Management of Coral Reefs in Gulf of Mannar
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Management of Coral Reefs in Gulf of Mannar

Girija Phadke and Khaimesh Sehgal*

*Central Institute of Fisheries Education, Seven Bunglows, Versova, Andheri (W.), Mumbai, Maharashtra, India 400061.

*Corresponding Author: sehgal.cife@gmail.com

 

Introduction:

Coral reef system as also the ecosystem of the tropical rain forest, are the most matured marine ecosystems of our planet. The coral reefs characterize an ecosystem of high biological diversity, having the greatest number of species of any marine ecosystem. Livelihood of many millions of people is dependent on this unique ecosystem as a considerable proportion of their food and earning from the productivity of coral reef. Coral reefs are considered as one of the most important critical resources for various ecological, environmental and socio-economic reasons.

Uses of Coral reefs:

They play an important role in global biochemical processes and in the reproduction of food resources in the tropical regions. The people living along the coast obtain a considerable proportion of their food and earnings from the productivity of coral reefs. Coral reefs act as a barrier against wave action along coastal areas thus preventing coastal erosion. In addition, coral reefs protect mangroves and seagrass beds in certain areas, which are the breeding and nursing grounds of various economically important fauna. Coral reefs are also important breeding, spawning, nesting, and feeding areas for many economically important varieties of fishes and other marine organisms.

Gulf of Mannar:

Gulf of Mannar, extends from Tuticorin to Rameswaram Island in the SW-NE direction, lies between 78° 5' & 79°30' E longitudes and 8°47' & 9°15' N latitudes, to a length of about 140 km. There are 21 low islands situated at an average distance of about 8 km from the coast and running almost parallel to the coastline. These islands are broadly grouped into Tuticforin, Vembar, Keelakkarai and Mandapam groups. These areas are endowed with a combination of ecosystems including mangroves, seagrass, seaweeds and corals reefs. Different types of reef formations have also been observed along the periphery of the islands viz. fringing reef, patch reef and coral pinnacles.

In Gulf of Mannar, the coral reefs have been used as a source of calcium carbonate, building blocks and rubble for construction of roads. In general, coral reefs in Gulf of Mannar can be categorized as "degrading", and hence, monitoring and management of these valuable marine resources are of prime importance.

Blasting and dredging activities result in high sedimentation on the coral reef of Gulf of Mannar there by leading to its degradation. The coral reefs of Tuticorin group of Islands in Gulf of Mannar have been damaged due to the discharge of effluents from petrochemical and other industries along the coast, and fly ash discharges from thermal plants.

Techniques of study:

To study coral reefs area effectively and to monitor changes over time, accurate, rapid and cost-effective mapping techniques are required. Satellite remote sensing is widely used as a tool in many parts of the world for the management of resources and activities within the continental shelf containing reefs. Preliminary studies carried out in India by Space Application Center (SAC) Ahmedabad, have proved the importance of remote sensing data in mapping and monitoring of the coral reef.

 

Objectives of the Study:

 

The objectives of the present study are to:

  • Assess the current status of coral reefs in Gulf of Mannar based on remote sensing and GIS approach
  • Assess the changes that have occurred in the spatial distribution of coral reefs during the period 1988-1998 using multidate remote sensing data
  • Study the coastal geomorphology and its influence on the coral reefs
  • Study the influence of land use / land cover changes on the coral reefs
  • Suggest suitable management measures for sustainable management and preservation of coral reefs in Gulf of Mannar region

Materials and Methods:

 

To cover the objectives of this study, six types of approaches have been attempted are listed below:

  • Analyse and interpretation of optical remote sensing data for coastal geomorphology
  • Analyse and interpretation of optical remote sensing data and Survey of India Topographic sheets (1969) for shoreline change mapping.
  • Interpolation of Bathymetry using Naval Hydrographic charts and bathymetry survey
  • Analyse and interpretation of multidate optical remote sensing data for coastal land use/land cover change deduction
  • Socio-economic studies
  • Analyse and interpretation of multidate optical remote sensing data for coral reef change deduction.

Areal distribution of coral reefs in Gulf of Mannar:

 

A chain of 21 low islands has been observed along the offshore region of Gulf of Mannar (Table 1). It extends from south of Rameswaram to Tuticorin. All islands are made up of a calcareous framework of dead reef and sand. They have a low and narrow sandy coast and some of them have rocky coast. Around all offshore islands, well-developed coral reefs have been noticed. Geomorphologically, coral reefs in this area are of fringing type, though some patchy corals are also observed in between Appa Island and Pilliyarmuni Island, and in some areas like Bharathi nagar coast and southeast coast of Kariya Shuli Island.


Table 1: Areal Distribution of Offshore Islands and Types of Reefs and their Areal Extent

 

Sr. No

Islands

Island Area(km²)

Coral Reef

Reef type

Area(km²)

1

Van Island

0.245

Fringing type

1.090

2

Koswari Island

0.241

Fringing type

1.474

3

Kariya Shuli Island

0.166

Fringing type

0.702

4

Uppu Tanni Island

0.377

Fringing type

0.644

5

Shalli Island

0.126

Fringing type

0.249

6

Nalla Tanni Island

1.248

Fringing type

1.250

7

Anaipar Island

0.229

Fringing type

0.888

8

Palliyarmunai Island

0.134

Fringing type

0.687

9

Appa Island

0.410

Fringing type

3.612

10

Talairi and Valai Island

1.072

Fringing type

9.268

11

Muli Island

0.170

Fringing type

1.208

12

Musal Island

1.836

Fringing type

27.73

13

Manalli Island

0.353

Fringing type

14

Manalli Putti Island

0.037

Fringing type

15

Pumurichan Island

0.187

Fringing type

8.320

16

Kovi Island

0.482

Fringing type

17

Kursadi Island

0.740

Fringing type

18

Shingle Island

0.191

Fringing type

1.023

19

Near Bharathi nagar

---

Patchy type

0.131

20

Between Appa Island and Pilliyarmunai Island

---

Patchy type

3.981

21

Southeast of Kariya Shali Island

---

Patchy type

0.969

 

The areal distribution of coral reefs, reef vegetation, degraded coral reef, etc for the years 1988 and 1998 are shown in Table 2.

 

Table 2: Areal distribution of coral reefs and its changes observed during the period from 1988 to 1998

 

Category

Area (km²)1988

Area (km²)1998

Changes1988-1998

Reef area

73.70

48.18

-25.52

Reef vegetation

12.31

10.15

-2.16

Degraded reef (Coral mining)

-

2.68

+2.68

 

 

Degradation of coral reefs:

 

The degradation of coral reefs in the Gulf of Mannar is quit severe due to the human stress (anthropogenic) and also by natural agents. The coral mining for lime, sand mining, pollution, sedimentation, fisheries, population pressure, commercial shell collection and industrial development has led to the increase in coral reef degradation in India.

 

The total coral reef area in Gulf of Mannar based on the present study (1998) is about 61.01km², of which reef area covers 48.18 km², reef vegetation covers 10.15 km² and degraded coral occupies 2.68 km².

 

The analysis of multi-date satellite data indicates that nearly 25.52km² area of coral reef was lost over a period of ten years (1988-1998). Through remote sensing the demarcation of dead coral is possible within 2 feet beyond not possible. During the time of ground truth it was identified that nearly 67.2% of the corals were dead corals, 13% of coral reef are directly removed by coral mining and the remaining 19% were live corals.

 

Coral reef ecosystems are very sensitive to external impacts both natural and manmade, which violate their homeostasis. The majority of damage to coral reefs around the world has been through direct anthropogenic stress. 57% of the world’s coral reefs are potentially threated by human activity such as coastal development, destructive fishing, over exploitation, marine pollution, runoff from deforestation and toxic discharge from industrial and agricultural chemicals. As a result of the increasing human population along the coastal area, anthropogenic impacts on the coastal zone have become severe over the past few decades. Coral ecosystem also face many threats, of which some are of natural origin like storms and waves particularly tropical storms and cyclones that cause major intermittent damage to reefs. The majority of damage to coral reefs around the world has been through direct anthropogenic stress.

 

During the time of ground truth, the following activities were observed to be the major causes in degradation of coral reefs in the study area:

(I) Anthropogenic Stresses (Human Activities)

(II) Natural Stresses

1. Anthropogenic stresses (Human activities):

 

Increase in human population and economic activities in the study area have increased the pressure on the adjacent reefs. The major causes of coral reef degradation are:

  • Over fishing and destructive fishing practices
  • Sea weed collection
  • Commercial shell collection
  • Coral mining
  • Poor land use practices
  • Coastal urban development
  • Harbour and dredging activities
  • Industrial development and pollution

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. The marine fish catch from Gulf of Mannar during 1992-1996 was between 0.55 and 1.02 lakh tons, with an average of 78, 511 tons. The pelagic fishes formed 54.38%, demersal fishes 34.85%, crustaceans 5.69% and molluscs 5.08%. Nearly 20% of total fishes in Tamilnadu are being caught from Gulf of Mannar, of which 44,600 tons of pelagic fishes and 35,200 tons of demersal fishes. The fishermen collect these holothurians by diving in shallow reef areas. 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.

 

 

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 in the study area 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 was estimated to be about 2.16 km² area over ten years (1988-1998). The decreasing of seaweeds may lead to coastal erosion and removal of the coral reefs.

 

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.

 

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 the study area 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. The net result of removal of corals is that the depth has increased to about 1m. This has also been confirmed by the bathymetry map pertaining to Tuticorin group of islands.

 

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. In the present study it was observed that nearly 470.22 km² of land behind the coast of Gulf of Mannar has been under agriculture land, of this nearly 417.66 km² area fall under the category of cultivated land. Cultivation in this area is poor and intensive method of cultivation is generally practiced during rainy season. 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 major cause for the coral reef degradation is changing land use practice. The analysis of multidate satellite data indicates that nearly 2.45 km² of cropland has been converted in to salt pan (1.86 km²) and aquaculture pond (0.59 km²); 2.74 km² of agriculture plantation has been converted into cropland (1.77 km²) and salt pan (0.97 km²); 10.74 km² of forest plantation has been converted into agriculture plantation (6.19 km²) and forest blank (4.54 km²); Nearly 68.78 km² of scrubland has been converted into settlement (7.10 km²), saltpan (2.59 km²), aquaculture ponds (0.18 km²), forest plantation (4.31 km²), agriculture plantation (12.10 km²), fallow land (7.60 km²) and cropland (34.90 km²) over a period of ten years (1988-1998) along the coast of Gulf of Mannar.

 

 Another important human activity encountered in Gulf of Mannar is destruction of natural forest and island vegetation. Through remote sensing it has been clearly identified that nearly 3.80 km² of natural forest has been destroyed for fuel wood and cultivation by human beings along the coastal plains of the study area, and nearly 1.53 km² of island vegetation has been cut down for fuel wood over a period of ten years (1988-1998). These changing land use practices, destruction of natural forest and island vegetation accelerate the soil, coastal and island erosion and increases the input of inland-suspended soil into coastal waters. The increase in suspended soil and other suspended materials in coastal water smothers the reefs and causes their degradation.

 

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 increased in area from 22.07 km² to 34.92 km² over a period of ten years (1988-1998). As a result of this rapid urbanization there is an increase in the amount of discharge of sewage waste into coastal waters, which has caused the death of corals in Gulf of Mannar.

 

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. In the study area the New Tuticorin harbour has been constructed at the upstream side of the Tuticorin group of island. This harbour has a breakwater, which has changed the current flow pattern and sediment movement along the Tuticorin coast and islands. 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.

 

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, since the New Tuticorin port became operational. The industrial development has 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. These effluents contain mercury, sodium carbonate, ammonium chloride and sodium bicarbonate etc. which are harmful to the coral reefs and the environment.

 

II. Natural stress:

 

Natural problems are those that are not caused by man, but occur naturally over the long history. Natural problems such as storms, waves, sea level variation, fresh water runoff, volcanic activity etc cause the degradation of coral reefs. Various authors have studied coral degradation due to natural activities in the Gulf of Mannar. Through remote sensing and extensive ground truth it has been identified that natural activities such as monsoons, waves, currents, tides and sea level fall have caused the coral reef degradation in Gulf of Mannar.

 

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. Several corals die yearly during northeast monsoon period. In addition to this, during the low tide period, the tidal current moves towards south and transport some amount of sediment to Gulf of Mannar. 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. The prolonged and repetition of such action, result in the area between island and reef edge to became shallow. The multi-date bathymetry map analysis indicates that nearly 0.24m depth has decreased over a period of 24 years on the seaward side of Pumurichan, Kovi and Kursadi Island. This process has also caused the coral degradation. This problem is observed mainly in the Mandapam group of islands.

 

The sea level in Gulf of Mannar is on a regression phase. The sea level fall causes rising of coral reef. The raised reefs are the reliable indicator of sea level fall. Raised reefs have been observed in many places of Gulf of Mannar. The multi-date shoreline map and bathymetry map indicate that sea level has fallen by 0.36m over a period of 24 years (1975-1999) and the shoreline has migrated 143.11m towards the seaward side over a period of 30 years (1969-1998).

 

Sedimentation is a major factor controlling the distribution of reef organisms and overall reef development. The reduced level of light due to suspended sediment in the water column can reduce coral growth and has an impact on natural zonation patterns. Excessive sedimentation can also discourage the settlement of coral larvae. Most of the studies on the effects of turbidity on corals have concentrated on anthropogenic-increased sedimentation and turbidity. According to present study, nearly 67.2% of the coral reefs in Gulf of Mannar is not in living condition due to sedimentation and turbidity caused by anthropogenic and natural activities. The anthropogenic activities like destructive fishing methods, seaweed collection, commercial shell collection, coral mining, intensive agriculture, changing land use practices, deforestation and industrial waste input etc. and natural activities like monsoon, wave action, ocean current and tides were identified as the agents that increase the sedimentation and turbidity in coastal waters of Gulf of Mannar.

 

Recommendations:

Based on the present study some of the followings are the recommendation for manage the coral reefs in Gulf of Mannar

  • It is necessary to create awareness among the coastal communities in the study area, in order to protect and conserve the coral reefs through effective involvement of educational institutions and NGOs.
  • Stringent measures need to be under taken with immediate effect to ban coral mining and to take into task those involved in or those who encourage the exploitation of corals for any purpose. Patrolling the coast to check coral mining should be carried out.
  • Law should be enacted to regulate and stop trawl boat operation in the zone earmarked for non-mechanized boat. The Department of Forest and the Department of Fisheries should take steps to stop anchoring of vessels on coral reefs, pair trawling and dynamite fishing.
  • Indiscriminate picking of budding seaweeds needs to be banned.
  • Commercial shell collection should be controlled and closely monitored.
  • Marine Resources Management Centers should be established to improve the skills of fishermen communities in areas other than coral mining, which in turn will lead to efficient management of coral reefs.
  • Initiatives to train the coastal fishermen in mechanized boat operation, shell collection, seaweed collection and conservation of coral reefs need to be taken up so that they could find alternate sources of livelihood.
  • Deforestation along the coast and islands of Gulf of Mannar should be banned. The Forest Department should take up afforestation along the coast and islands of Gulf of Mannar to protect soil erosion.
  • Discharging of untreated sewage and urban wastes into the coastal waters should be totally banned.
  • Dumping of any kind of material that would affect the coral reef ecosystem should be banned.

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