For almost ten years, a strange sound combining grunts and metallic pings has intrigued scientists exploring the depths of the Pacific near the Mariana Trench. This sound, nicknamed ‘biotwang’, has been attributed to Bryde’s whales, thanks to advances in artificial intelligence (AI). Solving this mystery opens up a new era in the study of discreet marine species using new technologies.
by Laurie Henry
Cover photo : © NOAA Fisheries
Bryde’s whales (Balaenoptera edeni) are medium-sized cetaceans belonging to the rorqual family. They generally measure between 12 and 16 metres and can weigh up to 25 tonnes. Found in tropical and subtropical waters around the world, they prefer warm regions where temperatures exceed 20°C. They do not migrate over long distances, but their routes remain unknown. Their discreet behaviour and constant mobility complicate their observation and study, leaving many grey areas about their ecology. However, recent research by Ann N. Allen et al, 2024 has lifted some of the veil by combining underwater acoustic recordings and artificial intelligence.
Unique sounds revealing the marine ecosystem
In 2014, researchers captured an enigmatic sound in the depths of the North Pacific, near the Mariana Trench, which they named ‘biotwang’.
This complex sound combines a low tone with metallic pings. It was so atypical that it seemed artificial, more reminiscent of the sound of military sonar or a ship than the cry of a marine animal. Ann Allen, an oceanographer with the National Oceanic and Atmospheric Administration( NOAA), undertook years of analysis to unravel the mystery. By cross-referencing these recordings with visual observations of Bryde’s whales in this region, her team confirmed that these cetaceans were indeed the source of these vocalisations. Their identification was made possible by underwater acoustic sensors and rigorous tracking methods, shedding light for the first time on a little-known aspect of this discreet species.
Map of high-frequency acoustic recordings. © A. N. Allen et al., 2024
These vocalisations, which sometimes reach 8000 Hz, are unique among baleen whales and appear to be closely linked to their habitat. Bryde’s whales are heavily dependent on the Transition Zone Chlorophyll Front (TZCF), a zone where warm and cold waters meet and where an abundance of plankton, their main source of food, is concentrated. This essential zone for these whales fluctuates with the seasons and ocean currents.
Artificial intelligence, a revolutionary tool
Manually analysing more than 180,000 hours of acoustic recordings captured by the NOAA network of underwater hydrophones would have been a titanic task. Aware of this limitation, the researchers teamed up with Google to design an artificial intelligence model capable of recognising cetacean vocalisations, particularly those of Bryde’s whales.
This innovative system, dubbed the ‘Shazam* for whales’, converts sounds into visual spectrograms and compares them with a learning database to identify patterns (or repetitive and characteristic patterns). Not only has this process considerably reduced the time needed to analyse the data, it has also improved the accuracy of the acoustic classifications.
To do this, the researchers relied on long-term acoustic databases collected by NOAA’s network of underwater sensors. Since 2005, this network has accumulated more than 500 terabytes of data covering almost 200,000 hours of recordings at 13 different sites in the North Pacific. This approach is revolutionising the passive monitoring of marine species, particularly those living outside direct observation.
Recording frequency of biotwangs at four sites: (A) Wake, (B) Pagan, (C) Saipan, and (D) Tinian. This is expressed as a percentage of the daily recording time when these sounds were detected. The grey areas indicate the periods during which recordings were made.© A. N. Allen et al., 2024
In addition, during at-sea missions carried out between 2018 and 2021, researchers spotted 10 groups of Bryde’s whales near the Mariana Trench. On nine occasions, biotwangs were recorded simultaneously with visual observations of these whales, confirming their link.
Thanks to this combination of techniques, researchers have been able to reveal seasonal migratory patterns in Bryde’s whales in the north-western Pacific, with a notable concentration of biotwangs in plankton-rich areas. These areas, often located around climate transition fronts, play a key role in the feeding and movements of these cetaceans. As these fronts are modified each season, depending on currents and water characteristics (such as temperature), the whales follow complex routes. These changes highlight a wider reality: the growing impact of climate change on whale habitats and behaviour.
The impact of climate change on marine populations
Climate change is having a profound effect on the dynamics of marine ecosystems, and Bryde’s whales, which are closely linked to their feeding environment, are feeling the repercussions. Climate phenomena such as El Niño and La Niña are modifying the position and intensity of the TZCF. This zone is moving increasingly northwards in response to global warming, forcing whales to travel longer distances to reach their feeding grounds. These longer migrations increase their energy expenditure, reducing the reserves available for reproduction and survival. As Ann Allen points out, these extra efforts, combined with the instability of ocean currents, risk weakening the resilience of Bryde’s whale populations in the long term.
Acoustic analyses also reveal marked inter-annual variability in whale calls, influenced by ocean conditions. In 2016, for example, biotwangs were recorded in large numbers, corresponding to a period when the TZCF was particularly productive due to favourable currents.
This link between climate fluctuations and whale activity provides a better understanding of the impact of environmental changes on their feeding and migratory behaviour. On the other hand, years of low marine productivity, caused by increased climatic disturbances, could reduce the availability of food resources and threaten the ecological balance of these mammals. All this data from monitoring the behaviour of marine mammals is essential for modelling the long-term effects of climate change and guiding conservation policies adapted to the growing ecological pressures.
* Shazam : mobile application that quickly identifies songs by recording a sound extract via the microphone, then comparing it with a vast music database. In a matter of seconds, it provides information on the title, artist and album, facilitating instant music discovery.
Source : Ann N. Allen et al., “Bryde’s whales produce Biotwang calls, which occur seasonally in long-term acoustic recordings from the central and western North Pacific”, Frontiers in Marine Science (2024).