Soviet whalers wiped out blue whales from part of the Indian Ocean 50 years ago. Underwater audio reveals the gentle giants have returned.

• Blue whales near the Seychelles were wiped out half a century ago by whaling.
• After sightings, scientists set up audio recording devices on the seafloor near the island nation.

A population of blue whales near the Seychelles islands in the Indian Ocean that was wiped out half a century ago has returned after the area was protected, according to a new study.

Scientists confirmed some blue whales, the largest animals on Earth, were spending part of the year back in the area by taking underwater audio recordings that captured the whales' distinct songs, according to the study published this month in the journal Endangered Species Research.

"It turns out if you stop killing animals on mass scales and you give them a chance to rebound, they can recover," Kate Stafford, a professor at Oregon State University's Marine Mammal Institute and a lead author on the study, told BBC News.

Like many whale species, blue whale populations were depleted by commercial whaling in the 20th century. In the 1960s, a fleet of whalers from the Soviet Union killed 500 blue whales near the Seychelles, according to the study, essentially wiping out the animal from the area.

But thanks to global conservation efforts and whaling bans, blue whale populations are increasing worldwide. After several reported sightings of blue whales near the Seychelles, scientists set out to determine if the species had really returned.

Researchers set up a "sound trap" on the ocean floor near the island nation, complete with recording devices and batteries. The equipment recorded sounds for 15 minutes of every hour, every day, for a year. When scientists reviewed the recordings, they discovered the blue whales' signature song.

The songs, which are such a low frequency they cannot be heard by the human ear, were observed from December to April, and were believed to have come primarily from males.

"They sing during the breeding season and we think it's probably the males who are singing, based on what we know about other whales," Stafford told the BBC, adding: "This means the Seychelles could be really important for blue whales."

The scientists are still trying to understand the importance of the Seychelles to the blue whales, including if it is an area they use for breeding.

About 154,000 square miles of sea surrounding the Seychelles are protected thanks to a "debt for climate" swap in which the country agreed to protect its oceans in exchange for having its national debt paid off.

Despite making population gains, blue whales are still listed as endangered by the International Union for the Conservation of Nature and under the Endangered Species Act in the US.