Fish population dynamic in the newly impounded Nam Theun 2 Reservoir (Lao PDR)

Abstract

A fish population monitoring was carried out in the reservoir and tributaries of the hydropower project Nam Theun 2 (Lao PDR) at the end of the warm and dry seasons and at the end of the warm and wet seasons between 2008 and 2013. The study focused on three aquatic systems (river upstream, transition area, and reservoir) and different drowned habitats (dense/light forests and agricultural soils) associated or not with the presence of villages. The reservoir fish population was expected to follow three distinct phases, which should be respectively a rapid increase in biomass and abundance (trophic upsurge) followed by a decline and a stabilization of the population. This succession appeared to be different to the commonly observed scenario in terms of biomass, abundance and taxonomic richness compared to other temperate and tropical reservoirs. The indicator species (H. macrolepidota and P. carinatus) did not show changes in their size range since impoundment. The presence of structured habitats (e.g. inundated forest) favoured the fish population in terms of biomass and abundance. The fish population in the transition area and the river upstream improved quickly after the impoundment and presented higher taxonomic richness compared to the reservoir. Finally, the transition area appeared to be the most productive area within the Nam Theun 2 Reservoir especially if it remained outside of fishing activities.