The Great Barrier Reef sounds sick, so baby fishes aren't attracted to it

The fewer fish that return to the reef, the less able the reef is to recover.
The area at Lizard Island, Northern Great Barrier Reef, Australia, has been heavily degraded in the last five years by severe coral bleaching and tropical cyclones. ( Tim Gordon, University of Exeter)
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The Great Barrier Reef is in trouble. Not only do sections of it look bad, but they sound bad too. And this, it turns out, may be yet another barrier to reef recovery. 

In a new study published this week, a team led by Tim Gordon, PhD candidate in a marine biology from the University of Exeter, recorded sounds made by coral reefs before and after they were damaged and tested how the sounds changed the response of fish searching for a reef to settle on. 

What they found was a vicious feedback loop that made a bad situation for the reefs even worse.

What used to be a symphony of the sea has been silenced.- Tim Gordon, University of Exeter

"What used to be a symphony of the sea has been silenced," says Gordon. But it's the knock-on implications he's concerned about, "These fish that usually hear their way home based on this sound, we've shown now that this sound [of the damaged reef] is less attractive to new fish coming towards the reef. So it suggests that perhaps those fish will be less attracted [to the reef] fewer fish will arrive on the reef."

That could spell trouble, not only for the reefs and the fish that live there, but also for the billions of people around the world that rely on them for food and their livelihoods.

A tourist swims on the Great Barrier Reef in this undated file picture. (Great Barrier Reef National Park Authority/Reuters)

"Coral reefs are some of the most beautiful and the most valuable ecosystems that we have in the world," says Gordon. "Not only is there this incredible diversity of color and life, but also they provide livelihoods for millions of people around the world - both in terms of tourism and then also in these remote Pacific islands and islands all around the world there are communities that rely on the protein from coral reef fish. They're such an integral part of their diet. If the fish go, if the reefs go, those communities will have nothing."

Sound of a healthy reef compared to a degraded one

Gordon and his colleagues recorded the sound of the Great Barrier Reef in 2012 and then went back four years later to record the difference. 

In 2012, the sounds they recorded were rich in texture and volume due to the the activity of a range of invertebrates that live inside the coral. 

"You never see them but there's all these shrimp that snap their claws and sea urchins scraping across the substrate and, you know ,all of that noise makes this crackling sound when all of those thousands of individual snaps come together."

Between 2012 and 2016, the area of the Great Barrier Reef where they recorded saw two cyclones, one the strongest that had ever hit that part of the reef. On top of that, there was a record-setting heat wave that according to a recent study in the journal Nature, killed almost a third of the reef's coral. 

When Gordon went back to the reef in 2016, he says what he saw was heartbreaking. "Where there used to be colour and life in this incredible vista in every direction, you look it really is like diving in a graveyard now."

Not only has the volume gone down substantially, but also the quality of the sound has changed, the acoustic complexity has gone down, there's less going on. You can hear what is a dying reef.- Tim Gordon, University of Exeter

The sound of the reef had also significantly changed. "Not only has the volume gone down substantially, but also the quality of the sound has changed," says Gordon. "The acoustic complexity has gone down, there's less going on. You can hear what is a dying reef."

Fish need the reef as much as the reef needs fish

Fish are a central part of the reef's ecosystem. Gordon says, "Without fish, the whole system collapses. Fish help with nutrient cycling, they keep food webs in balance, they form associations with anemones, and most importantly, they graze away this sort of harmful macro algae that tends to grow on damaged reefs. It's almost like this sort of slimy seaweed that covers over the top when the corals die."

Degradation caused by bleaching and cyclones changes the soundscape of a reef. This makes reefs sound less attractive to young fishes, potentially reducing the populations of functionally important grazing species. Fewer fish and reduced grazing may exacerbate degradation, preventing recovery. (University of Exeter)

Without fish to graze the algae, new corals aren't able to settle or grow on the reef, so corals need fish to recover after damage to the reef. The fish, in turn, also need the reefs for their own survival. 

"Little baby fish start their life on the reef as an egg, but then almost immediately they're swept out into the open ocean to spend the first few weeks or months of their lives out there because there's fewer predators out there and they can grow up and develop a bit eating plankton."

From a long distance away, they can hear that sound, work out where the reef is, and also start to pick out things about the quality of the habitat, the health of the reef, to assess whether or not they want to go there.- Tim Gordon, University of Exeter

Once the fish larvae grow into their juvenile stage, they come back to the reef by hearing their way home. Once at the reef, they can use the coral structure to hide from predators coming from the open ocean.

"From a long distance away, they can hear that sound, work out where the reef is, and also start to pick out things about the quality of the habitat, the health of the reef, to assess whether or not they want to go there," says Gordon.

The effect a healthy versus degraded reef sounds on fish

Gordon wanted to test how important he reef sounds were to whether or not fish returned to the reef. He and his colleagues built experimental reefs on sand flats around the bay in Australia. 

Using underwater loudspeakers, they played the sounds coming from the healthy reef recorded in 2012, the degraded reef from 2016, as well as sounds of the open ocean as a control.

What we found was that on our reefs playing the sound of a degraded reef, there were 40 per cent fewer fish attracted there, settling there than the sound of the healthy one.- Tim Gordon, University of Exeter

"What we found was that on our reefs playing the sound of a degraded reef, there were 40 per cent fewer fish attracted there, settling there than the sound of the healthy one."

Gordon says this work definitely raises the possibility that playing the sounds from healthy reefs could help attract more fish, which would further protect the coral and help it regrow to become healthy again. But he says even if that does become a possibility, it would only be a small part of the solution.

"The only way we can really protect it, if we're really serious about keeping all of the wonder that it hosts, is to get serious about reducing our [greenhouse gas] emissions."