The year the sea fell silent

During the lockdowns of 2020, global shipping was severely restricted and ocean noise pollution all but ceased. Instead the songs of fish filled the sea.

Crackles, snaps, pops and clicks – that is the noise of a thriving underwater soundscape.

"All of the individual sounds, when they add up, become this orchestra – thousands of different instruments all playing at the same time," says Steve Simpson, a marine biologist at the University of Bristol.

For decades, many people believed the ocean was silent, limited by what our own ears could detect beneath the surface. But in the early 20th Century, when hydrophones – underwater microphones – were introduced to monitor ocean acoustics, we discovered that marine species use a huge variety of sounds.

As human-produced sounds become more prevalent underwater, important animal behaviours such as communication, breeding and feeding may be disrupted. So, since 2010, scientists have been wondering how they could quieten the seas, as a testbed for the impacts of noise on marine species. 

Then a pandemic happened – and we were given a rare moment of quiet. 

As shipping and commercial boating came to a halt, we could finally hear what the ocean sounded like without the hubbub of manmade noise pollution. Researchers are still studying this year of quiet, investigating the extent to which louder oceans harm marine species.

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Peter Tyak, a professor of Marine Mammal Biology at the University of St Andrews, was a founding member of the International Quiet Ocean Experiment (IQOE), a global scientific research programme.

"The core idea of the International Quiet Ocean Experiment was that rather than adding sounds and seeing what happens, maybe you have to go to places and reduce sounds," he says. However, turning the ocean's volume down on a global scale proved to be a costly, logistical nightmare. 

But in 2020, Covid-19 put the brakes on shipping and tourism, leading to a 4.1% decrease in global maritime trade. In some economic zones, marine traffic hit lows of 70%, with models estimating a 6% reduction in shipping noise energy worldwide.

This allowed for a natural worldwide experiment of the impacts of sound on marine life.

Across the world, scientists listened to the ocean soundscape before, during and after lockdown, using 200 ocean hydrophones that were already in place around the global ocean.

When New Zealand entered lockdown on 26 March 2020, boat traffic in the Hauraki Gulf Marine Park – the country's busiest coastal waterway – almost completely stopped. Underwater noise dropped to about one-third of normal levels within 12 hours – allowing the communication ranges of fish and dolphins to increase by up to 65%. For dolphins, that meant their calls could travel around 1 mile (1.5km) further than when hampered by shipping noise.

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"Sound is the primary modality for most animals in the ocean," says Miles Parsons, a researcher at the Australian Institute of Marine Science. And the ocean is a noisy place. A recent study suggested that of the 20,000 fish species estimated to be found in the ocean, over two-thirds produce sound. 

"[Sound] is used for communication, foraging, reproduction, territorial interactions and all sorts of reasons over vast distances," says Parsons. Some animals, such as whales, operate in what scientists call the deep sound channel, a horizontal layer of water in the ocean in which sound can travel for thousands of kilometres.

Other species, such as snapping shrimp, communicate in shallower waters. Snapping shrimp quickly close their claws to produce a loud "snap" sound – creating a bubble that collapses and makes a noise, as a way to stun their prey and deter predators. These noise levels can exceed 210 decibels – louder than a rock concert.

But the ocean is steadily growing noisier – with an increase in anthropogenic noise, as well as contributions of geophysical sources, such as sea ice and storms that are driven by climate change. Ocean noise from human activity has been rising for decades, mostly because of the growth in global shipping. About 90% of consumer goods are carried by sea on huge container ships that run day-and-night across the world.

Plus, degraded and poorly managed ocean ecosystems have altered the marine biophony. With the loss of kelp forests and coral reefs, melting glaciers, overfishing and underwater mining, essential habitats for sea life are stripped away – reducing the abundance of noise-producing animals.

AIMS/ Daneil Estcourt

According to Simpson, the sounds of today's ocean are very different from those of preindustrial times. "The ocean soundscape used to be made up of the biophony – the animal sounds – and the geophony – the sounds of the Earth, rain, wind and currents. But now it's the anthrophony as well, for example the sound of motorboats on coral reefs." 

Anthropogenic ocean noise – for example, from vessels or construction – can mask animal noises, preventing them from communicating with one another, just as it would above the surface. "It's like being in a bar," explains Parsons. "If there's no one else in the bar, you can probably hear your friend. But if the bar is full of people, the distance over which you can hear each other is a lot smaller," he says.

It's not just the frequency or intensity of the noise, it's also the unpredictability of the sound's source that can be disruptive. "If we live in a city, we're not surprised by traffic noise, but if you live in the ocean and a boat suddenly drives over the top of you, you wouldn't be expecting it," says Simpson.

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In the Colombian Pacific, studies found that during the breeding season, humpback whales fed from the seafloor less frequently and at shallower depths as ship noise levels increased.

In Australia, researchers found migrating humpback whales spent more time underwater and extended their dive durations when vessels were nearby, and calf pods were noticeably more sensitive to vessel presence than non-calf groups. Rest time for both mother and calf southern right whales, meanwhile, was drastically reduced when whale watching vessels were nearby.

In addition, military sonar – a technology that detects and tracks underwater objects by emitting sound pulses – has been linked to whale strandings.

Beaked whales, thought to have stranded due to the use of military sonar, showed common injuries including "bubble lesions", similar to decompression sickness symptoms, and acoustic traumas. Some of the whales were also found to have suffered haemorrhaging around their ears and brains, which could have affected their hearing.

Just like humans, areas heavy with noise pollution can have a negative impact on the mental health of whales, too, resulting in conditions such as chronic stress.

Stress can shorten lifespans and reduce reproductive outcomes of fish as well, says Parsons. For example, sound on coral reefs can have a negative impact on nesting reef fish – increasing stress levels, reducing the ability to look after eggs, and masking predator-prey communication, says Parsons. In tests on the Ambon damselfish, a reef fish where males guard the eggs, motorboat noise made the males more cautious. On the Great Barrier Reef, Australia, they showed 34% more vigilance and spent 17% longer staying alert than fish in quiet conditions.

More like this:

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• The people eavesdropping on the ocean

The study found "detrimental impacts" of motorboat noise on some aspects of parental care in fish – for example less feeding, less interaction with offspring, and altered defensive behaviour.

The data gathered as part of the IQOE, during New Zealand's shipping ban of 2020, indicated even small changes in vessel activity can greatly affect underwater sound – showing how noise from small boats, as well as large ships, can limit how marine animals hear and communicate.

AIMS/ Jo Hurford

Understanding the ways marine life uses sound, though, has also meant we can use underwater sound as a tool for ocean restoration. By playing recordings of healthy reef soundscapes through underwater speakers, scientists can attract fish and other marine life back to damaged reefs, helping them recover faster and become vibrant ecosystems once again. "We false advertise that they are thriving neighbourhoods," says Simpson. 

One outcome of the IQOE was the establishment of the annual World Ocean Passive Acoustics Monitoring (WOPAM) Day. Started by Parsons and Simpson, on 8 June 2023, the initiative captures underwater soundscapes – from the canals in London to the ponds in France.

"Like many good ideas, WOPAM started over a beer with Miles at a conference in Germany," said Simpson. "We put a call out, hoping that we might get a reply from somebody, and we ended up with over 200 replies. In 2023, it became the first year where we had hundreds of people around the world, sharing their recordings, sharing what they were finding, sharing why they do their research, and sharing the joy of listening to the ocean.

"We realised that it was just this magic moment where we get to eavesdrop on another universe and share it across the world."

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