(CNN) Marine biologists have for the first time recorded the song of the world's rarest large whale, the eastern North Pacific right whale, of which fewer than 30 individuals remain.

The calls, which researchers have been trying to capture and identify for years, are thought to be the cry of lone males trying to attract mates. In the remote Bering Sea, it is an increasingly difficult task as the population of the extremely endangered whale dwindles.

A team from the US National Oceanic and Atmospheric Administration (NOAA) used moored acoustic recorders to capture repeated patterns of calls made by the whales. It is the first time any population of right whales has been known to break into song, the scientists say -- it is well documented that Southern and North Atlantic right whales restrict themselves to individual sounds.

First recording of rare North Pacific Right Whale song Courtesy NOAA Fisheries

"During a summer field survey in 2010, we started hearing a weird pattern of sounds," said marine mammal scientist Jessica Crance. "We thought it might be a right whale, but we didn't get visual confirmation. So we started going back through our long-term data from moored acoustic recorders and saw these repeating patterns of gunshot calls. I thought these patterns look like song."

Stout, slow-moving mammals which grow up to 18 metres (59 ft) and float after being killed, right whales around the world have been hunted to the brink of extinction. North Pacific Right Whales are believed to number in the low hundreds, and the eastern subset, with 30 or less according to NOAA, is the rarest of them all

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