Underwater speakers broadcasting the commotion of thriving corals could breathe life back into more damaged and degraded reefs that are at risk of becoming ocean graveyards, researchers say.
Scientists working off the US Virgin Islands in the Caribbean found that coral larvae were up to seven times more likely to settle on a troubled reef where they broadcast recordings of snapping, moaning, grunting and scratches that form the symphony of a healthy ecosystem.
“We hope this will be something we can combine with other efforts to put the good things back on the reef,” said Nadège Aoki of the Woods Hole Oceanographic Institution in Massachusetts. “You could leave a speaker off for a while and it could attract not only coral larvae, but also fish to the reef.”
The world has lost half of its coral reefs since the 1950s due to the devastating effects of global warming, overfishing, pollution, habitat loss and epidemics. These significant declines have fueled efforts to protect remaining reefs through approaches ranging from replanting nursery-raised corals to developing resilient strains that can withstand warming waters.
Aoki and his colleagues took another approach, building on previous research that showed that coral larvae swim towards the sounds of the reef. They installed underwater speakers on three reefs off St John, the smallest of the US Virgin Islands, and measured how many coral larvae, contained in sealed containers of filtered seawater, deposited on pieces of rock-like ceramic in containers up to 30 meters from the enclosures.
While the researchers installed speakers at all three sites, they only broadcast the sounds of a thriving reef at one: the degraded Salt Pond Reef, which was bathed in the marine soundscape for three nights. The other two sites, the degraded Cocoloba Reefs and the healthier Tektite Reefs, were included for comparison.
When coral larvae are released into the water column, they are carried by currents and swim freely before finding a place to settle. Once they fall to the ocean floor, they settle in place and – if they survive – become adults.
Write in the Royal Society Open Science journal, the researchers describe how, on average, 1.7 times more coral larvae settled on the Salt Pond reef than at other sites where no reef sounds were emitted. Settlement rates at Salt Pond decreased with distance from the speaker, suggesting that emissions were responsible.
While the results are promising, Aoki said additional work is underway to understand whether other coral species respond in the same way to reef noises and whether the corals thrive after they settle. “You have to be very thoughtful about the application of this technology,” she said. “We must not encourage them to settle where they will die. It really needs to be a multi-pronged effort with measures in place to ensure these corals survive and grow over time.
Professor Steve Simpson, a marine biologist at the University of Bristol who was also involved in the study that found coral larvae responded to reef sounds, has been using audio recordings to attract fish larvae to reefs for 20 years. He said the work was “exciting” and demonstrated how acoustic reading could promote coral colonization in reef habitats.
“We are in a race against time to secure the future of coral reefs while striving to reach net zero and begin climate change,” Simpson said. “Coral reefs are the first marine ecosystems we could lose to climate change, which means they are also the first we can save. If we can save the reefs, we can save everything.