- Marine scientists have documented Thailand’s coral reefs in unprecedented detail, providing a crucial baseline against which reef managers can measure future change.
- The surveys indicate that, as in other parts of the world, Thailand’s reefs are losing structural complexity, becoming dominated by simpler boulder-forming corals, while staghorn and branching species die out.
- Experts say the new baseline can help steer future strategies to prepare for future bleaching events through reef restoration and assisted reproduction.
- The surveys were conducted just before the full effects of the 2024 global bleaching event were felt in Thai waters, which will have inevitably taken an as-yet-unquantified toll on the region’s reefs.
Marine scientists compiling the most holistic “snapshot” of Thailand’s coral reefs to date have uncovered evidence of a long-suspected reality: Thailand’s coral reefs are losing structural complexity.
Home to more than 300 species of reef-building corals, Thailand’s reefs have been hit repeatedly by mass coral bleaching triggered by extreme marine heat waves. The stress of these events has likely prompted shifts in the species that make up coral communities, with knock-on effects across entire marine ecosystems, experts say.
The new study, based on underwater surveys carried out between 2022 and early 2024, just before the effects of the fourth global coral bleaching event were widely reported in Thailand, documents fringing reefs and offshore pinnacles across eight provinces on the Gulf of Thailand and Andaman Sea coasts. The 2024 bleaching event will have inevitably taken an as-yet-unquantified toll on the region’s reefs, the authors note.
“Having this map of what corals are represented across the region gives us a starting point for conservation,” said Rahul Mehrotra, research director at the Aow Thai Marine Ecology Center (ATMEC) and a co-author of the study. “We hope that this baseline will [motivate] more nuanced assessments.”
While nationwide studies have previously attempted to assess coral health at long-term monitoring sites in Thai waters, the majority of evaluations have been “highly localised and sporadic in nature,” the study says.
The new coast-to-coast data represent a fresh baseline against which reef managers, researchers and policymakers can measure future change, Mehrotra said.
Globally, coral reefs cover only 1% of the ocean floor, yet they support one-quarter of all marine species at some point in their lifecycles. In Thailand, reefs provide coastal protection, livelihood security and dietary protein, and they are a pillar of Thailand’s tourism industry, which brings in one-fifth of the country’s GDP.
Corals are notoriously vulnerable to environmental change, however. As in other parts of the world, mass bleaching events in 1998, 2010, 2016 and 2024 took a severe toll on Thailand’s reefs. During these events, corals turned ghostly white as extreme ocean temperatures caused them to eject the colorful symbiotic algae living within their limestone skeletons. The 2024 event alone affected up to 80% of corals in some areas, according to local media reports, although many areas reportedly recovered afterward.
“Many coral reefs are no longer as healthy or as beautiful as they once were, especially in areas [impacted] by human activities,” said Niphon Phongsuwan, a veteran marine scientist whose 2013 study documented long-term changes in Thailand’s coral communities. “After experiencing multiple coral bleaching crises, we observed that even when reefs recover, the coral community may change, and the biodiversity (coral species richness), can shift.”
Niphon, who wasn’t involved in the new study, welcomed the nationwide mapping as valuable data that can steer future reef management. “Coral reefs are constantly changing — they are dynamic,” he said. “Therefore, we need to survey as many areas as possible.”
‘Homogenized’ reefs
To assess Thailand’s reefs as extensively as possible, the team of researchers from ATMEC and Thailand’s Marine and Coastal Resources Research Center surveyed 99 nearshore reefs and 16 offshore pinnacles between September 2022 and April 2024. They recorded coral cover and the coral genera present at a total of 17 kilometers (10.6 miles) of reef, at depths ranging from 4-20 meters (13-65 feet). They also collated information from previous reef assessments in each province to draw comparisons between current and historic coral health.
The team report that even before the full effects of the 2024 bleaching event, many reefs in both the Gulf and the Andaman Sea appear to be more simple ecosystems than in decades past, dominated by species with lower structural complexity.
Staghorn and branching types of coral that offer a plethora of niches for fish and other reef organisms, such as the Acroporas — a group known to be particularly vulnerable to thermal stress — were recorded less frequently than in previous studies. Groups like the branching fire corals (Millepora species) were also noticeably absent from sites in the Gulf of Thailand, where they had once been abundant.
“Many major coral types have largely disappeared or are in the process of being lost under our noses without anyone noticing or explicitly documenting,” Mehrotra said, adding that the new baseline will enable scientists to quantify and document any future declines. “Now, any future changes are going to be more easily identifiable.”
Conversely, the researchers found that reefs on both sides of the country were dominated by Porites, a group of boulder-forming corals that typically offer fewer nooks and refuges for marine life.
While Porites has been dominant since the earliest coral assessments in Thailand in the 1980s, the study says its resilience in the face of repeated coral bleaching events appears to have increased its dominance, leading to a “widespread homogenization” of Thailand’s reefs. Stress-resistant coral species are often less structurally complex, Mehrotra said, leading to a loss of habitat complexity.
Scientists have documented reduced structural complexity and species diversity on reefs in other parts of the world, such as the Red Sea, the Caribbean and the eastern Pacific, prompting concern that they might no longer support as wide a variety of life as they once did.
“The moment we start to lose species, it has major knock-on consequences for the biodiversity on the reef,” Mehrotra said. “We are finding increasing numbers of invertebrates and fish life that need a single species of coral for their survival. The absence of that coral means that’s it for that species.”
Species diversity, marine connectivity are key
Niphon Phongsuwan said management interventions might be required in certain areas where reefs have become overly homogenized. “When only one or two species dominate the entire area, [reefs] become more vulnerable to unforeseen threats,” Niphon said. “For example, if these species have low resistance to disease, then an outbreak could wipe out the entire reef system all at once.”
A 2024 study from the Gulf of Thailand found that Porites corals can indeed be susceptible to a broad range of diseases present in Thai waters, such as white plague and pink line syndrome.
“The key is to maintain high coral species diversity,” Niphon said. “Ecosystems with high species diversity have lower risk of collapse when disturbances occur.”
Measures such as reducing the impacts of fishing, tourism and pollution; expanding marine protected areas; long-term monitoring; and diversifying coral restoration approaches can all help, Niphon added. The presence of nearby healthy mangrove forests and seagrass beds is also crucial. “Connectivity with these ecosystems enhances [reefs’] ability to recover quickly,” he said.
Reef restoration insights
The team also found reefs in the Gulf of Thailand had higher coral cover than sites in the Andaman Sea. Coral cover in the Gulf averaged 54%, compared with 35% in the Andaman.
This finding warrants further study, Mehrotra said, not least because the Andaman Sea is often associated with higher coral species richness and is typically perceived as more “pristine” and the focus of conservation attention.
“The Gulf of Thailand consistently gets ignored,” Mehrotra said. “But that there appears to be more coral cover there should have people stop overlooking [it].”
While the factors underlying the Gulf’s relatively high coral cover need further investigation, Mehrotra suggested it could come down to heavy human pressure along the Gulf’s densely populated coastline. Challenging conditions for marine organisms, such as high nutrient levels and poor water clarity due to sedimentation, might have forced corals to adapt to a harsher environment, potentially increasing their resilience to stress.
The data also revealed some insights that could have direct implications for the design of reef restoration plans. The surveys found coral reefs were richest in the top 10 m (33 ft) of the ocean, with the greatest coral cover recorded at shallower depths, averaging 52% at 5 m (16.4 ft), compared with 29% at 20 m across all sites.
Yet efforts to create artificial reefs in Thailand often focus on depths beyond this range, Mehrotra said. “The fact that the majority of Thai artificial reefs and substrates are at the deeper end of this range (most being deeper than 10m) raises the question of how we’re supporting (or not) the future of coral reefs in the country.”
Sexual reproduction offers hope
With the coral baseline completed, the team now plans to deepen their research on artificial reef management and other topics that will help Thailand come up with proactive strategies to prepare for future bleaching events.
Ultimately, reef restoration needs to be evidence-based, Mehrotra said, and practices have to evolve alongside new scientific understanding. Techniques that have been used for decades, such as transplanting asexual coral clones, might no longer be appropriate at certain sites, he added.
Reef scientists in Thailand are increasingly turning their attention to coral sexual reproduction as a way of increasing genetic diversity and building resilience into reefs by focusing on assisting the proliferation of stress-resilient corals. Therefore, Mehrotra and his colleagues are also studying coral dispersal and juvenile survival in the Gulf of Thailand. “If some corals are successfully reproducing, we have hope, because the juveniles will be strong,” he said.
With the new coral map depicting the current state of reefs in Thailand, “the sequel has to be what is going to come next, and what can we prepare for.”
Banner image: Structurally complex branching, staghorn and tabulate coral growth forms provide an assortment of niches for a vibrant array of marine life. Image courtesy of Aow Thai Marine Ecology Center (ATMEC).
Carolyn Cowan is a staff writer for Mongabay.
Citations:
Monchanin, C., Desmolles, M., Chankong, A., Nilkerd, N., Sangsawang, L., & Mehrotra, R. (2025). Establishing a baseline for coral reef community structure across Thailand with a review of earlier assessments. Regional Studies in Marine Science, 91, 104577. doi:10.1016/j.rsma.2025.104577
Phongsuwan, N., Chankong, A., Yamarunpatthana, C., Chansang, H., Boonprakob, R., Petchkumnerd, P., … Bundit, O. (2013). Status and changing patterns on coral reefs in Thailand during the last two decades. Deep Sea Research Part II: Topical Studies in Oceanography, 96, 19-24. doi:10.1016/j.dsr2.2013.02.015
Kritsanapuntu, S., Poepetch, S. (2024) Coral diseases outbreaks for massive coral (Porites lutea) at reef communities of Samui Island, and the adjacent area, Surat Thani province. Ramkhamhaeng International Journal of Science and Technology 7(3). https://ph02.tci-thaijo.org/index.php/RIST/article/view/255989
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