Blue whales may be the largest animals in the world, but they are also some of the hardest to find.
Not only are they rare (less than 0.15% of southern hemisphere blue whales are estimated to have survived whaling), but they are also reclusive by nature and can cover large areas of the ocean.
But now, a team of scientists led by UNSW Sydney are convinced they’ve discovered a new population of pygmy blue whales, the smallest subspecies of blue whales, in the Indian Ocean.
And it was the powerful song of the whales – recorded by underwater bomb detectors – that betrayed them.
“We found a whole new group of pygmy blue whales right in the middle of the Indian Ocean,” said Tracey Rogers, UNSW professor, marine ecologist and lead author of the study.
“We don’t know how many whales are in this group, but we think it’s a lot by the huge number of calls we hear.”
The discovery was made possible by data from the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), an organization that monitors international nuclear bomb tests.
Since 2002, CTBTO has used advanced underwater microphones (called “hydrophones”) to detect sound waves from possible nuclear bomb tests. The recordings – which pick up many other detailed ocean sounds – are available to scientists for their marine science research.
The UNSW-led team was studying the data when they found an unusually strong signal: a whale song that had already been identified in the recordings, but of which scientists still knew little. After closely studying its composition (details like the structure, frequency and tempo of the song), they realized that it belonged to a group of pygmy blue whales, but not to those previously recorded in the area. .
“I think it’s pretty cool that the same system that protects the world from nuclear bombs allows us to find new populations of whales, which in the long run can help us study the health of the marine environment,” says Professor Rogers.
Pygmy blue whales are the smallest members of the blue whale family, but that’s the only little thing about them – they can grow up to 24 meters in length, almost the length of two standard buses.
If visual observations confirmed this new population, they would become the fifth population of pygmy blue whales to be discovered in the Indian Ocean.
The results, recently published in Scientific reports, arrived in time for World Oceans Day.
“The blue whales of the southern hemisphere are difficult to study because they live offshore and do not jump. They are not show ponies like humpback whales, ”explains Professor Rogers.
“Without these audio recordings, we would have no idea that there was this huge population of blue whales in the middle of the equatorial Indian Ocean.”
A choir of whales
Dr Emmanuelle Leroy, lead author of the study and former postdoctoral researcher at UNSW Science, is a bioacoustician, a person who studies how animals create and receive sound. She was studying the CTBTO data when she noticed the emergence of a particular pattern.
“At first, I noticed a lot of horizontal lines on the spectrogram,” says Dr. Leroy. “These lines at particular frequencies reflect a strong signal, so there was a lot of energy there.”
To find out if the signal was a random blip or something more, Dr Leroy and the team scanned 18 years of CTBTO data – all of the data available since recording began – to look for larger patterns. .
They discovered that the songs weren’t just a random event.
“Thousands of these songs were produced each year,” she says. “They were an important part of the acoustic ocean soundscape.
“The songs couldn’t be from a few whales, they had to be from an entire population.”
Sing a simple tune
Like many other whales, blue whales are powerful songsters: scientists estimate that their songs can travel between 200 and 500 kilometers. These songs are very low frequency (barely audible to the human ear) and have a different structure from the songs of other whales.
“Humpback whales are like jazz singers,” says Professor Rogers. “They change songs all the time.
“Blue whales, on the other hand, are more traditional. They sing very structured and simple songs.”
The style of music can even change within a species of whale: each of the known populations of pygmy blue whales in the Indian Ocean sings slightly different melodies. Professor Rogers says these musical differences are similar to generational slang between humans.
“We still don’t know if they were born with their songs or if they learned it,” she says.
“But it is fascinating that in the Indian Ocean animals cross paths all the time, but whales from different regions still retain their distinctive songs. Their songs are like a fingerprint that allows us to follow them when they are travel thousands of kilometers. “
Dr Leroy compared the acoustic characteristics of the song with the other three types of blue whale songs known in the Indian Ocean, as well as four types of whale songs from Omura (another whale in the area) – but the evidence indicated that this was a whole new population of blue whales.
The team named the newly discovered population “Chagos” after the archipelago they detected nearby.
“We suspect that the whales singing the song of the Chagos move at different times across the Indian Ocean,” says Professor Rogers.
“We have found them not only in the central Indian Ocean, but as far north as the Sri Lankan coast and as far east in the Indian Ocean as the Kimberley coast in northern India. Western Australia.”
While the team is confident in their findings, Dr Leroy says it’s impossible to confirm the species without visual observation. Visual sightings for such an elusive animal can be difficult and expensive to fund, so it’s unlikely to be verified anytime soon.
“If it’s not a blue whale, it definitely sings like one,” says Dr Leroy.
A great find for conservation
The discovery is big news for marine conservation, as blue whales were brought to the brink of extinction after whaling in the 20th century.
And unlike many other types of whales in the southern hemisphere, their numbers have not increased.
“Discovering a new population is the first step in protecting it”, explains Dr Leroy.
Acoustic information hidden in whale songs can also tell us more about animals, such as their spatial distribution, migration patterns and numbers. A previous study by Dr Leroy even found that the changing tone of blue whale songs could be a response to the sound of cracking icebergs.
Professor Rogers is now leading a team using CTBTO data to study the evolution of the Chagos population over time. The results could tell us how whales have adapted to warming ocean temperatures over the past 18 years – and how they might fare in the future.
“The largest animal in the world is one of the most difficult to study,” says Professor Rogers.
“There are many more of these blue whales than we thought – and we were only able to find them with the help of this international infrastructure.”