Abstract
Killer whales (Orcinus orca) are top marine predators occurring globally. In Antarctic waters, five ecotypes have been described, with Type C being the smallest form of killer whale known. Acoustic recordings of nine encounters of Type C killer whales were collected in 2012 and 2013 in McMurdo Sound, Ross Sea. In a combined 3.5 h of recordings, 6386 killer whale vocalizations were detected and graded based on their signal-to-noise ratio. Spectrograms of the highest-quality calls were examined for characteristic patterns yielding a catalogue of 28 call types (comprising 1250 calls). Acoustic parameters of each call were measured and summarized by call type. Type C killer whales produced complex calls, consisting of multiple frequency-modulated, amplitude-modulated and pulsed components. Often, two components occurred simultaneously, forming a biphonation; although the biphonic components did not necessarily start and end together, with one component lasting over several others. The addition and deletion of components yielded call subtypes. Call complexity appears stable over time and may be related to feeding ecology. Characterization of the Type C acoustic repertoire is an important step for the development of passive acoustic monitoring of the diverse assemblage of killer whale ecotypes in Antarctica's rapidly changing marine ecosystems.
Highlights
Killer whales (Orcinus orca) are top marine predators occurring globally
Acoustic recordings were collected during nine separate encounters with Type C killer whales that were confirmed to ecotype by diagnostic features
This study yielded the first comprehensive description of the call repertoire of Type C killer whales, based on a unique dataset comprising concurrent visual and acoustic recordings that confirmed the ecotype over nine encounters with a summed total of 392 individuals
Summary
Killer whales (Orcinus orca) are top marine predators occurring globally. In Antarctic waters, five ecotypes have been described, with Type C being the smallest form of killer whale known. Type C killer whales ( recognized as Ross Sea killer whales; [11]) are known mainly from summer sightings off eastern Antarctica [2], where they occur in continental shelf waters along the fast ice edge. They occur deep into ice leads where they hunt for fish, such as the Antarctic toothfish (Dissostichus mawsoni) and additional fish species as documented by stable isotope analysis [12,13] and field observations [14,15,16]. There are very few records of Type C killer whales in other seasons [25], resulting in knowledge gaps and uncertainty relating to year-round usage of the Ross Sea, in general, and the Ross Sea Region Marine Protected Area (RSRMPA)
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