Coral reefs are getting quieter

08 May 2018

Interview with

Tim Gordon, University of Exeter

Coral reefs are the most biodiverse ecosystems in the oceans. They make up less than 1% of area but host a quarter of all life. And this makes for a noisy atmosphere but within the past 5 years, they've become significantly more quiet.  A group from the University of Exeter has been listening in to the lifebeat of the great barrier reef, and found that it’s about a quarter of the volume it used to be. And this is going to have a devastating knock on effect. Georgia Mills got the story from first author Tim Gordon.

Tim - Reefs are really noisy places. They’re full of shrimps clicking their claws and fish chatting and chirping and whooping, and those sounds are used for all sorts of different things. Fish alarm call to warn each other about predators, they communicate with each other, they hunt together using sound. But what we’ve seen recently is that where we work on the Great Barrier Reef has been decimated by climate change. Tropical cyclones are happening more frequently and more strongly, and coral bleaching is wiping out whole areas of the ecosystem.

When we went back recently and listened again to some of the areas that we’d worked in five years ago, it broke our heart. What used to be a raucous symphony of noise from the whole orchestra of animals, is being silenced.

Georgia - Oh wow! And what kind of impact is this going to have?

Tim - It’s really sad in of itself to hear that degradation. But it’s more than that actually because the sound of a reef is really important for attracting fish to it. Fish start their lives as larvae out in the open ocean where they’re avoiding predators and eating plankton and then once they’ve developed into juvenile fish, they then hear their way back home again. They come and find a reef to settle on and they listen out from miles away in the open ocean to hear that reef.

Now we did an experiment where we showed that the sound of reefs today, that new quieter degraded sound, is much less attractive to juvenile fish and fewer fish are able to hear their way home to it.

Georgia - How did you test that?

Tim - What we did is we built a lot of experimental replica reefs all around a bay and on some of the reefs we put loudspeakers playing the sound of healthy reefs from five years ago, and on some of the reefs we put loudspeakers playing the sound of today’s degraded reefs. What we found was that on the reefs that played today’s degraded sounds, we got 40% fewer fish settling to those reefs than the ones playing the sound of a healthy reef.

Georgia - Right. So this is really making a big difference then in how many fish are coming back. What kind of a knock-on effect would that have on a reef?

Tim - It’s worrying because reefs really need their fish or the whole system starts to collapse. You see fish help with nutrient cycling, they keep food webs in balance, they form associations with anemones but, most importantly, they graze away this harmful macroalgae that tends to grow over degraded reefs. This stuff sort of like slimy seaweed and when corals die, it grows over the top of the whole thing and chokes the reef. It stops any new corals from settling, any new corals from growing back, and it really slows down recovery. If there are fish on reefs, they eat away at the algae; that clears bare patches and allows coral to grow back again. But if reefs don’t have healthy fish populations, they’re really stuck on what we call this ‘slippery slope to slime.’

Georgia - Is there anything that we can do about this?

Tim - Absolutely. I think it is crucial that we remember that there is still hope. There’s renewable energy technologies that are advancing all the time and emissions are falling as a result. In the UK, we’ve recently broke our record for the longest run without using coal power in the UK. Our Government's discussing plans to go carbon zero by 2050, so there is already progress in reducing our emissions. I just think it needs to happen faster.

Georgia - While we’re waiting, can we use your experiment method to lure the fish back in while we wait?

Tim - This definitely raises that possibility and that is research that we’re actively doing at the moment. There’s the possibility that we might be able to use sound to help with reef restoration. If we can attract fish in using loudspeakers, it might help reefs to rebound quicker. But, like we say, the most pressing issue really is reducing emissions because any restoration efforts that we can produce will only be feasible on small scales, and will only partially help to restore reefs. So if we’re really serious about protecting what is the most beautiful and most valuable ecosystem in the world, then we really need to start addressing our emissions more seriously.


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