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The Science and Research Fund

Interspecific interactions between social top predators of the marine environment: the role of acoustic cues

Anna Selbmann, PhD student at the University of Iceland

Description of project

Anna Selbmann, PhD student in Biology at the Universty of Iceland (Háskóli Íslands), recieved a grant from the Science and Research Fund of South Iceland in 2022 for her research concerning interspecific interactions between social top predators of the marine environment: the role of acoustic cues.

Here, she briefly introduces her research:

The aim of this project is to investigate the role of acoustic communication in mediating interspecific interactions between marine mammals by focusing on interactions between long-finned pilot whales (Globicephala melas, grindhvalir) and killer whales (Orcinus orca, háhyrningar) in south Icelandic waters.

In different locations in the North Atlantic, including in the south of Iceland, killer whales have been observed avoiding pilot whales, sometimes fleeing at high speed. The killer whale is an apex predator and so these interactions are quite puzzling and much remains unknown about the behaviour during interactions, why they occur, and potential impacts on either species.

Anna Selbmann

The first objective of the project was to describe the occurrence of both species in Icelandic coastal waters to investigate possible overlap in time and space, which would provide potential for interactions. We found that killer whales frequently occur around Vestmannaeyjar in the south of Iceland and in Breiðafjörður in the west. These are known herring grounds and herring is a major prey for killer whales in Iceland. Pilot whale occurrence increased since 2014, in particular in the south of Iceland, leading to frequent interactions between the two species. These interactions appear to be antagonistic but seem to be more complex than previously described and appear to vary in intensity, with killer whales sometimes avoiding pilot whales but other times a high-speed chase between the two species ensues.

How sound can affect behavior of pilot and killer whales

These results led us to question what triggers the interactions. We hypothesise that sound should play an important role, as sound is very important for many marine mammals. Pilot whales and killer whales rely heavily on sounds to communicate, navigate and find food. We are testing the role of acoustic cues in the interactions by playing the sounds of pilot whales to killer whales and recording their response. The experiments were completed in 2023 and analysis of the results is currently underway.

We are currently looking at whether killer whales change their movement trajectory when they hear pilot whale sounds and whether they change their own acoustic behaviour. This data will give us an indication of how a top predator, like the killer whale, responds to a potential threat, which can be used as a yardstick to assess the impact of human activities, such as ocean exploration, marine construction or naval exercises.

The biological importance of the oceans in South Iceland

Knowledge on the occurrence of cetaceans in the south of Iceland, including the Vestmannaejyar archipelago, is limited. Killer whales are known to seasonally visit the area during summer when herring is spawning and other known prey species of cetaceans, such as capelin and krill are abundant. South Iceland is therefore likely of biological importance to cetaceans, including the Icelandic killer whale population. This was confirmed in the first part of the project, as Vestmannaeyjar had high sighting rates for killer whales and pilot whales and the occurrence of pilot whales was shown to have increased in the south of Iceland. Correspondingly, there were increases in the number of antagonistic interactions between pilot whales and killer whales. These interspecific interactions can carry consequences for both species and changes in prey availability, climatic conditions and human disturbance can exacerbate such effects.

There is a need for increased understanding of the importance of south Icelandic waters to cetaceans to support the management and protection of marine and coastal ecosystems. Conservation management plans, such as the proposal for a Marine Protected Area in the Vestmannaeyjar archipelago, are mostly based on seabirds due to a lack of knowledge on marine mammals. The conservation of aquatic environments is of upmost importance, as reflected in national legislation and international conventions. The community in South Iceland is strongly linked to the ocean due to its long history in fishing and the importance of the fishing industry to the local economy. Nature-based tourism plays an increasing role in the local economy. Therefore, knowledge on the occurrence, behaviour and ecology of cetaceans in the area is important to the local community and economy, as well as for national and international policy and legislation. 

New questions arise

The main results from the project so far are on the occurrence of killer whales and pilot whales in Icelandic coastal waters and their interspecific interactions, emphasising the importance of the South.

The results were published in the journal Acta Ethologica.

From this work arose the question what triggers the interactions, leading to playback experiments of pilot whale sounds to killer whales. Analysis of these results is currently underway but preliminary results already raised new questions. For example, pilot whale vocalisations seem to vary when they approach killer whales compared to other contexts. Therefore, it would be interesting to further test, whether killer whales only respond to specific sounds produced during interactions or to any type of pilot whale vocalisation. This would have implications for the complexity of the interaction and the degree of disturbance experienced by killer whales due to pilot whale presence.

Further publications from the project included work on the baseline acoustic behaviour of killer whales in the absence of interactions with pilot whales. We described the call repertoire and how it varies across regions in Iceland in Marine Mammal Science, as well as how killer whales combine sounds into sequences in Nature (Scientific Reports).