Bomb detectors have exposed a hidden population of pygmy blue whales in the Indian Ocean. A team of researchers led by UNSW Sydney was analyzing recordings captured by underwater microphones when they heard the blue whales singing.
“We’ve found a whole new group of pygmy blue whales right in the middle of the Indian Ocean,” said study senior author Professor Tracey Rogers. “We don’t know how many whales are in this group, but we suspect it’s a lot by the enormous number of calls we hear.”
The data was obtained by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), which monitors international nuclear bomb testing.
Since 2002, the CTBTO has used advanced hydrophones to detect soundwaves from potential nuclear bomb tests. The recordings are made available to scientists to use for marine science research.
The UNSW team detected an unusually strong signal in the recordings, which they recognized as a mysterious whale song. The song structure, frequency, and tempo revealed that the songs belonged to a group of pygmy blue whales – but not to any of the individuals previously recorded in the area.
“I think it’s pretty cool that the same system that keeps the world safe from nuclear bombs allows us to find new whale populations, which long-term can help us study the health of the marine environment,” said Professor Rogers.
Even though pygmy blue whales are the smallest members of the blue whale family, they are still massive, reaching up to 24 meters in length.
“Blue whales in the Southern Hemisphere are difficult to study because they live offshore and don’t jump around – they’re not show-ponies like the humpback whales,” said Professor Rogers.
“Without these audio recordings, we’d have no idea there was this huge population of blue whales out in the middle of the equatorial Indian Ocean.”
Study lead author Dr. Emmanuelle Leroy is a bioacoustician who studies how animals create and receive sounds. When Dr. Leroy and the team scanned 18 years of CTBTO data, they discovered that the songs were not just a random occurrence.
“Thousands of these songs were being produced every year. They formed a major part of the ocean’s acoustic soundscape,” said Dr. Leroy. “The songs couldn’t have just been coming from a couple of whales – they had to be from an entire population.”
Songs produced by blue whales can travel anywhere between 200 and 500 kilometers. They are performed at a very low frequency and have a different structure compared to other whales’ songs.
“Humpback whales are like jazz singers,” said Professor Rogers. “They change their songs all the time. Blue whales, on the other hand, are more traditional. They sing very structured, simple songs.”
“We still don’t know whether they’re born with their songs or whether they’ve learnt it. But it’s fascinating that within the Indian Ocean you have animals intersecting with one another all the time but whales from different regions still retain their distinctive songs. Their songs are like a fingerprint that allows us to track them as they move over thousands of kilometers.”
The researchers named the new population “Chagos” after the archipelago close to where they have been detected. Professor Rogers is currently using the CTBTO data to study how the Chagos population has changed over time.
“We suspect that the whales singing the Chagos song move at different times across the Indian Ocean. We found them not only in the central Indian Ocean, but as far north as the Sri Lankan coastline and as far east in the Indian Ocean as the Kimberley coast in northern Western Australia,” said Professor Rogers.
“The largest animal in the world is one of the hardest ones to actually study. There are many more of these blue whales out there than we’ve realized – and we’ve only been able to find them with the help of this international infrastructure.”
The study is published in the journal Scientific Reports.