Call of the Ross Sea Killer Whale.

The Killer whale is a marine top predator with a cosmopolitan distribution found in all corners of the ocean, from the Antarctic to the Arctic. The Antarctic residents have been classified under 5 genetically and morphologically distinct eco-types, known as Type A, B1 (larger), B2 (smaller), C and D.

The Type C killer whale is the smallest of all known Killer whale eco-types, reaching lengths of up to 6.1m. It is easily distinguished by its unique narrow and slanted eyepatch, with a diet composed of fish, such as the Antarctic toothfish. In summer, they take advantage of the ice-break to forage. They are found in large numbers, at an annual average of 470 individuals in the McMurdo Sound, Ross Sea in Eastern Antarctica. Yet, despite advances in photo-identification and telemetric studies shedding light on its abundance, distribution, ecology and behaviour, knowledge gaps remain on their year-round occurrence. The mobile nature of the Killer whale complicates the ability for direct monitoring. Passive acoustic monitoring offers the opportunity for year-round occurrence and relative abundance data.

Killer whales have distinct dialects as well as shared calls, as a result of learned behaviour. Groups that are more closely related have increased shared vocal commonalities than those geographically and relatively distant. A shared commonality amongst all is that their acoustic repertoire is broadly categorised into clicks , whistles, and burst-pulse sounds .

Echolocation clicks are used for foraging and navigation, whilst whistles and burst-pulse sounds are thought to be forms of communication. Additionally, sub-categories exist known as graded types that lie on a spectrum from whistles and burst-pulses. Through analysis of burst-pulses, clicks, whistles and graded types, the acoustic repertoire of the Killer whale can be achieved.

During the study period, a total of 9 encounters of the Type C killer whale of 392 individuals ranging in group size from 8 to 125 individuals was recorded. Their behaviour was seen to be a combination of socializing, foraging and/or travelling.

The acoustic data was captured on site with a hydrophone handheld below water between 3-4 metres depth paired with a recording unit (M-Audia Microtrack 24/96) and an in-line amplifier. Acoustic analysis of over 3 hours of recording was conducted visually and aurally using an acoustic software.

The complexity and acoustic repertoire of the Type C killer whale could reflect its feeding ecology and/or behaviour at the time of recording. Killer whale eco-types that differ in terms of foraging behaviour and diet have different acoustic repertoires and acoustic activity. In particular, mammal-eating killer whale eco-types maintain long periods of silence and have fewer complex calls, only found to regain acoustic activity after predation and during social interactions. In contrast, fish-eating killer whale eco-types display continuous complex acoustic repertoire throughout activities. This could be as a result of prey having more (e.g. whales, dolphins) or less (e.g. fish) acoustic ability, in turn, shaping the Killer whale’s vocal behaviour. While its behavioural state may influence call type complexity, higher rates of calls have been recorded during foraging and social interactions compared to traveling. Special consideration in call analysis must be made to their feeding ecology and behavioural context.

Antarctica is one of the last remaining pristine environments, but is rapidly changing due to increased environmental and human-induced threats. Its remoteness hinders monitoring of the ecosystem. Top predators, such as Killer whales are important ecosystem indicators, not only of their own health, but of the environment that supports them. This can be crucial to inform decision making, in particular for the Ross Sea Region Marine Protected Area (RSRMPA), the largest marine high-seas protected area.

This study through PAM and visual surveys was able to quantify and establish the Type C killer whale’s unique and complex acoustic repertoire, providing a step forward in passive acoustic monitoring and understanding its behaviour and ecology. Identification of different Killer whale dialects could help gain knowledge on cultural evolution, phylogenetic relationships, communication methods, as well as aid not only in their conservation, but aid the supporting ecosystem of the Southern Ocean.

Reference:

Wellard, R., Pitman, R.L., Durban, J. and Erbe, C. 2020. Cold Call: the acoustic repertoire of Ross Sea killer whales (Orcinus orca, Type C) in McMurdo Sound, Antarctica. R.Soc. open sci , 7 :191228.