- A recent study of the islands in the Gulf of Mannar has found evidence of reduction in the surface area since 1973.
- Accretion and erosion across these islands are mediated by ocean swells and rising sea levels.
- Rising sea levels and frequent ocean swells are also causing a “migratory response” in the islands.
Afloat the turquoise waters in the Gulf of Mannar, along the Tamil Nadu coast from Rameswaram to Tuticorin, are islands that house the country’s richest marine biodiversity. More than 4,223 species of flora and fauna are found here. The region was named a Marine National Park in 1986 and the country’s first Marine Biodiversity Reserve three years later.
But there’s now trouble in this marine paradise. Two out of 22 islands have already vanished, with erosion threatening the ones that remain. A recent study of the islands in the Gulf of Mannar by the CSIR-National Institute of Oceanography found that approximately 5.40% of the overall surface areas of these islands has reduced since 1973.
Additionally, many of these islands are also experiencing erosion and accretion due to rising sea levels and ocean swells.
How islands erode
The islands in the Gulf of Mannar are spread across 10,500 square kilometres and divided into five main archipelagos: Rameswaram (one island), Mandapam (seven islands), Keelakarai (seven islands), Vembar (three islands), and Tuticorin (four islands). Poovarasanapatti, an island in the Keelakarai group, disappeared before 1973, and Villanguchalli in the Tuticorin group disappeared completely by 2000.
Accretion (growth) and erosion across these islands are mediated by a number of factors, including wave dynamics, wind speeds, the frequency and intensity of storms. Among the biggest factors, however, are ocean swells. “Ocean swells are long duration waves with higher energy compared to regular waves. We have seen a rise in the number of swells reaching the Gulf of Mannar in recent decades,” said Mani Murali R., research scientist at the NIO and co-author of the paper.
The study found that the southern Tuticorin island groups had the most reduction in their surface areas from 1973 to 2020, reducing by 63.97 hectares or 83.04%. This was followed by the Keelakarai islands (74.93 ha, 33.35%), Mandapam islands (153.92 ha, 29.60%), Vembar islands (56.48 ha, 28.14%), and Rameswaram islands (325.87 ha, 3.43%). Overall, the mean erosion rate across the examined shorelines was calculated to be 1.7 metres per year.
Rising sea levels and frequent ocean swells are also causing a “migratory response,” in the islands, says the study. The effect of swells “manifests as erosion of the seaward shoreline and accretion of the adjacent continent-facing shore,” says the study, adding, “Consequently, the islands exhibit a net inward migration toward the adjacent continental shore. It is essential to highlight that this migratory response is associated with a substantial decrease in island size.”
Years of coral reef mining in the 1980s have helped accelerate island erosion, said Gladwin Asir, assistant professor at the Suganthi Devadason Marine Research Institute (SDMRI) in Tuticorin, which studies the islands and works on reef restoration. “Coral reefs act as a barrier and reduce the velocity of a wave,” he explained, adding, “When a wave comes from the sea, it moves up a sloped area as it approaches the land. Without the coral reefs, the height of the wave is increased, and its impact is harsher.”
Islands that experienced accretion, or an overall growth in their planform areas, is likely due to sediment redistribution from the changing wave dynamics, said Mani. The sediment is generated from dead coral reefs in these islands.
The role of reef restoration
According to the Intergovernmental Panel on Climate Change’s latest assessment reports, the Gulf of Mannar is already experiencing an annual sea level rise of 3.38 mm per year, driven by global warming. “Collectively, under the scenarios of one metre and two metre-sea level rise, submergence may occur in an area measuring 627.30 hectare and 1284.21 hectare,” says the study. The total area of the islands together is 12,497.25 hectares. “The findings further highlight that sandy areas, mangroves and tidal flats will experience the most significant impacts, followed by land vegetation, barren land, and built-up areas,” the study adds.
Coral reef restoration presents an opportunity for mitigation, the study says. An experiment by SDMRI, deploying trapezoidal artificial reefs in the Vaan Island of the Gulf of Mannar saw high rates of survival among transplanted corals. Researchers transplanted 50 fragments of live corals using artificial reef modules at a depth between 1.5 and 2.5 m with the help of scuba divers. The transplanted corals had an average survival rate of 62%, 22 months after being transplanted. “Interestingly, the surviving fragments have withstood two mild bleaching events in 2019 and 2020,” says a paper documenting the experiment.
“It’s very important that nature-based solutions be deployed, and not engineered concrete structures to prevent erosion,” said Murali, adding, “These are fragile ecosystems which are facing erosion despite being uninhabited, without any anthropogenic pressure. Deploying nature-based solutions such as artificial reefs can help achieve Sustainable Development Goals as well.”
According to the study, restoring coral reefs can help achieve sustainable goals related to clean water and sanitation, climate action, life below water and life on land.
Read more: Coral transplantation helps, but not the only solution
Editor’s note: The story was updated on March 17, 2025, to reflect the phenomenon of ocean swells and their response more accurately.
Banner image: An aerial view of Rameshwaram island at the southeastern tip of Tamil Nadu, which straddles the Gulf of Mannar Biosphere. Image by Tracy Hunter via Wikimedia Commons (CC BY 2.0).
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