Every year humanity discovers new species or populations of existing species we had no idea about previously. You might think, however, that these are all creatures small enough to hide from our eyes, not the largest animal that has ever lived. Nevertheless, a previously unknown population of blue whales has been discovered living in the western Indian Ocean, one that may even represent a distinct subspecies of these ocean giants.
The story began in 2017 when scientists from the African Aquatic Conservation Fund (AACF) detected a whale song they had never heard before in the waters between Mozambique and Madagascar. Whale songs can be detected up to 150 kilometers (90 miles) away in good conditions, so with the right equipment are much easier than visual detections of their makers. Blue whale populations have recognizably different dialects to their songs and researchers use these to identify a whale's tribe, but this was different from anything previously recorded.
The AACF Dr. Salvatore Cerchio was also involved in studying humpback whales off the coast of Oman. Reviewing recordings made some years earlier he recognized a very similar song. Based on similarity to other song-types and sightings soon after two of the Oman recordings, Cerchio concluded the song came from blue whales but of a previously unknown population. "It was quite remarkable," Cerchio said in a statement. "To find a whale song in your data that was completely unique, never before reported, and recognize it as a blue whale."
This set off a quest to trace the whale song's range. Researchers from the University of New South Wales heard Cerchio's report to the Scientific Committee of the International Whaling Commission and realized the recordings they had made east of the Chagos Archipelago in the central Indian Ocean were of the same song. In Endangered Species Research, scientists from both teams report their findings and the implications. The paper presents evidence it was members of this population that were hunted by Soviet whaling ships in the 1960s, rather than the whale population with a song referred to as Sri Lankan. It is believed 1,294 blue whales were taken in this slaughter, reducing an already small population to near the brink of extinction.
We don't yet know how many whales have this song, and whether their numbers are recovering from past culling. However, the new paper notes “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 authors plead for more information about their numbers and range.
Blue whales are no longer hunted for oil or meat, but they face threats from global heating and ship collisions. Living in unusually warm waters crossed by major shipping routes, the new population may be particularly vulnerable.
Whales play an outsized role in maintaining ocean ecosystems, redistributing nutrients to the areas that need them most. Blue whales may be preeminent at this.
The population is likely to be either part of the brevicauda subspecies (also known as pygmy blue whales because they are smaller than their Antarctic counterparts) or more closely related to these than the larger subspecies. Either way, they are almost, but not quite, the largest animals on Earth.