There are other sources of intense sound in the sea. Physicists working with U.S. defense agencies in the 1990s came up with the idea of using sound transmission to measure the temperature of the sea and thereby track global warming. The idea called for 195 decibel pulses of sound to be produced three thousand feet down between Hawaii and California for twenty minutes every four hours… continuously. Test transmissions were made between 1996 and 2006. Whose brilliant idea was this? If noise traveled as far above the sea as below, and with as little attenuation, we would have dismissed the scheme after little more than a moment’s contemplation. But physicists are sometimes inclined to forget that there is a living world. In one spectacular example, in 2005 they sank an array of highly sensitive hydrophones—instruments that record underwater sounds—into the Mediterranean off the coast of Sicily.10 They hoped to listen for the sounds that tiny subatomic particles called neutrinos might make as they smacked into water molecules. Because neutrinos are vanishingly small, the chances of that happening are low, even in a huge volume of sea. More than a hundred physicists collaborated on the project, but the possibility that background noise might interfere was not even mentioned until late in the day. Everyone assumed that the deep sea would be virtually silent. Of course, there was far too much noise, even before boat traffic was factored in, to detect the sound of a colliding neutrino. Instead, the hydrophones proved adept at pinpointing sperm whales using echolocation clicks to hunt squid in the liquid darkness. At least the physicists discovered something: There were more whales around than anyone had suspected.
The tests proved the viability of using sound as an acoustic ocean thermometer, but the method remains controversial. Research showed that humpback whales seemed to avoid the sound sources, but nobody can say for certain what the long-term impacts of these sounds might be.11 Since physicists still have plans to create a web of sound throughout the world’s oceans to measure water temperature, this question needs an answer.
The loudest noises in the sea cause the most dramatic effects and have attracted the most concern. But the rise in background noise from boat engines and now wind farms is probably more significant. Sound is one of the most important means of communication for creatures underwater. In the clear water of a coral reef you can see 150 to 200 feet at best. Usually visibility is far less; in the dark and muddy waters of estuaries it can shrink to under three feet. Because sound travels so far and fast, it is the best way to communicate. Many marine animals therefore have excellent hearing and use sounds to visualize their surroundings, to avoid predators, to attract mates, and to find prey. Background noise can mask important sounds and make life more difficult and dangerous.
Whales and dolphins, as their amazing vocalizations suggest, will likely number among those worst affected by noise. The din of boats is shrinking their world. Before we invaded the sea with our noise, fin whales could hear each other over distances of at least six hundred miles. It seems that only male fin whales produce very loud calls, perhaps to find and attract mates over these vast distances.12 Given the spectacular decline in whale populations caused by two centuries of commercial whaling, it must be much harder now to find mates. Many kinds of whale have nowhere near recovered from whaling, so long-distance calls could be a vital way to reach out to receptive mates. Today noise from ships and wind farms fills the sea with a low-frequency rumble that masks their calls beyond distances of just six miles. Because area scales as the square of distance, the space over which they can contact one another has shrunk ten thousand times.
Sarasota’s dolphins are famous for more than just the chemical toxins in their bodies. They know one another by “name.” Each dolphin has a distinctive whistle that others recognize.13 When boats pass by, the dolphins whistle to each other more often and move closer together, which suggests they worry about traffic. Several have been badly injured by boat strikes, so they have good cause. Since boats pass within three hundred feet of a dolphin roughly once every six minutes during daylight hours, they could interfere with the dolphins’ ability to feed, breed, and look after their young.14 Another explanation for the more frequent calls when boats are about is that the deafening sound of engines makes it harder to stay in touch.
Fish exposed to boat noise or other loud sounds suffer stress. Smallmouth bass live in the lakes and streams of North America. The hearts of captive fish beat faster when they were exposed to the sounds of boats, and engine noise produced a stronger and more prolonged response than the sound of a paddled canoe. Several kinds of freshwater fish exposed to boat noise have been found to react by producing more cortisol.15 This is the fight-or-flight hormone that increases blood sugar levels. It is found in a remarkable array of species throughout the animal kingdom, from fish and birds to ourselves. Animals release it in response to acute danger to mobilize energy, sharpen thinking, and dull pain. But they also produce it in response to longer-lasting stress. Wired executives and harassed mothers have raised cortisol levels. This is when benefits turn to costs. Cortisol can wreak havoc within our bodies. It raises blood pressure, increases risk of heart attacks and strokes, and suppresses the immune system. Fish and other animals in noisy, stressful places may also suffer health problems.
In an experiment that entertained me mightily, Greek researchers played Mozart to captive carp for hours every day over several months.16 Fish grew faster in the presence of Mozart than with the usual background noise of pumps and filters. The researchers thought that Mozart might have soothed away their stress. Whether or not you believe this, I am sure the composer himself would have had a laugh.
Wind farms are a relatively new source of sound, but given the push for green energy, one that is set to become ubiquitous throughout large regions of shallow sea. Pile-driving noise during construction is loud enough to damage the hearing of dolphins or other marine mammals that stray within a thousand feet. During operation wind farms produce a low-frequency underwater rumble whose volume depends on wind speed. These noise levels are probably similar to those produced by ships, but the difference is that they will affect the same area continuously. There has been very little research on how marine life might respond. On the one hand, it might render large areas of habitat unsuitable for sensitive animals. On the other hand, the footprints of wind farms often exclude fishing and so might benefit some fish and shellfish. It is too soon to tell for sure.
Fish and many other forms of marine life use sounds to visualize the world around them. Think of the way that sound enriches your own appreciation of where you are. The tapping on the window tells you it is raining outside. The whirr from the kitchen lets you know the dishes will soon be clean. The sound of footsteps upstairs shows the kids are in their bedrooms, while the light patter from the corridor tells you to expect a hungry cat to come and chance its luck. We may not think of ourselves as being particularly aural creatures compared to bats or owls, but our ears give us a good sense of the world around us.
The noisy underwater world sets the scene for marine animals in much the same manner but with an important difference. The far greater density of water means that sounds move particles in ways that can be felt. Fish have evolved the lateral line organ along their flanks to sense this particle displacement. Like us and other vertebrates, fish have ears with internal “bones” made of crystalline calcium carbonate to hear sounds, too. They share with us the ability to pick out specific sounds from background noise, just as we can hear the voice of a friend through the buzz of a party or tell which instruments are being played in an orchestra. For a time it was a mystery why goldfish, which don’t make noises themselves, had such finely tuned hearing.17 Trapped within their paradigms, scientists sometimes fail to notice the obvious. Clearly, there are benefits from listening to sounds other than your own voice!
The soundscape underwater has been called “acoustic daylight” for the way it reveals things about the world that cannot easily be seen. Larvae of fish from coral reefs must escape the reef for life in open water where t
hey grow for a period of days to months before they are ready to return as fully formed juveniles.18 If they cannot find a reef at this point, they die. Many larvae follow sounds they might have heard before they hatched from the egg to guide them back when the time is right—the whisper of breaking waves, the crackle and rasp of shrimp and crab, the scrape of parrotfish beak on coral. Even some coral larvae possess the ability to hear reef sounds.19 It is easy for us to picture how animals make sense of these sounds, but particle motions from underwater sound are harder for us to comprehend. Different sources probably produce distinctive particle motions that paint the surrounding scene with details we can only imagine.
Most marine animals use sound to tell when predators are about: the whoosh of a school of baitfish fleeing the thunderclap of a predator’s strike; the crack of a snail being crushed; the pistol shot of a snapping shrimp (actually, their claws move so fast to smash prey they produce a sonic boom!20). Some also call for mates, or use sounds to weigh the risk of a challenge for territory. Drums, as their name implies, are fish that form noisy spawning aggregations in estuaries, thumping out the rhythm of procreation on their swimbladder instruments. Red hind groupers in Puerto Rico surprised researchers by shouting “whoo hooo” as they rushed to spawn.21 Background noise might make it harder for larvae like those of coral reef fish to follow sounds to the right habitat. Already the number of offspring produced by many fish and invertebrates has fallen precipitously because of overfishing. Noise is yet one more way in which the viability of their populations may be compromised. Few other species have been tested to see how important noises are to navigation, but it seems unlikely that only coral reef animals have this ability, so the problem could be widespread. There can be more immediately harmful consequences of background noise. Hermit crabs in the Caribbean, for instance, became less attentive to possible predators when they were distracted by the sound of passing boats.
The underwater world is now awash with noises that evolution has not prepared marine animals for. The maigre is a kind of drum, a lithe fish of metallic gray that lives in Mediterranean estuaries. In the days before intense fishing, it grew nearly as big as the average man, but these days few reach more than three feet. Maigres exposed to the noise of a powerboat in experiments were one hundred times less responsive to the sound of other maigres. Like right whales, the seascape over which they could communicate would shrink ten thousand times when boats pass nearby.
Male toadfish “sing” by vibrating their swimbladders with “sonic muscles” to entice choosy females to their nests and warn off rivals. Lusitanian toadfish are flat-headed and have wide rubber lips that make them look like their heads have been run over by something heavy. They live along the coasts and estuaries of the east Atlantic and Mediterranean, where males set up territories among rock piles from which they glower at one another. There they display to female fish with a buzz called a “boatwhistle,” though a better description would be an extravagant fart. The most appealing males persuade females to leave behind a hoard of perfectly round, golden eggs for them to fertilize and protect. When captive toadfish were played the sound of ferryboats in a 2005 experiment they found it hard to hear one another.22 Someone has turned up the stereo at the party and now it is impossible to hold a conversation anymore. Just as revelers can switch to sign language, there may be other ways that animals can communicate, but at the expense of subtlety and with an increased risk of misinterpretation.
The Pacific coast of North America is home to a relative of the toadfish whose vocal dexterity throws light on the evolution of sound communication in backboned animals such as ourselves. Midshipmen are fish that look a bit like overgrown tadpoles, with round, flat heads and narrow tails. Some are drab, but others blaze as if sprinkled with gold dust. Lines of glowing photophores decorate their bodies with points of light, like Christmas lights. These fish live in the intertidal zone, where males set up mating territories in pools beneath rocks. From these muddy grottos they sing love songs to passing ladies and grunt warnings to other males. Their songs are short on melody and make Tuvan throat songs seem operatic by comparison. Mostly they just buzz, like lawn mowers drifting through summer suburbs, sometimes interspersed with boinnngggs reminiscent of twanged rubber bands. But inside their heads, the part of the brain that controls their songs turns out to be the same part that frogs, reptiles, birds, and people use, suggesting that the ability to “speak” to one another evolved some four hundred million years ago.23 These fish can get so boisterous that they keep houseboat owners in San Francisco Bay awake at night.24 One resident described the sound as akin to the noise of a generator; another said it was like ten electric razors run at once.
The clutter of the low-frequency hum we create in the sea overlaps pretty closely with the hearing ranges of fish and whales. There are signs that some try to compensate. Northern right whales call louder when there is background boat noise.25 The more noise there is, the louder they yell. They also call about an octave higher than they did in the 1950s, probably in an effort to be heard above the low-frequency drone of engines. But there is a limit to how loud you can shout. As noise levels continue to rise, whales will become increasingly isolated. Since there are fewer than four hundred northern right whales left, isolation could seriously threaten their survival. Alternatively, they might be able to compensate by calling during quiet periods or by making longer calls. Killer whales in three British Columbia pods made longer calls when surrounded by boats of whale watchers than when left in peace, which suggests that they struggle to communicate above the clamor.26
Birds try to compensate for noise in similar ways. Great tits in cities sing shorter, faster songs at a higher pitch than those in the countryside.27 Birds next to busy roads often have lower breeding success and are less fit than birds in quiet places. Oven birds nesting in Canada close to a noisy generator couldn’t get their messages across and attracted fewer mates than those farther away.
Noise pollution interferes with feeding. If an animal stops foraging every time a boat bears down on it, it may soon go hungry. It also affects species more directly if they use sound to hunt. Beaked whales hunt by echolocation in the deep sea. They produce a stream of clicks, made by blowing air through a structure in their blowpipe affectionately known as “monkey lips.” The noise is beamed through enlarged lobes of acoustic fat in their heads, and they receive echoes back in the flesh beneath their jaws. Beaked whales are particularly fond of squid. Aside from a thin, horny internal sheath, squid have a similar consistency to water, which makes them hard to detect. I imagine that you would need to listen closely to find a squid in the dark. A Cuvier’s beaked whale recorded hunting when there was background noise had less than half the success others had with no sound interference.28
Some whales and dolphins use echolocation to find their way around. If their hearing is damaged they may be prone to stranding in shallow water, or to entanglement in fishing nets. Experiments with captive bottlenose dolphins exposed to high-intensity sonar showed short-term hearing loss.29 Longer exposure may cause permanent damage. When stranded animals rescued in North America and the Caribbean were tested, a third of rough-toothed dolphins and over half of bottlenose dolphins had levels of hearing loss that would have been judged severe or profound in humans.30 Endangered northern right whales thread their way through some of the busiest shipping lanes on the planet as they migrate up America’s east coast to summer feeding grounds. If noise impairs their hearing, they may more easily fall victim to a ship strike. This is now one of the two main causes of death for these whales, the other being entanglement in fishing gear. Even whales with normal hearing find it impossible to hear one another when ships are closer than a mile or so, which is the case much of the time.
Human noise in the oceans is a relatively recent phenomenon. For most of history the only sounds of any note would have come from the clatter of oars against wood and the hiss and slap of boat hulls as they scythed through waves. That changed in the middle
of the nineteenth century, when the first paddle steamers were launched. But it was in the last two decades of the 1800s that the peace was broken in a major way, as tens of thousands of steamboats were built. The first offshore oil wells were drilled off Santa Barbara in California in the 1890s, starting a raucous rush for oil that shows no sign of letting up. Since then, background noise has grown inexorably, tracking global economic growth. Of goods traded internationally, 80 percent are now shipped by sea. As trade volumes increase, so does noise.31
It would be a mistake to suggest that all creatures in the sea are adversely affected by noise. Some like to gather beneath floating objects, even if they are noisy ships. And although sounds travel much farther in the sea than on land, there are still plenty of peaceful places in the oceans away from shipping lanes, ports, wind farms, and oil and gas fields. But it is hard to escape the conclusion that the rising clamor is but one more stress piled onto life in the sea. If nothing changes in the way we build or run ships, the din beneath the sea will only get louder.
The great majority of noise in the sea is from boats, and there seems little prospect of hushing them for the sake of marine life alone. Motives for developing quieter boats are more likely to be fuel economy or the comfort of the crew and passengers. There are simple ways to reduce ship noise. The loudest sounds come from propeller cavitation. As a propeller spins it creates a low pressure zone behind each blade. The faster it rotates the lower the pressure, until a point is reached where a gap opens up between the water and the blade. As these cavities spin off they slam shut with an audible slap. When propellers turn at low speeds there is no cavitation, so boats run more quietly. A simple way to reduce noise is just to slow down. That is exactly what the world’s merchant shipping fleet has done in recent years to combat the soaring cost of fuel, because cruising slowly is also cheaper. A 20 percent cut in speed can save 40 percent in fuel costs. In fact, many shipping lines have dropped steaming speeds from twenty-five knots to twelve knots, about the speed reached by sailing clippers, the workhorses of international trade and communication in the late nineteenth century.
The Ocean of Life Page 19
