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Scientists pinpoint source of 'bio-duck' sound

Mystery solved A puzzling sound heard for decades in the waters of the Southern Ocean and off the Australian west coast has finally been identified as belonging to Antarctic minke whales.

The findings, reported today in the Royal Society journal Biology Letters, bring to an end a mystery that began five decades ago.

The sound was dubbed the "bio-duck" by millitary personnel, who were the first to describe it in the 1960s.

"It's a repetitive signal [ranging from 50 to 300 Hertz] with a slight downsweep that happens, typically every two or three or four seconds," says co-author of the new study Dr Nick Gales, Chief Scientist in the Australian Antarctic Division.

The distribution of the unique sound, heard mainly in winter and spring, suggested that it may be coming from Antarctic minke whales (Balaenoptera bonaerensis), some of which are known to migrate north to give birth.

But until now, there has been no proof that the whales were the source of the bio-duck sound.

"We thought it was likely to be them, but no one knew. There was a lot of speculation," says Gales.

Acoustic activity

Gales and colleagues confirmed it was the minke whales when they recently tagged two with a recording device.

One of the tags stayed on a whale for nearly 24 hours and its recordings are the major focus of the new paper.

"We got six bursts of acoustic activity from the whale all just before it was about to feed," says Gales.

After comparing the recordings to archived sounds collected over the decades by military and researchers, the researchers confirmed it was indeed the bio-duck sound.

Gales says researchers can now go back to routine recordings that are made of the ocean and identify Antarctic minke whale sounds.

This will help them estimate the abundance of the animals and their movements, he says.

"A large part of ocean science now uses acoustics for any animals that are making noise because we can discriminate between different populations of animals on the basis of sound and their seasonal occurrence and absence from places, as well as a whole lot of behaviours."

The prevalence of the bio-duck sound in Antarctic waters during winter suggests that many whales live in ice-covered waters, says Gales.

Recordings of the sound off Western Australia at the same time supports the idea that others migrate to lower latitudes at this time, he adds.

Wallet-sized recorder

The wallet-sized recorder was plonked on the back of a whale from a boat using a carbon fibre pole and attaches via four suction cups, says Gales.

The suction cups stick to the whale for up to 24 hours before falling off. Once the recorder floats to the surface, it gives off a signal so it can be retrieved.

As well as sound, the recorder captures information on the whale's acceleration, pitch, roll and depth of water.

"You plug it into your computer and you can recreate in four dimensions the movement of that animal up and down through the water column and the sound it makes at the same time," says Gales.

He says the technology is routinely used on big hump back whales, southern right whales and blue whales.

"But minke whales were a huge challenge," says Gales. "They're very difficult to approach closely because they're fast. They're like big dolphins - they zip around."

Retrieving the recorder in rough icy seas off the western Antarctic peninsula was also a challenge, he adds.