Kampong Cham JEP2795

Existing dams on the Mekong are not the root cause of fish decline, lower water levels in Tonle Sap

Vientiane, Lao PDR, 29 Oct 2018

Vientiane, Lao PDR, 29 October 2018 – Existing dams on the Mekong mainstream are not causing major impact on fish production or lowering water levels in Cambodia’s Tonle Sap River and Tonle Sap Lake, says the Mekong River Commission (MRC).

Though a Mekong forum held in March this year heard that existing dams in China, Lao PDR, and Cambodia have caused “significant decline in fish stocks and even the disappearance of many species” and made water levels in the Tonle Sap Lake lower, MRC’s analysis on fish production and water levels in both the Tonle Sap River and Tonle Sap Lake shows otherwise.

The analysis indicates that the change in fish production in the Tonle Sap Lake is due to three key factors: fishing activities, infrastructure development, and climate change. For example, based on MRC fisheries monitoring since 1994, there has been no compelling evidence of the disappearance of any fish species or a reduction in total annual fish catch from the lower Mekong River basin in general or from the Tonle Sap River and Tonle Sap Lake in particular.

However, unsustainable fishing practice has contributed significantly to the decline in fish catch, including fish size. For instance, a national survey in Cambodia shows that more people in Cambodia are fishing not only to support their own livelihoods but also to sell at the market; many of them are using exploitative fishing gear. This has made fish catch to decline and the current catch is characteristically dominated by undersized fish, with a notable reduction in large sized fish in the catch.

Moreover, the analysis indicates that from 2015 to 2017, the lower Mekong basin was experiencing a drier condition, especially at the onset of the flood season. Water levels in the wet season during these years were lower than the long-term average but they were higher than the long-term average in the dry season. The years also saw low rainfall.

This means there was low reversed flow of water from the Mekong mainstream to the Tonle Sap Lake, whose system is strongly impacted by two factors that determine the Lake’s water levels – this reversed flow from the Mekong mainstream, and the inflow from tributaries and the Lake itself caused by local rainfall.

In addition, the same MRC’s analysis reveals that the hydrological impacts of reservoir operations in China on the flow regime of the Mekong mainstream waters manifested only at the upstream of Lao PDR’s Vientiane, but they did not influent the Mekong’s lower reaches, including the Tonle Sap Great Lake.

This means that recent lower water level in the Tonle Sap Lake in the dry season appears to be due to low reversed flow from the Mekong mainstream during the wet season with its short duration and early ending time. The reversed flow is generally influenced by the rainfall over the lower Mekong basin and operation of existing dams of the Lancang cascade and tributaries.

While the Commission’s current data indicates that existing dams on the Mekong mainstream are having minor impact on fish production and water levels in the Tonle Sap River and Tonle Sap, in the near future – if no mitigation is taken – there would be adverse impact. This is already indicated by the Council Study or “Study on Sustainable Management and Development of the Mekong River including Impacts of Mainstream Hydropower Projects”, which was published late last year.


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