Coral reefs in Mauritius are turning ghostly white. Can nurseries rescue them from climate change?

For Nadeem Nazurally, snorkelling off the coast of Mauritius lately has become disheartening. 

The coral reefs that once glowed in vivid greens, blues and pinks now stretch out below him, faded and ghostly.

“When I see … all white, it means there is a big problem,” Nazurally, an associate professor at the University of Mauritius’ faculty of agriculture, told What on Earth’s Laura Lynch. “We are losing all these colours, we are losing life, we are losing those important corals,” which not only act as natural buffers against the island’s frequent cyclones, but are also vital to tourism, fisheries and support a wide range of marine life.

In Mauritius, home to nearly 250 species of coral and 150 kilometres of reef, the decline has been stark. The island nation off Africa’s southeast coast has lost roughly half of its coral cover since the 1970s, according to the International Union from the Conservation of Nature, enduring multiple bleaching events and a devastating oil spill in 2020. Across the western Indian Ocean, rising ocean temperatures driven by climate change are triggering mass coral bleaching events on an unprecedented scale. The Intergovernmental Panel on Climate Change (IPCC) warns the world’s coral reefs would virtually vanish if global warming exceeds 2C above pre-industrial levels.

But restoration efforts are beginning to show promise. In 2019, with the help of a $10-million US grant ($13.75 million Cdn) from the Adaptation Fund, supported by the United Nations Development Programme, the Mauritian government, working alongside scientists, and other non-governmental organizations, launched a project to restore roughly three hectares of degraded reef by December.

Two paths to restoration

Meera Koonjul, director of fisheries at the ministry of agro-industry, food security, blue economy and fisheries in Mauritius, says restoration centres on two approaches: sexual and asexual reproduction.

Sexual reproduction, known as “coral spawning,” occurs when corals release eggs and sperm — gametes — into a water column in which they fertilize and develop into larvae, eventually forming new coral colonies, Koonjul says.

Meanwhile, asexual reproduction involves corals creating genetic clones. One common method is fragmentation, where a broken branch of coral grows to form genetically identical colonies.

Using this approach, Koonjul says the project established roughly 100 coral nurseries. Fragments were grown for eight months to a year before being transplanted into the seabed in Mauritius’ Blue Bay Marine Park, a 353-hectare protected marine area, known for its rich coral diversity.

“We have been able to plant more than 100,000 coral fragments in [this] area,” said Koonjul. “In this way, we have been building capacity, we have involved the communities and we’ve been able to restore the coral reef.”

Nazurally, who founded the non-profit EcoMode Society in 2012, has been employing the same method in his own restoration work after noticing corals growing on abandoned fishing basket traps. Instead of allowing the coral fragments — broken by waves or boat damage — to suffocate under the sandy sea floor, he collects the orphaned fragments known as “corals of opportunity” and then puts those abandoned baskets to good use.

“We fix [the coral fragments] on those structures,” said Nazurally. “We build them as a metal frame, two metres by one metre. We call it a nursery and we fix it on rope and we take care of them.” 

Marie Annielle Espiegle has seen first hand how combining both sexual and asexual methods can accelerate the recovery of degraded reefs.

As regional training coordinator and restoration technician for U.S.-based non-profit SECORE International, she has worked on coral sexual reproduction projects in the Caribbean for over a decade. These projects involve collecting gametes, raising larvae in floating tanks and eventually outplanting them onto degraded reefs.

While asexual methods can restore reefs more quickly, Espiegle says sexual reproduction produces more resilient corals due to the mixing of genetic material from male and female gametes.

“For example, if there’s a disease that killed the parents, the offspring — that is the new baby corals — might not be affected at all because they’ve developed some resilience,” she said.

For that reason, she says the most effective restoration strategies use both methods, “increasing the coral surface, but also introducing new genetics on the reef.”

The clock is ticking

Ahead of its December deadline, the island has restored just over 50 per cent of its target, Koonjul says. She adds the ministry of fisheries has asked the Adaptation Fund for an 18-month extension to reach its goal of restoring just over three hectares — something she is confident they can achieve.

“We have already started growing the corals in the nursery,” said Koonjul. “So we would have quite an amount of corals and we would be restoring them very quickly.”

For Espiegle, the work goes beyond protecting an ecosystem — it’s about preserving Mauritius’ identity.

“We’ve known the coral reefs since we’ve been born,” she said. “The island is what it is because there has been the reefs.”