Life for whales in the Arctic is becoming increasingly difficult due to climate change. Rising temperatures and dwindling sea ice are leading to changes in food availability on the one hand, and longer ice-free conditions are allowing human activities to expand on the other. Commercial shipping, fishing, and oil and gas exploration pose major threats to Arctic marine mammals. As activities expand, their risk of becoming victims of ship collisions, acoustic disturbances, and oil and chemical spills increases significantly. Underwater noise, generated by engines, propellers or airguns (sound cannons), is one of the greatest threats to the animals. In a recent study, an international team of researchers has for the first time closely examined the echolocation of free-living belugas in West Greenland in order to learn more about the acoustic field of vision of the animals.
Beluga whales(Delphinapterus leucas) have a circumpolar distribution and spend the whole year in the Arctic. Their position at the top of the Arctic food web makes them an important indicator of changes in the Arctic. In order to determine these changes and possible effects of climate change on the animals, researchers use so-called passive acoustic monitoring (PAM), in which hydrophones (underwater microphones) record the whales’ sounds.
Belugas and other toothed whales emit sounds at different frequencies for communication and echolocation to detect prey and orient themselves in the water and under the ice. The white whales are also called the “canaries of the sea” because they are so communicative and social. Their acoustic repertoire includes pulsed sounds, combined sounds, whistles, and echolocation clicks over a wide range of frequencies from 100 hertz to 20 kilohertz, with some echolocation clicks as high as 120 kilohertz. The quality of the acoustic image of the environment depends on the so-called sonar beam. In earlier studies, researchers found that toothed whales can actively control their sonar beam by adjusting the volume, beam width and beam direction. The narrower the beam or the smaller the angle, the more detailed the image the animals receive, but this results in a smaller scanned volume, meaning they only receive a smaller image of the environment. For toothed whales, the measured beam width ranges from 5 to 14 degrees.
In the current study, the international team with scientists from the USA, Germany and Iceland recorded the sounds of two beluga groups at two locations in Baffin Bay off West Greenland. This is the first detailed study of the echolocation of belugas in the wild, which makes the data obtained particularly valuable. Based on the data set, the researchers were able to get an idea of the acoustic field of view of the animals. The recordings show that, on the one hand, the belugas are able to spatially filter their surroundings and cover larger distances with a highly directional sound beam and high intensity of sounds. On the other hand, they scan their surroundings to increase the acoustic spatial coverage and obtain a larger image.
The researchers suggest that these features are eco-evolutionary adaptations in the acoustically complex Arctic in order to reduce unwanted echoes and, especially in winter when searching for openings in the pack ice, to navigate effectively.
Based on their results, the scientists estimate the acoustic beam width of belugas to be 5.4°, similarly narrow as in narwhals, where an angle of 5° was measured. Sperm whales(Physeter macrocephalus) and bottlenose dolphins(Tursiops spp.), for example, have larger acoustic angles of 8 to 9°, or harbour porpoises(Phocoena phocoena) 11 to 13°. The beam width of the Ganges or Amazon river dolphins is even higher at up to 14.5°, which may be an adaptation to the relatively shallow water depth of their habitat.
Because belugas inhabit many different habitats within the Arctic and subarctic, the authors hypothesize that there may be great variability among populations in terms of their echolocation. In their study, they demonstrated that belugas emit click sounds with high directionality, high intensity and a long range in water several hundred meters deep. For other beluga populations living near shore in shallow water, the characteristics of their echolocation clicks may be more similar to those of river dolphins.
Unlike other toothed whales, belugas (and narwhals) exhibit high sound source levels and narrow acoustic fields of view relative to their rather small body size. According to the researchers, this suggests that the two Arctic species have specific biosonar adaptations to increase their range, reduce surface reflections in their ice-dominated environment, and navigate effectively through openings in pack ice. Because both species rely on finding openings in dense pack ice, there is a high degree of selection pressure here.
In addition, the scientists’ recordings confirmed the high range of frequencies in the clicking sounds, with characteristic peaks at 90 and 150 kilohertz. This wide spectrum is consistent with the characteristic spectral patterns for species of the family Delphinidae. However, the spectra of belugas show a broadband signal with a unique spectral club-shaped pattern that distinguishes them from other delphinids. The authors think that their results provide important information that allows discrimination from other species with similar acoustic profiles.
The scientists also found that belugas scan their environment in a similar way to bats to increase acoustic viewing angle to compensate for reduced spatial coverage due to narrow beam width.
With their work, the authors have provided important data and insights for future programs using passive acoustic monitoring, which can help provide important data on the ecology of cetaceans and their habitats and improve their conservation.
Julia Hager, PolarJournal
More on the subject:
