Bryde's Whale Biotwang Mystery Finally Solved

A baffling booming noise rising from far beneath the ocean surface has at last been linked to an intriguing origin.

First detected in 2014 in the western Pacific, the "biotwang" has been identified as a call made by Bryde's whales (Balaenoptera brydei) as they travel across vast stretches of open sea. The methods that revealed the sound's source have also produced a new way to study whale populations, including how they travel through and occupy Earth's little-understood oceans.

Bryde's whales and the biotwang

"Bryde's whales occur worldwide in tropical and warm temperate waters, but their population structure and movements are not well understood," writes a team led by biological oceanographer Ann Allen of NOAA Pacific Islands Fisheries Science Center in Hawaii.

"Our results provide evidence for a pelagic western North Pacific population of Bryde's whales with broad distribution, but with seasonal and inter-annual variation in occurrence that imply a complex range most likely linked to changing oceanographic conditions in this region."

ORCAA_Lab · BioTwangExample

The sea contains an abundance of unusual sounds, yet biotwangs - odd, mechanical-sounding signals occurring above a particular frequency - had proved especially perplexing. They were initially detected around the Mariana Archipelago in an autonomous acoustic survey by Oregon State University. Two years afterwards, working solely from the recordings, researchers concluded that a baleen whale was the probable source.

In 2018, NOAA researchers carrying out a marine-mammal survey in the Mariana Archipelago believed they had come closer to a firm identification. Over the month-long expedition, they observed 10 individual Bryde's whales and captured biotwang recordings apparently associated with nine of them.

That was compelling evidence, although a little more confirmation was still required.

Using AI to track the Bryde's whale call

Studying the ocean is far from straightforward. It is an inhospitable setting for people, and researchers cannot simply investigate it directly. NOAA has, however, monitored the ocean continuously for many years, installing a new system off the Californian coast 23 years ago to remotely follow changes in its soundscape.

Through long-term passive acoustic recorders, the agency has monitored marine-mammal populations in remote places that people struggle to reach. As sound travels farther in water than in air, scientists can gather a broad range of acoustic data from across great distances of ocean.

If Bryde's whales truly produced the biotwang, the monitoring programme should have recorded these calls, and the detections ought to align with the whales' migration routes.

There was, though, a major obstacle: several recording devices operating for more than two decades had generated an enormous quantity of data. To search through it and isolate the desired signals, Allen's team worked with Google to create an algorithm capable of examining the recordings and extracting the biotwangs.

"Thanks to AI we were able to do it in a matter of hours, rather than years," NOAA wrote in a blog post.

"We identified a consistent seasonal presence of biotwangs only in the Mariana Archipelago and to the east at Wake Island. This suggests the biotwang may be a call specific to a western North Pacific population of Bryde's whales… The seasonal occurrence of biotwangs is consistent with Bryde's whales migrating between low and mid-latitudes. There is a small peak between February and April, and a larger peak between August and November, as the whales travel past the recording sites."

A distinct western North Pacific whale dialect

Scientists have recorded calls from other Bryde's whale populations over the years. Those made by the western North Pacific group appear to resemble a whale dialect, one not used by any other population documented so far.

Identifying the origin of the sound provides researchers with a fresh means of following and understanding the global distribution and migration of Bryde's whale populations. The AI tool can also be adapted to recognise other sounds and their timing patterns, helping establish where other marine mammals travel and when.

Researchers hope the resulting information will, in turn, help them find ways to safeguard the remarkable giants living in the world's untamed oceans.

The research has been published in Frontiers in Marine Science.