By Chris Barlow, Lee Baumgartner, and Dr Michael Raeder for Australian Centre for International Agricultural Research.

There are nine large hydropower dams planned for the mainstream of the Mekong River in Lao PDR, and two more in Cambodia. The dams have divided public opinion. On one hand, there are those who clearly see the benefits of power generation for regional consumption and poverty reduction. On the other, there is concern about the impacts on the livelihoods of people currently dependent on the river, and the difficulties of mitigating those impacts. This is especially the case for the barrier effect of dam walls blocking fish migrations, which in turn interrupt life cycles and breeding of fish, thus potentially diminishing an important source of food and income for many.

The fishery in the Mekong is immensely important, as it provides approximately 50% and 80% of the animal protein consumed in Lao PDR and Cambodia respectively. In the Mekong catchments in Lao PDR, Thailand, Cambodia and Vietnam the fishery yield is estimated at 2.3 million tonnes. By comparison, the amount of wild caught fishery products in Australia is about 150,000 tonnes.

The first two dams, at Don Sahong and Xayaburi, in Lao PDR, are under construction and will be completed soon. The Don Sahong is located near the Lao-Cambodia border, on the 11 km wide fault line commonly known as Khone Falls. The dam blocks one of many channels through the fault line, all of which have waterfalls a few metres high. The major fish migration route – the Hou Sahong channel – will be blocked by Don Sahong. The developer’s mitigation plan is to modify nearby channels to reduce vertical rises, which it claims will enable fish to swim up them, as they previously did in the gently sloping Hou Sahong. The effectiveness of the mitigation measures will only be known after the dam has been completed.

Xayaburi HPP in northern Lao is different. It blocks the entire width of the Mekong river with a dam wall in excess of 30 m, presenting an impassable barrier to all fish species. Construction of Xayaburi HPP, which started in 2012 and is now approximately 80% complete, will cost approximately US$3.7 billion. The first turbines are due to be operational in 2019.

As planning for the dam and associated public and regional government discussion intensified through 2007-2014, the developer (Xayaburi Power Company Limited – XPCL) increased its commitment to environmental mitigation. Significant investment in design changes, to provide for sediment transport and fish passage, were incorporated into the final designs.

The level of investment, and the complexity of the fish passage facilities, are unprecedented anywhere in the tropical or sub-tropical world. In fact the entirety of fish passage facilities are matched only in North America, where investment targets salmon, a group of powerful-swimming fishes renowned for their ability to move up waterfalls. But Mekong fish are completely different to strong swimming salmon.

The Xayaburi facilities have been designed to pass large biomasses of more than 100 species of fishes varying in size from a few centimeters to more than one meter. It is a formidable technical challenge. Whether the fish passage facilities will be effective is a question that the developer, scientists and downstream communities would like answered.

If the critically important fishery in the Mekong is to be maintained, then all other planned dams both upstream and downstream in the mainstream cascade will need to match or exceed the fish passage performance at Xayaburi. The weakest link in the chain will be the one that impacts fish the most.

From a fisheries management perspective, no further mainstream dams should be built in the lower Mekong basin until evaluation of the performance of the Xayaburi facilities is available to guide future fish passage design. Indeed, this argument was put forward by participants at a recent Mekong River Commission meeting to assess the design of the Pak Beng Dam, the next in the Mekong river cascade.

The fish passage facilities at the Xayaburi site provide an unprecedented research opportunity of local and global relevance. Research and operational modification will be required to optimize performance and to validate the extent to which the elaborate passage facilities actually work.

Recognizing this, XPCL has recently invited a group of Australian, US and Lao researchers experienced in fishway design to work with the XPCL environmental team to test and optimize the operation of the fish passage facilities. The information generated will provide valuable input into operations at Xayaburi, future hydropower development on the Mekong and similar projects elsewhere in the tropical world.

From a development perspective, energy generation is needed in the emerging economies of the Mekong countries. Dams will continue to be built in regions where poor people are dependent on natural resources, despite the tragic history world-wide of the negative effects of hydropower development on the maintenance of the ecosystem benefits of rivers. In that context, the Xayaburi facilities provide a unique opportunity to determine if better approaches can be demonstrated, and thereafter adapted and applied to other sites.

Photos by Australian Centre for International Agricultural Research.