MARINE MAMMAL SCIENCE, **(*): ***–*** (*** 2007) C 2007 by the Society for Marine Mammalogy DOI: 10.1111/j.1748-7692.2007.00109.x CETACEAN OCCURRENCE PATTERNS IN THE AMUNDSEN AND SOUTHERN BELLINGSHAUSEN SEA SECTOR, SOUTHERN OCEAN DAVID G. AINLEY H. T. Harvey and Associates, 3150 Almaden Expressway, Suite 145, San Jose, California 95118, U.S.A. E-mail: [email protected] KATIE M. DUGGER Department of Fisheries and Wildlife, Oregon State University, 104 Nash Hall, Corvallis, Oregon 97331-3803, U.S.A. VIOLA TONIOLO PRBO Conservation Science, 4990 Shoreline Highway, Stinson Beach, California 94970, U.S.A. IAN GAFFNEY 1028 Greenwich Street, San Francisco, California 94901, U.S.A. ABSTRACT We conducted 239.5 h and 3,494 km of cetacean surveys in the Amundsen and Bellingshausen seas, from 15 February to 31 March 1994; most of the area, the large portion of which was ice covered, had never before nor has it since been surveyed for cetaceans, even to the date when this paper was prepared (2006). Logistic regression and an information-theoretic approach related the occurrence of Antarctic minke whales Balaenoptera bonaerensis (the most abundant species) to whether we were in open- or pack-ice-covered pelagic or neritic waters, in or out of the marginal ice zone (MIZ), and north or south of the Antarctic Circumpolar Current southern boundary. Other variables included date and distance to the MIZ and shelfbreak front. Statistical analysis showed that the probability of sighting a minke, as well as killer whale—but not the case for an index to whale density—was related to the proximity of coastal polynyas in early autumn, switching offshore to the MIZ once waters within the pack began to freeze persistently later in the season. Probability of detection was higher with distance into the MIZ. Supporting these findings, the density index was strongly related to ice concentration in an inverse relationship. The strong relationship to polynyas and the MIZ indicate that sea-ice divergence altered by decadal or longer-term climate change, as described in the recent literature, could well affect any apparent, long-term trends evident in this species’ abundance 1 2 MARINE MAMMAL SCIENCE, VOL. **, NO. **, 2007 if surveyed only in open or near-to-ice waters. We speculate on how the minke whale’s pagophilic nature (1) could have been encouraged by large-scale industrial whaling and by competition with species more characteristic of open waters and the outer MIZ, and (2) may have protected the population somewhat during industrial whaling resulting in the much greater abundance of this species now compared to other targeted species. Key words: Amundsen Sea, Bellingshausen Sea, cetacean abundance, cetacean ecol- ogy, marginal ice zone, killer whale, Orcinus orca, minke whale, Balaenoptera bonaeren- sis, polynya, sea ice, Southern Ocean. Most Antarctic cetacean surveys, except for the few that used icebreakers (e.g., Ribic et al. 1991, Aguayo-Lobo 1994, Thiele and Gill 1999), have been conducted in open waters or the outer marginal ice zone (e.g., Kasamatsu et al. 1996, 1998; Thiele et al. 2000, 2004; Murase et al. 2002; Friedlaender et al., 2006). As a result, the current perception in the research community is that species such as the Antarctic minke whale (Balaenoptera bonaerensis) or certain types of killer whale (Orcinus orca) are creatures mainly of near-to-ice and marginal-ice-zone (MIZ) waters (see also Brown and Lockyer 1984, Costa and Crocker 1996). This perception may be biased by survey effort that favors open waters. After all, the Antarctic minke whale is perfectly suited to exploit pack-ice habitat, having a small, slim, compact body and small appendages that allow it to fit into narrow leads between ice floes without catching on ice and a hard, sharp rostrum for breaking through newly formed sea ice in order to breathe (Thiele et al. 2000, Ainley et al. 2003). The form of the killer whale found at highest latitude, “type C,” is also small bodied to exploit narrow passages in the ice (Pitman and Ensor 2003). The aim of the major Southern Ocean cetacean surveys, that is, the IDCR/SOWER circumpolar effort, has been to track trends in the size of the minke whale population, a task that can be fulfilled only by the sort of repetitive sampling thus far employed (e.g., Branch and Butterworth 2001). Unfortunately, these surveys are conducted from vessels that are not ice capable and the Antarctic marine climate has not remained constant. Over the period of these cruises (since the 1970s), portions of the Southern Ocean have seen significant fluctuations in sea-ice concentration and extent, longer or shorter sea-ice seasons, and changes in the prevalence of coastal polynyas (or post- polynyas [Arrigo and van Dijken 2003]: open waters remaining from winter-time polynyas; e.g., Jacobs and Comiso 1989, 1993, 1997; Stammerjohn and Smith 1997; Jacobs and Giulivi 1998; Parkinson 2002; Zwally et al. 2002). Realizing this chang- ing environment, recent authors in the proceedings of the International Whaling Commission (IWC) have questioned both the degree to which minke whales occur in pack-ice areas and, if they are prevalent therein, the potential effects of such occur- rence on trend analyses (e.g., Shimada et al. 2001, Shimada and Murase 2003, Murase and Shimada 2004). Attempts have been made to grapple with the problem and to better understand the importance of a changing sea-ice regime on minke whale dis- tribution and assessment (Branch and Butterworth 2001, Tynan 2002, IWC, 2005, Thiele et al. 2005), as this is not a trivial matter, especially given the size of long-term, open-water data sets and the huge expense in time and money required to develop valid correction factors to account for whales in an ever-changing pack-ice habitat. Moreover, the standard line-transect surveys typically used in cetacean stock assess- ments, where constant effort must be maintained, are problematic in the pack ice. AINELY ET AL.: CETACEAN OCCURRENCE PATTERNS IN THE SOUTHERN OCEAN 3 There, even icebreakers must constantly and dramatically change course and speed in order to maintain headway; otherwise, line transect data in the pack ice can be collected only by using very expensive aircraft that can fly straight lines but with reduced sighting probability due to their rapid speed relative to the whales’ diving behavior. As a contribution to better understand the cetacean community within the South- ern Ocean pack ice, herein we report results of an icebreaker-based survey conducted in the pack ice and adjacent waters of the Amundsen and southern Bellingshausen seas during autumn 1994. Owing to the heavy, persistent pack ice in this area, much of what we surveyed (and report) had never before and has not since been visited by cetacean observers, even as of the date when the analysis presented here was completed (2006). Our purpose, therefore, is to describe the factors that affected the occurrence patterns of cetaceans and especially of minke whales, which were by far the most abundant whale species encountered. We also make comparisons to the results of other surveys both in the deep and peripheral ice pack. As was the case for seabirds (reported previously: Ainley et al. 1998), we hypothesized that cetaceans were more likely to be seen in polynyas and that this relationship might change over time as cetaceans moved north into the MIZ, away from coastal polynyas as the season pro- gressed. Such a hypothesis was consistent with the main purpose of our cruise: to determine why the pack ice of the study area had become more divergent in recent years. METHODS Surveys Observations were made on the research icebreaker Nathaniel B. Palmer, cruise 94- 02. On this cruise we completed a series of meridional sections extending between the coast and points well beyond the shelf break beginning at King Edward VII Peninsula, Marie Byrd Land, ca. 150◦W, on 15 February 1994, passing east along the coast of Ellsworth Land, and ending at Marguerite Bay, Antarctic Peninsula, ca. 70◦W, on 31 March (Fig. 1). Thus, the first section, a short one owing to the narrow shelf there, was actually at the eastern boundary of the Ross Sea. The majority of effort was in the Amundsen and Bellingshausen seas and waters to the north. Eleven sections crossed the shelf break and large-scale pack-ice edge with eight of these also crossing the southern boundary of the Antarctic Circumpolar Current (ACC). Five sections passed through coastal polynyas/postpolynyas. Conductivity-temperature- depth (CTD) casts were made at intervals along these sections in order to investigate ocean structure and specifically to detect the intrusion of warm Circumpolar Deep Water onto the continental shelf. Thus, most sections passed over depressions that projected across the shelf break toward the inner shelf. CTD data for our purposes were used to identify the Antarctic Slope Front and the southern boundary of the ACC. Because much of the cruise occurred in uncharted waters, depth was monitored continuously along most of the track using a precision depth recorder. Cetaceans, seals, and seabirds (see Ainley et al. 1998) were counted using the methodology described in Ribic et al. (1991), as follows. Two observers worked together at 12 m above the sea surface. They simultaneously scanned, using 8× binoculars and the naked eye, a 90◦ area 800 m forward and to the side from the bridge wing whenever the ship was underway during daylight. We observed on the side 4 MARINE MAMMAL SCIENCE, VOL. **, NO. **, 2007 Figure 1. Cruise track in the Amundsen and Bellingshausen seas, 15 February to 31 March 1994, showing only segments on which cetacean surveys were made. Light levels were too low for adequate viewing on other segments.