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The Impact of Dams on humans and Biodiversity along the Lower Mekong Basin
Global Change Biology, School of Biology, Victoria University of Wellington
The Mekong River is one of the largest rivers and the second richest in biodiversity in the world. Because of the demand in electricity for social economics development, the lower Mekong riparian countries have constructed and proposed more than one hundred dams. This river is currently under threat as the dams will block migratory fish species from spawning to feeding habitats, and disturb non-migratory fish species’ habitats. These will lead to a decline in fish recruitment and food security. The dams will also risk the extinction of the critically endangered Irrawaddy dolphin and the Mekong Giant catfish. Moreover, they will seriously affect people’s livelihood by destroying agricultural lands and damaging water quality. Consequently, it is important to have an international standard of social and environmental impact assessment before building the dams. Governments should avoid building dams which are located in biodiversity hotspots. Small dam projects should be considered reducing the impact on fish migration by introducing with fishways. Ultimately, the governments should consider the long-term costs and benefits of these development projects on both people and biodiversity.
Due to the increase in energy consumption globally, hydropower is a source of renewable energy that could meet the demand of electricity and reduce impacts on climate change [1-3]. More than 105,000 megawatts of electricity are estimated from hydropower around the world . Currently, the world’s largest hydropower dam is the Three Gorges dam in China , which generates 22,400 megawatt of electricity per year .
The Mekong River is one of the largest rivers and is the second riches of biodiversity after Amazon River . It is 4,200 km long and flows across six countries, China (the upper Mekong River), Cambodia, Laos, Myanmar, Thailand and Vietnam (the lower Mekong River) . At least 1,200 fish species have been recorded in this river . Besides this high diversity, it is also recognized to be important for fish migration [5, 8]. Fish is the most important source of income and is a staple die for approximately 65 million people . Fish production in the lower Mekong basin have been estimated more than two million tonnes
Global Change Biology, Victoria University of Wellington annually, which is worth between US$1.4 to 2 billion per year (first sale value) and between US$4.2 to 7.6 billion per year (retail value) .
The Mekong River has been considered a source of electricity among the riparian countries, thus more than one hundred dams are being proposed along the lower Mekong basin . At the same time, the negative impacts of dams should not be neglected. The dams will alter water flow regime, trapping sediment, threatening to biodiversity, blocking fish migration, and possibly creating serious flooding, all of which have been recorded after dam constructions . This paper will focus on the impacts of constructed and proposed dams on fish diversity, human populations and the risk of extinction of the critically endangered Irrawaddy dolphin (Orcaella brevirostris) and the Mekong Giant catfish (Pangasiandon gigas) in the Mekong basin. The ultimate goal of this paper is to discuss on what can done to reduce impacts on people and biodiversity.
The water of the Mekong River originates from the Tibetan Himalayas Mountain in south China and flows in 4,600-km length toward south China sea [5, 10]. The lower Mekong River is divided into two land types: upstream mountains (Laos and less hill in Thailand) and downstream lowlands (Cambodia and Vietnam) . It is connected with the Tonle Sap Lake (Great Lake) in Cambodia. During the rainy season, the water flows from the Mekong River into the Tonle Sap Lake, while in the dry season, the water flows back from the lake into the Mekong River .
The Mekong River Commission (MRC) was established in 1957 by the four members of Mekong riparian countries. The main aim of this establishment is to coordinate water resource management in the lower Mekong River. Due to China and Myanmar are not interested in this committee, the four members including Cambodia, Laos, Thailand and Vietnam . This committee is a part of the United Nations’ Economic Commission for Asia and the Far East and the first UN that involve in international river management. . The members will meet and discuss if there are any development projects including dam development project are planning to conduct along the river. Finally, the members will give suggestion or recommendation whether projects should be implemented .
Due to the demand in social and economic development, Cambodia, Laos, Thailand and Vietnam have considered building hundred dams along the Mekong basin . More than 100 dams are proposed, and some of them are completed [14, 15]. Xekong, Se San, Stung Treng, and Sambor dams are proposed in the mainstream and tributaries of Cambodian Mekong River [14, 16]. Pak Bang, Luang Prabang, Xayaburi, Pak Lay, Pak Chom, Sankham, Ban Koum, Latsu, and Dan Sahung are proposed in the mainstream of the Mekong River in Laos [16, 17]. Among those Xayaburi dam is under construction . Pak Mun dam, located in Thailand, was completed in 1994 . The Yali dams is located in the tributaries of the Mekong Data in Vietnam, which was completed in 1996 . The dams which are proposed along the lower Mekong basin are projected to generate 14,697 megawatt of electricity per year . The economic value of these dams is projected about USD$ 235 million per year .
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Figure 1: The constructed and proposed dams along the Mekong mainstream. Source: Cronin and Hamlin (2010). These dams obviously show that will block fish migration from upper to lower Mekong River. Note: this map does not show dams along the tributaries of the Mekong River. 
The monsoon floods have merged several different habitats (lakes, rivers, ponds, streams, and flooded forests) to a single river polyclone. This phenomenon provides suitable habitats for fish migration from one location to another. Whenever the Mekong tributaries begin to flood, fish migrates into the flooded forests for breeding, spawning, and feeding . Once they become adults, many species move back to the mainstream of the Mekong River and live permanently [11, 18].
Many fish species in the Mekong basin migrate for reproduction and feeding [11, 18], and approximate 70 species of 229 fish species in the Mekong basin are migratory fish species . Although there is no evidence about specific spawning sites for Mekong fish species, as the vast majority of fish species migrate into many flooded areas of the Laos, Cambodia and Thailand it is expected that these may be the spawning habitats . Some species are found to spawn in the Tonle Sap Lake in Cambodia . The most important of feeding habitats of most Mekong fish are in floodplains of Cambodia, Laos and Thailand .
illustration not visible in this excerpt
Figure 2: The migration patterns of the three migratory species (a-Botia modesta, b-Bangana behri and c-Catlocarpio siamensis) along the Mekong basin. Source: Poulsen, A. F., et al (2004)
For example, Red-tail loach (Botia modesta) feed mainly on benthic invertebrates. The spawning habitats are not exactly sure, possibly in the flooded forests which are located in Laos, Cambodia and Vietnam. In in rainy season, they migrate to the flooded forests for spawning, breeding and feeding. Then when they became adults, they move back to the mainstream of the Mekong river and live permanently in the deep pools (Fig 2-a) . Two- headed carp (Bangana behri) exists only in a few locations in the lower Mekong River, which is located in northern Cambodia. The species feeds on phytoplankton, prerphyton and algae. It migrates from Khon Falls (Laos) to StrengTreng (Cambodia) to their spawning habitats. In the dry season, two-headed carp lives in the deep pools and large tributaries of the Mekong basin (Fig 2-b). Giant barb (Catlocarpio siamensis) is the Cambodian National fish, and this species now under threat due to overfishing. It mainly feeds on algae, phytoplankton, fruits and terrestrial plants, and small fish. This species distributes only in the lower Mekong basin from Laos, Cambodia and Vietnam. The possible spawning habitats may be in flooded forests of Cambodia and the feeding habitats are in the floodplain Laos. It lives permanent in the deep pools of the Mekong in northern Cambodia (Fig 2-c) .
The model predicting the impacts of dams in both Mekong mainstream and tributaries showed all those proposed dams would have serious negative impacts on biodiversity [5, 9]. The dams would alter hydrology potentially delaying flood arrival and shorting floods in the floodplain regions . This shift would have a negative impact on the river ecosystem and fish habitats [9, 20]. Moreover, dams would block at least 70 migratory fish species from moving upstream to downstream [9, 21]. This will decrease fish recruitment . Moreover, the non-migratory species may being also impacted due to their habitats being destroyed .
The dams along the mainstreams of the Mekong River may also cause extinction for the critically endangered Irrawaddy dolphin (Orcaella brevirostris) and the Mekong Giant catfish (Pangasiandon gigas). The Mekong Dolphin Conservation Project has conducted surveys of the threats on population as well as awareness raising activities aiming to conserve this species [19, 22]. The population of the Irrawaddy dolphin has declined significantly in the last few years, mainly because of killing by large-gillnets using for fishing [19, 22]. Moreover, the most important habitats of this species are found in small regions between the Cambodia and Laos border . If the two large dams are completed in both Stung Treng and Sambor, Cambodia, this dolphin risks extinction. The dams would separate the remaining population of dolphin in Cambodia, and block this species from moving to the deep for reproduction .
The Mekong Giant catfish is listed as the world’s largest freshwater fish in the Guinness Book of World Record . It can grow up to three metres long and weigh up to 300 kilograms . Currently, the IUCN red classified this species as critically endangered . The Mekong Giant catfish is reported to have spawning habitats in Cambodia, but it migrates to the feeding habitats of the Tonle Sap Great Lake in Cambodia and Mekong Deta in Vietnam, then lives in the deep pools of Kratie and Stutreng province in Cambodia and Xayabury province in Laos . The population of this species has declined dramatically because of overfishing and habitat destruction . If the dams in the main streams are completed, they would block migration of the Mekong Giant catfish from the spawning areas in the Tonle Sap Lake to the Mekong River, this will resulted in decline population of this species and may lead to extinction .
The Mekong fish species is a main source of food security and income for people living along the Mekong basin . The construction of mainstream dams will decrease migratory fish production between 0.7 and 1.6 million tonnes per year . Even though the non-migratory fish production is not documented, the loss of migratory and non-migratory fish species is estimated to be approximately USD$ 476 million. Moreover, dams will change seasonal flooding, which will have an effect on the most productive rice-growing areas in Cambodia and Vietnam and by causing severe floods . For example, due to the water released from YaliFalls dam in Vietnam, there were serious effects on people who live in the lower Mekong basin, Cambodia. In 1999, thirty two people were reported to have died by downing and there were 952 further deaths because of poor drinking water .The flood had destroyed several cultivation plots and killed thousands of livestock .
It is hard to stop hydropower developments along the Mekong basin. This is due to the fact that all the riparian countries need electricity to develop their social economies. However, Mekong River Commission (MRC), Would Wildlife Fund (WWF) and other international NGOs have cooperated with the Mekong riparian countries aiming to reduce the impacts of dam projects on both people and biodiversity . There are four suggestions and recommendations should be considered to have better management of the Mekong basin.
Firstly, the Social and Environmental Impacts Assessment (EIA) should be conducted with high standards which are mentioned in the United Nation Environmental Program (UNEP) and Wetland International. It is very important to conduct surveys on the impact on biota before the dams are going to be constructed , particularly along the Mekong River where the knowledge of these species is poor. Moreover, experts in relevant fields are needed for these surveys  in order to produce reliable reports. However, some dams along the Mekong basin do not meet the international standard of the EIA. For instance, the evaluation of the impact of fish migration in the Xayaburi dam (in Laos) is unacceptable by WWF, Food and Agriculture Organization (FAO), MRC, and WorldFish . The EIA report described only five fish species, while 229 fish species will be disturbed and 70 species of them are migratory fish species .
Secondly, the proposed dams should be considered for fish migration by introducing effective fishways. These tools have been used for reducing the impacts of fish migration which allow them to migrate across dams . Consequently, in 2008, the MRC called fisheries scientists and fish passage engineers from Australia, Southeast Asia, North and South America and Europe to discuss how to mitigate dams impacts on Mekong fish . However, due to the lack of basic information regarding the fish species, fish behaviour, migration timing, and life cycle [26, 28], it is hard for those scientists to design effective fishways . Moreover, it seems to be less effective to introduce fishways with the dams higher than 12 metres , particularly those proposed dams along the Mekong mainstream such as Xayaburi dam and Sambor projects . However, it is important to design fishways in small dams which are less than 6 metres . For example, the vertical-slot (fishways) has been introduced as the most effective fishways in many rivers because it is appropriate for most migratory fish species . However, in order to have better management and success designing fishways, three important things are needed: identification of the community fish migration, conducting experiment on fish migration on fishways, and the design and building of the fishways .
Thirdly, locations of the dams along the Mekong basin should be avoided in biodiversity hot spots . For example, the Sambor dam which is located in Cambodia should not be constructed as this dam will strongly affect the critically endangered Irrawaddy dolphin  and the Mekong Giant catfish . Although the fishways are possible tools for fish migration, it is likely that the Irrawaddy dolphin and the Mekong Giant catfishare too large for migration.
Lastly, the governments of the riparian counties should consider the long-term effect of those dam projects on both people and biodiversity. It is clear that approximately 65 million people who live along the Mekong basin rely on fish production. The total of the Mekong fish production is about 2.2 million tonnes per year, which is equal to US$2.2 and US$ 7.8 million. Moreover, the remaining Cambodia Mekong Irrawaddy dolphin are only approximately 100 individuals, however, this small population provide sustainable benefits to local people regarding eco-tourism project . Although the projection of electricity of dams along the lower Mekong basin is estimated to generate 114,697 megawatts and worth
Global Change Biology, Victoria University of Wellington about US$ 235 per year, this does not mean that this money can sustain the development in those countries.
The Mekong basin is considered as the main source of renewable energy that could meet the demand of electricity in the Mekong riparian countries. Although dams provide positive impacts on social economic developments, the negative impacts should not be neglected. The dams will block fish migration, disturb fish’ habitats and risk the critically endangered Irrawaddy and the Mekong Giant catfish extinction. Moreover, the dams will seriously affect people livelihoods. However, better management of hydro developments which mentioned in the UNEP and Wetlands International should be considered among those countries. The Laos government should resurvey the social and environmental impact assessment on the Xayaburi dam. The Cambodia government should cancel dam which is located in the sambor and Stung Treng districts. The Thailand and Vietnamese governments should monitor the impacts of the constructed dams on environment and people.
I would like to thank Professor Kevin Gould and Associate Professor Ken Ryan for comments and suggestions on my mini-thesis since at the being until the end. I also would like to extend my sincere thanks to Student Learning Support Service for checking my grammatically error and sentence structures. Finally, I would like to thank to my classmates in mini-thesis for their comments on my mini-thesis proposal.
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