For eighteen years, underwater microphones recorded a whale that researchers have great difficulty seeing.
Omura's whale remains one of the least-known large whales. It is elusive, lives in warm seas and does not necessarily follow the long migrations of some other whales. In northwestern Australia, its appearances at the surface are so rare that it is difficult to know precisely where it lives.
An Omura's whale (Balaenoptera omurai) photographed off northwestern Madagascar. This species is difficult to track at sea, which makes its underwater songs especially valuable.
Photo: Salvatore Cerchio and colleagues, Royal Society Open Science (2015), CC BY 4.0.
Fortunately, this whale sings. Its low-frequency sounds travel far through the water and can be recorded by devices placed on the seafloor. Scientists from Curtin University therefore revisited old sound archives collected between 2005 and 2023, sometimes for other research.
These recordings came from 35 sites off Australia. The team searched for the species' characteristic calls and compared them with ocean conditions: water temperature, winds, depth and the likely presence of food.
The songs were detected throughout the year. Their frequency nevertheless varied according to the seasons and locations. They were more common in warm waters and in areas where currents bring nutrients up from the depths.
These upwellings promote the growth of plankton, which feeds small marine animals. This is where the whales might find their prey more easily. The researchers also observed more calls during the southern summer, with detections shifting northward during winter.
Unlike whales that travel thousands of kilometres between their feeding and breeding grounds, Omura's whale appears able to remain in tropical seas year-round. It may feed regularly in regions where food remains relatively predictable.
One important limitation remains: a song cannot be used to count the whales present. A silent whale may pass nearby without being detected. The microphones therefore mainly provide information about where and when the species vocalizes, not about its entire population.
This new information could help protect a species about which data are still scarce.