The subtropical sea surrounding Hong Kong is a noisy place: A flow of massive freight ships chug through constantly; high-speed ferries jet time-pressed businesspeople to meetings in towers; and the coastal development is never-ending. It is one of the most densely-urbanized areas on the planet.

Meanwhile, the surrounding Pearl River Delta is home to somewhere around 2,000 Chinese white dolphins (Sousa chinensis), likely the world’s largest discrete population. But they are declining every year due to “severe human disturbance,” according to the WWF, a conservation group.

And the number within Hong Kong—mostly inhabiting the waters south of Lantau Island—has dropped by over 80 percent in the past 15 years, according to WWF.

“The dolphins are being suffocated by man-made noise,” says Doris Woo, project manager for cetacean conservation at WWF Hong Kong.

Since 2016, Woo and her team have been carrying out acoustic monitoring at around a dozen locations in the area and found that noise disturbances—a cacophony of buzzing propellers, submarine drilling, industrial trawlers, and more—have shrunk the animals’ communication range by up to 45 percent.

A short respite

The dolphins are sometimes known as the “giant panda of the sea,” and inhabit shallow, estuarine waters near shore. They face threats such as prey depletion, habitat loss, water pollution, and ship strikes, but a growing body of research into the damaging effects of noise pollution has set alarm bells ringing.

These animals rely on sound for feeding, socializing, and navigation, and are sensitive to loud noises, which can cause hearing loss or even death. As a result, the two cetaceans local to Hong Kong, Sousa chinensis and the Indo-Pacific finless porpoise (Neophocaena phocaenoides) have been rated “vulnerable” on the IUCN Red List since 2017.

Research by the WWF in collaboration with Seamar, StylesGroup Underwater Acoustics, the University of Victoria, and Oceanway Corp found that during the pandemic—when ferry traffic halted—the Chinese white dolphin’s levels of foraging jumped, seen during 70 percent of researchers’ observations—up from 8.5 percent before. Socializing rose nearly four-fold.

Similar research in New Zealand found that during the Marc 2020 pandemic lockdown, ambient sound levels in shipping channels dropped nearly threefold within 12 hours. This increased the communication range of fish and dolphins by up to 65 percent.

“What was surprising was the scale and how quickly it happened,” says Matt Pine, lead author on the study. “The benefits are immediate.”

The bane of noise 

Noise pollution has been overlooked, experts say. A 2021 literature review published in the journal  revealed that 90 percent of the 500 studies analyzed found excessive noise caused “significant harm” to marine mammals, such as whales, seals and dolphins, and four-fifths of fish and invertebrates.

“It’s a matter of urgency we take action on noise,” says Carlos Duarte, professor of marine science at King Abdullah University of Science and Technology in Saudi Arabia, who led the review. “All the focus is placed on climate change, overfishing, plastic, and habitat loss. But really the threat to the ocean soundscape has not been receiving the attention it should.”

Duarte and his team discovered that ocean soundscapes are changing because of declines in the number of “sound-producing” animals and increases in man-made racket. Over the past 50 years, shipping has increased low-frequency noise on major routes 32-fold, they found.

But beyond charismatic species such as white dolphins, the effects of noise pollution could reverberate all across the animal kingdom, says Benjamin Colbert, a researcher at the University of Maryland Center for Environmental Science.

“Clearly there are some species—dolphins, tooth whales, killer whales—that use echolocation, sound for hunting prey, and quieter oceans are important for them,” says Colbert. “But there are so many animals that haven’t been studied at all.” This applies, for example, to nearly all the world’s 30,000 fish species.

In the ocean, sound travels further than light, and is used by everything from krill to stingrays. Humpback whales sing complex mating songs with regional dialects; some shrimps produce a “snap” sound to stun prey; and the Oyster toadfish, the subject of Colbert’s current study, deploys a curious mating call.

Needed change

On a recent ride along with Woo on her small research vessel, she gestures up at the 34-mile-long Hong Kong-Zhuhai-Macau Bridge, the longest sea crossing in the world. The $20 billion structure, which required the creation of two islands and an underwater tunnel, was opened in 2018 to connect three Asian cities as part of the region’s Greater Bay Area. The decade-long construction created a lot of noise pollution, and traffic sends reverberations into the sea.

“This construction caused so much damage and disruption,” says Doris Woo, project manager for cetacean conservation at WWF Hong Kong. “But we’ve yet to fully understand the extent of it. And things are only going to get worse.”

Unless, of course, something changes. The WWF has proposed several noise management practices: construction bans in dolphin habitat areas, nature-based coastal restoration, a crackdown on illegal fishing, introducing speed limits to ferries and a cut in the frequency of journeys. A survey by the Hong Kong Public Opinion Research Institute found that to protect dolphins, people are willing to pay in terms of increased fares and travel time.

Other solutions exist. The electrification of shipping fleets and the rollout of more efficient propellers have already begun. More radically, air bubbles are being used to act as noise barriers for seabed piledriving and fish are being drawn to degraded coral reefs via loudspeakers playing sounds of healthy reefs. And the EU’s regulatory framework sets a world-first goal to cut man-made ocean noise.

But that is easier said than done in Hong Kong, where ongoing development projects include a 93-kilometer submarine gas pipeline and Lantau Tomorrow, a reclamation project to create a 1,700-hectare chunk of land off Hong Kong.

“We’re fighting against the current,” says Woo, as she pulls up an acoustic monitor from the choppy waters of the South China Sea. “But there’s still time.”

Duarte agrees. “Covid-19 provided a compelling, unplanned experiment in recovering marine life,” he adds. “The benefits are almost immediate once the sources of the noise are removed. It will be far less costly to address this now, before more damage is done.”