Deep Sea Research Part II: Topical Studies in Oceanography
Seasonal variability in whale encounters in the Western Antarctic Peninsula
Introduction
The core objective of the International Whaling Commission's (IWC) collaborative research in the Antarctic is to investigate how spatial and temporal variability in the physical and biological environment influences cetaceans (IWC 2000). The IWC participated in the Southern Ocean Global Ocean Ecosystems Dynamics (SO GLOBEC) studies in the Western Antarctic Peninsula (WAP) to contribute to this objective. The multidisciplinary framework and scale of the SO GLOBEC program provided a rare opportunity to investigate cetacean patterns of occurrence concomitant with data collected across physical and biological domains.
Programs of cetacean research have been conducted along the Antarctic Peninsula for many years, but most have focused on areas north of the SO GLOBEC study site (Stone and Hamner, 1988; van Franeker, 1992; Pankow and Kock, 2000; Olavierra et al., 2003). Cetacean sightings data have also been collected from the offshore, shelf and fjord regions to the north and west of Marguerite Bay during the austral summer, as part of the United States (US) Long Term Ecological Research (LTER) program (LTER unpublished data) and also on the IWC International Decade of Cetacean Research (IDCR) and Southern Ocean Whale Ecosystem Research (SOWER) circumpolar surveys (Kasamatsu et al., 2000). These surveys included portions of the WAP region (IWC Management Area I 60°W–120°W), in the austral summers of 1982/83, 1989/90, 1993/94, 1999/00, 2000/01 (Branch and Butterworth, 2001). Marguerite Bay has never been surveyed during IWC circumpolar surveys (Branch and Butterworth, 2001; Kasamatsu et al., 2000; Matsuoka et al., 2003). Those IWC surveys rarely approached the shelf in this area, and no cetacean sightings have been reported from the WAP SO GLOBEC study area (Kasamatsu et al., 2000). Reports of cetacean surveys in this region outside the spring and summer seasons or within the pack ice are rare (Joiris, 1991; Aguayo-Lobo, 1994).
Marguerite Bay is located in the central Western Antarctic Peninsula (WAP) (Fig. 1). It is a large embayment open to the continental shelf (∼100 nm wide) to the west, abutting a steep shelf slope and waters over 3000 m. The bay is bordered to the north by Adelaide Island, and to the south by Alexander Island. The eastern edge of the bay is the main Peninsular land mass. A coastal fjord system, formed by near-shore islands close to the steep mountains and glaciers of the continent, extends from the east side of Adelaide Island to Anvers Island in the north. Marguerite Bay is bisected in the south by a deep trough (Marguerite Trough) that extends across the shelf in the west and lies along the northern edge of Alexander Island. This trough extends all the way into the bay and ends in a glacier-filled sound (George VI Sound). The overall bathymetric environment of Marguerite Bay is complex, and comprises many shoals, minor troughs and deep holes and ridges. The southern boundary of the Antarctic Circumpolar Current flows in a northwesterly direction against the continental shelf here, causing warm-water intrusions throughout the year (Smith, D. A. et al., 1999).
The combination of complex bathymetry and oceanographic processes in this region produces very high-velocity currents and small gyres in some parts of the bay (Smith et al., 1999, Beardsley et al., 2004), that may concentrate and entrain zooplankton such as krill in particular areas (Lawson et al., 2004; Ashijan et al., 2004). This area was chosen for multidisciplinary research into the physical and biological factors that contribute to enhanced krill growth, reproduction, recruitment and survivorship (Hofmann et al., 2002) for two reasons: unusually high krill production (Lascara et al., 1999) and predictable winter sea-ice cover (Jacobs and Comiso, 1997).
Krill (Euphausia superba and E. sp.) form a significant component of baleen whale diet in the Antarctic (Kawamura, 1994), and relationships between the distribution of these predators and that of their prey have been shown at broad (∼100 to 1000s of km) spatial scales for the Antarctic (Tynan, 1998; Reid et al., 2000; Thiele et al., 2000; Murase et al., 2002). So far, strong associations between baleen whale and krill distributions have not been shown at smaller (10s of km) spatial scales (Pankow and Kock, 2000; Reid et al., 2000). This may be due to the difficulty of integrating data for predator and prey species that both have patchy distributions (van Franeker et al., 2002). In the Antarctic, the patchiness of whale distributions is further compounded by low densities of many species due to the depletion of populations during commercial whaling.
Krill population dynamics have been linked to variability in sea ice conditions in a number of studies (Fraser and Hofmann, 2003; and summarized in Constable et al., 2003), and winter ice extent is known to affect krill survival and abundance in the following season (Daly, 1990; Siegel and Loeb, 1995; Loeb et al., 1997; Nicol et al., 2000; Constable et al., 2003). High densities of krill, baleen whales and other predators have been observed at ‘ice margins’ or ‘ice edges’ (de la Mare, 1997; Ainley et al., 1998; Ferguson et al., 2000; Brierly et al., 2002; van Franeker et al., 2002), and particularly where ice habitat coincides with certain physical features and biological processes such as complex bathymetry, gyres, eddies, warm-water intrusions, high-velocity currents, high productivity, troughs, ridges, slopes, shelf edges, and other features which, alone or in combination, produce upwelling or concentration of nutrients and prey (Daly and Macaulay, 1991; Ribic et al., 1991; Plotz et al., 1991; Ichii et al., 1998; Murase et al., 2002). In general, habitat complexity is reported to result in increased diversity and abundance for many species (Bartholomew et al., 2000). Marguerite Bay and the surrounding area have complex habitat features, as well as a predictably high krill abundance. We therefore expected that significant numbers of baleen whales would be present there, and that the distribution of krill and krill predators would reflect the spatial complexity of the Marguerite Bay area.
Our central objective here was to investigate seasonal changes in distribution and concentration of minke and humpback whales, and to relate this to variability in feeding habitat associated with bathymetric and sea-ice features. Our analyses were focused on minke and humpback whales because they are often found in association with sea ice, and because of the relatively low numbers of other cetacean species observed. Sea-ice conditions and krill abundance differed greatly between 2001 and 2002 (Ashijan et al., 2004; Lawson et al., 2004), which provided a strong foundation for comparison of cetacean distribution and relative abundance between years.
Section snippets
Materials and methods
The SO GLOBEC research program in the WAP was conducted over two years commencing March 2001 and finishing in March 2003 (Table 1). Three sampling protocols were developed for research cruises: survey grid, process studies and mooring deployments (see Hofmann et al., 2004, for detail of cruise tracks and descriptions of survey types). Four survey cruises were conducted aboard the RVIB Nathaniel B Palmer (NBP), comprised of multidisciplinary sampling of the entire study area along pre-determined
Results
Ten cetacean species (fin whale Balaenoptera physalus, sei whale Balaenoptera borealis, blue whale Balaenoptera musculus sp., minke whale Balaenoptera bonaerensis and B. acutorostrata sub species, killer whale Orcinus orca, humpback whale Megaptera novaeangliae, sperm whale Physeter macrocephalus, hourglass dolphin Lagenorhynchus cruciger, Ziphiidae and various categories of ‘unidentified’ cetaceans) were detected on nine cruises (Table 2). Humpback and minke whales were by far the most
Discussion
The ecology of Marguerite Bay is probably influenced both by habitat complexity and by the effects of regional oceanographic and sea ice processes (e.g., Antarctic Circumpolar Current intrusions onto the MB shelf, timing, nature and extent of sea ice) on the fauna. The marked seasonal and interannual variability in this environment observed over the two years of this study had apparent effects on cetacean distribution in and around the bay.
The results of these surveys showed humpback and minke
Acknowledgements
We acknowledge the enthusiasm and support of Eileen Hofmann and Uli Bathmann in this work. The International Whaling Commission, the US SO GLOBEC Steering Committee and the National Science Foundation provided berths, funding for observers and support. A number of IWC observers contributed to the collection of data (Debra Glasgow, Rebecca Pirzl, Francisco Viddi) and deserve mention, as do the passive acoustics personnel and our colleagues from the many disciplines involved in the WAP study. The
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