 2615 km traveled, two hotspots for top predators were found, coinciding with frontal zones: one in the shelf-break front and the other in the Southern Front of the Antarctic Circumpolar Current. The highest bird diversity and the greatest cetacean concentrations were recorded in the polar regime in the presence of low ice-field debris (5%). The results suggest that at the end of the austral summer, the distribution of top predators in this section of the South Atlantic Ocean is highly unequal. Some oceanic areas have a few species aggregations which contrast with the vast pelagic areas that have scarce species presence and activity. The hotspots were associated with high-productivity areas, but it is likely that they were also facilitated by the time of year (post-reproductive season), as most of the species were concentrated and had fed prior to their migrations