The wide-scale bleaching of the planet’s coral reefs is one of the most visually arresting symbols of climate change. Previously colourful and diverse tropical reef systems have turned ghostly white due to rising sea temperatures caused by global warming.
But now a new study has provided hope for coastal communities that rely on these reefs for food. Scientists have discovered that fish living in bleached coral reefs can remain rich sources of micronutrients, and in some cases, they can even increase in nutritional value.
What is coral bleaching?
Microscopic algae called zooxanthellae give coral reefs their vibrant colours. Reefs and algae enjoy a symbiotic relationship, helping each other to thrive. When sea temperatures rise, however, corals are put under stress, which drives the algae away, resulting in the corals looking like they have been bleached.
While corals can recover if water temperatures lower again, if they don’t, the algae can’t return and the corals will die. Corals can also be bleached as a result of pollution or too much sunlight as well.
The National Oceanic and Atmospheric Association estimates that between 2014 and 2017, 75 per cent of the planet’s tropical reefs were put under heat stress, which triggered bleaching.
How does coral bleaching affect wildlife?
Coral bleaching can have a devastating impact on wildlife. Reef systems provide shelter, protection and spawning grounds for a huge number of marine species, including crabs, jellyfish and turtles. Without these vital support networks, many species will face extinction.
Scientists from Lancaster University, along with an international team from Seychelles, Australia, Canada and Mozambique used 20 years of monitoring data in their study. They focused on a mass bleaching event in Seychelles in order to collect information on the nutritional value of fish species found in the surrounding reefs.
The reefs in the Indian Ocean archipelago were damaged by a large bleaching event in 1998, which killed an estimated 90 per of the corals.
Around 60 per of the coral systems recovered, but 40 per cent were transformed into reefs dominated by seaweed. This provided the scientists with a natural way to compare and contrast nutrient levels in the fish in both of these climate-shaped systems.
How does coral bleaching affect local communities?
The study, published in the journal One Earth, states that more than six million people work in small-scale fisheries worldwide, relying on tropical coral reefs for their livelihoods. Their catches feed hundreds of millions of people, particularly in regions with high levels of malnourishment.
Until now, it wasn’t known whether coral bleaching affected the micronutrients available in fish caught in these reefs.
“Fish are now recognised as critical to alleviating malnutrition, particularly in the tropics where diets can lack up to 50 per cent of the micronutrients needed for healthy growth,” says Professor Christina Hicks, a co-author of the study.
“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.”
What did the scientists discover?
The international team found that reef fish are important sources of zinc and selenium (which plays a critical role in thyroid function). They also contain iron and omega-3 levels comparable to chicken and pork.
The scientists discovered that iron and zinc levels are higher in fish caught on reefs that have been affected by coral bleaching and have become dominated by seaweeds. Many of these seaweeds have higher mineral compositions, which the fish are ingesting, making them potentially more nutritious to humans.
“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,” says Dr James Robinson from Lancaster University, who led the study.
However, the researchers also struck a note of caution. While these coastal reef fisheries are proving more resilient to climate change than expected, more studies of the long-term impacts of coastal bleaching are urgently needed.