Listening beneath the waves
To human ears, the deep ocean seems a place of near-total silence. In reality, it is anything but. Sound travels roughly four times faster in water than in air and can carry for hundreds of kilometres with little loss of energy, so the sea is full of noise: the crackle of snapping shrimp, the rumble of underwater earthquakes and the long, low calls of whales, some of whose songs last for hours and are repeated, with slight changes, throughout a breeding season. For many marine animals, which live where light fades within a few hundred metres of the surface, sound is not merely useful but essential. It is how they find food, avoid predators, navigate and locate mates.
Over the past century, however, this acoustic environment has been transformed by human activity. The most widespread source of noise is commercial shipping. Most of the sound produced by a large vessel comes from cavitation: the formation and collapse of tiny bubbles around its propeller as it turns. The resulting noise is concentrated at low frequencies, precisely the range used by many large whales to communicate. Measurements taken in some busy shipping regions suggest that background noise at these frequencies rose markedly during the second half of the twentieth century, roughly in line with the growth of global trade.
Shipping is not the only culprit. Seismic surveys, used to search for oil and gas beneath the seabed, involve firing powerful blasts of compressed air at regular intervals, sometimes for weeks at a time. The construction of offshore wind farms, ironically a cornerstone of efforts to reduce carbon emissions, requires steel foundations to be hammered into the seabed, a process known as pile driving, which produces intense bursts of sound. Even recreational boats, though individually far quieter than cargo ships, can dominate the soundscape of busy coastal waters in summer. Military sonar, though less widespread, has been linked to several mass strandings of whales.
Establishing exactly how this noise affects marine life has proved difficult. Scientists cannot easily observe animals in the open ocean, and it is rarely possible to compare a noisy area with an otherwise identical quiet one. Occasionally, however, events have provided an unplanned experiment. In the days after the September 2001 attacks in the United States, shipping traffic off the east coast of Canada fell sharply. Researchers who happened to be studying a population of endangered whales there found that levels of stress-related hormones in the animals were noticeably lower than at the same time in other years. A similar opportunity arose in 2020, when the pandemic briefly reduced shipping in many regions, allowing scientists to record what some called a 'quieter ocean'.
The evidence that has accumulated points to a range of effects. Some whales have been recorded calling louder, more slowly or at different pitches when ships pass, much as people raise their voices in a crowded restaurant. Others appear to stop calling altogether. Young fish, which rely on the sounds of coral reefs to find a place to settle, can be confused by boat noise, and experiments have shown that some fish exposed to noise are slower to respond to predators. Not every species seems to be affected, however, and the long-term consequences for whole populations remain uncertain.
Unlike many forms of pollution, noise has one significant advantage: it disappears almost as soon as its source is removed. This makes it, in principle, one of the more tractable environmental problems. Ships can be fitted with quieter propellers, or simply made to travel more slowly, and some ports now offer reduced fees to vessels that slow down or meet noise targets. At wind-farm sites, curtains of air bubbles released around the foundations during construction can absorb a considerable proportion of the sound. International guidelines on reducing underwater noise from ships have also been published, although they are voluntary.
Whether such measures will be adopted widely enough to make a difference is less certain. Quieter technology often costs more, and there is no binding international limit on how much noise a ship may produce. Yet the direction of the research is clear. For decades, the ocean was treated as somewhere sound simply vanished. Scientists are only beginning to understand how many creatures were listening.