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.

"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

The North Atlantic right whale, the favourite prey of American whalers until a ban was introduced in the 1930s, has an estimated 450 individuals; the World Wildlife Foundation has said that its population has shown "no signs of recovery" after years of uncontrolled hunting. Ocean warming and the proliferation of industrial fishing lines have led to a further decline in recent years.

North Atlantic and Southern right whales have been found to use single gunshot calls, upcalls, screams, and warbles instead of the patterned phrasing that constitutes singing.

From initial field surveys in the Bering Sea, the NOAA researchers thought that the sound patterns they heard could be song coming from the right whale, but struggled to link the song to the rare mammal. After seven years of documenting sound patterns and combing through data collected from different locations in the Bering Sea, scientists were finally able to visually identify the right whales, confirming their theory.

"We heard these same songs during a summer survey in 2017, and were able to localize the songs to male right whales," said Crance. "We can now definitively say these are right whales, which is so exciting because this hasn't been heard yet in any other right whale population."

Scientists think that these songs may be part of a reproductive display.

"We have direct evidence of male right whales singing, and we think this may be exclusive to males, but we have very limited data on vocalizing female right whales," said Crance.

"With only 30 animals, finding a mate must be difficult. Lone male right whales tend to gunshot more frequently than females. Perhaps the 2:1 male ratio in the North Pacific has led to our males singing to attract females. But we may never be able to test that or know for sure."

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