When global lockdowns brought much of the world to a standstill in 2020, international shipping slowed dramatically — and, with it, the constant rumble of ocean noise. For the first time in decades, scientists were able to hear the sea as it might once have sounded, dominated not by engines but by the crackles, chirps and rhythmic pulses of marine life.
“Imagine thousands of individual sounds building into one great underwater orchestra,” says Steve Simpson, a marine biologist at the University of Bristol. For years, many assumed the deep ocean was largely silent, but early-20th-century hydrophones revealed a world alive with acoustic signals. Fish, shrimp, whales and countless other species rely heavily on sound to communicate, mate, hunt and survive.
As human-made noise has intensified, researchers have long sought ways to measure the effect on marine behaviour. Since 2010, scientists had been debating how they might temporarily quieten the seas to study the impact. Then Covid-19 unexpectedly delivered the opportunity.
With cargo traffic and tourism slashed, global maritime trade contracted by around 4%, and in some regions vessel numbers crashed by as much as 70%. Models estimate a roughly 6% drop in worldwide shipping noise — enough to create a global natural experiment.
Peter Tyack, professor of Marine Mammal Biology at the University of St Andrews and a founding member of the International Quiet Ocean Experiment (IQOE), says the pandemic achieved what would have otherwise been an impossible task. “Instead of adding noise to test reactions, we needed to remove it,” he explains. “In 2020, the world effectively flipped the switch for us.”
Hydrophones positioned across the globe captured the transformation. In New Zealand’s Hauraki Gulf Marine Park, one of the country’s busiest waterways, underwater noise fell to a third of normal levels within hours of the national lockdown. Fish and dolphins suddenly found themselves able to communicate far more effectively — some dolphin calls travelled over a kilometre farther than usual.
Sound is the ocean’s primary language. Two-thirds of the world’s fish species are estimated to make noise, from the deep-channel songs of whales to the explosive snaps of shrimp whose claws create shockwaves loud enough to stun prey. Yet these natural soundscapes are increasingly drowned out. With 90% of consumer goods transported by sea and marine ecosystems damaged by climate change, the modern ocean bears little resemblance to its preindustrial past.
Today’s seas are filled not only with the biophony of marine animals and the geophony of waves and weather, but with the constant hum of engines, drilling and construction. This noise can mask essential signals, similar to trying to have a conversation in a packed bar, says researcher Miles Parsons. The unpredictability of vessel noise can be particularly stressful.
Studies have revealed significant behavioural changes. In the Colombian Pacific, humpback whales fed less often and at shallower depths during noisy breeding seasons. Australian research showed migrating whales stayed submerged longer when ships passed nearby, and mother-calf pairs were especially sensitive. Whale-watching boats have also been linked to sharply reduced rest times for southern right whales.
Military sonar poses an even greater threat. It has been tied to strandings of deep-diving beaked whales, many of which show signs of decompression-like injuries and acoustic trauma.
The effects extend to mental health. Chronic noise can cause stress in whales and fish alike, reducing lifespan and reproductive success. On coral reefs, noise from passing boats has been shown to make male reef fish more anxious and less attentive to their nests. Experiments on the Great Barrier Reef found that Ambon damselfish spent significantly more time keeping watch and less time caring for their eggs when exposed to engine noise.
For Simpson, the brief quiet of 2020 offered something precious: a rare opportunity to “eavesdrop on another universe” and share it with the world.
