Abundance and population status of Ross Sea killer whales (Orcinus orca, type C) in McMurdo Sound, Antarctica: evidence for impact by commercial fishing?
For over a century, the Ross Sea killer whale (RSKW; Orcinus orca, Antarctic type C), a fish-eating ecotype, has been commonly reported in McMurdo Sound (McM), Ross Sea, Antarctica. However, a significant population decline reported at Ross Island after 2006 has been linked to a commercial fishery that began in the Ross Sea in 1996–1997 and targets large Antarctic toothfish (Dissostichus mawsoni)—the presumed primary prey of RSKW. We assessed RSKW population abundance and trends using photo-identification data collected in McM during seven summers from 2001–2002 to 2014–2015. We identified 352 individual RSKWs and estimated an average annual population of 470 distinctly marked whales. Using a Bayesian mark–recapture model, we identified two population clusters: ‘regulars’ showed strong inter- and intra-annual site fidelity and an average annual abundance of 73 distinctive individuals (95% probability: 57–88); ‘irregulars’ were less frequently encountered but comprised a larger population with an annual estimate of 397 distinctive individuals (287–609). The number of seasonally resident regulars appeared to be stable over the period of purported RSKW decline, with the estimated annual number of deaths (6; 95% probability: 1–22) offset by the number of recruits (6; 2–19). As an alternative to the decline-due-to-fishery hypothesis, we suggest that the presence of mega-iceberg B-15 at Ross Island during the “iceberg years” (2000–2001 to 2005–2006) could have temporarily disrupted normal RSKW movement patterns, resulting in an apparent decline. Continued population monitoring of toothfish and their predators will be important for assessing ecosystem impacts of commercial fishing in the Ross Sea.
KeywordsAntarctic toothfish Fishery impact Iceberg B-15 McMurdo Sound Orcinus orca Population assessment Type C killer whale
This work was supported by the National Science Foundation under Grants to R. Pitman (OPP-0338428) and Stacy Kim (ANT-0944747), and grants from the US Marine Mammal Commission, the International Whaling Commission—Southern Ocean Research Partnership, and a private grant from E. R. Johnson. We would like to thank The Natural History Museum, London, for permission to reproduce Fig. 2 and would also like to acknowledge the staff at McMurdo Station for their enthusiastic support, especially the helicopter pilots and the ice safety crew. For field assistance, we thank R. Andrews, D. Baetscher, L. Ballance, T. Cheeseman, M. P. Heide-Jørgensen, S. Kim, D. LeRoi, D. Mahon, and W. Perryman. Comments by D. Ainley, L. Ballance, and G. Watters helped to refine our arguments. Figure 1 was prepared by B. Saenz with bathymetric data provided by J. Black. Research was conducted under Antarctic Conservation Area Permit 2009-013, and Marine Mammal Protection Act Permits 774-1714 and 14097 issued to NOAA, Southwest Fisheries Science Center. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
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Conflict of interest
The authors declare that they have no conflicts of interest.
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed.
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