Lessons from Damming in the Mekong Tributaries: Effects on Riverine Fish Community and Biology

Fish monitoring was conducted in three Mekong tributaries in Thailand that share a similar natural environment but differ in hydrological regulations. Despite similar historical fish assemblages reported prior to the regulation, these tributaries currently support contrasting assemblages for both adults and larvae. Fish assemblage in an unregulated tributary with no existing dams along its mainstem showed a highest species diversity with a clear seasonal fluctuation in the number of species. Fish assemblage in a tributary having a series of low-head dams was characterized by low species diversity and less seasonal fluctuation in the diversity. Lastly, fish assemblage in a tributary with a large hydropower dam near the Mekong confluence changed according to the operation of the dam. Short-distance as well as a few long-distance migratory species dominated the assemblage during a period when sluice gates were opened in the rainy season.

A change from lotic to lentic systems and associated alternations to river flow by dams impact a life history of fishes, especially that of facultative riverine species. Although these fish are able to sustain populations in a lentic system, they typically exhibit lower productivity, slower growth rates, smaller maximum sizes and delayed maturation ages compared to their lotic counterparts.