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.