The super El Niño is cooking coral reefs — can lullabies and probiotics save them?

It’s peak spawning time for coral in the Caribbean Sea and there’s something new in the water this year. Underwater speakers at several locations are serenading clouds of floating larvae as they search for a place on the reef to permanently settle. Sounds recorded at vibrant reefs — such as the clicks of snapping prawn and the thrum of fish — fill the water at spots where coral reefs are struggling because of high temperatures, pollution and other stresses.

The hope is that these sea songs will lure floating coral larvae into settling on the degraded reefs and help them to recover. This project, run by researchers at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts, relies on the discovery that coral larvae can sense sound and use it as a cue to follow as they search for a permanent home¹.

“Healthy reefs are naturally broadcasting sound all over the place,” says Aran Mooney, a marine bioacoustician at WHOI who leads the project. He’s part of a small army of researchers stepping up their efforts to save threatened corals. In addition to playing lullabies to restore damaged reefs, they are implementing other emergency measures, such as moving corals from shallow locations to deeper, cooler water and inoculating reefs with protective microorganisms.

The planet’s corals need all the help that they can get. This year’s record-breaking ocean temperatures, resulting from human-caused climate change and a super-strong El Niño brewing in the Pacific Ocean, have spurred fresh alarm about the future of coral reefs. Elevated water temperatures can cause coral bleaching, which happens when the usually richly coloured organisms expel the symbiotic algae that they rely on for nutrients; this can ultimately kill the coral.

As of late August, coral bleaching had been observed in corals around Florida, the Pacific coast of Panama and Kiribati in the Central Pacific. The Coral Reef Watch of the US National Oceanic and Atmospheric Administration found that, in large swathes of the oceans, the water had reached temperatures that placed corals at risk of bleaching and heat mortality. Temperatures are expected to rise even further in 2027 as the El Niño reaches its full strength, and that will raise the risks for corals.

Coral reefs can recover from bleaching and their ability to bounce back has often surprised scientists. But recovery takes time and corals can’t catch a break: this year’s bleaching event comes less than a year after the last global one ended in 2025.

“It’s sort of like having COVID, also getting the flu, and at the same time being stressed about your job,” says Amy Apprill, a marine geochemist who leads WHOI’s coral-restoration team. She says that the accumulated stress is likely to make the heat this year and in 2027 even more harmful for corals than previous events.

Michael Sweet, a coral biologist at the University of Derby, UK, says that, despite the depressing outlook, many researchers are focusing on what they can do to help. “You swing between almost giving up, to going, ‘I’m going to fight to my dying breath to try and make things a bit better’.”

Lost reefs

The twenty-first century has not been kind to coral reefs. Four times since 1998, hot oceans — fuelled by global warming and boosted by El Niño events — have caused corals to bleach worldwide. Those bleaching events have taken a toll. According to a major report by hundreds of scientists published in August², overall hard coral cover between 2020 and 2024 was 9.5% lower than between 1980 and 2009. Much of the decline occurred during the record heat in 2024, which caused bleaching at nearly all of the world’s reefs, and saw hard coral cover fall by 8.9% compared with 2023, the largest single-year drop so far (see ‘Rough times for reefs’).

“What that decline tells us is that the recovery is never enough before that decline happens again,” says report co-author Manuel González-Rivero, a coral researcher at the Australian Institute of Marine Science (AIMS) in Cape Cleveland.

Projections beyond this year’s El Niño event also look bleak. “Even under the optimistic models for the future ocean temperature, we expect massive bleaching everywhere” over the next decade, says Giovanni Strona, an ecologist who studies corals at the European Commission’s Joint Research Centre in Ispra, Italy.

“By the end of the century, many projections predict almost complete annihilation of coral reefs worldwide,” adds Strona. A 2025 study he co-authored argued that, barring massive increases in funding or some major technological advance, the scale of the problem is now too great for restoration efforts to prevent huge declines³.

Coral scientists are unequivocal that the priority for protecting corals must be slashing greenhouse-gas emissions and protecting reefs from local stresses such as overfishing and pollution. Climate models suggest that limiting global warming to 1.5 °C relative to the pre-industrial average would spare some coral reefs from repeat bleaching events⁴. But in a 2 °C warmer world, nearly all reefs would be subjected to severe bleaching regularly — eventually leading to the corals’ death.

Scientists working on coral restoration insist that much can still be saved. “Coral reefs have not crossed a point of no return,” states the report². However, as the world looks set to overshoot both those temperature thresholds, researchers say that the heat is increasingly challenging reefs’ natural recovery capacity and that more-direct interventions are necessary to keep coral reefs alive.

“It flips between being an amazing time to be a coral scientist and a biologist in general and having the opportunity to develop and establish these really cool techniques, against the fact that we have to do it” because corals are in trouble, says Sweet, who studies ways to use the coral microbiome to help reefs.

That work includes a growing set of techniques that show promise for helping corals, although difficulties scaling them up and a lack of funding remain persistent issues. Some approaches are relatively low-tech. Long-standing coral-gardening projects, for example, are getting better at breeding corals with improved tolerance to heat and acidity⁵. Other approaches range from treating corals with microbial cocktails and micronutrients to luring larvae using underwater sounds of a healthy reef.

Coral lullabies

For Mooney, the fact that coral larvae — invertebrates less than one millimetre long — could be sensitive to sound came as a big surprise. In 2012, he and his colleagues were listening for differences in the soundscapes of healthy and degraded reefs near the US Virgin Islands. The Caribbean has seen some of the world’s most-severe declines in coral cover due to rising ocean temperatures and other local pressures, such as overfishing, sediment pollution and disease outbreaks.

In 2012, it was well known that fish were attracted to the sounds of a vibrant reef⁶. And Mooney had found that squid, too, were sensitive to sound⁷, as were crabs⁸. It occurred to him that other invertebrates, such as corals and oysters, might also use sonic cues to assess a reef’s state.

Mooney and his colleagues began conducting experiments to learn more about how corals might use sound. They found that the larvae were sensitive to sonic cues and settled more often when exposed to the ambient sounds of healthy reefs⁹.

Then, in 2022, they began testing whether reef soundscapes might be harnessed to help these ecosystems recover by broadcasting recordings of a vibrant reef in places where the reef had gone silent. The team placed coral larvae of the mustard-yellow stony coral Porites astreoides at degraded reefs in special cups that vibrated to let in sound, but that blocked other sensory information, such as light and chemical cues. The researchers then used underwater speakers to broadcast audio recordings of a healthy reef. They found that larvae exposed to the healthy-reef recordings settled almost twice as often, on average, as did the ones without the sound treatment¹⁰. “Oh wow. This actually worked,” says Mooney.