Whoops, croaks, groans and growls are the sounds of a healthy coral reef

Researchers used a new tool to understand how marine life was returning to a new reef and captured a rich soundscape of noises made by fish and invertebrates. Their underwater microphones recorded sounds, including many they'd never heard before, including whoops, croaks, groans, growls, even sounds akin to raspberries, produced by marine life.  

Central Indonesia, in particular near the island of Sulawesi, is home to an area known as the "coral triangle" and includes spectacular reefs rich in biodiversity. Sadly many of these reefs have been damaged or destroyed by destructive dynamite fishing, a practice that uses explosives to kill fish that then float to the surface.

Tim Lamont, a marine biologist from the University in Exeter in England, described one of those reefs to Quirks & Quarks host Bob McDonald. 

"It was absolutely hammered," he said "All that's left is the skeletons of old dead corals blowing around on the bottom of the seabed."

Reef stars and heroes

When the reefs are that badly damaged, it is almost impossible for them to recover naturally as waves and currents trundle loose fragments of coral over the seabed, making it difficult for new corals to settle and grow.

That's led to restoration projects, like the one led by the Mars Coral Reef Restoration, sponsored by the Mars food conglomerate. Together with help from local communities living on the islands around the reefs, they developed a system for stabilizing the seabed and encouraging the rapid growth of new coral.

"It is a modular metal frame that they call a 'reef star,'" said Lamont. "It's a hexagon about the size of a small coffee table. They fit together really well so you can put loads and loads of these hexagons out. They stop everything from moving around and they create a strong platform on which coral can start to grow."

Within a couple of years, what was once barren and lifeless was transformed into an almost complete coverage of coral. 

'Choral' reef

But restoring the reef is only one part of the full recovery. It was only when other marine animals — fish and invertebrates — return to the reef that the ecosystem is recovered.  However, monitoring the return of life to the reef using traditional methods like diving and netting is labour intensive and invasive. 

In a new study Lamont and his colleagues tested whether they could monitor the recovery of the reef with sound. Using hydrophones (underwater microphones), Lamont and his colleagues recorded the reef 24 hours a day over several weeks in two consecutive years.

"What we found to our delight was that the restored reefs sounded almost identical to these very healthy reefs nearby," Lamont said. "And completely different from the degraded reefs that they had used to be like."

Quirks and Quarks0:13Sounds like these mean fish have returned to the reef

Some of these sounds are new to scientists, and they don't know which creatures are making them.

What they heard was a full range of sounds mostly made by fish — some they were familiar with and some they had never heard before. Among the familiar sounds was a sharp tapping produced by clown fish banging their teeth together. Another was a high-pitched whoop sound made by a tiny amber damselfish defending its territory. But others were both mysterious in terms of who was making them, and what they mean. 

Quirks and Quarks0:14The diverse sounds from a healthy reef

Sounds from a restored coral reef in Indonesia

The sound of recovery

Regardless, Lamont was just happy to hear those sounds because they suggested that not only had the coral recovered, but so did the ecosystem the reef supports.

"When you hear the busy, bustle of a very busy soundscape like this, you're hearing a reef that is healthy and full of animals and functioning as it should," he said. "Reefs that are loud and diverse and have lots of different sounds are more likely to attract the next generation of animals toward them."

Lamont said the lesson to be learned from the success of this reef project is that others around the world can also be restored and their ecosystems returned to normal. But he cautions that such restorations will only work as long as the original problem is removed forever. Coral reefs are not to be restored so they can be destroyed again.

"The dynamite fishing in this area is a thing of the past through various interventions," he said. "It is no longer a threat to these rebuilt reefs, which is exactly as it should be. Otherwise, we're going to be wasting our time and our effort and our resources, trying to restore systems that will just get degraded again."

Produced and written by Mark Crawley.