- The Mesoamerican Reef, the longest barrier reef in the Western Hemisphere, stretches 1,000 kilometers (620 miles) along the Caribbean coasts of Mexico, Belize, Guatemala and Honduras.
- The latest instalment of the Mesoamerican Reef Report Card, a periodic health assessment, finds that in 2024, the worst coral bleaching event on record reduced the reef’s coral cover.
- Although the overall health of the Mesoamerican Reef remains “poor,” according to the report, its health actually improved for the first time in five years.
- The report attributes this positive development to an increase in fish populations due to effective enforcement of fisheries rules by regional authorities.
2024 brought the hottest Northern Hemisphere summer ever recorded, only narrowly surpassing the record set in 2023. The trend, driven largely by human-caused climate change, according to NASA, caused the worst bleaching event on record among delicate coral populations around the world. This includes the Caribbean Sea, home to the Mesoamerican Reef, the longest barrier reef in the Western Hemisphere.
The latest instalment of the Mesoamerican Reef Report Card, a periodic health assessment, finds that the 2024 bleaching event reduced the reef’s coral cover. But though the overall health of the Mesoamerican Reef remains “poor,” its health actually improved for the first time in five years, a positive development the report attributes to an increase in fish populations due to effective enforcement of fisheries rules.
The Mesoamerican Reef stretches 1,000 kilometers (620 miles) along the coasts of Mexico, Belize, Guatemala and Honduras. It is considered a biodiversity hotspot and features more than 60 types of corals and more than 500 fish species, along with endangered sea turtles and the world’s largest congregation of whale sharks (Rhincodon typus). This biodiversity attracts millions of tourist divers every year. Nearly 2 million people also depend on the reef for their livelihoods.
The Mesoamerican Reef Report Card is the result of a comprehensive monitoring and evaluation effort by Healthy Reefs for Healthy People, an international partnership program to assess and protect the region’s coral reefs. The group released the first report card in 2008 and issues updates every 2-3 years, drawing on collaborative monitoring that uses standardized methods to gather data from hundreds of sites. For the 2024 report card, 93 data collectors from 42 reef conservation organizations monitored a total of 286 sites.
Bad bleaching, bad water quality
The report finds that 62% of the 286 Mesoamerican Reef sites monitored are either in “poor” (39%) or “critical” (23%) condition and that coral reef coverage declined from 19% as of the 2022 report card to 17%, not counting mortality that occurred after the survey ended.
A key reason for the decline is bleaching, in which increased water temperatures cause corals to expel the symbiotic food-producing algae that live in their tissues. Bleaching causes corals to turn white, weakens them and can kill them if conditions do not improve so they can recover.
This year, “the reef endured the worst bleaching event to date with approximately 40% of corals affected,” the report states, adding “severe mortality occurred at several of our most iconic reef sites.”
In Tela, Honduras, the sight of the bleached reefs was “haunting,” Paolo Guardiola, a biologist and dive instructor with Coral Reef Alliance, a U.S.-based NGO that assists local reef conservation groups, told Mongabay. According to Guardiola, who volunteered to gather data for the report, in just the short span between the first data-gathering dives in March and the latest ones in October, bleaching and algae had killed vast swaths of the reef he was surveying.
“It felt like floating over an endless graveyard,” Guardiola told Mongabay.
Daniel Garcia, head of the Department for Protected Areas in Honduras’ National Institute of Forest Conservation and Development, Protected Areas and Wildlife, said the historic bleaching event of 2024 is only the beginning. “When we see global warming, we will see bleaching; they are so intimately connected,” he told Mongabay.
According to Garcia, who was not involved in producing the report card, the 2024 bleaching should inspire collaboration to reduce climate change-related stressors as much as possible.
Another factor in the Mesoamerican Reef’s coral cover decline was poor water quality, the report states. It notes “fair to poor or unacceptable nutrient levels” across the reef and “fair to poor” oxygen levels at 64% of sampled sites. Moreover, 74% of samples contained human sewage pathogens that exceeded acceptable limits under the Cartagena Convention, a treaty to limit marine pollution in the Caribbean that took force in 1986.
According to Guardiola, a major driver of water quality changes affecting the Mesoamerican Reef are the many Central American rivers that discharge wastewater into the ocean during heavy rainfalls.
The report also finds that 1.4% of surveyed corals were affected by viral and bacterial diseases. It notes that the figure appears low but reflects a loss of sensitive corals and subsequent rise of resistant weedy coral species.
A beacon of hope
Despite high mortality in some regions and the bleaching, the report also finds that the health of some reef sites improved for the first time in five years, with sites in “fair” and “good” condition having risen from 20-28% and 5-9% since 2022, respectively.
This positive development correlates with an increase in herbivorous fish populations in many places. According to the report, greater enforcement and closer regulation of fishing has allowed herbivorous species, such as parrotfish (family Scaridae), to increase their biomass by 30% since 2022 in three of the four surveyed countries. This in turn reduced the cover of marine algae, which can suffocate corals and hijack access to critical nutrients.
However, 90% of parrotfish are still under 30 centimeters (1 foot) long, and the species that eat the most algae remain rare, according to the report, so efforts to protect them from overfishing must continue.
The report stresses that these positive signs amount to a small step forward in a massively affected and vulnerable region, and the Mesoamerican Reef’s “overall health remains ‘poor.’”
What next?
The report underlines the extreme unfolding effects of global warming and the critical need to reach for the climate target of limiting warming to 1.5° Celsius (2.7° Fahrenheit) above pre-industrial temperatures, as established in the 2015 Paris Agreement. However, the report also emphasizes the tangible benefits that active reef conservation and effective fishing enforcement can have.
Guardiola praised the report for bringing important issues to light, but he said its greatest value would be if it can incentivize action. “We have to move beyond just having policies on paper and actually enforce coral protection; this report shows that it works,” he said. The report’s findings must reach beyond “coral reef enthusiasts,” Guardiola said, and help educate communities and get more people involved in coral conservation.
But conservation specialists like Garcia face an uncomfortable reality. “There is very little that we could do, as a small country like Honduras, to stop the rise of temperatures due to climate change,” he said. These countries must find a firm posture in international forums to hold the industrialized countries that are largely responsible for global warming accountable and find serious solutions to slow rising temperatures, Garcia said.
“If global warming continues, bleaching and extinction of species will continue, there is no doubt about it,” Garcia said.
Banner image: A hawksbill sea turtle in Grand Cayman’s coral reefs, part of the Mesoamerican Reef. Image by Jason Washington / Ocean Image Bank.
Lab-grown corals resisted bleaching during Caribbean’s worst marine heat wave
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