Until now, if researchers wanted to record whale songs, they had to rely on individual underwater microphones. An innovative technology now makes it possible to detect the sounds of the giant marine mammals on a much larger scale: with the help of the fiber optic cables already laid on the seabed. The cables are so sensitive to the tiniest vibrations that even the sound waves emitted by whales are reflected in the signal. In an experiment in the Arctic Ocean, researchers were able to use this data to locate whales to within a few meters.
Despite their size, whales are often difficult to track down in the vast oceans. While local sightings and satellite images of surfacing whales allow researchers to draw conclusions about the occurrence and migration of the giant marine mammals, they only provide an incomplete picture. Underwater microphones, so-called hydrophones, which record whale songs, only cover a small area at a time. Especially in view of the changing activities of whales and humans in the course of climate change, it is important for researchers to get more precise data on the whereabouts of whale populations in order to be able to protect the animals more effectively.
“Satellites” in the ocean
A team led by Léa Bouffaut from the Norwegian University of Science and Technology in Trondheim has now successfully tested a new approach that could make it possible to track whales in large areas of the ocean based on their songs and possibly even monitor them in real time. To do this, the researchers used existing fiber optic cables laid on the seabed and tapped into them using a technique called Distributed Acoustic Sensing (DAS). The DAS technology makes it possible to send light pulses through unused fibers of the fiber optic cable with the help of an interrogation device and to evaluate their transit time. Vibrations in the cable affect the propagation time of the signals. The system is therefore already being used to detect earthquakes.
Bouffaut and her team focused on much weaker tremors: those that cause the sound waves of whale calls. “I believe that this can change the field of marine bioacoustics,” says Léa Bouffaut. “The use of hydrophones is extremely expensive. But fiber optic cables are located all over the world and are easily accessible. In my view, this system could become something like satellites in the ocean.”
New kind of data
The research was carried out in the Svalbard archipelago in Norway. In this part of the Arctic Ocean, baleen whales like blue whales forage during the summer. For 44 days, the researchers recorded the signals from the fiber optic cables laid there and received a huge amount of data of around seven terabytes a day. “If anything moved or made a sound near this fiber buried in the seabed, we could measure it,” reports Bouffaut’s colleague Martin Landrø. “So we saw a lot of shipping traffic, of course a lot of earthquakes, and we could even see distant storms. And finally: whales. In total we registered at least 830 whale calls.”
One challenge at first was interpreting the signals correctly. “We were looking for signals without knowing exactly what to expect,” explains Bouffaut. “It’s new technology and a new type of data that no one has used before to search for whales.” However, by analyzing frequency, pattern and repetition, they were able to clearly identify the whale songs in the data make. With this knowledge, it will be possible in the future to train models for machine learning and thus simplify and automate data analysis.
Contribution to the protection of whales
“With this system we have the possibility to cover a large area for surveillance,” says Bouffaut’s colleague Hannah Joy Kriesell. “And since we receive the sound from multiple angles, we can even determine the position of the animal. If we take it even further, which still requires some additional work, it could be done in real time, which would really be a big step forward for acoustic whale monitoring.” transmit and thus avoid collisions with whales. Especially if shipping traffic in the Arctic increases as the ice sheet melts, this could be of great importance for the protection of whales.
Source: Léa Bouffaut (Norwegian University of Science and Technology (NTNU), Trondheim) et al., Frontiers in Marine Science, doi: 10.3389/fmars.2022.901348