Shore seine operation has been in practice for many years mainly around the islands of Gulf of Mannar and Palk Bay. As the reefs and seagrass beds are close to the shore, they are damaged by these trawls. Recruits on dead rubble are brought ashore, and killed instantaneously.
Inshore bottom trawling in the reef and seagrass areas is also a deleterious fishing method that damages the entire benthic community.
The indigenous fish traps made of bamboo and, currently, fibre are kept in reef areas to capture reef associated fishes. About 300 vallams (a type of boat used by small-scale fishermen) are engaged in trap fishing and each vallam carries 20-25 traps. The traps are set close to the reef or between reef-covered areas. Corals are purposely broken to set the traps firmly, and during trap retrieval the reef areas are again disturbed. Reef dwelling herbivores in particular parrot fishes are caught widely, which results in the proliferation of macroalgae over live coral colonies causing coral mortality and ecological imbalance.
There are several industries operating in the southern part of the Gulf of Mannar region. They include chemical industries, a major port, thermal power plants, and a chain of salt pans. The area between Pamban and Tuticorin basically suffers due to the discharge of untreated domestic sewage let out directly into the sea, which seriously affects the environmental health.
In Palk Bay, the untreated wastewater from aquaculture ponds and domestic sewage poses threats to seagrass beds, coral reefs and associated biodiversity.
The red alga Kappaphycus alvarezii is reportedly indigenous to Indonesia and the Philippines. K. alvarezii is a destructive as well as invasive species as per IUCN’s Global invasive species database, wherein this alga has been termed as “Destructive Invasive Species” and ‘Serious Danger’ to coral reefs.
This exotic invasive alga was introduced into the Gulf of Mannar for commercial cultivation in 1990s for commercial cultivation. The Government of Tamil Nadu issued orders in December 2005 [G.O. Ms. No.229, E & F (EC.3) Department dated 20.12.2005] that K. alvarezii can be cultivated only in the seawaters North of Palk Bay and the South of Tuticorin coast by members of Self Help Groups; to disallow the cultivation of K. alvarezii near the eco-sensitive zones of Gulf of Mannar and Palk Bay.
Since 2005 researchers have been cautioning of the impact of the introduction of the exotic K. alvarezii into Gulf of Mannar. In 2008 it was noticed that K. alvarezii had invaded the coral colonies of the islands of Shingle, Krusadai and Poomarichan. Thereafter, till now the invasion of K. alvarezii has caused impact on vast reef areas killing thousands of coral colonies (branching and non-branching) in the islands of Krusadai, Shingle, Poomarichan, Mulli, Valai and Thalaiyari in Gulf of Mannar.
Tamil Nadu Forest Department has taken positive management actions to protect the reefs from bio-invasion. As a first step, in 2010 the Ramanathapuram District Collector initiated the manual removal of K. alvarezii from the coral colonies in Krusadai Island. The Ministry of Environment, Forest and Climate Change (MoEF&CC), Government of India (GoI) also funded the Tamil Nadu Forest Department for the removal of K. alvarezii from the affected area from 2011 onwards. Even after removal this alga can re-grow from loose left-out fragments as small as 0.5cm. It is proven not only in GoM but also in other parts of the world that once this alga establishes itself in coral areas, it is not possible to remove it completely, either manually or using super-sucker machines. MoEF&CC, GoI, has funded a four-year (2013-2017) research project “Studies on the impact of exotic seaweed, K. alvarezii on corals, associated resources and management measures in Gulf of Mannar Marine National Park” to SDMRI. The findings of this study reveal that K. alvarezii has caused significant impact on coral communities. Damage to corals from overgrowth by K. alvarezii would be long-lasting and irreparable. Thus this invasive alga has become a persistent threat to the corals of the Gulf of Mannar.
Coral diseases are a rising problem on coral reefs worldwide. Disease outbreaks not only reduce coral cover but also can cause significant changes in community structure, species diversity and abundance of reef associated organisms. Incidences of disease outbreaks have become more frequent in Gulf of Mannar during the last few years.
Climate change has intensified the space competition between corals and other benthic organisms such as macroalgae, sponges and octocorals. Due to the bleaching driven coral mortalities in 2010 and 2016, algal cover in Gulf of Mannar increased considerably taking the space from corals. Space competing sponges have become a serious threat in Gulf of Mannar during the last decade as sponges can survive in extreme climatic conditions.
SDMRI Reef Research Team has been regularly monitoring the reefs in Gulf of Mannar since 2005. Significant rise in the surface water temperature during summer and the subsequent coral bleaching and mortality are being observed. Significant coral mortality (10% live corals) was only recorded in 2010, when elevated temperature (32.2 to 33.20 C) persisted for four months (April to July). In 2016, about 16% of coral mortality was recorded due to elevated temperature caused in conjunction with the 3rd Global Coral Bleaching event.
Blooms of Noctiluca scintillans have become frequent in Gulf of Mannar, and they have occurred during the same season (September-October) of the year consecutively in the past three years (2019-2021).
These blooms have caused significant fish kill by depleting dissolved oxygen levels and damaging the gills of fishes.
Low dissolved oxygen levels are detrimental to the corals too, and hence the blooms cause coral mortality.
N. scintillans blooms, like the coral bleaching events, have also become a major annual threat to corals.
There is urgent need to better understand the ecology of these blooms and to formulate mitigation measures for conserving the fragile coral ecosystem of Gulf of Mannar.
Gulf of Mannar has a chain of 21 low-lying uninhabited coral-reef islands, which were formed by complex physical and biological processes, starting from the interglacial period. In recent years, reef-building hard substrates present around these islands are affected by natural and anthropogenic activities. Boulders and branching coral species were extensively exploited for building materials and chemical industries in the Gulf of Mannar for more than a century. Mining of coral reef not only removes the hard substrates but also changes the reef geomorphic structures and alters the prevailing wave climate. As a result the shores of the islands have been exposed to direct assault of wave action leading to erosion. So far two islands have submerged and fifteen of them have reduced area cover.