DARLENE KETTEN hoped the morbid express delivery would finally answer some questions. The two beaked whale heads, packed in ice, were flown in from the Bahamas for autopsy in a lab in Boston. The pair had died in an unusual mass stranding of 17 whales on the Abacos Islands. Beached animals tend to be dead when they are discovered, but this time the whales were caught in the act. Local marine biologists returned more than half to the water and preserved the heads of some of those they couldn’t save. It was the first time a post-mortem had followed so swiftly after a beaching.
The incident coincided with a US navy sonar exercise nearby. Naval sonar tests have been blamed for driving thousands of whales and dolphins to their deaths. Ketten, who specialises in underwater hearing at Woods Hole Oceanographic Institution in Massachusetts, examined the whales’ ears with a CT scanner. She was looking for signs of nerve damage and hair-cell loss, which tend to happen after acute exposure to loud sounds. What she saw was more grisly – and not obviously linked to noise. In both cases, there was blood in the inner ear, but it was draining from a cavity on the surface of the brain. Had sonar really killed the Abacos whales?
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Human noise pollution stresses marine animals (Image: Tim Mcdonagh)
The Bahamas case is one of many. Marine mammals have been found stranded on numerous beaches from Hawaii to the Canary Islands, Greece to Madagascar – all when sonar tests or other noisy industrial activity are taking place nearby. For many, the evidence is cut and dried.
And it’s not just mammals that are at risk. Fish and marine invertebrates also seem to be greatly affected by sound. Loud, far-reaching noise – whether from sonar, pile drivers or acoustic guns – seems to cause stress and disrupt animal behaviour. Even the sound of ships could be a problem, especially in busy coastal regions. As ocean development continues, noise pollution is getting worse.
Explore the sounds of the sea in our interactive map: “Sounds of the Sea”
Muddy waters
The question is what to do about it. That’s where the waters are muddier. To manage the impact of human noise, we need to understand exactly how it affects marine life. But we are only just beginning to learn which sounds different underwater creatures can actually hear. Or, in some cases, whether they can hear at all. However, the more we tune in to the sounds of the sea, the better equipped we will be to exist in harmony with the animals that live there.
The sea is awash with sound. From whale song and dolphin chatter to the swish of approaching sharks and the gun-crack claws of pistol shrimp, there’s a lot for marine animals to listen out for. But there’s no question we are adding to this natural cacophony. Industries are continuing to expand into the sea, creating noise pollution from ship traffic, wind farm construction and the seismic air guns used to search for oil and gas. Some noises come in loud bursts, while others are more continuous, drowning out natural ambient levels. In certain areas, measurements suggest underwater background noise has .
Many human-made sounds are low frequency and in the hearing range of most aquatic animals (see diagram). What’s more, low-pitched sound can travel over huge distances underwater, which is why many animals favour it for communication. But that also means that human noise may have a more far-reaching effect than it would on land. Sudden sounds, like airguns, can scare animals away from their normal foraging areas, hampering their ability to find food. Drowning out mating calls can stop animals reproducing, and disrupting communication in general can cause animals stress.
“Human noise may have a more far-reaching effect underwater than on land”
After the 9/11 attacks in New York, commercial sea and air traffic halted in most of the world, creating a rare opportunity to look at stress levels in wild whales. Researchers who examined whale faeces in the Bay of Fundy, Canada, at this time found .
So what can marine animals actually hear? Seals are among the . They have developed different hearing mechanisms for land and sea and hear well in both environments. For example, seals have erectile tissue in their inner ear, which swells up with blood when they are underwater. “It’s like the penis of a man,” says Ron Kastelein at the Sea Mammal Research Company in Harderwijk, the Netherlands, who did the hearing tests. The blood in the engorged tissue helps conduct sound waves to the inner ear, allowing seals to hear a slightly greater range of frequencies in water than on land.
Other researchers are studying the effects of noise on fish and marine invertebrates. When scallop larvae were exposed to seismic pulses, for example, many developed significantly more slowly and some ended up with abnormal bodies. Harming these populations can have knock-on effects on the food chain, as well as human food supplies.
And , a colleague of Ketten’s at Woods Hole Oceanographic Institution, has identified the hearing range and sensitivity of cuttlefish for the first time by observing their reaction to sounds played in a tank. Cuttlefish will ink, jet, change colour or twitch to various degrees based on how threatened they feel. But without recognisable ears, some thought they might not hear at all.
His tests suggest that cuttlefish have a . They displayed colourful patterns and moved their fins at all frequencies and levels in this range. But high sound levels also provoked jetting, and loud, low-pitched sounds triggered inking, both escape responses to predators. “The pervasiveness and obviousness of the responses was a surprise,” says Mooney. “It was quite clear from the start that we were getting clear responses to sound.”
Most invertebrates detect sound by the way it buffets their bodies, rather than by sensing soundwaves directly. A cuttlefish ear, for example, contains a tiny structure that looks like a grain of sand and sits on top of hair cells that sway as the animal moves. The grain lags behind the hair cells as they sway – a motion that is detected and interpreted as sound. “Sound moves them back and forth and they feel the vibration,” says Mooney. “It’s like being close to a speaker.”
Tune in, zone out
One consequence of this is that invertebrates can hear only over short distances, so human noise is less likely to annoy them. Mooney also found that cuttlefish would get used to the repetitive tones he played underwater, dampening down their response over time. But it is hard to work out if that would be an advantage. “You can adapt to a loud rock concert but it might still deafen you,” says Mooney. “Behavioural adaptations don’t necessarily prevent damage.”
Certain whales also seem to be able to live with intrusive noises to some extent. Peter Tyack at the University of St Andrews, UK, and colleagues found that when noise from commercial boat traffic interferes with the sounds right whales use to communicate, they modify the sound levels and frequencies that they emit. But their ability to compensate has its limits as there is a physical threshold to the sounds a whale can make, says Tyack. If human noise continues to increase, adjusting their signals may no longer be enough.
However, responses to noise can differ greatly between close cousins and even between individuals of the same species. Ketten’s whales weren’t the only ones in the area at the time of the beachings. Other nearby whales, including beaked whales, were unharmed. In fact, instead of fleeing sonar, some whales – like pilot whales – are attracted to it and sperm whales mostly seem to ignore it, though it might reduce their ability to catch prey.
After examining the heads of her whales, Ketten ruled out sonar as the direct cause of death. The blood-filled ears were not the result of injury inflicted by sound. Nor was there evidence that their hearing had been dramatically affected. Instead, Ketten concluded that the haemorrhage was probably caused by a dive gone wrong – the whales failed to adjust properly to changes in pressure by ascending too quickly or remaining deep down for too long.
Even if sonar didn’t deliver the final blow, however, Ketten thinks it could have provoked unusual behaviour in the whales that ultimately led to their deaths. “My best guess is that the beaked whales were disturbed by the sound of the sonar and panicked,” she says. “I’ve seen the same type of bleed in humans: it can happen to haemophiliacs when they are upset.” While trying to escape the sound, the whales may have aborted a dive too quickly and ended up on land.
“Even if sonar did not deliver the final blow, it may have led to a fatally aborted dive”
So how can we protect sea creatures from our noise pollution? Kastelein and colleagues have developed a way to keep animals a few kilometres away from noisy areas.
They built three devices, each tailored for seals, porpoises or fish, that emit a randomly generated playlist of sounds through underwater speakers to make the animals swim away. The sounds are played at irregular intervals to avoid habituation and the volume is increased gradually to minimise any stress caused by the device itself. Ascending tones mimic the rise in frequency that would happen as something approaches. “Upsweeps sound awful to all mammals,” says Kastelein. “Alarms and cellphone ringtones use them to make you pay attention.”
So far, the fish deterrent is working. A Swedish dredging company that was exploding dynamite underwater to enlarge a harbour found no fish among the rubble. Similar endeavours have killed thousands. The porpoise deterrent has been used in the Netherlands during wind farm construction, where laws forbid the killing of porpoises, and the system has also been rented for seismic surveys.
Christ de Jong at the Netherlands Organisation for Applied Scientific Research is looking at noise created by ships. As part of the European , which aims to reduce noise due to cavitation, he is targeting propellers, the biggest source of underwater noise from most ships. In fast-moving vessels, propellers can produce cavitation bubbles – pockets of low pressure in a liquid that generate a shock wave when they implode. This effect is behind the gun-crack of a pistol shrimp’s claws, which can stun and even kill small prey. By redesigning ship propellers and hulls we can reduce the noise they make, says de Jong. “Designers need to label propellers in terms of noise levels so that shipyards have a choice of quieter alternatives.”
The SONIC project is also drawing up sound maps of the North Sea that track the location of ships, showing a vessel’s size, for example, and modelling the sound it radiates. “We use the maps to investigate how the total noise in the sea would be affected if shipping noise requirements were set to a certain level,” says de Jong. The maps will also incorporate information from biologists about where different species live, and at what depth, so that ships can exercise more caution in areas with sensitive species.
Kastelein also thinks we could tweak industrial noise to make it less disruptive. Airguns, for example, are already using narrower beams in seismic surveys to reduce the area of the seabed hit by the sound. In future, sonar on ships could also use descending tones rather than ascending ones, as animals find these less threatening.
At present, there are rules governing noise for certain activities that require permits, such as wind farm construction and seismic oil exploration. But there are no general international regulations and no rules about shipping. Ketten and de Jong are calling for better protection for marine life based on our growing understanding of the relationship between human noise and animal behaviour. This is at an early stage, though, says Ketten. For now, awareness is helping. In the US, for example, the military now watches out for beaked whales when using sonar at their bases. When an animal is spotted, they quickly reduce sound levels.
The sounds of the sea are less mysterious than they were a few years ago. But Ketten feels she has a responsibility to identify what she calls the perfect storm of terrible conditions that can lead to disaster – such as multiple ships using loud sonar in an enclosed area where whales are trapped. Post-mortems are not likely to provide all the answers. “We have no relatives coming forward to tell us about an animal’s history,” she says. “We have to piece everything together as best we can.”
Hear today, hear tomorrow
Sea anemones listen out for prey with adjustable ears. Sugars given off by passing tiny organisms like shrimp plankton bind to bundles of hair cells on the anemone’s tentacles, making the hair cells grow. The longer hairs are better at detecting the low frequencies produced by swimming prey.
“It’s a lot like a slide guitar,” says at the University of Louisiana at Lafayette. “When the string is long it’s a low frequency,”
That’s not the anemone’s only trick. The vigorous movement of approaching prey often damages the bundles of hairs. But anemones have the tools to fix them: they quickly secrete a mixture of proteins that repair the bundles in about 4 hours.
The is an example of how indestructible animals can be, says Watson. It was the ability of some anemones to reproduce by ripping themselves in half that prompted him to investigate. He doubts anemones are affected by human noise (see main story). “They are incredibly resilient animals,” he says.
Indeed, Watson thinks humans could benefit from the sea anemone’s toolkit. By isolating the repair proteins, Watson found that he could regrow hair bundles in fish and he has new evidence that these proteins can repair hair cells in mammal ears, too.
Unlike fish, mammals don’t have the ability to repair their sensory cells, so exposure to extreme noise can lead to permanent damage. Since the proteins need to be injected before hair cells die, Watson thinks they could be given to us before we are exposed to loud noise as a form of protection against hearing loss.
This article appeared in print under the headline “Ocean commotion”
