Bleached coral reefs can still supply essential nutrients

New research on fish catches in tropical regions where coral reefs have been bleached brings unexpected hope to millions of people. After 20 years of monitoring bleached coral reefs in the Seychelles, a team of international scientists conclude that these damaged environments can continue to supply nutritious food to local communities.

The study, led by scientists from Lancaster University and involving researchers from the Seychelles, Australia, Canada and Mozambique, followed the recovery of coral reef communities after a 1998 bleaching event that killed off 90 percent of the corals. During this time, 60 percent of the coral reefs recovered to coral-dominated communities, but around 40 percent were transformed to reefs dominated by seaweeds. 

This situation provided a natural experiment for the scientists to compare the catches and micronutrients available from fisheries on coral reefs with those available in different climate-driven ecosystems. 

More than six million people work in small-scale fisheries that rely on tropical coral reefs. Their catches help to feed hundreds of millions of coastal people in regions where malnourishment is present. However, until now the nutritional composition of coral reef fish catches, and how climate change might affect the nutrients available from reef fisheries, was not known.

The scientists used a combination of experimental fishing, nutrient analysis, and visual surveys of fish communities in the Seychelles, to calculate the contribution of micronutrients to the diets of local people. They found that reef fisheries can remain rich sources of micronutrients, even following bleaching events that kill the coral and transform the composition of reef ecosystems. In some instances, the nutritional value of the reef fish even increased after the corals were killed by warmer water. 

The findings, published today in the journal One Earth, show that reef fisheries can remain rich sources of micronutrients such as selenium, zinc, calcium, iron and omega-3 fatty acids, despite the destruction of the corals by bleaching events. 

The researchers even found that levels of iron and zinc are higher in fish caught on reefs that have been transformed after coral bleaching and are now dominated by macroalgae such as Sargassum seaweeds. These seaweeds contain high levels of minerals which, researchers believe, is the reason that the algal-feeding herbivorous fishes – found in greater numbers on transformed reefs – contain higher levels of iron and zinc.

The results are unexpected and mean that local communities can continue to benefit from the consumption of nutrient-rich fish and other seafood in their diets, despite the impacts of climate change. This provides a small ray of hope as continued global warming means coral bleaching events will become more frequent and more severe, placing greater stress on these vulnerable ecosystems.

Dr. James Robinson, who led the study, said: “Our findings underline the continuing importance of these fisheries for vulnerable coastal communities, and the need to protect against over-fishing to ensure long-term sustainability of reef fisheries.”

The researchers caution that, although their findings indicate coral reef fisheries may be more resilient to the effects of climate change than expected, these fisheries need to be protected and managed sustainably to provide essential nutrients to local people. To achieve this, data from other regions are urgently needed, along with continued, long-term monitoring of the impacts of climate change on these fisheries. 

“Coral reef fish contain high levels of essential dietary nutrients such as iron and zinc, so contribute to healthy diets in places with high fish consumption. We found that some micronutrient-rich reef species become more abundant after coral bleaching, enabling fisheries to supply nutritious food despite climate change impacts. Protecting catches from these local food systems should be a food security priority,” said Dr. Robinson.

“Fish are now recognized as critical to alleviating malnutrition, particularly in the tropics where diets can lack up to 50% of the micronutrients needed for healthy growth,” said study co-author Professor Christina Hicks. “This work is promising because it suggests reef fisheries will continue to play a crucial role, even in the face of climate change, and highlights the vital importance of investing in sustainable fisheries management.”

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By Alison Bosman, Earth.com Staff Writer