Status and changes of mangrove forest in Mekong Delta:Case study in Tra Vinh, Vietnam Phan Minh Thua,* , Jacques Populusb a Institute of Oceanography, 01 Cau Da, Nha Trang, Khanh Hoa, Vi
Trang 1Status and changes of mangrove forest in Mekong Delta:
Case study in Tra Vinh, Vietnam
Phan Minh Thua,* , Jacques Populusb
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
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
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
led to an increased need for land for agricultural production
In addition, environmental degradation and sedimentation
1996; Le and Munekage, 2004)
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
www.elsevier.com/locate/ecss
Trang 2generated 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 selecdetec-tion 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
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
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, 1992epers 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 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
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
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 resres-olution So, they could help to recognize the distribution of mangrove forests
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 sensanalyz-ing 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
process-ing was implemented by ArcView 3.2 and ENVI 3.4
Trang 32.3.1 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
For vegetation areas, including mangrove forest, NDVIs al-lows cataloguing into 3 classes: low, moderate and high
that the classification of mangrove forests could be identified
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).
Trang 4(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
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
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
Trang 5performed 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.
Trang 6Further, comparison between mangrove forest layers with
NDVI classes permitted us to classify mangrove forest layers
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 informainforma-tion 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 rede-forestation 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
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) )
Table 3 Characteristics of distribution of Mangrove forest in Tra Vinh
Unit: (ha)
Trang 7constantly 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
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
Son-neratia covered more than 25%, Excoecaria agallocha was
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
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
2001 were distributed in Tra Cu district but they were lower
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
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
than the total area of mangrove forests of high and moderate
most low density of mangroves might not be forest but just
(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
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 (: Reduction, þ: Increasing)
Table 5 Characteristics of mangrove forest changes in Tra Vinh during 1965e2001
Unit: ha
Unchanged mangrove
Unchanged mangrove
7013
Trang 8compensating 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 5andFigs 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
area reduction of the mangrove forests in Tra Vinh was 1.4%,
northeastern part, the rate of destruction of the mangrove forests
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.
Trang 9period (the rate was 13.1% eTable 4andFig 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
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
5 and Fig 5) It was demonstrated that some small natural
Most of these areas were mangrove forests in moderate and
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
DoF, 1999, 2000, 2001) However, these areas were not
Most natural mangrove areas in Tra Vinh were replaced by
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
(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
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).
Trang 10the 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
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
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
been degraded and shrimp ponds have become bare-land
of shrimp farming in Tra Vinh, and the area under shrimp
shrimp areas have increased again Shrimp farmers cut down
the mangrove forest to make new shrimp ponds in Duyen
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
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0
5
10
15
20
25
0 10 20 30 40 50 60
Shrimp farm
Fig 6 Relationship between mangrove forest areas and shrimp farm areas (NE
part: Northeastern part, MF: Mangrove Forest).