With a little help, Mannar’s corals stand strong in the face of growing threats

It was a sight of devastation that met 22-year-old Gilbert Mathews when he dove into the clear waters near southern Vaan, a coral island off the Tamil Nadu coast in the Gulf of Mannar, in 2002.

“Broken coral skeletons lay everywhere. And barely any fish,” he recalled.

That was because coral mining for limestone, used as a construction material, was then common in the Gulf (the government banned it in 2005). Climate change was beginning to take a toll, too, with researchers already reporting traces of coral bleaching.

Those threats were the reason Mathews was exploring the Gulf, having joined the Suganthi Devadason Marine Research Institute (SDMRI) in Thoothukudi to study how corals were faring.

Now, nearly 20 years later, corals are thriving in those reef patches near Vaan, said Mathews, currently an associate professor at SDMRI. The corals support a diversity of reef fish as well.

This recovery – not just around Vaan Island but also in other parts of the Gulf – has come about despite several setbacks, the most prominent of them destructive fishing practices, marine debris and two debilitating bleaching events in 2010 and 2016.

While climate change still remains the chief worry, reefs here are showing immense resilience, researchers said. And this didn’t happen on its own.

Reviving biodiversity

The impact of a changing climate on corals worldwide first came to light in the 1980s and 1990s, when scientists discovered some reefs bleaching – turning pale as they lost the algae that gave them their distinct colours and helped them photosynthesise. This happens when the ocean warms – and warming oceans is one of global warming’s major consequences.

This said, bleaching alone is not a danger; algae can regrow on coral given the right conditions. The real problem is prolonged exposure to warm water, which can kill off corals entirely. Picture a field of ghostly white skeletons unable to sustain reef fish or other marine life – that’s what it looks like.

Since 1998 cientists have observed both bleaching and coral death in the Gulf of Mannar, which comprises a group of 21 coral islands. The most significant bleaching events here happened in 2010 and 2016. In the latter year, scientists reported that almost 50% of corals in a few islands off Thoothukudi had bleached. Some patches across the Gulf displayed similar effects.

Ailing corals support fewer species and quantities of reef fish, or drastically affect fish communities. This in turn hits coastal communities. More than 100,000 fishers in coastal villages near the Gulf depend on reef-associated fishery, according to J.K. Patterson Edward, the director of SDMRI.

As part of a study to improve biodiversity and enhance fish catch, Edward and his team placed ‘artificial’ reefs underwater near Vaan Island in 2002. These are tent-like contraptions devised out of three cement slabs fused shoulder-to-shoulder. By 2004, plankton, barnacles and a host of other marine life had latched on to these slabs, as did four types (or genera) of corals. In 2017, the team reported 14 types of corals growing on the slabs.

“In India, artificial reefs are usually deployed to enhance fish production because fish aggregate at reefs. But they can also aid resilience in coral communities,” Edward said. Currently, they have deployed around a thousand artificial reefs in the Gulf and the adjacent Palk Bay, including near Mandapam town and Thoothukudi port. Since 2002, his team has also been restoring some reefs by attaching live coral fragments to concrete frames.

This process, called coral transplantation, has been fairly successful: 80% of the team’s transplants have survived. Other teams have been monitoring coral health and conducting coral transplantation exercises of their own. In 2018, Chennai’s National Centre for Coastal Research (NCCR), affiliated to the Ministry of Earth Sciences, conducted a coral transplantation campaign in six islands in the Gulf.

As they grew, the transplanted coral secreted calcium carbonate – or lime, which helps corals adhere to a substrate. The team is considering how they might replicate their success in other Indian reefs, said T. Shunmugaraj, an NCCR scientist leading the project.

Monitoring environmental impacts

The coral resilience prevailing in the Gulf today reflects the resilience many reefs around the world have put on display, said NCCR director M.V. Ramana Murthy. In an effort to support United Nations Sustainable Development Goal No. 14 – conserving underwater life – his team will study the environmental factors that support this resilience and growth, he added.

And there are many such factors. Resilience can be species-specific: For example, some branching corals in the Gulf didn’t bleach during a warming event in 2016 while others did.

Even the weather plays a role, said Edward. In the summer of 2020, when the Gulf experienced more coral bleaching, cyclones, low pressure events and depressions pulled these temperatures down. As a result, Edward added, “all the bleached corals recovered faster than they usually would.”

Similarly, near Hare Island in 2019 a lack of sea currents, which could have displaced warmer sea water and decreased the sea surface temperature, could have promoted the bleaching that year, said T. Shunmugaraj. Strong wave action can also erode coral islands.

This is one important reason, together with the coral mining before to the 2005 ban, that Vaan Island was eroding at an accelerating pace, scientists noted. So in a state-backed project in 2015, SDMRI introduced a different set of artificial reefs near Vaan to stem this problem. It worked. By 2017, Vaan Island had grown by 2.24 hectares in low tide, as The Wire has reported. Sources at SDMRI said this figure has since increased to 3 hectares.

Competition between corals and other marine life – and how climate change alters these interactions – could be yet another factor in coral resilience. Sponges and some species of algae, for instance, can easily grow over bleached or injured corals. Scientists at SDMRI have spotted the yellow spot sponge competing with bleached corals off Thoothukudi this way. But they have also found that a native species of mollusk – the tiger cowry – feeds on this sponge.

While corals may have an ally in the tiger cowry, these mollusks are threatened by human activity: The locals harvest them in huge numbers to craft shell jewellery popular among tourists. Protecting or at least regulating these harvests could boost coral resilience to some degree, researchers have said.

Protection, enforcement and awareness

Edward has been observing the Gulf for more than 30 years. And while coral restoration is a great tool, he said the one thing that triggered enormous recovery in the Gulf was the coral-mining ban.

“That’s when coral recruits increased. From 2005 to 2009, there was so much resilience until the 2010 global mass-bleaching. We lost some corals then,” he said. “After that also there was some resilience. After the 2016 bleaching, too, there was resilience – though not as much as before.”

Climate change is still very much the primary concern – but sustained law enforcement by the forest department is an important reason many existing reefs still stand, Edward added. The department intensely monitors and patrols the Gulf of Mannar Marine National Park. Anti-poaching sea and island patrols include around a hundred local fishermen as watchers. Separate teams engage in shore patrols to ensure no illegal marine catch is landed, and a mobile ‘forest squad’ helps with daily enforcement activities.

Most importantly, community engagement – like the formation of eco-development and marine conservation committees in 252 villages – and awareness programmes have been paying off. Local fishers are cooperating with the department, even informing them of any violations, according to A. Marimuthu, the park’s wildlife warden.

“The biggest stakeholders are the local people. Unless they feel that it is an important resource [that] should be conserved for their own well-being, no conservation is possible,” Marimuthu said.

Aathira Perinchery is a wildlife biologist-turned-journalist who writes about wildlife and conservation science in India.

Banner image: Coral transplantation activities undertaken by the NCCR in the Gulf of Mannar / Credit: NCCR Mandapam.