Changes in distribution, relative abundance, and species composition of large whales around South Georgia from opportunistic sightings: 1992 to 2011

Abstract

To examine the general population trends of large whales in South Georgia waters, 2 opportunistic data sets of sightings of large whales from 1991 to 2010 around South Georgia were analyzed: the South Georgia Museum log of whale sightings and the British Antarctic Survey whale sighting reports from the Bird Island research station. Bird Island abuts the northwest tip of South Georgia. The 4 most reported species in both data sets were southern right whale Eubalaena australis, humpback whale Megaptera novaeangliae, minke whale Balaenoptera bonaerensis, and killer whale Orcinus orca. These totally independent data sets showed comparable changes in abundance through time; thus, despite a lack of sighting effort records, inferences could be drawn about changes in relative abundance. The number of reported sightings per 5 yr period from both data sets increased from the 1991 to 1995 period through the 2001 to 2005 period and has since decreased. Species composition of reported sightings has changed over time; southern right whales have become the most sighted species in both data sets, with a peak of reported sightings in the 2001 to 2005 period. Sightings were concentrated around Shag Rocks, at the northwest tip of South Georgia, and along the north/east coastlines of South Georgia; sightings in the bays around South Georgia have increased over time. In an area such as the Antarctic, which poses many difficulties when conducting research, opportunistic data sources such as these, although not ideal, can become invaluable, since such information would otherwise be unattainable.