Healthy, young, bred coral during last summer’s bleaching in the Caribbean on the reef. Credit: Raul Tecalco Renteria/ SECORE International.

Propagated corals reveal increased resistance to bleaching across the Caribbean during the fatal heatwave of 2023.

SECORE International’s Coral Seeding approach utilises assisted reproduction, the breeding of corals, for reef restoration. This approach is realised within a training and partner network throughout the Caribbean.

Now, a peer-reviewed study shows that all the effort was worthwhile: during the devastating heatwave in the Caribbean in 2023, the young bred corals out on the reef stayed healthy while most of the remaining wild corals bleached and many died in the aftermath.

Elkhorn corals bleached heavily during 2023’s fatal heat wave in the Caribbean Sea. Credit: Sandra Mendoza Quiroz/ SECORE International.

Deadly

The summer of 2023 was deadly for many corals in the Caribbean Basin. An unprecedented heatwave, in intensity as well as in duration, hit the Caribbean with catastrophic effects.

Coral bleaching spread in the Caribbean Sea, and as high (seawater) temperatures persisted, many weakened corals died.

Sandra Mendoza Quiroz, SECORE’S restoration co-ordinator in Mexico, was the first to discover a glimmer of hope during that desperate time.

When Mendoza Quiroz and her colleagues set out for a routine monitoring dive to check on the health of their out-planted corals, expectations were low.

But then, amidst the bleached corals on the reef, she spotted the first young corals that they had grown, and they appeared to be completely healthy.

A similar observation was made by SECORE’s team on Curaçao: bred corals of a different species were withstanding the elevated temperatures as well. 

SECORE research director, Dr Margaret Miller, said: “We were excited to observe this pattern showing another benefit of using assisted coral recruits in restoration.

“[We] gathered data on the health status of several species and cohorts of our outplanted corals. Then we contacted partners throughout our Caribbean Restoration Network to see how widespread and consistent this pattern was.

“Fundación Dominicana de Estudios Marinos, the Nature Conservancy in the Caribbean and Reef Renewal Foundation Bonaire contributed additional data.”

Miller added: This provided confirmation that assisted recruits of six species of reef building corals at 15 individual reef sites in five nations throughout the Caribbean Basin showed the same pattern: young corals bred for restoration are a lot more resistant to bleaching under extreme levels of heat stress than the prevailing corals on the reef.”

Most corals spawn once a year, each species releasing eggs and sperm synchronously. Credit: Paul Selvaggio/ SECORE International.

Improved resistance

It is the first scientific evidence showing that restored corals from approaches using natural reproduction have improved resistance over natural and small fragmented corals under extreme seawater temperatures well above bleaching thresholds.

Miller advised: “I have been working on breeding corals in the Caribbean over the past thirty years, while simultaneously witnessing tremendous coral loss – due to disease, hurricanes and heat waves – and the unravelling of the communities that depend on them.

“These results provide a lot of encouragement and confirmation that restoration using assisted coral recruits can play an important role in orchestrating coral persistence into our warmer future. Nonetheless, truly securing the future of coral reefs is absolutely dependent on humankind’s success in controlling global warming.”

Just a decade ago, corals were propagated only by fragmentation, that is, breaking a fragment off a source colony to grow a new colony which is its clone. Small coral fragments are grown in nurseries and transplanted onto the reef manually.

Today, Coral Seeding – breeding corals for restoration – is implemented throughout the Caribbean by SECORE‘s Training and Capacity Building Program.

SECORE’s Coral Seeding approach involves collecting coral spawn from wild corals, fertilising the eggs and sperm in the lab (or on a boat or beach), and thus producing millions of coral embryos.

The developing coral larvae are grown in enclosures in the ocean and settled on special substrates. After the corals have reached a certain size, the substrates are outplanted onto the reef. 

Every time a population reproduces, new offspring receive newly mixed sets of genes through recombination, making them different from their parent colonies and thus enabling adaptation.

Although naturally occurring offspring could perform similarly under elevated temperatures, general recruitment failure of reef-building species in the Caribbean means hardly any natural offspring occur.

Important contribution

SECORE founder and executive director, Dr Dirk Petersen, said: “I am excited about the very positive results of this large study since it shows that our Coral Seeding approach is an important contribution to help coral reefs dealing with climate change.

“Our investment over the past five years to build a large network for coral restoration in the Caribbean has paid off.

“This network not only produces and outplants tens of thousands of corals every year but could also immediately assess how these corals responded to this unprecedented heat wave.

“Our priority is now to further scale efforts to the ecosystem level.”

The study is published in PLOS ONE.