Eavesdropping on whales off Svalbard at the speed of light | Polarjournal
Blue whales regularly forage off Svalbard during the summer. Thanks to a successful experiment by researchers at the Norwegian University of Science and Technology, Trondheim, it is possible to eavesdrop on them continuously in the future – by tapping into underwater fiber optic cables. (Photo: Julia Hager)

Studying whales in the world’s oceans is no easy task. Scientists around the world are making an immense effort to locate the animals so that they can learn more about them. To do this, they use data from satellites, aerial photographs, hydrophones, or try to sight them directly. But these are usually only snapshots; longer-term monitoring is only possible with animals equipped with GPS transmitters. Now, for the first time, researchers at the Norwegian University of Science and Technology (NTNU), Trondheim, have succeeded in continuously eavesdropping on and locating baleen whales in near real time by tapping into underwater fiber optic cables off Svalbard.

The entire world ocean is crisscrossed by some 1.2 million kilometers of fiber optic cables, which are used for telecommunications and hold enormous potential for whale research. The technique is called Distributed Acoustic Sensing (DAS) and uses an onshore interrogator to tap into a fiber optic system and turn it into a virtual array of hydrophones – underwater microphones.

Thanks to the existing global submarine cable network, DAS can be deployed in many regions. However, in the polar regions where baleen whales feed during the respective summer, there are only a few cables, or none at all in Antarctica. Map: TeleGeography

One of the few submarine cables in the Arctic is the one between Longyearbyen and Ny Ålesund on Spitsbergen, which went into operation in 2015 and was used by the research team for the study, which was published in the journal Frontiers in Marine Sciences. In and off Isfjorden, blue whales and other baleen whales go in search of food during summer. For forty days in June 2020, two female researchers were able to use the interrogator and listen to the 120-kilometer fiber optic cable.

Blue whales are the largest animals that have ever lived on earth. In the last century, they have been hunted so heavily that the global population is now only between 5,000 and 15,000 adults, 3 – 11 percent of the 1920 population size. Photo: NOAA

“I believe this can change the field of marine bioacoustics,” says lead author Léa Bouffaut, who was a postdoctoral fellow at NTNU and now continues her research as a postdoctoral fellow at the K. Lisa Yang Center for Conservation Bioacoustics at Cornell University. “The use of hydrophones is extremely expensive. But fiber optic cables are all over the world and accessible. This could be similar to the coverage of the Earth by satellite imagery, which has allowed scientists from many different fields to do many different kinds of studies of the Earth. To me, this system could become something like satellites in the ocean.”

In addition, hydrophones are installed in only a few locations, they cover only a limited area, and they are not evenly distributed across the ocean, making it difficult to study migration routes, for example.

“By studying their sounds, their calls and their vocalizations, we can learn a lot about them. We can find out where they hang out at different times of the year and how and where they migrate. So we get a lot of information by listening to them,” says Hannah Joy Kriesell, a postdoctoral researcher at NTNU and co-author of the study.

Distributed Acoustic Sensing (DAS) can be used to monitor baleen whales. (A) Onshore, an interrogator is connected to one end of an existing fiber optic cable to convert it for DAS. (B) It interrogates the fiber by sending laser pulses that are backscattered from anomalies while these defects simultaneously shift under the influence of the incoming acoustic waves. (C) The interrogator calculates the time delays of the backscattered response at regular intervals along the fiber, called channels. The time delays are averaged over the measurement length and converted to longitudinal strain waveforms corresponding to acoustic pressure. (D) The two-dimensional DAS data are transmitted in near real time to a remote data processing center. Graphic: Bouffaut et al. 2022

Distributed Acoustic Sensing, on the other hand, allows researchers to not only detect whale sounds, but they can use the fiber optic cable network to determine the position of whales spatially and temporally at unprecedented resolution, Bouffaut said.

“With this system, which we can basically call a hydrophone array, we have the ability to cover a much larger area for monitoring. And because we’re receiving sound from multiple angles, we can even tell where the animal was – the position of the animal. And that’s a big advantage. And if we go even further, which requires some additional work, this could be done in real time, which would really be a big step forward for acoustic monitoring of whales,” Kriesell explains.

Distributed Acoustic Sensing also enables “listening” to other sounds transmitted through the water, such as large tropical storms, earthquakes, and ships. “If anything moves near this fiber buried in the seafloor or makes an noise, we can measure that,” says Martin Landrø, a geophysicist at NTNU, head of the Center for Geophysical Prediction and co-author of the study. “So what we saw, of course, was a lot of ship traffic, a lot of earthquakes, and we could also see distant storms. And last but not least: Whales. We spotted at least 830 whale sounds in total.”

The data transmission capacity of the Norwegian Research and Education Network (Uninett) is very high and allowed the transfer of raw data from Longyearbyen to Trondheim in near real time. “The researchers were able to start studying the signal records from the sea off Svalbard almost immediately from Trondheim. This is a very good example of a paradigm shift in distributed data collection,” says Olaf Schjelderup, head of Sikt – the Norwegian Agency for Shared Services in Education and Research – and co-author of the study.

The whale sounds were recorded starting from Longyearbyen over a distance of 120 kilometers. (B) The DAS fiber optic cable is located 1 – 2 meters deep in the seafloor at an average depth of 216 meters. (C) number of recorded sounds and (D) Visualization of the sounds in the spectrogram. Graphic: Bouffaut et al. 2022
Recording of a North Atlantic blue whale that emitted both stereotyped sounds and D sounds. Audio: Bouffaut et al. 2022

Bouffaut and Kriesell analyzed a total of 250 terabytes of data over 40 days. The challenge was not only the amount of data, but also that “we are looking for signals without knowing exactly what to expect. It’s a new technology and a new type of data that no one has used to search for whales before,” Bouffaut said. Although the work was arduous, she still described it as “very, very exciting,” especially when they saw whale signals.
With the data now available, machine learning models could be trained to simplify and automate data analysis.

The two researchers identified so-called stereotypic sounds of North Atlantic blue whales outside Isfjorden. They also discovered so-called D sounds, in which the sound resonates downward, and which can come from males, females and calves.

The Arctic is currently changing very rapidly due to climate change, and so is its use by animals and humans. While whales may visit polar waters for longer periods in the future as the ice recedes, new opportunities are simultaneously opening up for shipping, fishing and tourism.

“If the whales change their behavior in this area and maybe stop using it just for foraging or for activities where they are very vulnerable, then this kind of technology can help us monitor those changes,” Bouffaut said.

The comparatively slow-swimming baleen whales are frequent victims of ship collisions, like this blue whale. Photo: Craig Hayslip via Wikipedia (CC BY-SA 2.0)

Bouffaut and Kriesell hope to set up the DAS system so that data can be analyzed in real time. Then the information could be relayed to ships transiting waters with whales, reducing the risk of collisions. “The ice in the Arctic is melting, and shipping in the Arctic has increased dramatically. And that’s a problem for the animals. So if we had a way to inform ships in real time about the whales’ whereabouts, we could prevent or at least reduce the risk of ship collisions,” Kriesell says.

Julia Hager, PolarJournal

Link to the study: Léa Bouffaut, Kittinat Taweesintananon, Hannah J. Kriesell, et al. Eavesdropping at the Speed of Light: Distributed Acoustic Sensing of Baleen Whales in the Arctic. Frontiers in Marine Science, 2022; 9 DOI: 10.3389/fmars.2022.901348.

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