Soundscape ecology is a non-invasive method to monitor the diversity and health of ecosystems, but the process behind it is still very time-consuming.

By: Charlotte Hu | Published 3:04 PM, December 9, 2021

When a reef fish is born, it is immediately drawn into the open ocean, where life is full of uncertainty. But if it is alive and matures from larva to larva, the coral reef will always call it to continue its reproduction cycle. 

There are many ways to get fish out of the coral reef and return. These underwater communities are very noisy places full of sound of marine animals and invertebrates. Given that the sound travels far under the water, small fish can judge the quality of coral reefs through traffic reports. A good-sounding environment also tends to attract more new animals. 

A healthy coral reef has a rich soundscape. "Many invertebrates and fish make weird and wonderful sounds for all sorts of weird and wonderful reasons. When you record, you will hear all these pops, hums, trills, and whines," Said Tim Lamont, a marine biology researcher at the University of Exeter. On the other hand, degraded coral reefs have fewer creatures around them, so they are much quieter. "If you are in the ecosystem restoration business, it is a very good thing to be able to create coral reefs that sound great," Lamont explained. 

Lamont and his colleagues have always been interested in studying the relationship between sound and underwater creatures. But this is by no means a simple process. Usually, you need to manually clean up the background noise in the recording, annotate the audio, and mark all the different sounds. Although attempts have been made to automate this task, there are still many technical limitations. 

This became apparent in the recent cooperation with Mars Inc., which proactively contacted and asked them to use soundscape ecology to monitor the progress of coral reef restoration. (Chocolate and pet food companies have been working with scientists and local communities to restore damaged coral habitats around the world as part of their larger efforts to offset some of the negative environmental impacts. Mars also partially funded this recent soundscape Project.) Corresponding research on this effort published this week in the Journal of Applied Ecology found that according to its voice, Indonesian coral reefs damaged by blast fishing have been well restored after the restoration work. 

Coastal communities rely on coral reefs for food, etc. When these structures collapse due to human activities (such as fishing with explosives), they can have a devastating effect on the people who depend on them for their livelihoods. Since the natural recovery of these coral reefs is slow, restoring the corals that form the basis of these ecosystems can bring back fish and other marine life. However, it is not always easy to determine whether habitat restoration needs to be carried out. "Planting a few corals is different from bringing back the entire ecosystem," Lamont said. In addition to checking whether corals regenerate, ecologists must also test whether new coral reefs can support marine life, suppress wave energy, control carbonate budgets, and provide food for coastal communities. 

This is where the soundscape analysis comes in. They are a promising indicator of the diversity of the overall ecosystem because they can detect more organisms than images and visual observations; for example, biologists can hear hidden or well-disguised fish. In addition, with the help of sound, experts can monitor the habitat around the clock. "You can measure different aspects of the soundscape," Lamont said. "You can measure its complexity, loudness, degree of change over time, or the degree of change of the pass frequency band at different pitches."

[Related: What underwater sounds can tell us about the condition of coral reefs]

In order to collect all the necessary data, the team installed underwater microphones or hydrophones around the degraded, healthy and restored coral reef. They recorded the sound of coral reefs during dawn, dusk, midnight, noon, full moon, and new moon for two years. "We want to create a very good picture," Lamont said. They found that although restored coral reefs "sound different from healthy coral reefs, they sound very similar" and "very different from degraded coral reefs." The planted corals are full of marine life and tell biologists that many small animals have agreed to build. 

Then there is the tedious part of the Lamont team: sitting in the sound room, separating percussions, grunts, croaks, growls, and shouts. This is similar to unraveling a single instrument from a complicated orchestral arrangement. 

"This is a very time-consuming science. Through this research, I spent a few months listening carefully to all these recordings with headphones," Lamont said. "It's numbing sometimes."

The team is now trying to automate this process by "making computers do the same work," Lamont said. But delegating to a machine is a daunting task. Because the coral reef ecosystem is very busy, there are many background sounds that can bias the analysis toward rough wild animals. "When you listen to these recordings, you hear a lot of invertebrate noises [such as] shrimps," which sounds like static crackling or frying bacon, Lamont said. In fact, these sounds are so loud and common that during the Second World War, the army used to hide the submarine on the coral reef because the shrimp effectively masked the sound of the submarine. 

The existing calculation technique used to measure soundscapes, also known as the acoustic index, is mainly designed for terrestrial habitats, such as listening to birds or bats in the forest. Nevertheless, Lamont saw many similarities between forests and coral reef communities. On the one hand, different animals seem to be active during a certain period of time, which can help scientists classify sound libraries by space, time, and frequency. "We have tried to apply these indexes underwater," Lamont said. "But of course, sometimes there are fundamental differences between the types of soundscapes you get in different habitats, so they may not work properly."

[Related: Birders watch: Cornell University’s Merlin app is now a one-stop shop for bird identification]

Scientists studying forest soundscapes have proposed creative solutions to build machine learning algorithms that do not require recording stations in the woods. Lamont pointed out that in the PNAS paper published in July last year, researchers from Imperial College London, the University of Sydney and Cornell University used Google's AudioSet data training algorithm to identify the sound differences between individual forests. 

The Google Voice Compilation, which is a mixture of human speech, music, and machine noise, first taught the algorithm to distinguish various types of noise. Then when the system is applied to the forest, it can classify the sounds in the forest. The author writes that one day it can be used to detect irregular activities such as illegal logging and hunting.

Lamont imagined that he could reuse similar algorithms to classify the ocean sounds he had to analyze. 

Another more classic way to power useful machine learning algorithms for submarine soundscapes is to collect large amounts of data. All parts of the world have been working hard to build ocean sound sets, but the cost of equipment may be a limitation. For example, the price of a high-quality hydrophone with a large-capacity memory card is approximately US$3,000.

"They are usually very expensive, and if we want more than just well-funded scientists to be able to do this, that's a problem," Lamont said. In October, he and his colleagues published a study on Ecological Indicators, which found that the audio quality of Go-Pro cameras (priced at about $500) was in many cases comparable to the data they obtained using hydrophones. 

[Related: These free-floating robots can monitor the health of our oceans]

"Trying out these low-cost recordings has opened the door for more people to participate," he said. "This will allow us to collect more data, which will be fed into these automated analysis techniques so that we can [easily] obtain useful information." 

Marine monitoring technology has made great strides. About ten years ago, the hydrophone was running on a tape reel and had to be connected to a cable hanging from the ship’s side. A non-waterproof recording station was installed on the ship. They are now completely wireless and can be placed on the seabed for weeks or months at a time-until data needs to be collected. 

"We can make long-term, high-quality recordings underwater, which is a very new thing," Lamont said. "That's part of the reason why we keep discovering all these new things that no one has ever recorded before." He recalled that when he listened to the recording from Indonesia, he heard completely incredible reef noises, including the grunting of soldier fish. , The scraping of the parrot fish and the cry of the damselfish. But there are some sounds, such as the "laugh" that usually appears at sunrise, and he can't be completely placed on a specific species. 

"There is an interesting mysterious element in it," Lamont said. "Considering that this field is still young, its prospects are exciting."

Charlotte Hu is the assistant technical editor of Popular Science. She covers social internet, artificial intelligence, privacy, security, human-computer interaction, digital economy and general technology news. She has a master's degree from Columbia Institute of News and Information, and her work has previously appeared in GenomeWeb, Business Insider, and Discover magazines. Contact the author here.

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