Threats to Gulf of mannar coral reefs

1. Gulf of Mannar coral reef diversity,
Threats and conservation measures
prepared by:
B. Bhaskar
FRM
Acknowledgement: ICAR

2. Gulf of Mannar
• The Gulf of Mannar reefs are located in southeastern India, mainly
between Rameswaram on the north and Tuticorin on the south.
• The south side of the reef area is having several industries and a major
port.
• The Gulf of Mannar reefs are formed around the 21 islands situated
between lat. 8’ 47 - 9’ 15’ N and long. 78’ 12 ‘ - 79’ 14’ E and the area
comes under Gulf of Mannar Marine National Park.
• For administrative purpose and also to provide better managerial
service, the 21 islands are grouped in to four viz. Mandapam (7
islands), Keezhakkarai (7 islands), Vembar (3 islands) and Tuticorin (4
islands) based on the geographic distribution.
•

4. Areal distribution of offshore islands ,
types of coral reefs and their extent

5. Reefs in Gulf of Mannar
• Different types of reef forms such as shore, platform, patch, and
fringing type are observed in the Gulf of Mannar (Venkataraman et
al., 2003).
• The islands have fringing coral reefs and patch reefs around them.
Narrow fringing reefs are located mostly at a distance of 50–100 m
from the islands. On the other hand, patch reefs rise from depths of
2–9 m and extend to 1–2 km in length with width of as much as 50
m. Reef flat is extensive in almost all the reefs in the Gulf of Mannar.
• Pillai (1986) had provided a comprehensive account of the coral
fauna of this region. There are about 94 species of corals belonging
to 32 genera in the Gulf of Mannar.
• The most commonly occurring genera of corals are Acropora,
Montipora, and Porites.
• Extensive sea grass beds are present; green turtles, olive ridley
turtles, and dugongs are dependent on these sea grasses as their
feeding ground.

6. Threats to Coral Reef in GOM:
I. Over fishing and destructive fishing practices
II. Sea weed collection
III. Commercial shell collection
IV. Coral mining
V. Poor land use practices
VI. Coastal urban development
VII.Harbour and dredging activities
VIII.Industrial development and pollution
IX. Natural threat

7. 1. Over fishing and destructive fishing practices:
• Fishery is the primary economic activity of the people of the
coast of Gulf of Mannar. There are about 50,000 fishermen
engaged in fishing activity. Nearly 20% of total fishes in
Tamilnadu are being caught from Gulf of Mannar.
• Much of the fishing is carried out on / and near the coral reef
area.
• In many areas of Gulf of Mannar, the majority of the reefs have
been destroyed by fishing due to modern fishing techniques like
mechanized launches with trawling net and gill net and also by
destructive fishing practices such as dynamite fishing near
Tuticorin area.
• These destructive methods cause direct damage to the corals
and indirectly affect their growth by increasing turbidity and
suspended sediment in the coastal waters and reducing the
clarity of seawater and increasing sediment loads on reefs.

9. OVERFISHING
Over-fishing and
overexploitation of coral
reef fisheries and coral rock
and sand resources.
Within the last two decades there
has been an increase in damaging
fishing activities involving the use
of home-made bombs, cyanide and
other damaging practices such as
muro-ami that involves dropping
weighted rocks onto corals to drive
fish into set nets.

10. Changing fishing practices
• What is causing marine debris, and why are islands closer to Thoothukudi bearing
the brunt? That could be because human population has increased near the city –
and in turn, the pressure on fish resources – said Patterson Edward.
• As per the last Census of India (2011), the population of Ramanathapuram district
(one of the two coastal districts that border the Gulf of Mannar) alone recorded a
growth of 14 per cent in ten years.
• The fishing industry here has changed too, explained Raj. Like in many other coastal
areas in the country, yields from the ocean have started decreasing over the past
few decades, while prices have increased.
• Fishing is now a far more lucrative industry: most of the catch is exported.
• This in turn has increased fishing, and the pressure on the Gulf’s fish resources, he
said.
• “The use of plastics in fishing has also increased significantly in recent times.
• Even traditional fishing traps that have been in use for several decades now are no
longer made of bamboo but plastic,” he said.
• However, of the total 16,685 square metres of coral reefs the team surveyed, only
1152 square metres were afflicted with marine debris: an area far smaller than the
size of a football field. However, it’s not about area but impact,

11. Tackling debris
• One of the first steps in tackling marine debris is knowing
more about it.
• In 2016, in his commentary on marine debris, A. Arun
Kumar pointed out how the characteristics of marine
debris, its accumulation and transport pathways along
India’s coastline are poorly studied.
• Four years later, not much has changed.
• While individuals and non-governmental organisations are
studying this in one-off projects, or clearing marine debris
when they can, there is no sustained, large-scale effort to
monitor it in India, said Kumar.
• The lack of a standardised procedure to quantify it makes it
impossible to compare debris levels across the country and
world too.
• “Incentivising the collection of used fishing nets could be
one way to tackle marine debris,”

12. • Marine debris such as abandoned nets, plastic fish traps, ropes and lines are becoming far
more common on the coral reefs of the Gulf of Mannar, along the coast of southern Tamil
Nadu.
• Though marine debris spans an area of only 1,150 square metres in the Gulf, it has an
impact on coral health, making them more susceptible to injury, fragmentation, and
disease. It also affects corals’ existing resilience to climate change, worry researchers.
• Marine debris is increasing in the Lakshadweep as well, and it represents exactly how
wide the footprint of the commercial fishery is, say scientists.
• While there are individuals studying marine debris for stand-alone projects, and NGOs
clearing debris when they can, tackling the issue could require more sustained action
including long-term monitoring.
• Dive into the clear, shallow waters of the Gulf of Mannar and you’d be met with more
than a hundred species of corals.
• Among these vivid corals of varying shapes and sizes live thousands of reef-fish that
locals depend on for food and income.
• However, corals in some parts of this narrow strip of ocean separating the south Indian
state of Tamil Nadu from Sri Lanka now don some very odd-looking ornaments.
• Ghost nets (as abandoned fishing nets are called) envelop the finger-like branches of
some, while torn lines, ropes and discarded plastic fish traps are littered on others.
• Around 1,150 square metres of reefs in the Gulf are covered in marine debris.
• That amounts to a very small portion of coral cover in the Gulf, but that hasn’t stopped
researchers from raising concerns about the threat it poses to the waters biodiversity.

13. Nets, traps and lines
• In the Gulf of Mannar, which is one of India’s only four major
coral reef formations, marine debris has been turning up on
corals for some time now, recollects Diraviya Raj of the
Suganthi Devadason Marine Research Institute in
Thoothukudi.
• In their diving and snorkel surveys, they found that ghost
nets, fishing traps, ropes, lines and other debris were indeed
present in some patches.
• These marine debris swathed both live and dead coral
colonies; and more than 40 per cent of all debris were ghost
nets.
• Vilanguchalli and Kariyachalli, small coral islands just off
Thoothukudi city, recorded the highest prevalence of ghost
nets: around 60 per cent of marine debris here consisted of
ghost nets alone.
• Other small islands in this area also showed higher levels of
marine debris.

14. • Marine debris damages corals by causing injuries, or tissue
damage. It can also fragment or break corals apart. Some
coral genera are more prone to this, and ghost nets tend to
do most harm. Raj and his colleages, for instance, found
that in the Gulf, acroporids – corals of the genus Acropora,
that have delicate, finger-like branches – faced the most
amount of fragmentation when they came in contact with
marine debris. Corals on almost all islands they surveyed
for the study faced both tissue loss and fragmentation.
• Such structural damage to corals can make them more
prone to diseases. Scientists who studied almost 160 coral
reefs in the Asia-Pacific in 2018 found that the likelihood of
disease increased from 4% to 89% when corals came in
contact with plastic.

15. Effects on coral resilience
• Most importantly, such injuries and fragmentation also affect corals’ abilities to fight
back and revive after climate change-associated set-backs.
• The coral reefs off Thoothukudi, for instance (where ghost nets dominated marine
debris) are among the most resilient patches in the Gulf, said Patterson Edward.
• They had revived well after consecutive bleaching events. So tackling the issue of marine
debris and especially ghost nets – in time, and well – could be crucial in retaining coral
resilience here, according to Edward.
• Marine debris appears to be emerging as a threat in some of India’s other reefs too.
Debris has certainly increased in Lakshadweep, wrote scientist Rohan Arthur, in an e-
mail.
• Arthur, who has been studying these reefs in the Arabian Sea for more than two decades
now, said that he began monitoring marine debris only recently, for he did not see it as a
major problem even five years ago.
• Here too, like in the SDMRI study, the majority of debris is in the form of abandoned
nets and entangled fishing lines; along a gradient of urbanisation, there are also other
debris that increase (such as lost cement bags, construction material and plastic bags),
he wrote. But the amount of direct damage this may do in the Lakshadweep is likely to
be small, he noted.
• It is a very good indicator of how sharply reef fishing has increased in reefs like the
Lakshadweep, he wrote.
• “What is astonishing here is that, even in the most remote atolls of the archipelago,
including the uninhabited atolls, these abandoned signs of fishing are rife.”

16. 2. Sea weed collection:
• Seaweeds form one of the most important
marine living resources in Gulf of Mannar.
• Because of their commercial value, the seaweeds
are harvested by fishermen for agar industry.
• More than 1000 fishermen and women are
engaged in seaweed collection Gulf of Mannar
and nearly 5000-7000 tons (dry weight)
seaweeds are harvested annually.
• The continuous harvesting of seaweeds results in
the decrease in areal extent of seaweed area,
which may lead to coastal erosion and removal of
the coral reefs.

17. 3. Commercial shell collection:
• Commercial shell collection is another human
activity which causes coral reef degradation to
some extent.
• Shell collection is one of the important
economic activities of the fishermen living in
the study area. Nearly 770 fishermen are
engaged in this occupation.
• This activity increases the sedimentation in
coastal water leading to the death of coral

18. 4. Coral mining:
• Coral mining activity has caused extensive degradation of reefs, coastal erosion
and sedimentation in a number of countries all over the world. In gulf of
mammar coral mining is the major human activity, which causes coral reef
degradation.
• Coral reefs are used on a large scale as raw material by the lime industries.
• In Tuticorin group of islands this activity is very active and the corals have been
used for building, industrial and chemical purposes.
• The rapid development of lime based industries in and around Tuticorin has
accelerated the coral mining.
• Nearly 250 skilled divers and about 50 boats are engaged in the coral reef
mining.
• The use of explosives for reef mining, has produced very serious problems to
the coastal and marine ecosystems like coastal and island erosion and
sedimentation, coral reef degradation and reduction of marine faunal
population.

19. CORAL MINING
Coral mining involves live coral
being removed from reefs for use
as bricks or road-fill, while sand
and limestone is made into
cement for new buildings.
Corals are also removed from
their habitats to be sold as
souvenirs to tourists and
exporters.

20. 5. Poor land use practices:
• Another cause for the damage of coral reefs is poor land use
practices, such as agricultural activities, changing land use
practices and deforestation etc, which increases land derived
sediment flowing onto coral reef.
• The poor agricultural practices increase the agriculture waste like
pesticides and fertilizers, which are dumped into the coastal water
through surface runoff during rainy season and through rivers.
This encourages rapid algal growth, which chokes coral polyps and
cut off the supply of light and oxygen. This problem is common in
Tuticorin group of islands, Vembar group of islands and
Keelakkarai group of islands.
• Another important human activity encountered in Gulf of Mannar
is destruction of natural forest and island vegetation.

22. 6. Coastal urban development:
• Increasing urban population along the coastal
area has lead to pollution due to sewage
discharge into the coastal waters.
• Discharge of sewage is responsible also for the
loss of coral reefs in large quantities.
• Along the coast of Gulf of Mannar, five coastal
towns are located and due to rapid growth of
population, the urban areas have increase in the
amount of discharge of sewage waste into
coastal waters, which has caused the death of
corals in Gulf of Mannar.

23. 7. Harbour and dredging activities:
• Dredging and other harbour related activities like
anchoring and ship grounding have increased
sedimentation in the coastal waters and caused
the degradation of coral reefs.
• The periodic dredging operations at the entrance
of the harbour and anchoring of ships have
increased the amount of silt plume, which in turn
has increased water turbidity, and lowered the
light intensity, leading to coral death.
• This problem has been extensively observed in
Tuticorin group of islands.

24. Careless boating, diving, snorkelling and fishing
can also damage coral reefs.
Substantial damage has been caused by people
touching the reefs, stirring up sediment, collecting
coral, or dropping anchor on the corals.

25. 8. Industrial development and pollution
• Industrial development and their waste discharge into
coastal water affect the coral reefs.
• The coast of Gulf of Mannar is experiencing an
accelerated growth in the rate of industrialization led
to marine pollution and coral reef degradation.
• The dumping of fly ash slurry into Karapad Bay by the
Thermal power station resulted not only in filling up of
an extensive portion of the bay, but also letting out of
ash directly in to the sea causing extensive damage to
the coral reef.
• Dharangadhare Chemical Works Ltd, Plastic Resins and
Chemical Ltd and Tuticorin Alkali Chemical Ltd are the
major chemical industries in Tuticorin area, which
discharge their effluents into open sea.

26. Natural threats
• Natural problems such as storms, waves, sea level
variation, fresh water runoff, volcanic activity etc cause the
degradation of coral reefs.
• During the northeast monsoon period highly turbid and
low saline water from Palk Bay is transported to Gulf of
Mannar by long shore current along Pamban channel.
• This increases the silt in Gulf of Mannar waters and causes
the death of coral reef.
• Towards the seaward side of the island, the high velocity
waves hit against the reef edge (wave breaker) and a fall in
the velocity of waves cause the wave induced current
forms which moves towards the island and drop the littoral
sediment on reef leading to coral degradation.

27. • Predation by Crown: The triton snail living on the ocean bed
feeds on the young ones of the crown of throwns starfish and
keep their numbers in check. Once commercial harvesting of
the triton shells has started, there were not enough shells to
feed on the starfish. This has led to build up of the star fish
which in turn fed on coral reefs.

28. PREDATOR PLAGUES
Predator plagues such as COTS
are increasingly reported around
areas of human activities, with
two strong hypotheses
advanced:
•the plagues may be initiated and
certainly exacerbated by either
over-fishing of key starfish
predators, and/or
•increases in nutrient runoff from
the landfavours the planktonic
stages of the starfish.

29. Others
• Another fast emerging threat to the Gulf of Mannar corals is
from Kappaphycus, a seaweed cultivated at the sea bottom.
• It invades the corals and suffocates them to death.
• Around two to three square kilometres of area in the sea
bottom here is now invaded by Kappaphycus in GOM.

30. CORAL DESTRUCTION AND CLIMATE CHANGE
These newer global threats (bleaching, disease and
predators) are increasing rapidly in frequency and
severity, coincidentally with direct human
disturbances.
Coral disease has caused major disruptions to coral
reefs in the Caribbean with a range of human
disturbances potentially implicated, and there are
now increasing reports of similar disturbances from
the Indo-Pacific region.
Evidence linking severe coral bleaching and
mortality to increasing rates of global climate
change attributed to rising levels of anthropogenic
greenhouse emissions is growing stronger.

33. Thank you all