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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 1. Introduction Tropical mangrove forest ecosystems play an important role in coastal zones, not only in the biogeochemical cycle but also in the economic life of the region through activities such as aquaculture and fishing. Mangrove forests in the Me- kong Delta used to cover more than 250,000 ha (Hong and San, 1993). War, forest fire, collection of fuel wood and other human activities have resulted in the reduction of the man- grove forests in the Mekong Delta. Especially, since the end of the 1990’s, mangrove forests have been cleared for shrimp farming in many areas (Hong and San, 1993; Hong, 1995; Hao, 1999). Despite the many factors that have affected the mangroves of the Mekong Delta, the most important factor that has contrib- uted to mangrove destruction is the shrimp culture activities. The herbicides sprayed by the USA in the war (1962e1971) destroyed about 104,939 ha, about 36% of the total mangrove area in South Vietnam (NAS, 1974). Population pressure has led to an increased need for land for agricultural production. In addition, environmental degradation and sedimentation have also negatively affected mangrove forests (Macintosh, 1996; Le and Munekage, 2004). Earlier studies (Hong, 1995; Macintosh and Zisman, 1995; Vits and Tack, 1995; Macintosh, 1996; Phuong and Hai, 1998; 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 brackish water animals. The mangroves also absorb waste * Corresponding author. E-mail address: phanthu@dng.vnn.vn (P.M. Thu). 0272-7714/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.ecss.2006.08.007 Estuarine, Coastal and Shelf Science 71 (2007) 98e109 www.elsevier.com/locate/ecss generated by shrimp farming (Hong, 1995; Macintosh and Zis- man, 1995; Macintosh, 1996; Lin, 2000; Gautier et al., 2001; Wosten et al., 2003). Shrimp culture supplies nutrients for mangrove forests through water and sediment discharge into mangrove areas. Nevertheless, the high economic returns in shrimp farming have resulted in thousands of hectares of man- grove forest being converted to shrimp ponds and the natural waterways blocks. The pattern of land-use in the Mekong Delta has been changed significantly over decades, and this has consequently affected the economic development in the region. Mangrove habitat maps have been used for three general management applications: resource inventory, change detec- tion and the selection and inventory of aquaculture sites. The mangrove distribution maps can be made from investiga- tion in situ or obtained from remote sensing images and GIS techniques (Aschbacher et al., 1995; Blasco et al., 1998; Dahdouh-Guebas et al., 2000; Kairo et al., 2002). Images used for the present study include SPOT XS (Multispectral mode imagery from Satellite Pourl’ Observation de la Terre), SPOT XP or SPOT Pan (Panchromatic mode imagery from SPOT), Landsat TM (Landsat Thematic Mapper), Landsat MSS (Landsat Multispectral Scanner), MOS-1 MESSR (Mul- tispectral Electronic Self-Scanning Radiometer carried out on the Marine Observation Satellite), JERS-1 (Japanese Earth Resources Satellite), ERS-1 SAR (Synthetic Aperture Radar carried on the European Remote Sensing Satellite), MK6 (Rus- sian Multispectral camera carried on the Salyut-7 Satellite), and KATE-140 (Soviet panchromatic large format camera). Aerial Photos were also involved. These together were used to map mangrove habitat with different image processing tech- niques, including Visual interpretation, Vegetation index (NDVI e Normalized Difference Vegetation Index, and BI e Brightness Index), Unsupervised classification, Supervised classification, Band ratios and Resolution merge between Landsat TM with SPOT Pan, Leaf area index (LAI) (Lorenzo et al., 1979; Bina et al., 1980; Untawale et al., 1982; Patterson and Rehder, 1985; Blasco et al., 1986; Ranganath et al., 1989; Roy, 1989; Chaudhury, 1990; Dutrieux et al., 1990; Gray et al., 1990; Vibulsresth et al., 1990; Jensen et al., 1991; Kay et al., 1991; Populus and Lantieri, 1991; Eong et al., 1992; Gang and Agatsiva, 1992; Loo et al., 1992; Mohamed et al., 1992; Palaganas, 1992 e pers. comm; Long and Skewes, 1994; Asch- bacher et al., 1995; Vits and Tack, 1995; Rasolofoharinoro et al., 1998; Blasco et al., 1998; Green et al., 1998; Dahdouh-Guebas et al., 2000; Kairo et al., 2002; Tong et al., 2004; Kovacs et al., 2005). These processing methods have been acceptable for application on mangrove habitat maps in management, including mangrove inventory and mapping, 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, 1994; Vits and Tack, 1995; Rasolofoharinoro et al., 1998; Green et al., 1998; Tong et al., 2004; Son and Thu, 2005). It is recognized that SPOT images can be classified for mangrove forest identification achieving an accuracy of from 81 to 95% (Palaganas, 1992 e pers. comm; Vits and Tack, 1995). The present study provides an overview of the mangrove forest distribution and changes in Tra Vinh province by using Remote Sensing (RS) and Geographical Information Systems (GIS). 2. Materials and methods 2.1. Materials Tra Vinh Province belongs to the Mekong Delta (Fig. 1), which is situated from 9  31 0 Nto10  04 0 N and from 105  57 0 E to 106  36 0 E. With a total shoreline of 65 km, it lies between two branches of the Mekong River (Co Chien River and Bassac River) and flows into the Bien Dong (South China Sea). The economy in Tra Vinh is based mainly on ag- riculture and aquaculture. Shrimp farming areas have devel- oped significantly and the mangrove forest has also changed accordingly. In 1943 the area of mangrove forest was about 65,000 ha (Hong and San, 1993), however by 1995 it was hard reduced to 6678 ha (Phuong and Hai, 1998). Data have been made available to this study from different sources. There were topographical maps in 1965 from US Navy maps which were established in 1967 (Scale map: 1:50,000 and UTM: Indian 1960, Zone 48 in Southern), and remote sensing images e SPOT image on February 04, 1995 with 3 bands and 20 m resolution (however, the 1995 image only intercepts the northeastern part of the study area) and SPOT4 image on January 22, 2001 with 4 bands and 10 m res- olution. These are images in medium resolution. So, they could help to recognize the distribution of mangrove forests with the high accuracy (Vits and Tack, 1995). 2.2. Field trips Four field trips were carried out at 20 stations, September 10e20, 2000; March 14e28, 2001; September 6e23, 2001 and March 2e20, 2002. At each station, one water and one sediment sample were collected for environmental factors an- alyzed in every survey, including salinity, the color of water and turbidity. In these surveys, salinity and turbidity were measured by YSI multi-parameter, and the color of water was measured by color scales. In addition, land-use classifica- tion and the structure of mangrove forests were identified. Structure, density, height, floristic composition and standing biomass of mangrove forests were studied, which helped rec- ognize training areas and to access accuracy ratio after analyz- ing the results of remote sensing to find out the distribution of mangrove forests at the studied areas. 2.3. Methodology to identify mangrove forest by GIS and RS The processing of the identification of mangrove forests was carried out step by step as shown in Fig. 2. This process- ing was implemented by ArcView 3.2 and ENVI 3.4. 99P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 2.3.1. Image registration Image registration is a process whereby an image is re- sampled to conform to another image or topographical map (e.g. US Navy maps). In this stage, the varying pixel sizes of the different images were changed into a common map grid based on a reference image/map. Evenly-distributed GCPs (Ground Control Points) were selected in the different images and registered with the reference images/maps. A RMS (Root Mean Square) error of less than 0.5 pixels was accepted for the transformation. Resampling is preformed by converting dif- ferent pixel sizes to the same final image pixel sizes. 2.3.2. Preliminary analysis After satellite images were geometrically corrected, prelim- inary analysis methods could be applied for image enhance- ment, filtering, unsupervised classification and NDVI computation (Son and Thu, 2005). For vegetation areas, including mangrove forest, NDVIs al- lows cataloguing into 3 classes: low, moderate and high den- sity. Blasco et al. (1986) and Chaudhury (1990) indicated that the classification of mangrove forests could be identified by NDVIs. According to Guyot and Gu (1994), NDVI of man- grove forest was higher than 0.13. Based on values of NDVIs Fig. 1. Study area (left) and SPOT image (January 22, 2001) displaying false color composite in Tra Vinh study area (right). 100 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 (Table 1), the training areas of different mangrove layers were selected to support for field trips. 2.3.3. Training area selection 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 on the experience gained from the visual image interpretation. Each parcel was captured from homogeneous areas and en- coded. Several parcels were selected per code. These training sites, therefore, were determined by the numbers of groups that retained to define the spectral space. The spectral signature for each group was defined by the means of each band reflectance and their standard deviation. Each training area was larger than 200 pixels (20,000 m 2 for image with 10 m in resolution and 80,000 m 2 for image with 20 m in resolution). 2.3.4. Realization of ground data Comparison between the training areas and the actual dis- tribution of the themes in the field trips was an essential ele- ment of any remote sensing work. By the end of this step, the whole spectral space was split into classes and each class represented one or several training areas, and each training site was assigned with a thematic code. The fieldwork also helped to ascertain training areas. 2.3.5. Supervised classification For any given theme the pixels of training sites were used to calculate a mean spectral reference value. A standard Maxi- mum Likelihood Classification with Bayesian variation was Geometric correction Preliminary analysis Definition of training area Field trip Supervised classification Post-classification Accuracy assessment Input of GIS Ground truth Overlay map in GIS Digital analysis Input information Labeling Mangrove classification maps in 1995 and 2001 SPOT image Topographical map Mangrove map in 1965 Mangrove changes Processing Material or production Fig. 2. Processing flowchart to map mangrove change by GIS and RS. Table 1 NDVI value of the training areas NDVI Mangrove class Code 0.13e0.42 Low density I 0.43e0.71 Moderate density II 0.72e1.00 High density III 101P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 performed on the image. Some post classification steps such as checking information and layers were carried out. 2.3.6. GIS database The contour of the dykes separating the area between ma- rine and freshwater were digitized from the topographical maps and imported to the images. Similarly, the networks of road and estuaries were digitized. The results from this proce- dure were used to eliminate interpretation/classification errors thus providing a more accurate stratification of mangrove, non-mangrove and other land-use areas. 2.3.7. Evaluation of classification results Results were calculated from the images obtained using field observations as reference. The accuracy of the classifica- tion results was evaluated by comparing the geographical data derived from ground truth. Randomly selected reference pixels (about 200 pixels) were inspected at the corresponding sites to verify the classification results derived earlier. 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. 102 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 Further, comparison between mangrove forest layers with NDVI classes permitted us to classify mangrove forest layers with their NDVI value in Table 1. 2.3.8. Classification of mangrove forest based on topographical maps Topographical maps were classified into a digital table and/ or automatic software (ArcView 3.2 software). Mangrove for- est, rice field paddy, human settlement, swamp, marsh and river areas were separated in different layers. These informa- tion layers were labeled with information from topographical maps and input to the GIS. 2.3.9. Mapping overlay All the mangrove layers from the various periods were overlaid in ArcView to assess mangrove changes as well as de- forestation and reforestation areas. The overlay method for vector maps was applied to establish mangrove deforestation, mangrove reforestation and unchanged mangrove areas. This method was based on the ‘‘Union two themes’’ and ‘‘Dissolve feature based on an attribute’’ functions of reprocessing in ArcView. 3. Results and discussion Overall, the results of surveys show that waters are highly turbid, highly concentrated in loamy and clayey particles, 0 5 10 15 20 25 Area (1000 ha) 1965 1995 2001 Southwest part Northeast part C D Fig. 3 (continued). Table 2 The distribution of mangrove Forest (ha) in Tra Vinh Province (Source: * Phuong and Hai (1998)) Year 1943* 1965 2001 Tra Vinh 65,000 21,221 12,797 Table 3 Characteristics of distribution of Mangrove forest in Tra Vinh Year Units Northeast part Total Unit: (ha) 1965 Total of mangrove forest 7877 21,221 1995 Total 7241 Low density 2596 Modulated density 3343 High density 1301 2001 Total 3122 12,797 Low density 2050 8666 Modulated density 910 2347 High density 162 1784 103P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 constantly redistributed in suspension by tidal currents and by speed boats used as transportation means prevailing along the coast and nearshore of the study area. The concentration of suspended sediment ranges from 22.5 to 1079.5 mg l À1 . The color of the water is often brown red which could be due to the erosion of red basaltic soils bearing rubber trees and up- stream. Nowhere old mangroves have been observed. They probably exist in very limited areas but are rare whilst human density is high almost everywhere. The salinity ranged from 13 to 20 in the dry season and from 0.1 to 10.6 in the rainy season. It is suitable for shrimp culture. This probably explains why Tra Vinh has been the best places for shrimp farming in the past years. Six groups of mangrove species are obtained in Mangrove forest in Tra Vinh. Along the water ways, Avicennia and Son- neratia covered more than 25%, Excoecaria agallocha was about 20%, Derris trifoliatta was about 15%, Phoenix palu- dosa was about 15%, Nypa ranged 15e20%, and Ceriops, Bruguiera and Xylocarpus was less than 5%. This abundance of mangrove species paid attention of the results of remote sensing analysis. 3.1. Status of mangrove forest in Tra Vinh By digitizing the 1965 topographical maps and analyzing the supervised classification of remote sensing images, man- grove forest areas were identified with different classes. The results showing the mangrove distribution in Tra Vinh indicate significant changes in mangrove forest coverage in Tra Vinh (Fig. 3 and Tables 2 and 3). In 1965 rice paddy was the most use of land in Tra Vinh. The mangrove forests were dis- tributed in Duyen Hai district (Fig. 3A). Out of the 38,000 ha 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 37% of the total of land-use area) with 3,122 ha in the north- eastern part (Fig. 3D and Table 3). Some mangrove areas in 2001 were distributed in Tra Cu district but they were lower in density (Fig. 3C). In the northeastern part of Duyen Hai District, the results show that the decrease of mangrove forest areas in the period between 1965 and 1995 was slower than that in the period from 1995 to 2001 (Fig. 3D). The total area of mangrove forest in 1965 was 7877 ha (Fig. 3A) and 7241 ha in 1995 (Fig. 3B) while in 2001 it was 3122 ha (Fig. 3C). Fig. 3A also indicates that a half of the areas of natural mangrove forests were distributed mainly in the northeastern of Duyen Hai. Hence, the changes of distribution of mangrove forests in the north- eastern part of Duyen Hai in 1965, 1995 and 2001 could rep- resent the varying status of mangrove forests in Tra Vinh (Fig. 3D). In addition, compared with research results of Phuong and Hai (1998), mangrove forests of Tra vinh in 1995 covered only 6678 ha (all area) (Fig. 3B), lower than that of the present study (Phuong and Hai, 1998) but higher than the total area of mangrove forests of high and moderate density (5939 ha) (Table 3). These results demonstrated that most low density of mangroves might not be forest but just mangrove trees (Hong, 1995). Also, according to Hong (1995), before 1995, shrimps were farmed in Can Long, and just began to expand in Tra Cu and Duyen Hai (after 1995). Thus, mangrove forests, which were low density in the north- eastern part, were cut down and converted to shrimp farms. Hence, most areas of mangrove forest, which were low in den- sity, were a mixture between mangrove trees and shrimp farms or canals. For all study areas, mangrove forests also varied greatly (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 were located in areas converted from paddy rice fields, which were one crop and low productivity, to shrimp farming. This has happened in the early 1990’s (Hong, 1995; Phuong and Hai, 1998). Especially, after the decision of Vietnamese Gov- ernment in 1999 about the changes of the structure of major land-use patterns in wetland and lowland areas, the converting to shrimp farms has occurred rapidly. According to the above results, mangrove forests in Tra Vinh have been decreasing rapidly. However, replanting activ- ities have been carried out in some places, but are far from 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 Northeast part Area (ha) 7877 7241 3122 À636 À4119 À4755 Annual rate (%) À0.2 À13.1 À2.5 Whole study area Area (ha) 21,221 12,797 À8424 Annual rate (%) À1.4 Table 5 Characteristics of mangrove forest changes in Tra Vinh during 1965e2001 Areas Status 1965e1995 1995e2001 1965e2001 Unit: ha Northeast part Deforestation 2235 4996 5619 Reforestation 1434 877 863 Unchanged mangrove 5642 2245 2258 All area Deforestation 14,208 Reforestation 5784 Unchanged mangrove 7013 104 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 compensating losses by deforestation. The details of mangrove changes are presented in the next part. 3.2. Mangrove changes in Tra Vinh To identify mangrove changes in Tra Vinh, mangrove for- ests in 1995 and 2001 were assumed to have similar density. Using GIS method, mangrove changes were indicated in Tables 4 and 5 and Figs. 4 and 5. Mangrove forests in Tra Vinh changed considerably but ra- tios of mangrove changes in the two periods were significantly different. In the period between 1965 and 2001, reduced area of mangrove forests was 8424 ha (Table 4). The annual rate of the area reduction of the mangrove forests in Tra Vinh was 1.4%, and in the northeast part was 2.5% (Table 4). However, in the northeastern part, the rate of destruction of the mangrove forests in the 1965e1995 period (the rate was 0.2% e Table 4 and Fig. 4A) was many times lower than that in the 1995e2001 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. 105P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 period (the rate was 13.1% e Table 4 and Fig. 4B). These results demonstrated that in the early period, the shrimp farming in Cau Ngang affected mangrove forest slightly, whereas they signifi- cantly increased the destruction of mangrove forests in the later period (Table 4 and Figs. 4C and 5). In addition, the activities of deforestation and re-planting caused mangrove changes in Tra Vinh. Most natural mangrove forests was cut down for firewood and converted to rice paddy fields and salt fields in the previous period, or converted into shrimp farms in the latest period. The total area of mangrove deforestation between 1965 and 2001 was 14,208 ha while mangrove reforestation was 5784 ha (Table 5 and Fig. 5). The total area of un-changed mangrove was 7013 ha (Table 5 and Fig. 5). It was demonstrated that some small natural mangrove forests were restored and/or protected (Fig. 4C). Most of these areas were mangrove forests in moderate and high density in 2001 (Figs. 3C and 4C). Fig. 5 shows that re- planting of mangrove in the southwestern part was higher than that in the northeastern part. The large replanting areas of mangrove forest have poor quality in soil nature and are lo- cated mainly in shrimp farming areas, which were converted from paddy rice fields having low productivity (Tra Vinh DoF, 1999, 2000, 2001). However, these areas were not for- ests. They only were mangrove trees (Hong, 1995). Most natural mangrove areas in Tra Vinh were replaced by shrimp culture areas (Tra Vinh DoF, 1999, 2000, 2001). Some natural mangrove areas existed but their quality waned consid- erably. Many planting areas have existed in shrimp culture but with low density. Only replanting areas, located nearly Dinh An estuary, were considered as ‘‘mangrove forests’’ (Fig. 4C) (Tong et al., 2004). 3.3. Future research in land-use in Tra Vinh Province Although mangrove replanting and deforesting activities have been carried out in parallel in the Mekong River Delta, Fig. 4 (continued). 0% 20% 40% 60% 80% 100% Northeast part All area 0 2 4 6 8 10 Mangrove areas (x1000 ha) 1965 -1995 1995 - 2001 1965 - 2001 Deforestation Unchange Reforestation 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). 106 P.M. Thu, J. Populus / Estuarine, Coastal and Shelf Science 71 (2007) 98e109 the overall mangrove areas have been decreasing rapidly. The main reason is reduction of mangrove forest is the conversion of land areas previously covered in mangroves to shrimp farms. Due to high profit, shrimp cultivation areas have in- creased rapidly in early 1990s (Fig. 6) but the shrimp farming areas were reduced by outbreak of disease during 1995e1997 period. However, after that, under effects of government’s con- trol, shrimp culture has developed again. Seemingly, the man- grove forests decreased as the shrimp farming expands (Fig. 6). In addition, because of a lack of information about environ- mental conditions, shrimp culture techniques and financial re- sources required, shrimp farming failed in some areas or shrimp ponds were used only in the short period (Hong, 1995). After few years, land has been degraded and farmers have continued to cut down mangrove forests and to make new shrimp ponds. Before 1995, shrimp farming was mostly 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 and/or dry (Hong, 1995). This might have caused the reduction of shrimp farming in Tra Vinh, and the area under shrimp cul- ture were at their lowest in 1997 (Fig. 6). After that, however, shrimp areas have increased again. Shrimp farmers cut down the mangrove forest to make new shrimp ponds in Duyen Hai (Tong et al., 2004; Son and Thu, 2005). The motivation for mangrove destruction and degradation is based on the short-term exploitation for immediate eco- nomic benefit, rather than longer-term and sustainable exploi- tation. These are major reasons of mangrove deforestation in the period between 1995 and 2001 in Tra Vinh. 4. Conclusion Combining GIS and RS techniques can help to identify the distribution of mangrove forests in the Mekong Delta in detail. The area of mangrove forests was 21,221 ha in 1965, while in 2001 it was 12,797 ha. In addition, RS analysis may identify 3 types of mangrove forests: low, moderate and high density. However, the mangroves in low density could not be forests but they were ‘‘mangrove trees’’. This GIS-combined study on historical changes in man- grove distribution in Tra Vinh of the Mekong Delta, Vietnam has demonstrated that over the past 36 years (1965e2001), about 50% of the mangrove forest area was lost, but the reduc- ing annual rate of the period from 1965 to 1995 was lower than that in the period from 1995 to 2000. In the period from 1965 to 2001, the total area of mangrove deforestation was 14,208 ha whereas mangrove reforestation was 5784 ha. These changes in mangrove forests cover were affected by two activities: deforestation and replanting, but planting ca- pacity was slower than deforestation. Recent mangrove changes are due mainly to shrimp farming expansion, which is developing in an unplanned way. Shrimp farm development and degradation also caused environmental and natural re- sources problems with socio-economic consequences such as land degradation, environmental pollution, the conflicts among natural resource users and the gap between the rich and poor. Reforestation of abandoned shrimp ponds might be a good so- lution to improve the sustainability of this ecosystem before a new government master plan of land use for the coastal zone can be developed. Acknowledgements This research was supported by The European Commission in the framework of the GAMBAS Project. Our deepest thanks and gratitude to Prof. Nguyen Tac An (on behalf of GAMBAS project) for his support for our study and field trips. We thank staff of GIS group at Institute of Oceanography, Nha Trang, for help in mapping and digital processing. We also thank Dr. Amararatne Yakupitiyage (Asian Institute of Technology, Thailand) and Dr. Trevor Charles Platt (Bedford Institute of Oceanography, Canada) for their suggestions. References Aschbacher, J., Ofren, R., Delsol, J.P., Suselo, T.B., Vibulsresth, S., 1995. An integrated comparative approach to mangrove vegetation mapping using advanced remote sensing and GIS technologies: preliminary results. Hydrobiologia 295, 285e294. 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