- In 2023, the Caribbean Sea experienced unprecedented heat: Beginning in March, sea surface temperatures throughout the region ranged from 1°-3°C (1.8°-5.4°F) warmer than normal.
- This unprecedented heat brought the worst coral bleaching event in the Caribbean’s recorded history, bleaching 60-100% of some reefs, and killing many patches.
- A new study found that certain species of coral propagated in the lab and then outplanted to restore reefs in five countries showed few signs of bleaching despite the prolonged marine heat wave, faring better than wild corals or corals propagated from fragments.
In mid-2023, the Caribbean Sea simmered as air temperatures soared, marking the hottest days ever recorded in Puerto Rico and Barbados. Beginning in March, sea surface temperatures throughout the region ranged between 1° and 3° Celsius warmer than normal (1.8°-5.4° Fahrenheit). This unprecedented heat brought on the worst coral bleaching event in the Caribbean’s history: It whitened 60 to 100% of the reef in some areas, and many patches died. But among the skeletons, a group of young corals kept their color, appearing to have not only survived bleaching, but resisted it altogether.
“It’s pretty devastating when [you look out at] reefs that you’ve been working on for many, many years and then you see the coral suffering,” Valérie Chamberland, a coral reef ecologist with the Miami-based conservation organization SECORE International, told Mongabay. “But seeing all of our young [coral] recruits faring pretty well was very encouraging.”
Under extreme thermal stress, corals expel the symbiotic food-producing algae that live in their tissue, turning pale or completely white. This bleaching can cause death when ocean temperatures surge 1°C greater than the historical maximum monthly average for two months. In 2023, abnormal heating lingered for more than five months in some parts of the Caribbean around Haiti, Jamaica, Nicaragua, Honduras and Costa Rica, the longest duration on record in the region.
In a study published Sept. 18 in the journal PLOS ONE that Chamberland co-authored, researchers found that certain species of coral bred for restoration showed few signs of bleaching despite the prolonged marine heat wave.
It’s the first scientific evidence that restored corals propagated through assisted sexual reproduction exhibit greater resistance to extreme ocean temperatures than naturally occurring corals or corals propagated from small fragments, according to a SECORE press release.
Until recently, the main conservation methods used to combat coral reef decline included transplantation and coral gardening, where small fragments of corals are brought to nurseries and grown to suitable size before outplanting on damaged reefs. However, scientists say these methods are insufficient to address the worsening impacts of climate-induced thermal stress on coral reefs.
Since the beginning of 2023, 75% of the world’s coral reefs have been impacted by bleaching-level thermal stress. According to the U.S. National Oceanic and Atmospheric Administration (NOAA), we are currently experiencing the fourth — and most extensive — mass coral bleaching event since 1998, when records began, as average ocean temperatures spike in every area of the globe.
If not killed by bleaching, some species of coral can take years to recover from thermal stress. A study published in June in Science Advances predicts that coral reefs near the equator will experience bleaching-level thermal stress for more than three months each year by as soon as 2035, making it nearly impossible for reefs to fully recover.
Therefore, some scientists say restoration methods that increase corals’ adaptive capacity are critical for their long-term survival and that of the marine life that depends on them.
In the new paper, researchers studied the survival of six species of reef-building corals they or their partners had bred using a method of assisted sexual propagation called “coral seeding,” and planted at 15 reef sites across five countries of the Caribbean between 2011 and 2022 (most between 2019 and 2022).
Coral seeding involves collecting spawn from a mix of parent colonies and bringing them to the lab for fertilization, producing millions of coral embryos with new combinations of traits. The developing larvae are then grown in enclosures in the ocean and settled on special substrates before being outplanted onto the reef.
Although coral may naturally pass on traits better suited to a warming marine environment during spawning in the wild, in the Caribbean, the natural sexual recruitment of major reef-building species is “poor to non-existent,” according to the new paper. To help the reef reproduce, scientists around the world have adopted coral seeding, a method that encourages genetic diversity and ultimately promotes adaptation.
During thermal peaks of the 2023 marine heat wave, the researchers surveyed the coral recruits — most aged between 1 and 4 years — and one or more nearby comparison populations composed of wild corals or corals propagated from fragments. Looking just at the six target species, they visually scored each colony as either having “healthy” coloration, or being “pale,” fully “bleached” or recently dead.
Ninety percent of the pooled sample of sexually propagated corals appeared visually healthy vs. just 24% of the pooled sample from comparison populations — nearly four times as much.
“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,” Dirk Petersen, SECORE’s founder and executive director, said in a statement.
One limitation mentioned by Mark E. Warner, a marine biologist at the University of Delaware who is not affiliated with the study, is the study’s method of gauging coral bleaching by the perception of color. “Humans are not the best at having equal perceptions of things like ‘healthy’ and ‘pale,’” Warner wrote in an email to Mongabay. In addition, “having a standard method, such as photographic analysis, helps others compare their data.”
Warner said he found it particularly interesting that the lab-crossed juveniles performed somewhat better than corals grown asexually from fragments. “This study provides a nice glimpse of the possible ways that coral breeding programs can try to compare the success of their efforts against other methods” of coral restoration, Warner wrote.
Another caveat is that young corals are generally more resistant to bleaching under moderate heat exposures than adult corals. The authors found that among propagated coral colonies aged 10 years or older, none retained healthy coloration and 32% (of 19 colonies) had already died due to bleaching at the time of survey. They write that the thermal tolerance of coral grown through assisted reproduction may diminish with age.
Chamberland said she is looking forward to seeing how the coral recruits do over time. But even if they do well, coral seeding “is just one tool in the toolkit,” she said. “It’s a necessary intervention but it’s insufficient in the long term if we don’t start reversing climate change.”
Banner image: Fish at a coral reef in Mexico. Image by Philip Hamilton / Ocean Image Bank.
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Citations:
Miller, M. W., Mendoza Quiroz, S., Lachs, L., Banaszak, A. T., Chamberland, V. F., Guest, J. R., … Petersen, D. (2024). Assisted sexual coral recruits show high thermal tolerance to the 2023 Caribbean mass bleaching event. PLOS ONE, 19(9), e0309719. doi:10.1371/journal.pone.0309719
Mellin, C., Brown, S., Cantin, N., Klein-Salas, E., Mouillot, D., Heron, S. F., & Fordham, D. A. (2024). Cumulative risk of future bleaching for the world’s coral reefs. Science Advances, 10(26). doi:10.1126/sciadv.adn9660
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