A mysterious quacking noise in the ocean that has baffled scientists for decades has finally been identified, researchers say. Acoustic recorders placed on Antarctic minke whales have produced what NOAA experts say is "conclusive evidence" that the sound is their chatter, the BBC reports. The sound was dubbed the "bio-duck" when it was first picked up by sonar operators on submarines in the '60s. It's since been heard in the Southern Ocean around Antarctica and western Australia, typically in winter and spring. Still a mystery: why the whales make the sound. Researchers did note, though, that the vocalizations were always recorded when the whales were close to the surface, before making deep dives to eat.
The discovery is good news for whale conservation, because acoustic monitoring "can give us the timing of their migration—the exact timing of when the animals appear in Antarctic waters and when they leave again—so we can learn about migratory patterns, about their relative abundance in different areas and their movement patterns between the areas," the lead researcher says. The hardest part of the study was attaching the recorders to the whales with suction cups, another researcher tells the Australian Broadcasting Corporation. Similar recording devices are used on other whales, "but minke whales were a huge challenge," he says "They're very difficult to approach closely because they're fast. They're like big dolphins—they zip around." (Speaking of whales, researchers recently discovered the planet's deepest-diving mammal.)