What impact do undersea cables have on cetaceans?

More than 1.4 million kilometres of undersea cables now line the ocean floor. They carry 99% of global internet traffic, along with a growing share of the electricity produced by offshore wind farms and international energy interconnectors. With the boom in optical fibre and offshore wind, the pace of installation has accelerated considerably in recent years, and its effects on cetaceans are beginning to worry scientists and judges.

The installation phase: massive noise

This is probably the most harmful phase. Before a cable is laid, vessels survey the seabed with high-resolution mapping sonars. These active sonars emit intense pulses, reaching up to 220 decibels at source, a level comparable to that of the military sonars known to cause problems. The effects on cetaceans are several: possible decompression sickness in deep divers (ziphiids in particular) startled by a powerful signal and surfacing in panic; disruption of communication, of hunting, of migration routes; acoustic masking that covers the biological signals cetaceans rely on.

Burying the cable also generates considerable disturbance: machines that excavate the sediment (jet plough), pile drivers for anchoring points, cable-laying vessels. The cumulative noise can disturb cetaceans over several tens of kilometres for weeks at a time.

The Bay of Biscay precedent

In August 2024, a French court suspended the installation of a power cable in the Bay of Biscay following complaints from marine conservation organisations. The ruling explicitly recognised the risk to 17 marine mammal species present in the area, among them the sperm whale, Cuvier’s beaked whale, the fin whale and the common bottlenose dolphin. It is a major legal precedent, and one that signals a growing awareness of these impacts among the authorities.

Several other countries have adopted preventive protocols. The United Kingdom now requires marine mammal observers on board installation vessels, able to stop work if a cetacean is spotted inside an acoustic exclusion zone. Germany and Denmark use bubble curtains around pile-driving sites: a curtain of air bubbles that attenuates the transmission of noise through the water by 10 to 20 decibels.

Electromagnetic fields: a more subtle threat

Once the cable is in place the noise stops, but another form of pollution takes over: the electromagnetic field it generates. Undersea power cables carry alternating or direct currents that produce magnetic fields measurable over several tens of metres around the cable. Telecommunications cables, for their part, emit far weaker fields.

Yet many cetaceans orient themselves, at least in part, by the Earth’s magnetic field. Several studies have correlated mass stranding sites with anomalies in the coastal magnetic field. The presence of powerful cables could in theory disturb this biological compass, especially in migration corridors. Direct evidence remains limited, but the hypothesis is the subject of active research, in particular in connection with offshore wind projects, which are multiplying high-voltage cables.

Direct physical risks

Beyond noise and fields, cables carry a third risk: entanglement. Though rare, cases of cetaceans caught in cables have been documented, particularly during the laying phase, when the cable is not yet buried. Young baleen whales on migration, whose filter-feeding dives bring them close to the bottom, are especially vulnerable.

Abandoned cables left on the seabed are another problem: in the absence of systematic recovery of obsolete cables, the ocean floor is accumulating what amounts to a network of cabled ghost nets. Currents can lift them, creating structures that are dangerous for wildlife.

Towards more protective protocols

Several levers exist in the face of these issues. The choice of route is crucial: avoiding marine sanctuaries (such as the Pelagos Sanctuary in the Mediterranean), known migration corridors and breeding areas. Seasonal timing of the work makes it possible to avoid peaks in the presence of sensitive species. Acoustic attenuation by bubble curtains is now technically well mastered. And real-time passive acoustic monitoring, networks of hydrophones that detect cetacean vocalisations so that work can be halted as soon as an animal approaches, is becoming an increasingly standard practice on responsibly run sites.

The challenge of the coming years will be to reconcile the necessary development of undersea digital and energy infrastructure with the protection of ecosystems already under severe strain. The Bay of Biscay precedent shows that the courts can now impose that reconciliation.

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