Artificial reefs bring a sinking island back to life in India's Gulf of Mannar

Vaan Island
The New Indian Express
,
Tamil Nadu, India

Artificial reefs bring a sinking island back to life in India's Gulf of Mannar

THOOTHUKUDI: A few years back, scientists thought Vaan Island, a coral formation in the Gulf of Mannar, would be completely underwater by 2022. Already, two of the 21 islands in the fragile Gulf of Mannar Marine Biosphere Reserve — Vallanguchalli and Poovarasanpatti — had been submerged, and the scientists expected it was only a matter of time before Vaan was also lost.

Data showed the coral island to be on the brink of collapse — its area had been reduced drastically from 20.08 hectares in 1969 to 1.53 hectares in 2015; a substantial 23.92 percent of the live corals on the islands were bleached during the global coral bleach of 2016; and the live coral cover that remained fell from 38.86 percent in 2015 to a mere 22.69 percent in 2016.

Then, a miracle happened, and today Vaan Island is teeming with colourful marine life. The shallow, bright blue waters surrounding it are home to healthy coral reefs. Since 2018, the reef cover has also seen a steady increase, with mortality rate dropping to negligible levels.

With several other islands in the same biosphere facing threats of extinction from ecosystem degradation, what saved Vaan could serve as a model that can be replicated to salvage the entire area, say experts. So, what exactly did Tamil Nadu do to save the dying island?

Decades of plunder

Island subsidenceThe Gulf of Mannar is the first marine biosphere in Asia. But since the 1969 Survey of India Toposheet, there had been no comprehensive data collected on the morphology of the islands in the Gulf. In 2017, to understand the morphological changes that had occurred there over the last five decades, the Union Environment Ministry supported a study by Suganthi Devadason Marine Research Institute (SDMRI).

The data the study produced was stark.

It showed that over period of 50 years, the Thoothukudi group of islands, including Vaan, had experienced the highest percentage of land cover reduction (71 percent), followed by Keelakarai (43.49 percent), Vembar (36.21 percent) and Mandapam (21.84 percent). There were multiple reasons why these islands were sinking — unsustainable and exploitative fishing practices, rising sea levels and surface temperatures due to climate change, and coral mining that went on for decades until it was stopped in 2005. The most significant of these factors in land reduction, the study found, was mining.

The corals were being mined for their rich lime content, which was in great demand in the limestone industry and played a major role in construction. In fact, many old houses in Thoothukudi that stand tall today were constructed using corals. Vaan itself is made up of calcareous frameworks of dead reefs and sand and was a minefield for coral hunters.

The demand was so high that in the early 1970s, around 25,000 metric tons of corals were mined every year from Palk Bay and the Gulf of Mannar combined. It was only after a good three decades, in 2001, that the central government included all corals under Schedule I of the Wildlife Protection Act. Subsequently, in 2005, the Supreme Court stayed a halt on coral mining activities. But the damage had already been done. By 2005, the Gulf of Mannar had lost a whopping 32 square kilometres of reef area.

Secret of success

Armed with this data, the Tamil Nadu government started on a first-of-its-kind mission to revive these drowning islands. It first sought financial support from the Environment Ministry, under the climate change adaptation fund. The ministry committed 25 crore rupees (US$3.44 million), and Vaan Island was chosen for the restoration project.

“The reason was because Vaan is closest to the mainland, barely five kilometres from the Thoothukudi coast,” said KV Giridhar, director of the Department of Environment. “Reviving Vaan was crucial because the island acts as a defence against waves, preventing coastal erosion.”

To save Vaan from drowning, we had to ensure the reef area surrounding the island was revitalised, added Edward Patterson, director of SDMRI.

“Corals need hard substrate to grow. Generally, coral larvae use dead corals as substrate, which unfortunately is not available in many islands of the Gulf of Mannar due to decades of mining," Patterson said.

So instead, his team of researchers helped deploy 10,600 artificial reef modules in three phases.

"Initially, we deployed artificial reefs with the main purpose of protecting the fast-eroding landmass, but in due course, found natural corals colonising in these artificial reefs. This helped Vaan to grow. Compared to 2015, Vaan’s land cover has increased by a whopping 54.51 percent from 2.33 ha to 3.60 ha in low tide."

Artificial reef
Fish and corals now teem in the waters surrounding Vaan Island / Credit: SDMRI

The latest morphological study, conducted by SDMRI in October 2020, shows abundant natural coral recruitment. A total of 10 coral species and 28 fish species were observed around the modules. The Indian Institute of Technology Madras (IIT-M) also had an important role to play. The IIT-M team worked alongside SDMRI researchers to design the artificial reef modules.

“After many trials, we worked out the design, the build, pattern and place of deployment of artificial reefs based on bathymetric wave studies," said Prof. SA Sannasiraj, head of thee Department of Ocean Engineering at IIT-M. "The holes in the concrete structure were to achieve good water circulation. The holes are also required for marine growth. The whole experience was unique and path-breaking as such a project was never tried before in India. It gives us great satisfaction that our efforts materialised and corals are thriving on these structures,” he said.

Small success, big lesson

Vaan, however, is not a destination but an example. There are many islands in the Gulf that need attention. Researchers say Koswari and Kariyachalli islands near Thoothukudi are facing an increased threat and need immediate intervention.

“If the current rate of erosion continues, Koswari and Kariyachalli islands are likely to be submerged by 2036; Vembar group [Upputhanni, Puluvinichalli and Nallathanni] between 2064 and 2193; Keelakarai group [Anaipar, Valimunai, Poovarasanpatti, Appa, Thalaiyari, Valai and Mulli] between 2032 and 2180; and Mandapam [Manoliputti, Poomarichan and Pullivasal] between 2140 and 2525,” said K Diraviya Raj, assistant professor at SDMRI.

Morphology and geomorphology studies reveal Koswari island has lost 60 percent of its land cover, and has shrunk from 19.59 hectares to 7.70 ha, while Kariyachalli island land mass shrunk by 71.37 percent, from 20.85 ha to 5.97 ha. The exceptions to this general rule are four islands of the Mandapam group — Hare, Manoli, Krusadai and Shingle — which saw their area increase by 16.44 percent. Hare island is the biggest of all, with an area of 152.31 ha.

However, Krusadai and Shingle islands face an increased threat from a newly-constructed fishing jetty by supported by Tamil Nadu's fisheries department. Despite opposition from the forest department, the fisheries department went ahead and finished the construction of the jetty, triggering erosion problems.

The scientists and researchers involved in the restoration project say that what the Vaan example indicates, it’s that the other islands, too, could soon be home to healthy coral reefs, if due attention is paid and action taken.

Climate control

Coral bleaching is another growing problem, and while coral reefs in the Gulf of Mannar have shown a great degree of resilience to climate change, experts say continuous monitoring is needed.

“Corals here bleach in summer if the water temperature surpasses 300 Celsius," said Marimuthu, wildlife warden at the Gulf of Mannar Marine National Park. "But they recover when the temperature comes down, which usually happens in August.

Last April, water temperatures reached 31.80 Celsius and partial coral bleaching (of less than five percent) was observed, Marimuthu explained. "The high temperatures persisted up to May 2020 [the highest level being 31.90 C], when widespread bleaching was witnessed. This triggered fears of mass bleaching and high mortality."

To measure the prevalence of coral bleaching, SDMRI carried out a comprehensive underwater assessment jointly with the Tamil Nadu forest department from April to June 2020. Eleven islands (Krusadai, Manoli, Hare, Valai, Thalaiyari, Poovarasanpatti, Puluvinichalli, Vilanguchalli, Kariyachalli, Koswari and Vaan) representing the three island groups were assessed.

Status of Vaan IslandThe assessment found average bleaching prevalence was 28.20 percent. The Thoothukudi group of islands had the highest bleaching prevalence (30.80 percent) followed by Mandapam and Keelakarai groups. Among the islands, Koswari has the highest bleaching prevalence (38.40 percent) followed by Vaan (34.80 percent) and Kariyachalli (34.70 percent). SDMRI director Patterson said despite such high temperatures, the corals have recovered fully and no mortality was reported.

“This is largely because of low pressure and cyclonic storms that brewed in the Arabian Sea and Bay of Bengal in April and May," said Patterson. "The water temperature dropped to 28.60 Celsius in early June, which helped the corals restore their zooxanthellae. The bleached corals would completely recover by the end of July.”

Significantly, the reduction in sewage inflow, industrial and human activity and halt of fishing during the coronavirus-induced lockdown assisted in the improvement of reef health, resulting in enhanced fish populations and faster coral recovery, Patterson added.

The dangerous traps that don’t just affect fish

Trap fishing for ornamental fish is practised near the reef area. Indigenously-fabricated fish traps are set in and around the reefs. To keep these traps safe, nearby live and dead corals are broken. Therefore, the reefs are disturbed when laying and retrieving the traps. Marine biologist G Mathews said, “In most cases, the traps are laid mainly to catch reef-dwelling herbivores such as the parrotfish, which in turn cause the proliferation of algae over live coral colonies due to the lack of predators, leading to coral mortality and ecological imbalances.”

Parrotfish are colourful and voracious herbivores that spend up to 90 percent of their day eating algae off of coral reefs with their beak-like teeth.

“Though [the parrotfish] is not consumed locally, it’s exported to China, among other countries,” said Mathews.

Trap fishing
Trap fishing, which damages nearby live corals / Credit: SDMRI

The Global Coral Reef Monitoring Network of the International Union for Conservation of Nature released a report in 2014 on Caribbean corals, which reportedly declined by more than 50 percent since the 1970s. The report says most Caribbean reefs have been dominated by algae, a phenomenon was linked to overfishing of parrotfish.

The healthiest Caribbean reefs are those that still have robust parrotfish populations, such as Bermuda and Bonaire, which restricted or banned fishing practices that harm parrotfish, including fish traps.

Forest range officer R. Raghuvaran said he and team --  internationally-certified scuba divers -- regularly patrol in reef areas.

"We regularly monitor the health of coral reefs and remove fish traps laid in the reef area," he said. "Fishermen are now aware that damaging coral reefs is an offence. Coral mining was completely stopped since 2005, but poaching of protected sea cucumbers still gets reported.”

An earlier version of this story was first published in The New Indian Express on 14 February 2021. This version has been edited for length and clarity. Thee story was produced with the support of Internews' Earth Journalism Network.

Banner image: An aerial view of Vaan Island with artificial reefs pictured underwater / Credit: SDMRI

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