The death song of the seas

Whales and dolphins rely on sound for communication, to hunt, navigate and mate. Now many marine mammals may be dying because of the racket made by shipping and construction industries and military sonar, says Sanjida O'Connell

Earlier this year, 16 whales and a dolphin were stranded on the northern coast of the Bahamas. Six of the whales and the dolphin died; autopsies showed that they had severe brain haemorrhaging. It is suspected that both the stranding and the bleeding were caused by sonar transmissions from US Navy ships engaged in anti-submarine exercises nearby.

There are growing fears that the ocean has become a booming, buzzing and confusing place for whales, dolphins and other marine mammals. Sound is their principal sensory medium. They use it to communicate, navigate, hunt and find each other in the vastness of the ocean. Blue whales are known to carry out "conversations" with each other over distances of up to 1,000km (600 miles).

Many mammals hear like humans do, and within a similar range of frequencies, but cetaceans (whales and dolphins) communicate at sound levels both higher and lower than we can typically hear. As a result, marine mammals are affected by a wider range of sounds, and each species is affected by sounds in a different audible range; for instance, sounds in the region of 140 to 180 decibels (dB) can cause discomfort for beluga whales, while exposure to sound levels of 195 to 210dB may damage soft tissue and the ear drums of baleen whales.

The military is one of many producers of underwater noise – jetskis and motor boats, shipping, off-shore construction and oil drilling are all contributors. "The seas are definitely more noisy than they were 50 years ago," says Doug Nowacek of the Woods Hole Oceanographic Institute in Massachusetts.

The cetacean deaths earlier this year were not, unfortunately, an isolated incident. Haemorrhaging can be caused in people as well as marine mammals by sonar, or by underwater explosions. When whales and dolphins dive, air from their lungs is forced into cavities in their bodies. The trapped air bubbles can magnify sound waves by up to 25 times, leading to the sort of internal bleeding found in the animals stranded off the Bahamas. This "resonance" leads to massive tissue damage at much lower sound levels and over a wider area of the sea than had been thought possible, says Ken Balcomb of the Center for Whale Research in Washington. Balcomb has been studying Cuvier's beaked whales in the Bahamas for 10 years.

The US Navy is now seeking approval for its new Surveillance Towed Array Sensor System, which would use four ships equipped with low-frequency sonar. They would be able to sweep 80 per cent of the world's oceans. In the proposed system, transmissions could be as loud as 230dB. The navy proposes to ensure, using observers and monitoring equipment, that no marine mammals are within a kilometre of the ships; beyond this distance, the noise will dissipate to a safer 180dB. But some scientists, including Balcomb, believe that 180dB could cause serious physiological damage, even at distances of up to 100km.

There is almost no data on the effects of low-frequency sonar on creatures such as whales and dolphins. "The proposed US low-frequency sonar worries us most," Nowacek says, "because it will affect diving mammals such as sperm whales, elephant seals and beaked whales, which descend to depths of 1,000m."

Britain's Ministry of Defence, in conjunction with QinetiQ (formerly part of the Defence Evaluation Research Agency), has developed a low-frequency sonar, called 2087, which is to be fitted to six anti-submarine ships. The ministry insists the sonar will be more dolphin- and whale-friendly. This seems unlikely; an MoD spokesman admits that "there's nothing magic" about the new system. "It's still sonar, it still transmits noise into the sea and not all sea animals are going to be enthusiastic about it," he says.

Balcomb says problems may still arise. "A low-frequency system should not cause resonance effects in a dolphin, but the total power of the sonar system may nonetheless be sufficient to cause traumas at close range." The MoD says it is aware of the environmental issues, and intends to operate the sonar initially at low power to enable whales to move away. "There's no evidence to say it works – I don't know, I'm not a dolphin – but we are giving them a chance to get out of the way," a spokesman says.

The plan has met with guarded scepticism from some sea-mammal scientists. "This 'ramp up' seems a common-sense precaution," says Jonathan Gordon, a sperm whale researcher at the University of St Andrews, "but no one has tested whether it actually achieves the desired effect. Without a test and observations it isn't safe to assume it offers real protection."

Nowacek warns that many marine mammals can become accustomed to loud sounds and might not get out of the way. "Whales may not see ships or subs as a threat. There is nothing in their world that is naturally bigger than them, and if an animal gets hit and dies, it won't have a chance to learn." Also, the desire for food may take precedence. Many fishing ships have "pingers" that emit loud sounds in an attempt to scare marine mammals away and prevent them from being caught in the nets along with fish, but when a whale or a dolphin is hungry, this strategy has limited effectiveness and may actually attract the mammals to fish-laden nets.

Apart from the resonance problem, no one knows how noise affects marine mammals in a general sense. The main sources of ocean noise are shipping and construction, which could block communication, Nowacek says. "It's like being at a cocktail party; you have to talk louder and louder to be heard. The problem of ocean noise is that it decreases the range of communication and means that whales and dolphins will have to shout louder."

A study by researchers at Woods Hole has shown that when breeding whales are in the presence of a large ship, they increase the noise of their own communications as if competing with the ship. "It could keep them from meeting up and mating, and if they can no longer hear they cannot navigate," Nowacek says.

Tony Heathershaw, the head of QinetiQ's Environmental Impact Assessment Centre at the Southampton Oceanographic Centre, advises the military and other agencies, such as oil and construction companies, on ways to manage underwater noise. He has developed a technique for assessing the impact of sound that takes into account frequency, intensity and duration and is based on health and safety regulations for humans in the workplace.

For instance, the lower the noise, the longer a person can continue working; the higher and louder the noise, the less time a person should spend near it. "We have every reason to believe that work on human beings can be applied to marine mammals as well," Heathershaw says. "We may not know what's in the ocean at any one time, and we may not know what the threshold for hearing in a particular species in the ocean at that time is. The approach we take is very precautionary."

Industry is starting to use Heathershaw's mitigation measures by operating noisy machinery only at certain frequencies and intensities for specific lengths of time or, if they are able to monitor the surrounding ocean, when they know large marine mammals are not present. The US has developed a "mitigation sonar" that can detect whether a cetacean is nearby even when a louder sonar is operating.

However, Heathershaw is realistic about the size of the oceans and the difficulty of detecting other creatures; fish and diving birds, for instance, may also be affected. "It's difficult to avoid situations where mammals are not affected by noise. There's always likely to be some risk, whatever man does in the environment. What one is trying to do is minimise that risk."