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Sử dụng kỹ thuật viễn thám và hệ thông tin địa lý để nghiên cứu hiện trạng và những thay đổi của rừng ngập mặn ở Trà Vinh trong giai đọan 1965-2001. Nguyên nhân chính gây ra sự suy giảm rừng ngập mặn là do chiến tranh, phá rừng lấy củi, phá rừng làm ruộng hoặc nuôi tôm

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  1. Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Status and changes of mangrove forest in Mekong Delta: Case study in Tra Vinh, Vietnam Phan Minh Thu a,*, Jacques Populus b a Institute of Oceanography, 01 Cau Da, Nha Trang, Khanh Hoa, Vietnam b IFREMER, Centre de Brest, Technopole Brest-Iroise, BP 70, 29280 Plouzane, France Received 9 August 2006; accepted 10 August 2006 Available online 28 September 2006 Abstract Because shrimp culture in the Mekong Delta develops rapidly, it has negatively impacted the environment, socio-economics and natural re- sources. In particular, mangrove forests have been altered by the shrimp culture. The area of mangrove forests in the region has been reduced and this is seen especially in Tra Vinh province. The results obtained from GIS (Geography Information System) and RS (Remote Sensing) show the status of mangrove forests in Tra Vinh province in 1965, 1995 (Northeastern part of Tra Vinh Province) and 2001. In 1965, the area of mangrove forests was 21,221 ha making up 56% of total land-use, while in 2001 it was 12,797 ha making up 37% of total land-use. Also based on GIS analysis, over the 36 years (1965e2001), the total coverage of mangrove forests have decreased by 50% since 1965. However, the speed of mangrove forest destruction in the period from 1965 to 1995 was much less than that in the period from 1995 to 2001. The average annual reduction in mangrove forest coverage in the first period (1965e1995) was 0.2% whereas it was 13.1% in the later period (1995e2001). For the long time, mangrove deforestation has been caused by war, collection of firewood and clearing for agriculture, and recently, shrimp farming has significantly contributed rate of mangrove destruction. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: mangrove forest; GIS; remote sensing; Mekong; mangrove changes; mangrove management Despite the many factors that have affected the mangroves of 1. Introduction the Mekong Delta, the most important factor that has contrib- Tropical mangrove forest ecosystems play an important uted to mangrove destruction is the shrimp culture activities. role in coastal zones, not only in the biogeochemical cycle The herbicides sprayed by the USA in the war (1962e1971) but also in the economic life of the region through activities destroyed about 104,939 ha, about 36% of the total mangrove such as aquaculture and fishing. Mangrove forests in the Me- area in South Vietnam (NAS, 1974). Population pressure has kong Delta used to cover more than 250,000 ha (Hong and led to an increased need for land for agricultural production. San, 1993). War, forest fire, collection of fuel wood and other In addition, environmental degradation and sedimentation human activities have resulted in the reduction of the man- have also negatively affected mangrove forests (Macintosh, grove forests in the Mekong Delta. Especially, since the end 1996; Le and Munekage, 2004). of the 1990’s, mangrove forests have been cleared for shrimp Earlier studies (Hong, 1995; Macintosh and Zisman, 1995; farming in many areas (Hong and San, 1993; Hong, 1995; Vits and Tack, 1995; Macintosh, 1996; Phuong and Hai, 1998; Hao, 1999). Lakshmi and Rajagopalan, 2000; Lin, 2000; Srinath et al., 2000; Yap, 2000) have demonstrated that mangrove and shrimp farming have shown a complex relationship. Mangrove forests serve as nurseries and food-supply base for marine and * Corresponding author. brackish water animals. The mangroves also absorb waste E-mail address: (P.M. Thu). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.08.007
  2. P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 99 generated by shrimp farming (Hong, 1995; Macintosh and Zis- The present study provides an overview of the mangrove man, 1995; Macintosh, 1996; Lin, 2000; Gautier et al., 2001; forest distribution and changes in Tra Vinh province by using Wosten et al., 2003). Shrimp culture supplies nutrients for Remote Sensing (RS) and Geographical Information Systems mangrove forests through water and sediment discharge into (GIS). mangrove areas. Nevertheless, the high economic returns in shrimp farming have resulted in thousands of hectares of man- 2. Materials and methods grove forest being converted to shrimp ponds and the natural waterways blocks. The pattern of land-use in the Mekong 2.1. Materials Delta has been changed significantly over decades, and this has consequently affected the economic development in the Tra Vinh Province belongs to the Mekong Delta (Fig. 1), region. which is situated from 9 310 N to 10 040 N and from Mangrove habitat maps have been used for three general 105 570 E to 106 360 E. With a total shoreline of 65 km, it management applications: resource inventory, change detec- lies between two branches of the Mekong River (Co Chien tion and the selection and inventory of aquaculture sites. River and Bassac River) and flows into the Bien Dong (South The mangrove distribution maps can be made from investiga- China Sea). The economy in Tra Vinh is based mainly on ag- tion in situ or obtained from remote sensing images and GIS riculture and aquaculture. Shrimp farming areas have devel- techniques (Aschbacher et al., 1995; Blasco et al., 1998; oped significantly and the mangrove forest has also changed Dahdouh-Guebas et al., 2000; Kairo et al., 2002). Images accordingly. In 1943 the area of mangrove forest was about used for the present study include SPOT XS (Multispectral 65,000 ha (Hong and San, 1993), however by 1995 it was mode imagery from Satellite Pourl’ Observation de la Terre), hard reduced to 6678 ha (Phuong and Hai, 1998). SPOT XP or SPOT Pan (Panchromatic mode imagery from Data have been made available to this study from different SPOT), Landsat TM (Landsat Thematic Mapper), Landsat sources. There were topographical maps in 1965 from US MSS (Landsat Multispectral Scanner), MOS-1 MESSR (Mul- Navy maps which were established in 1967 (Scale map: tispectral Electronic Self-Scanning Radiometer carried out on 1:50,000 and UTM: Indian 1960, Zone 48 in Southern), and the Marine Observation Satellite), JERS-1 (Japanese Earth remote sensing images e SPOT image on February 04, 1995 Resources Satellite), ERS-1 SAR (Synthetic Aperture Radar with 3 bands and 20 m resolution (however, the 1995 image carried on the European Remote Sensing Satellite), MK6 (Rus- only intercepts the northeastern part of the study area) and sian Multispectral camera carried on the Salyut-7 Satellite), SPOT4 image on January 22, 2001 with 4 bands and 10 m res- and KATE-140 (Soviet panchromatic large format camera). olution. These are images in medium resolution. So, they Aerial Photos were also involved. These together were used could help to recognize the distribution of mangrove forests to map mangrove habitat with different image processing tech- with the high accuracy (Vits and Tack, 1995). niques, including Visual interpretation, Vegetation index (NDVI e Normalized Difference Vegetation Index, and BI e Brightness Index), Unsupervised classification, Supervised 2.2. Field trips classification, Band ratios and Resolution merge between Landsat TM with SPOT Pan, Leaf area index (LAI) (Lorenzo Four field trips were carried out at 20 stations, September et al., 1979; Bina et al., 1980; Untawale et al., 1982; Patterson 10e20, 2000; March 14e28, 2001; September 6e23, 2001 and Rehder, 1985; Blasco et al., 1986; Ranganath et al., 1989; and March 2e20, 2002. At each station, one water and one Roy, 1989; Chaudhury, 1990; Dutrieux et al., 1990; Gray et al., sediment sample were collected for environmental factors an- 1990; Vibulsresth et al., 1990; Jensen et al., 1991; Kay et al., alyzed in every survey, including salinity, the color of water 1991; Populus and Lantieri, 1991; Eong et al., 1992; Gang and turbidity. In these surveys, salinity and turbidity were and Agatsiva, 1992; Loo et al., 1992; Mohamed et al., 1992; measured by YSI multi-parameter, and the color of water Palaganas, 1992 e pers. comm; Long and Skewes, 1994; Asch- was measured by color scales. In addition, land-use classifica- bacher et al., 1995; Vits and Tack, 1995; Rasolofoharinoro tion and the structure of mangrove forests were identified. et al., 1998; Blasco et al., 1998; Green et al., 1998; Structure, density, height, floristic composition and standing Dahdouh-Guebas et al., 2000; Kairo et al., 2002; Tong et al., biomass of mangrove forests were studied, which helped rec- 2004; Kovacs et al., 2005). These processing methods have ognize training areas and to access accuracy ratio after analyz- been acceptable for application on mangrove habitat maps in ing the results of remote sensing to find out the distribution of management, including mangrove inventory and mapping, mangrove forests at the studied areas. change detection and management of aquaculture activities (Blasco et al., 1986; Ranganath et al., 1989; Chaudhury, 1990; Palaganas, 1992 e pers. comm; Long and Skewes, 2.3. Methodology to identify mangrove forest by GIS 1994; Vits and Tack, 1995; Rasolofoharinoro et al., 1998; and RS Green et al., 1998; Tong et al., 2004; Son and Thu, 2005). It is recognized that SPOT images can be classified for mangrove The processing of the identification of mangrove forests forest identification achieving an accuracy of from 81 to 95% was carried out step by step as shown in Fig. 2. This process- (Palaganas, 1992 e pers. comm; Vits and Tack, 1995). ing was implemented by ArcView 3.2 and ENVI 3.4.
  3. 100 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Fig. 1. Study area (left) and SPOT image (January 22, 2001) displaying false color composite in Tra Vinh study area (right). 2.3.1. Image registration 2.3.2. Preliminary analysis Image registration is a process whereby an image is re- After satellite images were geometrically corrected, prelim- sampled to conform to another image or topographical map inary analysis methods could be applied for image enhance- (e.g. US Navy maps). In this stage, the varying pixel sizes of ment, filtering, unsupervised classification and NDVI the different images were changed into a common map grid computation (Son and Thu, 2005). based on a reference image/map. Evenly-distributed GCPs For vegetation areas, including mangrove forest, NDVIs al- (Ground Control Points) were selected in the different images lows cataloguing into 3 classes: low, moderate and high den- and registered with the reference images/maps. A RMS (Root sity. Blasco et al. (1986) and Chaudhury (1990) indicated Mean Square) error of less than 0.5 pixels was accepted for that the classification of mangrove forests could be identified the transformation. Resampling is preformed by converting dif- by NDVIs. According to Guyot and Gu (1994), NDVI of man- ferent pixel sizes to the same final image pixel sizes. grove forest was higher than 0.13. Based on values of NDVIs
  4. P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 101 Topographical map SPOT image Geometric correction Preliminary analysis Digital analysis Definition of training area Input information Field trip Supervised classification Labeling Post-classification Input of GIS Ground truth Accuracy assessment Mangrove classification Mangrove map in maps in 1995 and 2001 1965 Overlay map in GIS Processing Mangrove changes Material or production Fig. 2. Processing flowchart to map mangrove change by GIS and RS. (Table 1), the training areas of different mangrove layers were signature for each group was defined by the means of each selected to support for field trips. band reflectance and their standard deviation. Each training area was larger than 200 pixels (20,000 m2 for image with 10 m in resolution and 80,000 m2 for image with 20 m in 2.3.3. Training area selection resolution). The training areas were selected based on prior informa- tion, including the result of a preliminary analysis, topograph- ical maps and information gathered during the field trips, and 2.3.4. Realization of ground data on the experience gained from the visual image interpretation. Comparison between the training areas and the actual dis- Each parcel was captured from homogeneous areas and en- tribution of the themes in the field trips was an essential ele- coded. Several parcels were selected per code. These training ment of any remote sensing work. By the end of this step, sites, therefore, were determined by the numbers of groups the whole spectral space was split into classes and each class that retained to define the spectral space. The spectral represented one or several training areas, and each training site was assigned with a thematic code. The fieldwork also helped to ascertain training areas. Table 1 NDVI value of the training areas NDVI Mangrove class Code 2.3.5. Supervised classification For any given theme the pixels of training sites were used to 0.13e0.42 Low density I 0.43e0.71 Moderate density II calculate a mean spectral reference value. A standard Maxi- 0.72e1.00 High density III mum Likelihood Classification with Bayesian variation was
  5. 102 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Fig. 3. Mangrove forest in Tra Vinh province from 1965 to 2001. (A) In 1965 of all region with 21,221 ha of mangrove forest; (B) 1995 at the sourthern part with 2596 ha of low density, 3343 ha of moderate density and 1301 ha of high density of mangrove forest; (C) 2001 of all region with with 8666 ha of low density, 2347 ha of moderate density and 1784 ha of high density of mangrove forest; and (D) Comparing of mangrove forest during 1965e2001. performed on the image. Some post classification steps such as thus providing a more accurate stratification of mangrove, checking information and layers were carried out. non-mangrove and other land-use areas. 2.3.7. Evaluation of classification results 2.3.6. GIS database Results were calculated from the images obtained using The contour of the dykes separating the area between ma- field observations as reference. The accuracy of the classifica- rine and freshwater were digitized from the topographical tion results was evaluated by comparing the geographical data maps and imported to the images. Similarly, the networks of derived from ground truth. Randomly selected reference pixels road and estuaries were digitized. The results from this proce- (about 200 pixels) were inspected at the corresponding sites to dure were used to eliminate interpretation/classification errors verify the classification results derived earlier.
  6. P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 103 C D Southwest part 25 Northeast part 20 Area (1000 ha) 15 10 5 0 1965 1995 2001 Fig. 3 (continued). Further, comparison between mangrove forest layers with vector maps was applied to establish mangrove deforestation, NDVI classes permitted us to classify mangrove forest layers mangrove reforestation and unchanged mangrove areas. This with their NDVI value in Table 1. method was based on the ‘‘Union two themes’’ and ‘‘Dissolve feature based on an attribute’’ functions of reprocessing in ArcView. 2.3.8. Classification of mangrove forest based on topographical maps 3. Results and discussion Topographical maps were classified into a digital table and/ or automatic software (ArcView 3.2 software). Mangrove for- Overall, the results of surveys show that waters are highly est, rice field paddy, human settlement, swamp, marsh and turbid, highly concentrated in loamy and clayey particles, river areas were separated in different layers. These informa- tion layers were labeled with information from topographical Table 3 maps and input to the GIS. Characteristics of distribution of Mangrove forest in Tra Vinh Year Units Northeast part Total 2.3.9. Mapping overlay Unit: (ha) All the mangrove layers from the various periods were 1965 Total of mangrove forest 7877 21,221 overlaid in ArcView to assess mangrove changes as well as de- 1995 Total 7241 forestation and reforestation areas. The overlay method for Low density 2596 Modulated density 3343 Table 2 High density 1301 The distribution of mangrove Forest (ha) in Tra Vinh Province (Source: 2001 Total 3122 12,797 * Phuong and Hai (1998)) Low density 2050 8666 Modulated density 910 2347 Year 1943* 1965 2001 High density 162 1784 Tra Vinh 65,000 21,221 12,797
  7. 104 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Table 4 Changes of mangrove forest in Tra Vinh (À: Reduction, þ: Increasing) Regions Year Mangrove area Mangrove changes 1965 1995 2001 1965e1995 1995e2001 1965e2001 À636 À4119 À4755 Northeast part Area (ha) 7877 7241 3122 À0.2 À13.1 À2.5 Annual rate (%) À8424 Whole study area Area (ha) 21,221 12,797 À1.4 Annual rate (%) constantly redistributed in suspension by tidal currents and by distributed mainly in the northeastern of Duyen Hai. Hence, speed boats used as transportation means prevailing along the the changes of distribution of mangrove forests in the north- coast and nearshore of the study area. The concentration of eastern part of Duyen Hai in 1965, 1995 and 2001 could rep- suspended sediment ranges from 22.5 to 1079.5 mg lÀ1. The resent the varying status of mangrove forests in Tra Vinh color of the water is often brown red which could be due to (Fig. 3D). In addition, compared with research results of the erosion of red basaltic soils bearing rubber trees and up- Phuong and Hai (1998), mangrove forests of Tra vinh in stream. Nowhere old mangroves have been observed. They 1995 covered only 6678 ha (all area) (Fig. 3B), lower than probably exist in very limited areas but are rare whilst human that of the present study (Phuong and Hai, 1998) but higher density is high almost everywhere. than the total area of mangrove forests of high and moderate The salinity ranged from 13 to 20 in the dry season and density (5939 ha) (Table 3). These results demonstrated that from 0.1 to 10.6 in the rainy season. It is suitable for shrimp most low density of mangroves might not be forest but just culture. This probably explains why Tra Vinh has been the mangrove trees (Hong, 1995). Also, according to Hong best places for shrimp farming in the past years. (1995), before 1995, shrimps were farmed in Can Long, and Six groups of mangrove species are obtained in Mangrove just began to expand in Tra Cu and Duyen Hai (after 1995). forest in Tra Vinh. Along the water ways, Avicennia and Son- Thus, mangrove forests, which were low density in the north- neratia covered more than 25%, Excoecaria agallocha was eastern part, were cut down and converted to shrimp farms. about 20%, Derris trifoliatta was about 15%, Phoenix palu- Hence, most areas of mangrove forest, which were low in den- dosa was about 15%, Nypa ranged 15e20%, and Ceriops, sity, were a mixture between mangrove trees and shrimp farms Bruguiera and Xylocarpus was less than 5%. This abundance or canals. of mangrove species paid attention of the results of remote For all study areas, mangrove forests also varied greatly sensing analysis. (Table 3 and Fig. 3A,C,D). About 50% of mangrove forest area was destroyed or decreased in either quantity or density (Fig. 3D). In addition, most low-density mangrove areas 3.1. Status of mangrove forest in Tra Vinh were located in areas converted from paddy rice fields, which were one crop and low productivity, to shrimp farming. This By digitizing the 1965 topographical maps and analyzing has happened in the early 1990’s (Hong, 1995; Phuong and the supervised classification of remote sensing images, man- Hai, 1998). Especially, after the decision of Vietnamese Gov- grove forest areas were identified with different classes. The ernment in 1999 about the changes of the structure of major results showing the mangrove distribution in Tra Vinh indicate land-use patterns in wetland and lowland areas, the converting significant changes in mangrove forest coverage in Tra Vinh to shrimp farms has occurred rapidly. (Fig. 3 and Tables 2 and 3). In 1965 rice paddy was the According to the above results, mangrove forests in Tra most use of land in Tra Vinh. The mangrove forests were dis- Vinh have been decreasing rapidly. However, replanting activ- tributed in Duyen Hai district (Fig. 3A). Out of the 38,000 ha ities have been carried out in some places, but are far from of Duyen Hai district, the total area of mangrove forests was 21,221 ha (making up 56% of the total of land-use area) in 1965 including 7877 ha in the northeastern part, whereas in 2001 mangrove areas covered only 12,797 ha (making up Table 5 37% of the total of land-use area) with 3,122 ha in the north- Characteristics of mangrove forest changes in Tra Vinh during 1965e2001 eastern part (Fig. 3D and Table 3). Some mangrove areas in Areas Status 1965e1995 1995e2001 1965e2001 2001 were distributed in Tra Cu district but they were lower Unit: ha in density (Fig. 3C). Northeast part Deforestation 2235 4996 5619 In the northeastern part of Duyen Hai District, the results Reforestation 1434 877 863 show that the decrease of mangrove forest areas in the period Unchanged 5642 2245 2258 between 1965 and 1995 was slower than that in the period mangrove from 1995 to 2001 (Fig. 3D). The total area of mangrove forest All area Deforestation 14,208 in 1965 was 7877 ha (Fig. 3A) and 7241 ha in 1995 (Fig. 3B) Reforestation 5784 Unchanged 7013 while in 2001 it was 3122 ha (Fig. 3C). Fig. 3A also indicates mangrove that a half of the areas of natural mangrove forests were
  8. P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 105 Fig. 4. Map of mangrove forest changes in Tra Vinh. (A) In 1965e1995 at the sourthern part with deforestation, reforestation and unchanged mangrove areas are 2235 ha, 1434 ha and 5642 ha, respectively; (B) in 1995e2001 at the sourthern part with deforestation, reforestation and unchanged mangrove areas are 4996 ha, 877 ha and 2245 ha, respectively; and (C) in 1965e2001 of all region with deforestation, reforestation and unchanged mangrove areas are 14,208 ha, 5784 ha and 7013 ha, respectively. compensating losses by deforestation. The details of mangrove Mangrove forests in Tra Vinh changed considerably but ra- changes are presented in the next part. tios of mangrove changes in the two periods were significantly different. In the period between 1965 and 2001, reduced area of 3.2. Mangrove changes in Tra Vinh mangrove forests was 8424 ha (Table 4). The annual rate of the area reduction of the mangrove forests in Tra Vinh was 1.4%, To identify mangrove changes in Tra Vinh, mangrove for- and in the northeast part was 2.5% (Table 4). However, in the ests in 1995 and 2001 were assumed to have similar density. northeastern part, the rate of destruction of the mangrove forests Using GIS method, mangrove changes were indicated in in the 1965e1995 period (the rate was 0.2% e Table 4 and Tables 4 and 5 and Figs. 4 and 5. Fig. 4A) was many times lower than that in the 1995e2001
  9. 106 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Fig. 4 (continued). period (the rate was 13.1% e Table 4 and Fig. 4B). These results that in the northeastern part. The large replanting areas of demonstrated that in the early period, the shrimp farming in Cau mangrove forest have poor quality in soil nature and are lo- Ngang affected mangrove forest slightly, whereas they signifi- cated mainly in shrimp farming areas, which were converted cantly increased the destruction of mangrove forests in the later from paddy rice fields having low productivity (Tra Vinh period (Table 4 and Figs. 4C and 5). DoF, 1999, 2000, 2001). However, these areas were not for- In addition, the activities of deforestation and re-planting ests. They only were mangrove trees (Hong, 1995). caused mangrove changes in Tra Vinh. Most natural mangrove Most natural mangrove areas in Tra Vinh were replaced by forests was cut down for firewood and converted to rice paddy shrimp culture areas (Tra Vinh DoF, 1999, 2000, 2001). Some fields and salt fields in the previous period, or converted into natural mangrove areas existed but their quality waned consid- shrimp farms in the latest period. The total area of mangrove erably. Many planting areas have existed in shrimp culture but deforestation between 1965 and 2001 was 14,208 ha while with low density. Only replanting areas, located nearly Dinh mangrove reforestation was 5784 ha (Table 5 and Fig. 5). An estuary, were considered as ‘‘mangrove forests’’ (Fig. 4C) The total area of un-changed mangrove was 7013 ha (Table (Tong et al., 2004). 5 and Fig. 5). It was demonstrated that some small natural mangrove forests were restored and/or protected (Fig. 4C). 3.3. Future research in land-use in Tra Vinh Province Most of these areas were mangrove forests in moderate and high density in 2001 (Figs. 3C and 4C). Fig. 5 shows that re- Although mangrove replanting and deforesting activities planting of mangrove in the southwestern part was higher than have been carried out in parallel in the Mekong River Delta, 100% 10 Mangrove areas (x1000 ha) 80% 8 60% 6 Deforestation 40% 4 Unchange Reforestation 20% 2 0 0% 1965 -1995 1995 - 2001 1965 - 2001 Northeast All area part Fig. 5. Changes in mangrove forest in Tra Vinh from 1965e1995e2001 (Left: mangrove changes in northeast part in 1965e1996, 1995e2001 and 1965e2001, Right: Comparision of mangrove changes in two parts over period 1965e2001).
  10. P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 107 2001 it was 12,797 ha. In addition, RS analysis may identify 3 NF part of MF All MF types of mangrove forests: low, moderate and high density. Shrimp farm However, the mangroves in low density could not be forests 25 60 but they were ‘‘mangrove trees’’. Mangrove forest area (1000 ha) This GIS-combined study on historical changes in man- 50 Shrimp farm areas (1000 ha) 20 grove distribution in Tra Vinh of the Mekong Delta, Vietnam has demonstrated that over the past 36 years (1965e2001), 40 about 50% of the mangrove forest area was lost, but the reduc- 15 ing annual rate of the period from 1965 to 1995 was lower 30 than that in the period from 1995 to 2000. In the period 10 from 1965 to 2001, the total area of mangrove deforestation 20 was 14,208 ha whereas mangrove reforestation was 5784 ha. 5 These changes in mangrove forests cover were affected by 10 two activities: deforestation and replanting, but planting ca- pacity was slower than deforestation. Recent mangrove 0 0 changes are due mainly to shrimp farming expansion, which 1965 1975 1985 1995 2005 is developing in an unplanned way. Shrimp farm development Fig. 6. Relationship between mangrove forest areas and shrimp farm areas (NE and degradation also caused environmental and natural re- part: Northeastern part, MF: Mangrove Forest). sources problems with socio-economic consequences such as land degradation, environmental pollution, the conflicts among the overall mangrove areas have been decreasing rapidly. The natural resource users and the gap between the rich and poor. main reason is reduction of mangrove forest is the conversion Reforestation of abandoned shrimp ponds might be a good so- of land areas previously covered in mangroves to shrimp lution to improve the sustainability of this ecosystem before farms. Due to high profit, shrimp cultivation areas have in- a new government master plan of land use for the coastal creased rapidly in early 1990s (Fig. 6) but the shrimp farming zone can be developed. areas were reduced by outbreak of disease during 1995e1997 period. However, after that, under effects of government’s con- Acknowledgements trol, shrimp culture has developed again. Seemingly, the man- grove forests decreased as the shrimp farming expands This research was supported by The European Commission (Fig. 6). in the framework of the GAMBAS Project. Our deepest thanks In addition, because of a lack of information about environ- and gratitude to Prof. Nguyen Tac An (on behalf of GAMBAS mental conditions, shrimp culture techniques and financial re- project) for his support for our study and field trips. We thank sources required, shrimp farming failed in some areas or staff of GIS group at Institute of Oceanography, Nha Trang, shrimp ponds were used only in the short period (Hong, for help in mapping and digital processing. We also thank 1995). After few years, land has been degraded and farmers Dr. Amararatne Yakupitiyage (Asian Institute of Technology, have continued to cut down mangrove forests and to make Thailand) and Dr. Trevor Charles Platt (Bedford Institute of new shrimp ponds. Before 1995, shrimp farming was mostly Oceanography, Canada) for their suggestions. conducted in Cau Ngang, but after an outbreak of shrimp dis- ease in the period from 1995 to 1997 (Hao, 1999), land has been degraded and shrimp ponds have become bare-land References and/or dry (Hong, 1995). This might have caused the reduction of shrimp farming in Tra Vinh, and the area under shrimp cul- Aschbacher, J., Ofren, R., Delsol, J.P., Suselo, T.B., Vibulsresth, S., 1995. An ture were at their lowest in 1997 (Fig. 6). After that, however, integrated comparative approach to mangrove vegetation mapping using shrimp areas have increased again. Shrimp farmers cut down advanced remote sensing and GIS technologies: preliminary results. the mangrove forest to make new shrimp ponds in Duyen Hydrobiologia 295, 285e294. Bina, R.T., Jara, R.B., Roque, C.R., August 28e29th, 1980. Application of Hai (Tong et al., 2004; Son and Thu, 2005). multilevel remote sensing survey to mangrove forest resource management The motivation for mangrove destruction and degradation in the Philippines. In: Proceedings of the 20th on Mangrove Development, is based on the short-term exploitation for immediate eco- Research and Management. University of Malaya, Kuala Lumpar, nomic benefit, rather than longer-term and sustainable exploi- Malaysia. tation. These are major reasons of mangrove deforestation in Blasco, F., Lavenu, F., Baraza, J., 1986. Remote sensing data applied to man- grove of Kenya coast. In: Proceedings of the 20th International Sympo- the period between 1995 and 2001 in Tra Vinh. sium on Remote Sensing of the Environment Programme. Blasco, F., Gauquelin, T., Rasolofoharinoro, M., Denis, J., Aizpuru, M., Caldairou, V., 1998. Recent advances in mangrove studies using remote 4. Conclusion sensing data. Marine Freshwater Research 49, 287e296. Chaudhury, M.U., 1990. 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