Distributed acoustic sensing reveals temporal occurrence, movement and vocal behaviour of singing fin whales in theGibraltar-Alboran maritime corridor

The ocean is a vast, dark place where sound travels farther and faster than light. For many marine animals, especially the great whales, hearing is their primary way of navigating, finding food, and communicating across hundreds of miles. Among these giants, the fin whale is a particularly vocal traveler, known for producing deep, rhythmic calls that can be heard across entire ocean basins. Scientists have long known that these whales migrate through the Strait of Gibraltar, a narrow and busy waterway connecting the Atlantic Ocean to the Mediterranean Sea. However, understanding exactly when they sing, where they go while singing, and how their songs change over time has been difficult. Traditional methods, like sending out boats with hydrophones or waiting for clear days to spot them from a distance, often miss the big picture because the whales move quickly and the ocean is too large to cover completely.

To solve this, researchers turned to a technology originally designed for telecommunications: the submarine fiber-optic cable. These cables, which carry internet and phone data across the ocean floor, are now being used as giant listening devices. By sending laser pulses down the cable, scientists can detect the tiniest vibrations caused by sound waves hitting the glass fibers. This technique, known as distributed acoustic sensing, effectively turns the entire length of the cable into thousands of microphones, allowing researchers to listen to the ocean continuously for months at a time without needing to deploy expensive equipment that runs out of battery. In a recent study, scientists used this method to listen to fin whales in the Strait of Gibraltar and the nearby Alboran Sea, revealing a detailed story of their seasonal movements and vocal habits that was previously hidden from view.

The team monitored two existing fiber-optic cables stretching for about 95 kilometers across the region. One cable ran between Tarifa and Ceuta, and the other between Estepona and Tetouan. They listened for ten months, from June 2022 to February 2024, capturing the acoustic signatures of singing fin whales. The results showed that these whales are not just passing through; they are actively singing while they migrate. The study found that singing activity was not constant but followed a clear seasonal pattern. At the Tarifa cable, the whales began singing in mid-August, reached a peak in September, and then declined by October. At the Estepona cable, singing started in September, dipped in the middle of winter, and reached its highest level in January. This suggests that the whales continue their reproductive displays throughout their journey, rather than stopping to sing only when they reach a specific destination.

One of the most striking discoveries was how the whales moved while they sang. In August, the whales detected at the Tarifa cable were almost all moving westward, heading out of the Mediterranean and into the Atlantic. This matched what visual observers had seen in the past. However, as the months changed, the direction became less predictable. In September, most whales at Tarifa were moving eastward, back toward the Mediterranean. At the other cable, the whales moved in both directions throughout the study period, with no single dominant direction for any given month. This indicates that the Strait of Gibraltar is not just a one-way highway for these animals. Instead, it acts as a social bottleneck, a narrow corridor where males gather and sing, potentially competing for the attention of females or establishing their presence among other males. The researchers even detected instances where two whales were singing simultaneously just a few kilometers apart, confirming that these gatherings are social events, not just solitary migrations.

The study also looked closely at the songs themselves to see if they could tell the researchers which group of whales they were listening to. Fin whale songs are famous for their stereotyped structure, usually consisting of a low-frequency note followed by a pause, repeated over and over. The researchers found that the pitch of these notes was incredibly consistent, hovering around 20 Hertz, which is a very low, deep sound. This stability suggested that the whales in this region share a common vocal identity. However, the timing between the notes was much more flexible. The time between notes changed depending on the season and the year. In the late summer and early autumn, the intervals were shorter, while in the winter, they became longer. This variability was so significant that the researchers concluded that you cannot simply look at the timing of a song to decide which population of whales is singing. The songs are dynamic and change over time, meaning that a whale's identity is not locked into a single, unchanging rhythm.

This flexibility in song timing challenges the old idea that scientists can easily sort whale populations based on how fast or slow they sing. The study showed that the same group of whales can change their song speed over the course of a single season, and that different groups can end up singing with similar rhythms at different times. This means that conservation efforts need to be more nuanced. The Strait of Gibraltar is a place where these whales face intense pressure from human activity, including thousands of ships passing through every year. The noise from these ships can mask the whales' calls, making it hard for them to communicate. Furthermore, because the whales are often singing while moving in a directed path, they may be less likely to change course to avoid an approaching ship, increasing the risk of collision.

By using the existing cables as a listening network, the researchers were able to gather a massive amount of data that would have been impossible to collect with traditional methods. They could track the whales continuously, day and night, regardless of the weather or the time of year. This approach revealed that the whales are present and active throughout the migration season, singing as they travel. The study suggests that the Strait of Gibraltar is a critical area not just for travel, but for the social lives of these animals. It is a place where they gather, sing, and interact, making it a vital zone for conservation. The success of this project demonstrates that we can turn the infrastructure of human industry, like undersea cables, into tools for protecting the natural world. It offers a new way to listen to the ocean, providing a clear, continuous window into the lives of some of the planet's largest and most elusive creatures.