Unique Song of Indian Ocean Whales Suggests Previously Unknown Population

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A blue whale (Balaenoptera musculus) sighted off the coast of Sri Lanka (Photo: Andrew Sutton/Shutterstock)

A previously unknown population of blue whales has been discovered in the Indian Ocean thanks to the unique nature of its members' singing, according to a study of whale song recordings published in the online journal Endangered Species Research.

Despite being the largest animals known to have ever lived, blue whales (Balaenoptera musculus) are notoriously elusive creatures. The number of whales in the Indian Ocean is also thought to have been severely impacted by the whaling industry, particularly during an illegal Soviet operation cited by the report, which killed at least 1,294 blue whales between 1963 - 1967. Based on historic location data, the report's authors speculate that the newly discovered group may be the descendants of a population almost wiped out by the Soviet slaughter.

The recordings of blue whale song were taken from a study of humpback whales off the coast of Oman during 2011-2012 and a 2016-2019 study around the coast of Madagascar targeted at a number of different whale species, including southern hemisphere blue whales. A third dataset of whale song recorded between 2010 and 2013 was obtained from the International Data Centre of the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO), which had set up monitoring stations near the island of Diego Garcia, part of the Chagos archipelago in the central Indian Ocean and home to a joint US/UK military base.

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Despite being the largest animal to have ever lived, blue whales are also one of the most difficult to spot (Photo: Chase Dekker/Shutterstock)

Identification of different whale species is complex – based partly on the tones the whales are making; the location and depth at which the song is recorded; and failing all else – positive identification of a whale by sight. The songs that the whales sing, however, are unique to each distinct population. Once a 'dictionary' of sounds has been compiled, tones that vary from the established repertoire of a particular group indicate that a different gathering of animals may be responsible. Salvatore Cerchio, one of the lead authors of the report, compared it to differentiating between human musicians. 'It's like hearing different songs within a genre – Stevie Ray Vaughan versus B.B. King,' said Cerchio, 'It's all blues, but you know the different styles.'

According to the study, at least four populations of blue whales have been recorded throughout the Indian Ocean, but the new song was recorded more frequently off the coast of Oman, and less frequently in the southern reaches of the Indian Ocean. Similarly, the more familiar songs of the existing populations were far less frequently heard in the north. This, coupled with known sightings of blue whales during the times when the vocalisations were heard; a lack of sightings of other whale species and the location data retrieved from the 1963-67 Soviet hunt, led the scientists to provisionally conclude that the songs were those of a previously undocumented blue whale population.

Although much work must be done to prove conclusively that the song belongs to blue whales, as the report's abstract makes very clear – if there is a northern Indian Ocean subspecies of blue whale, 'the potentially restricted range, intensive historic whaling, and the fact that the song-type has been previously undetected, suggests a small population that is in critical need of status assessment and conservation action.'

 


The complete study 'A new blue whale song-type described for the Arabian Sea and Western Indian Ocean' by Salvatore Cerchio et al can be found at www.int-res.com/articles/esr2020/43/n043p495.pdf

 

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