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"Whooping, croaking and growling": Hear the underwater sounds of marine life at a restored reef in Indonesia
Scientists who "eavesdropped" on a restored coral reef in Indonesia say their recordings of fish "whooping, croaking and growling" are the reef coming back to life.
Over a decade, the reef has been re-seeded with new corals.
The researchers used underwater microphones to record at the site.
The sounds, some of which have never been recorded before, provide an audible measure of the health of the reef, researchers say.
The researchers deployed hydrophones on the restored reef
The team also compared the recordings they captured at the restored reef to healthy reefs nearby, and to very degraded reefs close to the same site.
Restored reefs sound "more like the healthy, thriving reefs", explained lead researcher Dr Tim Lamont from the University of Exeter.
"Our study shows that this restoration can really work, but it's only part of a solution that must also include rapid action on climate change and other threats to reefs worldwide."
Parts of the reef the researchers studied are being restored from rubble. Decades of dynamite fishing - where sticks of explosive are thrown into the reef and floating dead fish are collected from the surface - had "blown it to smithereens".
"What's left behind is a rubble field," Dr Lamont explained. "It's very difficult for coral to grow because there's no solid substrate on the seabed."
To repair the damage, that rubble is stabilised with metal frames, while small fragments of live coral are attached to those structures.
After three years, the restored reef looks and sounds healthier
Describing the recordings, Dr Lamont said the "backing track of the reef" was snapping shrimp.
"That sounds like a bit like the static on the radio or frying bacon. Then, through that sound, you'll occasionally hear little trills, whoops or croaks."
The species responsible for many of these unusual sounds remain a mystery. Dr Lamont said the diversity of the sounds that fish produced was "as much as the diversity of birdsong".
"In some cases we can make an educated guess about what animal is making the noise, and in other cases we have no idea," he told BBC Radio 4's Inside Science.
"For me, that's part of the excitement of this whole field - the the joy of knowing that you might hear something that nobody else has ever heard before."
Healthy coral reefs are colourful, but are also rich soundscapes
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Before and after footage of restored coral reef