New York, April 24 (IANS): They tracked this mysterious low-frequency sound in the southern ocean for decades but could not find its true source. Now, scientists are determined that this 'duck' like sound is actually coming from antarctic minke whale.

A unique rhythmic sound, recorded for decades in the Southern Ocean and called the “bio-duck”, was first described and named by submarine personnel in the 1960s.

“We do not know very much about antarctic minke whale (Balaenoptera bonaerensis), but now, using passive acoustic monitoring, we have an opportunity to change that, especially in remote areas of the Antarctic and Southern Ocean,” explained Denise Risch from the National Oceanic and Atmospheric Administration's (NOAA) Northeast Fisheries Science Center (NEFSC) in Massachusetts.

The bio-duck sound is heard mainly during the austral winter in the Southern Ocean around Antarctica and off Australia’s west coast.

The acoustic tags, which also recorded water temperature and pressure, were placed on the whales using a hand-held carbon fibre pole by researchers working from a rigid-hulled inflatable boat.

Animals were visually tracked from the boat during daylight hours to identify behaviour and group composition.

“No other marine mammal species were observed in the area when calls were recorded, providing further evidence that the recorded sounds were produced by the tagged whale or other nearby Antarctic minke whales,” Risch noted.

Identifying the bio-duck sound will allow for broader studies of the presence of minke whales in other seasons and areas, said the study published in the journal Biology Letters.