The worrying sound of silence

Timothy Lamont's first dive into the Great Barrier Reef in Australia eight years ago was an unforgettable experience — but not a happy one. Lamont, then a PhD student at the University of Exeter, U.K., and his teammates were eager to hear the lively orchestra of fish schools in the coral ecosystem. Instead, they found a starkly quiet seascape, far from the vibrant world they'd imagined. "I remember swimming around and just feeling so empty inside, and so defeated," Lamont, now a marine biologist at Lancaster University, recalls.

Typically, the end of a dive is filled with excited chatter as people share their experiences after being unable to speak under water for hours. This time, everyone quietly unpacked their gear and drove home in silence. The only thing they carried back with them was a recording of the soundscape. Comparing it to earlier data revealed a grim reality: the reef was dying.

As the reef degraded, the once-vibrant sounds of marine life faded. "The animals responsible for the noise had died or moved away," Lamont explains. This silence discouraged juvenile fish, which rely on reef sounds to find habitats, creating a cycle of decline. The experience left them asking: can a dying reef be restored, or is its silence final?

Healthy oceans are filled with vibrant sounds. Marine animals communicate, make friends, sing, mate, and navigate using sound. This natural orchestra reveals stories about ocean health and its changes. Once, there were only two types of sounds: biophony, or sounds produced by living organisms, and geophony, the natural sounds of the Earth. Over time, marine life adapted to these two sources of sound. However, human-made sounds, or anthrophony, introduced disruptive noises, disturbing marine life that caused the extinction of many species. For ocean scientists, sound is crucial for understanding the sea. Acoustic data offer insights into soundscape quality, species distribution, and community composition, especially when visual methods are not feasible.

Hydrophones allow scientists to study underwater ecosystems without disruptive noises, points out Bishwajit Chakraborty, a former marine scientist at the National Institute of Oceanography (NIO), Goa. Chakraborty has studied the Arabian coastal soundscape for over two decades with his teams. In 2023, in collaboration with Goa's agriculture department, NIO researchers deployed hydrophones 65 feet deep near Goa's Grande Island to listen to reef ecosystems. The devices recorded hundreds of sounds, from soniferous fish calls and snapping shrimp to boat rumbles. Using machine learning (ML), the team identified fish choruses, including those of groups such as Sciaenidae and species such as Terapon theraps. They reported in a study (bit.ly/Biodiversity-assessment) that plankton-eating fish synchronised their calls with lunar phases. The researchers were also intrigued by an unidentified buzzing sound, similar to that captured off North Carolina's coast. "Unknown sounds offer insights into soundscape richness. Understanding these sounds helps decode animal behaviour," Chakraborty adds.

As global biodiversity declines and climate change intensifies, documenting ocean sounds before they disappear is vital, the study emphasises. "Acoustic monitoring will enable us to know more about the climate and its related long-term changes in the physical and biological condition of the underwater environment/organisms," Chakraborty tells Shaastra. For example, he says, it is important to conduct acoustic studies on coral reefs, the hub of underwater biodiversity, to know the state and health of the reef and its inhabitants.

The varied whistles, squeaks, and chatters of marine species tell their own stories. Marine scientist Shyam Madhusudhana, who was a Research Associate at NIO in 2018, collaborated with Chakraborty to analyse acoustic data collected near Grande Island, where they identified Arabian Sea humpback whales. Globally, whale songs differ, with Arabian Sea whales showing unique patterns. Madhusudhana, now a Research Scientist at Curtin Mauritius, explains that humpback whales' notes are like "social sounds". Their songs consist of repeated sound patterns — akin to letters forming phrases, then themes, and ultimately full songs that last up to 20 minutes. While eastern and western Australian whales have distinctive songs, researchers observed western Australian whales' tunes in the eastern population, suggesting migration or song sharing. However, these whales are non-migratory, which led the scientists to wonder if the Arabian whales learnt new sounds from nearby groups or developed them independently.

In another study (bit.ly/Fish-activity), Chakraborty and his collaborators show that marine life, especially non-mammalian life, is directly impacted by a change in temperature, climate conditions, etc., because they are mostly ectothermic or cold-blooded, which means that they regulate their body temperature based on the surrounding temperature. The team observed that as temperatures rise, fish vocal activity increases. This is a sign of elevated metabolic rates and could lead to early ageing and premature death. "In such cases, climate change is likely to impact these organisms, altering their behaviour patterns," he says.

The impact of climate change became apparent when researchers observed Antarctica's snowy landscape fading into a less pristine and harsher environment. Using the region's soundscape, researchers established the relation between the presence of ice and the behaviour of habitats. "We use these data to look into the vocal presence of Antarctic seal species," says Ilse van Opzeeland, an acoustic ecologist at the Alfred Wegener Institute in Germany.

Opzeeland and her team anchored a network of 20 hydrophones in the Antarctic region. They acoustically monitored the vocal patterns of Antarctic seals' calls during the Austral summer mating season in 2022, following a similar study they'd conducted and published in 2016. Her team compared eight years of acoustic data for four seal species and correlated it with ice conditions. The data revealed a stable pattern where each species calls in a specific order influenced by ecological factors. For instance, crabeater seals are heard first, but fall silent or leave the area when predator leopard seals arrive, allowing the leopard seals to dominate acoustically. This sequence is consistent each year. The data also show the presence of Weddell seals.

However, one year with unusually low ice levels disrupted this pattern. Almost no seal calls were recorded, suggesting the seals had likely left the area. Additional data confirmed no significant ice patches nearby, reinforcing the absence of vocal activity. Opzeeland points out that this lack of acoustic presence signals a disruption in mating and reproduction. She stresses that if such low ice conditions persist, they could profoundly impact the seals, as they rely on stable ice for birthing, nursing pups, and mating.

"The biological world underwater has evolved to use sounds for everything," says Madhusudhana, stressing that studying ocean soundscapes is crucial for protecting ocean habitats from anthropogenic disturbances. For example, the excess movements of large ships and boats destroyed the natural life of the North Atlantic right whales. It is estimated that there are now only about 350 such whales because of low reproduction rates. Researchers are using underwater recorders to detect whale sounds and relay alerts to port authorities, instructing nearby ships to reduce speed, minimising the risk of fatal ship strikes.

Researchers use autonomous underwater vehicles to collect soundscape recordings from nearby hydrophones and relay the data to larger ships, allowing for real-time monitoring. Recent advances in recording technology have made data collection more accessible and affordable, with low-cost underwater recorders. This has led to a surge in data collection by research institutions globally. Additionally, artificial intelligence and ML techniques help analyse the ocean's acoustic data.

The Great Barrier Reef faces a tipping point due to human impact, as studied by Lamont and his team. He proposed restoring coral reefs by broadcasting healthy reef sounds using underwater loudspeakers (bit.ly/Reef-restoration). On a tight budget, Lamont and a friend improvised a device, using a baked beans can as a circuit board, a motorcycle battery for power, and a child's pool ring for flotation. They played recordings of a vibrant reef taken six years before the damage.

In a two-year experiment (2017-19), the team set up artificial reef patches, broadcasting either degraded or healthy reef sounds. Over six weeks, patches playing healthy sounds attracted twice as many fish, showcasing the potential of soundscape restoration. "Seeing a thriving fish community form before my eyes was amazing and deeply encouraging," Lamont says.

Installing soundscapes of healthy reefs to revive coral ecosystems is gaining traction. Researchers at the Woods Hole Oceanographic Institution, U.S., have been exploring the potential of these sounds as a tool for reef restoration. "Acoustic enrichment methods, while promising, are in their infancy, and effective applications will require considerable additional research," their study, published in October 2024 (bit.ly/Coral-sound), states.