Fish biodiversity declines with dam development in the Lower Mekong Basin

Abstract

Hydropower dams are a source of renewable energy, but dam development and hydropower generation negatively affect freshwater ecosystems, biodiversity, and food security. We assess the effects of hydropower dam development on spatial–temporal changes in fish biodiversity from 2007 to 2014 in the Sekong, Sesan, and Srepok Basins—major tributaries to the Mekong River. By analyzing a 7-year fish monitoring dataset, and regressing fish abundance and biodiversity trends against cumulative number of upstream dams, we found that hydropower dams reduced fish biodiversity, including migratory, IUCN threatened and indicator species in the Sesan and Srepok Basins where most dams have been constructed. Meanwhile, fish biodiversity increased in the Sekong, the basin with the fewest dams. Fish fauna in the Sesan and Srepok Basins decreased from 60 and 29 species in 2007 to 42 and 25 species in 2014, respectively; while they increased from 33 in 2007 to 56 species in 2014 in the Sekong Basin. This is one of the first empirical studies to show reduced diversity following dam construction and fragmentation, and increased diversity in less regulated rivers in the Mekong River. Our results underscore the importance of the Sekong Basin to fish biodiversity and highlight the likely significance of all remaining free-flowing sections of the Lower Mekong Basin, including the Sekong, Cambodian Mekong, and Tonle Sap Rivers to migratory and threatened fish species. To preserve biodiversity, developing alternative renewable sources of energy or re-operating existing dams to increase power generation are recommended over constructing new hydropower dams.

Prolonged and Severe Drought in the Most Dammed Tributaries of the Lower Mekong Basin

“…Aside from drought, streamflow alteration is caused by anthropogenic activities such as hydropower dam operations and water extraction for agricultural irrigation [57]. For instance, summer baseflow in dammed rivers is generally higher than free-flowing rivers [18,58], which is the case for the Sesan River [12]. Li et al [18] and Wild et al [58] reported that dams have significantly decreased streamflow in the wet season in the Mekong River and Tonle Sap Lake, and increased streamflow in the dry season.…”

“…A major tributary system of the LMB is formed by the Sekong, Sesan, and Srepok Rivers (the 3S Basin), which contribute up to 25% of streamflow and sediment to the mainstem Mekong River [11]. This basin is currently experiencing hydrologic alteration due to rapid hydropower development [12]. The 3S Basin has 51 operational dams as of 2021.…”

“…These two rivers are the focus of our study (Figure 1). The Sekong River is also part of the 3S Basin and has the fewest dams [12]. The Sesan and Srepok Rivers originate from the central highland of Vietnam, with peak elevations ranging from 2000 to 2400 m [13], and flow through the northeastern mountain area of Cambodia to join and enter the Mekong River in Stung Treng Province.…”

The Impacts of Dams on Streamflow in Tributaries to the Lower Mekong Basin

“…As of 2021, at least 51 dams were operating in the 3S Basin, with a combined generating capacity of 4,684 MW. The Sesan River has the most dams of the three basins, with 22 dams, and the Sekong has the fewest dams, with 14 dams [32]. Given on-going hydropower dam development, streamflow in 3S rivers have deviated from natural regimes [33,34].…”

Anthropogenic modifications: impacts and conservation strategies

“…de Silva et al found that in Asia, since 1700, more than 64% of suitable elephant habitat has been lost in conjunction with land use practices for agricultural purposes 10 . Finally Sor et al show how in the Lower Mekong Basin, dams built to produce renewable hydropower reduce fish biodiversity 11 . They therefore recommend using existing dams rather than building new ones, as well as diversification in the development and use of renewable energy sources.…”