Abundance and distribution of seabirds and crabeater seals during SO GLOBEC I and II

 Christine Ribic and Erik Chapman

We studied seabird and Crabeater Seal (Lobodon carcinophagus) abundance and distribution in and around Marguerite Bay on the Antarctic Peninsula during autumn (May-June) and a winter (July-August) cruises in 2001. We used the continuous survey method; the standard method for counting seabirds at sea in the Southern Ocean (BIOMASS Working Party on Bird Ecology, 1984). Strip transects were conducted simultaneously at 300 m and 600 m widths. Surveys were conducted continuously while the ship was underway within the study area and when visibility was >300 m. Presence of Crabeater Seals in the transect was also recorded using standard protocols (Marine Mammal Lab, pers.comm.). The autumn cruise took place prior to establishment of the winter pack ice and virtually no ice was encountered during this cruise. During the winter cruise, the pack was established and we encountered heavy ice coverage in the study area. During the autumn cruise, we surveyed 88 h over a distance of 938 km. During the winter cruise, we surveyed 99 h over a distance of 829 km. We observed 1,771 birds and 13 species during the autumn cruise. Snow Petrel (Pagrodoma nivea) was the most abundant species observed, followed by Southern Fulmar (Fulmarus glacialoides), Antarctic Petrel (Thalassoica antarctica), Blue Petrel (Halobaena caerulea), Cape Petrel (Daption capense) and Kelp Gull (Larus dominicanus). Bird distribution appeared to be even throughout the study area, with the exception of Snow Petrels that were concentrated along the north and west shore of Alexander Island. Crabeater Seals were also concentrated along this same area near Alexander Island. During the second cruise, we observed only 708 birds and 6 species. Snow Petrel was again the most abundant bird species, followed by Adélie Penguin (Pygoscelis adeliae), Antarctic Petrel, Southern Giant Petrel (Macronectes giganteus) and Emperor Penguin (Aptenodytes forsteri). Bird distributions were even throughout the study area, but, at a small scale, birds were found in association with leads in the pack ice. Crabeater Seals, however, had extended their range further away from shore and were concentrated just north and west shore of Alexander Island and south of Adelaide Island. The shift in seabird species assemblage between cruises reflected changes in ice conditions in the study area between autumn and winter cruises and largely supported habitat relationships suggested by previous studies in the Southern Ocean. However, Snow Petrels, which are believed to have a strong association with sea ice, were observed in both open water and habitats containing ice during the autumn cruise. Overall, pack ice appeared to push out open water, surface feeding species while extending the range of penguins, Snow Petrels and Crabeater Seals further out over the continental shelf. However, the density of Adélie Penguins, an important predator of Antarctic Krill, was low in Marguerite Bay during the winter cruise. We look forward to working with other SO GLOBEC investigators to interface our data with physical and other biological data to investigate hypotheses concerning the winter ecology of the Antarctic Peninsula marine system.