NANNING, June 6 (Xinhua) -- In the heart of south China's Guangxi Zhuang Autonomous Region, scientists have repurposed earthquake-monitoring gear into an AI-powered "marine stethoscope." By combining an island-based seismometer with advanced deep learning algorithms, this innovative system now tunes in to the elusive, low-frequency calls of an endangered whale species.
The breakthrough reveals that the whales "overstay" and breed in local coastal waters of the Beibu Gulf in the South China Sea for months longer than previously recorded.
The study, published this week in the journal Geophysical Research Letters, details how a research team led by Xiao Zhuo, an associate professor at Guangxi Minzu University, trained an AI model to isolate the calls of Bryde's whales from a full year of seismic recordings collected off Xieyang Island, a small volcanic outcrop off the coast of Beihai city.
The AI system classified the whale calls with 99 percent accuracy after being trained on a dataset of more than 1.7 million labeled seismic samples, according to the study.
"What genuinely astonished us was that the low-frequency sound energy emitted by Bryde's whales could travel kilometers to the island and still be received by the seismometer with such clarity," said Xiao.
The genesis of the discovery lies in a chance encounter. While Chen Mo, a marine biologist at the Guangxi Academy of Sciences and co-author of the study, had teamed up with Guangxi's earthquake monitoring authorities (Earthquake Agency of Guangxi Zhuang Autonomous Region) in 2021 to build a research station on nearby Weizhou Island, they had struggled to find a way to bridge seismology and marine science.
Then, Xiao approached Chen to see if seismic monitoring could be adapted for whale research. Chen shared insights from his fieldwork dating back to 2018, while Xiao mapped out the technical capabilities of seismic detection.
"That single conversation fundamentally changed our approach," Chen said. "We left that meeting and immediately got to work."
The calls captured by the device reached frequencies as low as 5 Hz -- far below the threshold of human hearing and the detection limits of most conventional hydrophones, underwater microphones, used in marine biology.
The rhythmic pulses, repeated at intervals of roughly five to 13 seconds, are consistent with the mating calls produced by male baleen whales during the breeding season.
"From the acoustic profile, these signals bear a very close resemblance to what is typically classified as whale song," said Xiao. "That leads us to think they may be connected to courtship or breeding behavior."
The most striking discovery, however, was just how long the whales remained in these waters. For years, coastal visual surveys indicated that Bryde's whales typically migrated out of the area by April.
The seismic record told a radically different story. In 2021, whale calls were detected continuously from January through July, peaking in February and March, rising again in May, and tapering off only gradually through midsummer.
This acoustic evidence was backed by local fishermen, who reported spotting a whale roughly 38 km southwest of the island as late as July.
"This discovery has sharpened our understanding of when Bryde's whales are present in the waters around the islands," said Chen. "It gives us stronger backing for conclusions we had already begun to draw about their migration patterns."
However, limitations remain. Acoustic data alone are unable to uncover exactly what the whales are doing during their extended stay -- whether they are feeding, socializing, or breeding. Still, Chen stresses that pinpointing their core activity zones is invaluable for effective habitat conservation.
The waters off Beihai, particularly around Xieyang and Weizhou islands, shelter China's only known stable inshore population of Bryde's whales, and one of the largest documented coastal groups of the species in the world.
Official figures show that more than 70 individuals have been photo-identified in recent years, up from just 10 in 2018.
As a national Class I protected species with one of the most restricted ranges of any large whales -- found mainly in the shallow coastal waters of South and Southeast Asia and parts of the western Indian Ocean -- the population grapples with persistent threats from commercial fishing, vessel traffic, and habitat degradation.
"We can now establish, with a far greater extent of certainty, when Bryde's whales are present, which hours of the day they are most active, and the range of their movements," Chen said.
Traditional monitoring of the species has depended on boat-based visual surveys, which are highly susceptible to weather conditions and confined to daylight hours. Hydrophones can record non-stop but are logistically demanding and prohibitively expensive to deploy, retrieve, and maintain.
Land-based seismometers bypass these constraints. Permanently tethered to local power grids and data networks, they run around the clock for years while transmitting data in real time. Furthermore, because these devices measure ground motion in three dimensions simultaneously, they offer scientists the potential to triangulate the real-time location of a vocalizing whale.
The science relies on the fact that low-frequency whale vocalizations do not remain trapped in the water column. The sound waves travel through the rocky seabed and up into the island's crust, where the seismometer registers them as distinct micro-tremors.
The frequencies that survive this subterranean journey are far lower than those captured directly by underwater microphones. This explains why the seismometer recorded acoustic energy concentrated between 5 and 15 Hz, compared with the 40 to 60 Hz range detected by a hydrophone recording the same calls simultaneously nearby.
The researchers acknowledged that their dataset, drawn from a single station over one year, is not yet sufficient to draw definitive conclusions about how the whales structure their calls throughout the day. While a tentative dip in calling during the early afternoon was observed, the daily rhythm remains too inconsistent to draw firm conclusions without further research.
Yet the implications for global conservation are vast. "Seismic monitoring stations already exist in many places, particularly on islands and in regions of significant tectonic activity," said Xiao. "By harnessing these existing networks, we can achieve continuous, round-the-clock monitoring at very little additional cost." ■
