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Remote Sensing for Mangrove Forest Management

L Kannan, T. T. Ajith Kumar and A. Duraisamy
CAS in Marine Biology, Annamalai University,
Parangapettai-608502, Tamil Nadu

Introduction
Mangrove forests are one of the most important coastal ecosystem in the world in terms f primary production and coastal protection. Distributed in the tropical and sub tropical regions, mangroves reach their maximum development and greatest luxuriance in Southeast Asia. Mangrove forests are now under stress in almost all tropical countries because of natural and demographic pressures. Along the Indian coasts, mangorve have been affected severly due to human induced stresses such as deforestation and other developmental activities. Therefore, there is a pressing need to have integrated approaches for coastal zone (mangrove) management as a means of achieving sustainable resurces development.

To study and evolve remedial measures to the extent possible, various organisations have beeen condutiong a variety of research and planning, implementation and monitoring activities. Remote sensing technology is an important tool in this assesment because of its ability to provide synoptic view of the earth which would not be possible from the ground without exhuastive field surveys.

Fundamentals of Remote Sensing
Remote Sensing is the science dealing with the acquisition, processing and interprtation of images and related data obtained from aircrafts and satellites tthat record the interaction between matter and electromagnetic radiation. When electromagnetic radiation falls upon a surface, some of its energy is absorbed, some is transmitted through the surface, and the rest is reflected. Surfaces also naturally emit radiation, mostly in the form of heat. It is that reflected and emitted radiation which is recorded either on the photographic film or digital sensor. Since the intensity and wavelengths of this radiation are a function of the surface in question, each surface is described as processing a characteristic “Spectral Signature”. If an instrument can identify and distinguish between different spectral signatures, then it will be possible to map the extent of surfaces using remote sensing.

Applications of Remote Sensing in Marine Research
Remote sensing applications, with the concerted efforts of the space scientist of our country during the past ten years, have proved very useful in resource surveys and management activities. These techniques have been applied successfully in forestry, agriculture, disaster managemnet, flood and drought monitoring, land use and land cover mappng, urban planning, mineral targeting, environmental impact assessment, coastal zone mapping etc. Application of remote sensing techniques to coastal and marine research has been now extended for the identification of Potenial Fishing Zone (PFZ), estimation of primary productivity using ocean colur data, identification of geomorphological changes of beaches, mapping coastal wetlands, distinguishing coral seagrass and mangrove habitats and monitoring marine pollution. The launch of IRS 1D satellite is yet another milestone in the space science history of our country. Different sensors of this satellite offer unique application opportunities and would near time information system for resources including marine wealth.

Remote Sensing in mangrove research
Wetland mapping should be done for the better understanding of various conditions of the wetlands and for the delineation of areal extent and boundaries of wetland especially coastal wetlands. These maps will serve as baseline data for classifying the coastal zones into preservation, conservation and development zones.

Remote sensing dat aderived from different satellite such as Landsat MSS data can provide us information about the areal extent, conditions and boundary of coastal wetlands. IRS LISS II Landsat TM data have also proved extremely useful for wetland mapping as well as for delineation high and low water lines Likewise, it is possible to distinguish mangroves from oyher plant communities (Nayak, 1993). Further, multidate satellite data could be used effectively to find out the changes in the arealextent of mangroves. For example, the 1986 TM and 1993 IRS LISS II data have helped to quantify the changes in the area cover of mangrovessince both the sensors have similar resolution (Krishnamoorthy, 1997).

False Colour Composites (FCC) derived from the green, red, and infrared bands of satellite data can be virtually analysd on 1:25000 or 1:50000 scale when information is reuired at the state/ national level. An image interpretation key indicating the tone colour, size, shape, texture pattern, location a nd association can be prepared for each category of vegetation including magroves using ground truth information, topographical maps, aerial photographs etc. The classification accuracy can be tested on a sample basis assuming binominal distribution for the pobaility of success/failure of sample tests. Sample size is decided using trhe look Up Table (LUT), prepared by employing a binominal probability model (Arnoff, 1982).

Case studies in Mangrove Using Remote Sensing
Estimates of mangrove cover are subjected to several sources of inaccuracy and confusion. If the mangrove canopy is dense, it obscures the treeless patches and channels so that the total mangrove cover is over estimated if the mangroves occur in either small patches or in low density. This is because small mangrove patches may be beyond the sensor’s spatial resolution.

State Area in sq. km(Govt. of India Status Report) Area in sq. km(IRS data)
West Bengal 4200 1619
Andaman & Nicobar 1190 770
Orissa 150 187
Andhra Pradesh 200 480
Tamil Nadu 150 90
Gujarat 260 1166
Maharashtra 330 138
Goa 200 5
Karnataka 60 19
Kerala Sparse Sparse
Total 6740 4474

The total area of mangroves in India was estimated to be 6740 sq km (Status Report, Government of India, 1987). But the Indian Remote Sensing data have shown that the total mangrove area in India is 4474 sq km (Nayak 1993) as detailed below:The National Remote Sensing Agency (NRSA), Hyderabad has recorded a decline of 70,000 ha of mangroves in India within a period of six years from 1975 to 1981 and Vedaranyam/Point Calimere coastal areas of Tamil Nadu have lost 40% of their mangroves with a reduction of 18% of fishery resources within a period of 13 years from 1976 to 1989.

Assessment of the degradation of the Pitchavaram mangroves in Tamil Nadu has been possible to a large extentdue to the information obtained from satellites. In 1987, the Pitchavaram mangrove forests were declared as reserve forests with an area of about 700 ha. Of this, nearly 62.8% of the mangrove area has been degraded between 1897 and 1994 as revealed by satellite data. Further, the remote sensing analysis has been much affected.

A comparison of the Survey of India (SOI) topo sheet and Satellite imagery of IRS IB-LISS II shows that the breadth of the beach area around Pitchacaram has been reduced by 550m between 130 and 1970 and further about 150m between 1970 and 1992 and the rate of erosion has been calculated as 13m a year. The comparison of toposheet and satellite imagery also shows that erosion and sedimentation occurs simultaneously in the Pitchavaram area, which is a serios problem. If this trend is not changed, mangroves may be completely wiped out from here soon. So, a long term management plan is the immediate necessity to save these mangroves. The information required for mangrove management will be mainly on the distribution and extent of mangrove areas, forest composition, degradation sites, drainage network, spread of coastal villages, other land uses with in and outside the mangroves etc. and it is essential to use high resolution remote sensing data to prepare large scale thematic maps on vegetation cover. For this, both remotely sensed data and Geographic Information System (GIS) can be used with advantage.

Conclusion
 Mangrove ecosystem which is fragile but yet highly productive is constantly undergoing changes (seasonal/ short-term and/ or succesional / long-term) due to its dynamic nature through various natural and biotic influences. Hence, an accurate and up-to-date information base on the status of mangrove vegetation, continually overtime, is a prerequisite for the sustainable management of mangrove forests. The required information cannot be obtained with traditional field surveys to be made inside the mangrove swamps as they are extremely difficult. Hence, Remote Sensing and Geographic Information System serves as valuable aids in providing fast, efficient and accurate information to detect the changes. The information thus gained can be utilised for the effective planning and management of mangrove forests so as to save these delicate and highly valuable ecosystems for posterity and sustainable utilisation.

References

  1. Ajith Kumar T. T., R. Kannan and L. Kannan, 1995, Remote sensing in mangrove research. Seshaiyana, 3: 46-48
  2. Green, E. P., P. J. Mumby, A. J. Edwards, C.O. Clark, 1996. A review of Remote Sensing for the assessment and management of Tropical Coastal Resources. International Journal of Marine Environment, 24: 1-40
  3. Krishnamoorthy, R., 1997. Remote Sensing for the assessment and management of mangroves: Indian Experience, (paper presented at the international workshop on “Application of Remote Sensing and GIS for sustainable Development at National Remote Sensing Agency. Hyderabad.
  4. Narayan L. R. A., 1997. Sentinel in the Sky.
  5. Nayak, S. R., M. C. Gupta and H. B. Chawan, 1986. Wetland and shoreline mapping of the part of Gujaratcoast using Landsat data, Scientific note (IRS-UP/SAC/MCE/SN06/86). Space Application Centre, Ahemdabad, 24 pp.
  6. Shailesh Nayak, R., 1993. Role of Remote Sensng Application in the management of wetland ecosystems with special emphasis on Mangroves (Lecture deliveredat the UNESCO Curriculum Workshop on “Management of mangrove Ecosystem and Coastal Ecosystem” at the Department of Marine Living Resource, Andhra University, Vishakhapatnam).