Researchers can now detect swimming blue whales off the coast of Svalbard even when the marine mammals are completely silent, using underwater fibre-optic cables and a 100-year-old physics equation. According to Martin Landrø, head of the Norwegian University of Science and Technology’s (NTNU) Centre for Geophysical Forecasting, animals moving through the water create very low-frequency pressure waves that standard telecommunications lines can pick up, opening new pathways for marine conservation.
Fibre-Optic Telecommunications Cables as Passive Listening Devices
Fibre-optic lines lace the planet’s oceans to carry telephone conversations and banking information. In 2020, NTNU researchers realized those same cables function as passive listening devices capable of capturing deep rumbles in frigid waters off Svalbard, according to project findings published in the Proceedings of the National Academies of Science (PNAS). While scientists previously relied on acoustic data to hear whale vocalizations, the latest study demonstrates that silent marine mammals register in the data streams through physical water displacement. According to Robin Andre Rørstadbotnen, a postdoc at the Centre for Geophysical Forecasting and first author of the new paper, the primary obstacle in detecting these signals is that low-frequency pressure waves decay very rapidly over distance. Because whales displace significantly less water than large commercial vessels, they must dive deeper into the water column for the seabed infrastructure to register their passage.
Using Ship Traffic and Lord Rayleigh Equations to Calibrate Sensor Data
To interpret faint pressure signatures, researchers turned to heavy vessel traffic cruising the waters around Svalbard along identical fibre-optic paths. Cargo ships and cruise liners are legally required to carry Automatic Identification Systems (AIS), which transmit exact location and speed data. By monitoring vessels like the cruise ship Le Commandant Charcot over three cruise seasons, scientists matched acoustic and pressure signatures against known vessel movements, according to Landrø. Investigators then applied a 1917 equation published by physicist Lord Rayleigh that originally described how bubbles in boiling water collapse. Although boiling water and swimming whales seem unrelated, the foundational physics governing pressure dynamics remain identical, providing a calibration method to isolate whale movements from background noise, according to the PNAS study.
Tracking a Blue Whale Dive Without Acoustic Signals
Field verification occurred when researchers observed a blue whale vocalizing near the surface before it stopped calling and executed a deep dive, according to Landrø. Investigators first tracked the animal using standard acoustic data from the seabed cables. Once the vocalizations ceased, the team analyzed simultaneous low-frequency pressure data calibrated with Lord Rayleigh’s equation. This combination allowed researchers to continuously follow the blue whale track as it descended, proving the method works independently of animal vocalizations. Commercial whaling during the twentieth century decimated global populations, leaving an estimated 5,000 to 15,000 mature blue whales today—roughly 3% to 11% of early 1920s levels, according to historical data cited by NTNU.
Did you know? Blue whales are the largest animals ever known to have lived on Earth, yet their massive size and far-ranging migratory habits make traditional population counts exceptionally difficult for marine biologists.
Frequently Asked Questions
How do fibre-optic cables detect whales without sound?
According to NTNU researchers, swimming whales push water ahead of their bodies, creating low-frequency pressure waves that disrupt seabed sediments and are registered by fibre-optic telecommunications cables.
What role did Lord Rayleigh play in the discovery?
Researchers used a 1917 equation by physicist Lord Rayleigh—originally formulated to describe collapsing bubbles in boiling water—to calibrate and interpret low-frequency pressure data gathered from ships and whales, according to the PNAS study.
Why is tracking silent whales important for conservation?
Most whale populations remain vulnerable following historic commercial whaling, but tracking them is difficult because they roam vast oceanic areas. Detecting silent marine mammals via existing subsea cables provides a new tool for population monitoring, according to Martin Landrø.