Underwater nuclear bomb detectors have picked up an increasing flurry of song from pygmy blue whales in the Indian Ocean, indicating numbers of the elusive species have rebounded after it was hunted to near-extinction.

But marine researchers have warned that warming ocean temperatures could deplete the whales’ food supply of krill.

This population of Chagos pygmy blue whales, named after the Chagos Islands, was discovered last year in the central Indian Ocean. Their presence was revealed through their song by a network of underwater listening devices originally set up to monitor for unapproved nuclear testing.

All blue whales sing at a very low frequency that is below that of human hearing. The recording of the pygmy blue whale song attached to this story has been sped up to make it audible.

Scientists from the University of New South Wales have analysed almost two decades of recordings to conclude the distinctive songs of these pygmy blue whales, a subspecies of blue whales, have increased in number.

Sightings of the pygmy blue whales are extremely rare, despite the whales reaching 24 metres long, but researchers are cautiously hopeful the increase in whale song means numbers have rebounded after the whales were hunted to near-extinction in the 20th century.

Professor Tracey Rogers, a co-author of the research and an expert on how mammals overcome the challenges of changing environments, said: “When we do the right thing – which is stop killing them and get out of the way – Mother Nature can fix the situation, even though they are still at catastrophically low numbers.

“The next concern for these guys is the warming oceans – they don’t like warm water and they need the cold upwelling in the oceans, which brings nutrients from the bottom.”