In 1998, a mass bleaching event hit reefs in the Seychelles, leading to a devastating loss of 90% of the African island nation’s live coral. While that event wasn’t caused by climate change (rather by El Niño, a recurring climate pattern that causes ocean warming every few years), global heating has increased the frequency of these harmful incidents, which strip coral of the microalgae coating that supplies sea life with a nutritious food source.
In turn, the 850 million humans who live close to reefs and rely on those reef fisheries for nutrition and their livelihoods are also put at risk. “It’s a huge food provisioning role that these reefs are playing,” says James Robinson, a research fellow at Lancaster University in the U.K. Almost all the fish that are caught in reefs around the world are consumed domestically by the local population. That’s why a new study, which Robinson coauthored, gives hope: Even reefs that haven’t recovered their original algae states after bleaching seem to provide nutrient-rich food to fish.
The Seychelles islands, which contain 1,700 square kilometers of reefs, have been particularly hard hit, making them a good case study for Robinson to determine exactly how climate events can affect the nutritional potential of reefs. And, more generally, to find out how nutrient-rich tropical reef fish are, in a place where it’s the primary source of protein, representing 47% of animal protein intake among residents.
Of the country’s bleached reefs, 40% experienced “habitat collapse and macroalgal overgrowth,” where the reef is overrun by seaweed, a relatively common outcome. That new food source favors tropical bottom-feeders, like rabbitfish and parrotfish, which have increased in biomass in the Seychelles, despite the coral collapsing. The team measured specific minerals in the muscles of these fish from the reefs, comparing them with ones from reefs that had recovered their coral. They then compared the mineral content to those of other common proteins, including chicken breast, ribeye, and tilapia.
They found the fish were nutritious, consistent with most seafood. But the content of minerals like iron and calcium was higher than in species like tilapia. Most surprising was how nutritious the fish from the unrecovered corals were: Iron and zinc content was higher in fish from those habitats versus the recovered reefs. “We didn’t see that coming,” Robinson says.
The researchers believe the reason is, quite simply, that the seaweed is richer in those minerals. “Sargassum seaweeds . . . have high levels of minerals and, when cover is high, may account for similar amounts of primary production as turf algae,” the report reads. And if the fish are now eating nutrient-rich seaweed, humans who eat those fish are also benefiting. “We think this higher contribution of a new energy source provides nutrients in the entire food web,” Robinson says.
That finding could be vital in the Seychelles and similar tropical countries, given malnutrition struggles due to deficiencies of iron, zinc, and calcium, which leads to stunted growth and anemia as well as food insecurity. “These results suggest that coral reef small-scale fisheries may play a vital local role in public health,” the study reads.
Coral bleaching is still reducing biodiversity, and the biomass of piscivores (fish-eaters) that rely on a more diverse source of prey than the seaweed feeders. “It’s bad from a coral reef biodiversity perspective,” Robinson says. “There’s no doubt about that.” But the nutrient-packed herbivorous fish that are caught for food have increased in biomass, which is a boon for the local population. Around the world, there are similar trends, Robinson says: The Great Barrier Reef, which experienced the largest mass bleaching event since the Seychelles, has also attracted more herbivores.
“Despite loss of coral, you can still produce food,” Robinson concludes, noting that the main takeaway should be the importance of managing reef fisheries in a more sustainable way. “Reefs do play an important role in local food security in the tropics,” he says. “And they can still do so. We just can’t overfish.”