The sea around Kariyachalli Island, off the Thoothukudi coast, holds evidence of an ecological crisis unfolding largely out of sight. From above, the island appears as a shrinking strip of land surrounded by turquoise waters. Beneath the surface, damaged coral reefs and dwindling seagrass meadows reveal why saving this fragile landform requires more than protecting its shoreline.
TNIE travelled to Kariyachalli with scientists, divers and forest officials and undertook a couple of dives to witness the restoration work first-hand. Despite poor underwater visibility, transplanted coral fragments and restored seagrass beds were visible offering small signs of recovery in an ecosystem under pressure. Nearby, thousands of artificial reef modules now lie on the seabed, intended to reduce wave energy, encourage sediment accumulation, attract corals and provide habitats for marine life. The deployment of all 8,500 multipurpose artificial reef modules was completed this week under the Tamil Nadu Sustainably Harnessing Ocean Resources and Blue Economy (TNSHORE) project, funded by the World Bank. The intervention also includes restoration of two acres of degraded coral reefs and three acres of seagrass beds.
The scale of the challenge is enormous. Chief Wildlife Warden Rakesh Kumar Dogra told TNIE Kariyachalli’s area declined from 20.85 ha in 1969 to just 5.97 ha in 2018 and further reduced to 3.14 ha in 2025, a loss of more than 85%. Conservation assessments have warned that the island could be completely submerged by 2036 without effective intervention. “The decline is evident in the data on coral cover around Kariyachalli as well. The reef area shrank from about 500 ha in 2005 to 197 ha in 2021, with 79 hectares of the remaining reef area classified as degraded. Live coral cover, which stood at 46.6 per cent in 2005, rose to 51.9 per cent in 2009 before bleaching in 2010 brought it down to 37.1 per cent. It went down to a record low of 26.2 per cent after a massive bleaching event in 2024. There was only a marginal recovery to 26.7 per cent in 2025,” Dogra said. With two of the original 21 islands in the Gulf of Mannar already lost, Kariyachalli has become a test of whether engineering and ecological restoration can reverse an apparently relentless erosion trajectory.
Rebuilding the island’s natural defences
Kariyachalli is part of the Gulf of Mannar Marine National Park, where islands are closely linked to the condition of surrounding coral reefs and seagrass meadows. Reefs act as natural breakwaters, dissipating wave energy before it reaches the shore. Seagrasses help bind sediments, stabilise the seabed and support marine organisms.
When these habitats deteriorate, the island loses part of its natural protection. Waves and currents can reach the shore with greater force, while the loss of sediment makes it harder for the landform to maintain its area.
Historical coral mining weakened the reef system, and recurring bleaching events have added to the damage. K Diraviya Raj, associate professor and coral reef researcher at the Suganthi Devadason Marine Research Institute (SDMRI), said bleaching had become a major threat after coral mining stopped.
“Coral mining was an issue before 2004. But in recent decades, coral bleaching has become the primary threat, not just in the Gulf of Mannar, but globally,” he said and added that severe bleaching events in 1998, 2010, 2016 and 2024, which, according to the assessment he described, had reduced coral cover by almost half. As the super El Nino is getting supercharged this year, the coming summer is going to be critical. There is a growing threat of another mass bleaching event.
SDMRI director Edward JK Patterson said the restoration effort was aimed at rebuilding the living ecosystem, not merely placing structures underwater. “What we are doing is restoring corals to keep the biomass intact.”
Of the original 21 islands in the region, only 19 remain. The loss makes it urgent to understand how the surviving islands are changing and where conservation interventions could have the greatest impact.
Engineering designed for local conditions
The 8,500 artificial reef modules deployed around Kariyachalli were designed with technical support from the Department of Ocean Engineering at IIT Madras. Their positioning was informed by bathymetry (the underwater topography) and wave dynamics, which help determine where structures can dissipate wave energy and encourage sediments to settle. The modules were made using high-grade concrete, cured for 28 days and subjected to compressive-strength testing. From the construction site, they were transported to a storage area at Sippikulam village, taken to a jetty and loaded onto a barge before being carried to their designated locations. Each module had to be placed according to a deployment layout based on site conditions.
The IIT Madras team led by S A Sannasiraj, Chair Professor in the Department of Ocean Engineering at IIT Madras, said the design needed to account for differences between islands. “Each island in the Gulf of Mannar is different in terms of orientation and coral or seagrass distribution around it. Hence, the reef module has been redesigned considering this bathymetry,” the team said. The structures are intended to serve two purposes. They can reduce the energy of incoming waves and create conditions in which sediments accumulate. Their hard surfaces can also provide a foothold for corals and other marine organisms. But completing deployment is only the beginning. Monitoring will be needed to establish whether the structures remain stable, alter wave conditions as intended, encourage sediment deposition and support biological colonisation without damaging existing habitats.
Akhil Tambi, Wildlife Warden, Gulf of Mannar Biosphere Reserve, told TNIE: “Continuous monitoring will be done for four years.”
How coral fragments become colonies
At the coral restoration site, Diraviya Raj explained how small branches from healthy donor colonies are secured to slabs and placed underwater to grow into new colonies.
“We have transplanted it. We collected a small branch from the donor colony and tied it with this cement slab,” he said, pointing to the coral fragments attached to the slabs.
The slabs are placed inside concrete frames measuring one metre by one metre. Each frame can accommodate 10 to 12 slabs carrying coral fragments. The frames are then deployed at selected underwater sites and monitored for attachment and growth. The initial months are crucial. The fragments are expected to attach firmly to the substrate within two to three months, allowing them to develop into separate colonies. The survival of each fragment will depend on factors including water quality, temperature, sediment conditions and wave action.
Scientists take only a small portion, about 3 per cent to 5 per cent, from a healthy donor colony, with care taken to avoid significant damage. Branching corals are prioritised because they grow relatively quickly and can help rebuild coral biomass. Slow-growing, massive corals require a different approach.
“Those are very slow-growing, and for slow-growing corals, we are putting artificial reefs,” Diraviya Raj said, explaining the two strategies. The reef modules can also encourage natural coral recruitment. Corals release gametes that fertilise and develop into larvae called planulae. These larvae remain free-swimming for around four to 10 days before settling on a suitable hard surface. Artificial reef modules can provide that surface, allowing new colonies to establish without fragments being taken from donor corals. Transplantation and natural recruitment therefore serve complementary purposes, one introduces coral fragments directly, while the other provides conditions for the ecosystem to regenerate.
Seagrass and blue carbon
The restoration programme also aims to rehabilitate three acres of degraded seagrass beds. Seagrasses are important not only as habitats and feeding grounds for marine life like critically endangered Dugongs, but also because their roots and rhizomes bind sediments and their meadows store carbon.
For transplantation, seagrass sprigs are tied to casuarina frames using jute twine. Local women help prepare the frames, which are transported by boat and anchored underwater at selected sites. Scientists then monitor whether the shoots survive and spread.
As the plants establish themselves, they can help stabilise the seabed and slow water movement close to it. Their contribution complements the wave-dissipating role of the coral reefs and artificial structures.
The project also includes a blue-carbon assessment. Scientists collect sediment cores and seagrass biomass samples to estimate the organic carbon stored in vegetation and seabed sediments. The work could provide a broader picture of the benefits of restoring marine habitats from biodiversity and fisheries to coastal protection and carbon storage.
Meanwhile, the Tamil Nadu government is also planning to carry out a comprehensive assessment for the Gulf of Mannar and Palk Bay. Deepak S Bilgi, Chief Mission Director, Tamil Nadu Coastal Restoration, said a proposal was under review to help the government make informed decisions on future conservation and restoration.
Akhil Tambi said a detailed survey was needed to map coral reefs, seagrass meadows and underwater topography across the islands. The proposed assessment would combine satellite observations with extensive field surveys to identify vulnerable habitats and prioritise future interventions.
(S V KRISHNA CHAITANYA is a resident fellow of the Climate Change Media Hub at the Asian College of Journalism. The programme is supported by Interlink Academy Germany)