Etude de la diversité génétique et de l’état des stocks des populations de barbeaux et de hotus en Wallonie. Amélioration des techniques d’élevage en vue de repeuplements raisonnés et de transferts de connaissances vers les pisciculteurs | Condition and genetic structure of nase (Chondrostoma nasus) and barbel (Barbus barbus) populations in South Belgium. Toward a rational management of conservation restocking through the control of production techniques and know-how diffusion.

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. • Development of fish production techniques. The complete control of fish farming is necessary to produce high quality juveniles for restocking. All the steps of the production cycle were addressed: broodstock management and reproduction, egg incubation, larval rearing and grow-out. Production systems ranging from extensive pond culture to intensive RAS were tested and the optimal farming conditions were identified (temperature, density, feeding, tank volumes, …). • Adaptation assessment of farmed fingerlings to natural conditions. Growth and survival performances of captive farmed fish were assessed in an experimental environment that mimics natural conditions. The results suggested that the more efficient practice for restocking would be based on operations performed in spring with large juveniles (3 to 50 g). • Know-how diffusion toward fish farmers. All the breeding and grow-out techniques developed for nase and barbel production are the subject of two handbooks. These documents, attached to the report, will be published and distributed to fish producers.