Do military sonars kill cetaceans?

On 15 March 2000, along the coasts of the Bahamas, seventeen cetaceans stranded within a few hours. Fourteen were beaked whales, species so elusive that they are almost never seen. The necropsies carried out over the following days by Ken Balcomb and Diane Claridge revealed strange lesions: haemorrhages in the inner ear, cerebral haemorrhages, damage to the paranasal sinuses. Nothing consistent with a natural cause. At the same moment, a few nautical miles away, the United States Navy and NATO were conducting a major naval exercise. For sixteen hours, mid-frequency sonars swept the waters at full power. It was the first time the scientific community was to connect these two events explicitly and publicly. It was also the start of a long legal and scientific battle that would lead, two decades later, to a certainty now established: yes, some military sonars kill cetaceans.

The reference events

Before the Bahamas in 2000, several incidents had already been suspected without the necessary physical evidence being available. In 1996, twelve Cuvier’s beaked whales stranded in the Gulf of Kyparissia in Greece, a troubling coincidence: NATO was at that moment running tests there of an experimental low-frequency sonar. In 1998, the same year in which the United States Navy deployed its new SURTASS-LFA system, several small stranding events were reported without any causal link being demonstrable, for lack of access to fresh animals.

The Bahamas event changed matters because Balcomb and Claridge, based on site for their own research, reached the carcasses within hours. They froze the tissues, alerted the international community, and made it possible for pathologists to carry out detailed necropsies. Their conclusions, published in 2001, were categorical: the lesions resembled no other known case of natural stranding. They point to an acute acoustic trauma followed by too rapid an ascent.

On 24 September 2002, the story repeated itself in the Canary Islands. Fourteen Cuvier’s beaked whales stranded within a few hours near the islands of Fuerteventura and Lanzarote during NATO’s “Neo-Tapon” exercise. This time it was the veterinary team of Antonio Fernández, at the University of Las Palmas, that took charge of the investigation. Their 2005 paper in Veterinary Pathology has become the reference in the field. It describes in detail, for the first time, a gas and fat embolic syndrome in beaked whales: the biological equivalent of decompression sickness in a human diver.

The mechanism: why the ziphiids?

To understand why these animals die, one has first to understand how they live. The beaked whales, Cuvier’s beaked whale, the various mesoplodonts, the bottlenose whales, are champions of deep diving. Cuvier’s beaked whale even holds the absolute records for depth (2,992 m) and for breath-hold (222 minutes, see our records page). To reach such performance they have developed an extremely refined diving physiology: their surface breathing cycles are precise, their dive profiles codified, their ascent stops observed.

That extreme specialisation paradoxically makes them vulnerable to any disturbance. If a Cuvier’s beaked whale diving at 1,500 metres abruptly hears an MFAS signal at 200 dB, the acoustic equivalent of an explosion at very short range, it may be seized by a panic that short-circuits its regulatory mechanisms. It then comes up too fast, sometimes in a matter of minutes instead of the usual hour. The pressure drops abruptly. The nitrogen dissolved in its tissues, as in the human diver who surfaces too quickly, forms bubbles in the blood and the organs. Multiple embolisms. Internal haemorrhages. Brain lesions. The animal, disoriented and injured, heads for the shore, where it strands.

The necropsies confirm this scenario. The ziphiids stranded near the Canary Islands in 2002 showed gas bubbles in the hepatic veins, the kidneys, the liver and even the bone marrow. No natural case had ever shown this picture. The diagnosis left no further room for doubt.

Species affected

The ziphiids are by far the worst affected, without it yet being perfectly understood why: perhaps because of their particularly extreme dive profile, perhaps because of an inner ear more sensitive to these frequencies. Sperm whales are disturbed as well but appear less at risk, perhaps thanks to their greater mass and a more robust diving physiology. Dolphins and other small odontocetes show flight reactions, but no series of fatal strandings.

The mysticetes (baleen whales) do not appear to be affected by MFAS sonars, since these sonars operate in a frequency band that mysticetes make little use of. Low-frequency sonars (such as SURTASS-LFA) at 100-1000 Hz, on the other hand, could affect them, within the band of their own songs. Studies on this point are less conclusive.

The legal battle

In the United States, the Natural Resources Defense Council (NRDC) has brought suit against the US Navy on several occasions, on the basis of the Marine Mammal Protection Act. In 2008, the United States Supreme Court ruled in the Navy’s favour on one specific point (exercises off California could continue, given the strategic urgency), but confirmed that the Navy had to assess the environmental impact of its sonars and apply mitigation measures.

Spain took a more far-reaching step in 2004: a ban on MFAS sonar exercises within a zone of eighty nautical miles around the Canary Islands. Since then, no mass stranding has been observed there. This is the most convincing indirect evidence of causality.

Today, NATO has instituted pre-exercise observation protocols (with marine mammal observers on board), seasonal restrictions in certain areas, and a gradual power build-up system (“ramp-up“) that allows any cetaceans present to move away before the signal reaches full power. These measures are judged insufficient by several NGOs, but they have probably prevented incidents.

Sources

• Fernández, A., Edwards, J.F. et al. (2005). “Gas and fat embolic syndrome involving a mass stranding of beaked whales (family Ziphiidae) exposed to anthropogenic sonar signals”. Veterinary Pathology, 42(4), 446-457.
• Balcomb, K.C. & Claridge, D.E. (2001). “A mass stranding of cetaceans caused by naval sonar in the Bahamas”. Bahamas Journal of Science, 8(2), 2-12.
• Cox, T.M. et al. (2006). “Understanding the impacts of anthropogenic sound on beaked whales”. Journal of Cetacean Research and Management, 7.
• D’Amico, A. et al. (2009). “Beaked whale strandings and naval exercises”. Aquatic Mammals, 35.
• NRDC v. United States Department of the Navy, US Supreme Court (2008), case law.

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Further reading