A COMPARISON OF THE GENETIC-VARIABILITY OF 2 FISH SPECIES, GRAYLING THYMALLUS-THYMALLUS AND ROACH RUTILUS-RUTILUS, IN A LARGE REGULATED RIVER

Both species inhabit the catchment of the French Upper Rhone above the city of Lyon (Southeastern France). They mainly differ by their genetic variability: 67 % of overall grayling variability occurs between populations, whereas 86 % of roach variability is to be found within the populations, with a higher diversity in the lateral parts of the system (lake and side arm). These differences are discussed in relation with habitat area and structure, as well as fish management and river equipment. Grayling is organized as small and localized populations, with greatest diversity in the middle of the upstream-downstream gradient, where the environment is most favourable. Backwaters act as genetic reservoirs of roach diversity, from which important but selective fluxes move into the channel and drift downstream, thus maintaining genetic uniformity between populations.