Fiber optic cables can be turned into hydrophones to track whales, study suggests

The DAS system can determine the vibrations or acoustic signals' location, frequency, and amplitude by analyzing the backscattered light. In this case, a device called an interrogator was used to send laser pulses down fiber-optic cables while continually recording the pulses that were returned.

The main drive for this work was to assist in tracking whale movements, specifically fin whales, to understand their habits better and to find a way to avoid accidental deaths from collisions with shipping.

As the researchers point out, Fin whales are being driven to colder waters in more northern latitudes to avoid the warming waters from climate change they more normally inhabit. Fin whales, the second-largest mammal after blue whales, have changed their eating habits due to rising temperatures and a dramatic decrease in ice cover, putting them closer to ships. The fin whale is already threatened by commercial whaling and is thought to be extinct except for about 100,000 individuals worldwide.

The Svalbard archipelago was of particular interest to them, an area severely impacted by rising sea temperatures, located in the Arctic Ocean well north of the Arctic Circle.

The first underwater fiber-optic cable was set up in the 1850s between Newfoundland and Ireland to carry telegraph signals. Nowadays, submarine telecommunications cables stretch 550,000 miles (885,139 km) of our oceans and seas, allowing data to be transmitted between continents and islands. For this study, the Norwegian researchers modified two 155 miles (250 km) long fiber-optic cables that extend from Longyearbyen, the world’s most northernmost settlement and the largest inhabited area of Svalbard, to Ny-Ålesand, a research outpost located in the northwest.

Using these cables, the researchers could follow the movements of eight fin whales over five hours using linked wires and their vocalizations. The whales were tracked throughout about 695 square miles (1,800 square km), and their whereabouts were accurate to within 328 feet (100 meters).

“This work demonstrates how we were able to simultaneously locate and follow these whales over a 1,800 square km area – with relatively low infrastructure investment,” said Martin Landrø, a co-author of the study.

“The capabilities demonstrated here establish the potential for a near-real-time whale tracking capability that could be applied anywhere in the world where there are whales and fiber-optic cables,” said the researchers. “Coupled with ship detection … a real-time collision avoidance system could be developed to reduce ship strikes,” they added.

You can view the study in the journal Frontiers in Marine Science.

Study abstract:

"Climate change is impacting the Arctic faster than anywhere else in the world. As a response, ecosystems are rapidly changing. As a result, we can expect rapid shifts in whale migration and habitat use concurrent with changes in human patterns. In this context, responsible management and conservation requires improved monitoring of whale presence and movement over large ranges, at fine scales and in near-real-time compared to legacy tools. We demonstrate that this could be enabled by Distributed Acoustic Sensing (DAS). DAS converts an existing fiber optic telecommunication cable into a widespread, densely sampled acoustic sensing array capable of recording low-frequency whale vocalizations. This work proposes and compares two independent methods to estimate whale positions and tracks; a brute-force grid search and a Bayesian filter. The methods are applied to data from two 260 km long, nearly parallel telecommunication cables offshore Svalbard, Norway. First, our two methods are validated using a dedicated active air gun experiment, from which we deduce that the localization errors of both methods are 100 m. Then, using fin whale songs, we demonstrate the methods' capability to estimate the positions and tracks of eight fin whales over a period of five hours along a cable section between 40 and 95 km from the interrogator unit, constrained by increasing noise with range, variability in the coupling of the cable to the sea floor and water depths. The methods produce similar and consistent tracks, where the main difference arises from the Bayesian filter incorporating knowledge of previously estimated locations, inferring information on speed, and heading. This work demonstrates the simultaneous localization of several whales over a 800 km area, with a relatively low infrastructural investment. This approach could promptly inform management and stakeholders of whale presence and movement and be used to mitigate negative human-whale interaction."