Nase (Chondrostoma nasus) and common barbel (Barbus barbus) are two rheophilic cyprinid fish naturally present in South Belgian rivers. During the last decades, the construction of dams together with changes in hydrological regimes, modifications of riverbed morphology and water pollution caused some local dramatic declines in their populations. However, recent improvements in terms of water quality and habitat fragmentation allow now to implement a rational restocking plan of locally endangered patrimonial fish species such as nase and common barbel. To reach this goal, this project (co-funded by the European Fisheries Fund and the Wallonia Public Service) proposed to develop five complementary parts with specific objectives: • Review of the knowledge on nase and barbel geographical distribution and stock health in Wallonia. This section presents the distribution and recent evolution of populations in Europe, and more specifically in Wallonia. Areas where population declines were reported are identified. Even if a weak population expansion was reported in some isolated cases, the global status of Walloon populations is still concerning. • Characterization of genetic structure and diversity of South Belgium populations. Restocking operations for a conservation purpose have to be based on the knowledge and the use of wild type genetic strains. Nase and barbel populations from South Belgium were genetically characterized by use of microsatellites. Globally, nase and barbel populations are structured on a basin scale. A slight genetic differentiation exists between populations from the Rhine basin and the Meuse basin, defining two conservation units, but no finer structure was observed among populations from the Meuse basin. Genetic variation was high within populations. Genetic structure of barbel populations is more complex due to past restocking operations with different genetic lineages. An analysis of mDNA identified 6 different haplotypes but was unable to categorize them as autochtone or allochtone. • ...