Navy’s sonar has a killer sound

The similarity between the sounds made by the killer whale and navy sonar may provoke panic in the mammal’s prey, causing mass strandings.

SCIENTISTS may have found the explanation for the role of naval sonar in the mass stranding of whales and dolphins with the discovery that ships’ equipment uses the same frequencies as killer whales.

They suggest that the bursts of sound emitted by warships to spot submarines resemble the noises used by killer whales to communicate as they track their prey. The similarity may deceive some species into thinking they are about to be attacked, so they swim for their lives.

The research, by a team including scientists from the sea mammal research unit at St Andrews University and the Woods Hole Oceanographic Institution in the United States, follows a spate of mass strandings around Europe and America.

In June this year 26 common dolphins beached themselves along the south coast of Corn-wall. Last week the Ministry of Defence admitted that 30 sonar-equipped naval vessels had been staging an exercise just beforehand.

Peter Tyack, a senior scientist at Woods Hole, said: “Some naval sonar uses pulses of similar frequency and duration to the pulses emitted by killer whales and is very loud. It seems to have a particularly strong effect on species, such as small beaked whales, of which killer whales are the primary predator.”

Suspicion fell on naval sonar after unusual mass strandings of beaked whales were first reported in the early 1960s, when such devices became commonly deployed. However, many nonnaval ships also use sonar without being linked to strandings. Additionally, some species beach themselves when there is no naval activity.

Tyack believes that deep divers such as Cuvier’s and Blainville’s whales are among those most likely to confuse sonar with predators. Dolphins may also be vulnerable.

On a research cruise last year he and his colleagues found a Blainville’s whale foraging over a part of the sea bed that had been equipped with hydro-phones - underwater microphones to detect movements and noises. They tagged the whale so that they could track the direction and course of its dives as well as the sonar pulses it emitted as it searched for food a mile beneath the surface.

The researchers played a recording of the sounds made by a pod of killer whales through underwater loud-speakers, and the Blainville’s whale suddenly switched to “stealth mode”. It stopped emitting the characteristic echo-location pulses normally used to find prey and then glided slowly and silently back to the surface for air before leaving the immediate area.

When the experiment was repeated, this time with naval sonar directed at the animal, it caused a similar disruption to its behaviour patterns.

Such research must be repeated with more animals to be conclusive, but Tyack believes other factors support the link with killer whales.

Naval sonar ranges include low frequencies of 3-10kHz, a range also used by killer whales. The MoD has confirmed that the sonar used in its June exercises had frequencies in the 2-50kHz range.

Civilian ships and fishing vessels, by contrast, tend to use sonar with shorter pulses and higher frequencies. This may explain why they appear not to cause beachings.

One positive outcome of the research could be that navies are able to recalibrate their sonars to avoid harming whales and dolphins.

In Britain the number of strandings of cetaceans has doubled to more than 700 in 10 years. Professor Ian Boyd, director of the sea mammal research unit at St Andrews, is leading the investigation into the cause of strandings aboard the research vessel Roger Revelle off the Bahamas. He is trying to repeat the whale-tracking experiments of last year.

In an e-mail from the ship he said: “One theory is that these whales confuse the sonars for killer-whale calls and have a magnified antipredator response that ends up with them becoming stranded.”

Tyack points out that a far larger threat is that of being hit by ships or caught in fishing nets: 1,000 sea mammals a day are thought to die that way.