Genetic integrity of the European grayling (Thymallus thymallus) populations within the Vienne River drainage basin after five decades of stockings
The upper Vienne River is the westernmost drainage in the natural distribution of European grayling Thymallus thymallus (Linnaeus, 1758). The grayling extension across this sub-basin of the Loire catchment has been dramatically reduced through habitat degradation, which began in the 19th century with the construction of milldams and eventually large hydroelectric dams, which flooded former grayling habitat (i.e. fast flowing waters) under increasingly larger reservoirs. Throughout the 20th century, habitat degradation, fragmentation and pollution have strongly affected the remaining isolated populations. Since the 1960s, fishing authorities have attempted to compensate for these declines with stocking programs. Most stocked fish stem from imported eggs or fry of unknown origin. However, the efficacy of this policy has never been assessed. Here, we aim to assess the genetic traces of these stocking programs as an indirect measure of the long-term survival of stocked fish. Sampled grayling from four localities in the Upper Vienne basin were analysed at both mtDNA (N = 37, control region) and nDNA levels (N = 88, 10 microsatellites), and compared to populations representative of surrounding drainage basins or fish farms. Microsatellite analysis demonstrated low genetic diversity within samples, typical for the Loire drainage. Both Factorial Correspondence and “Structure” analyses presented Upper Vienne gray- ling as a comprehensive genetic unit distinct from diverse samples from the Rhine, Rhone and Danube basins, as well as other regions of the Loire catchment and the sampled hatcheries. Within the Vienne, only the uppermost sample differed from the other sites. Among the 37 individuals sequenced from the Upper Vienne, only three control region haplotypes were identified, all closely related to other haplotypes observed in Loire and Allier (a Loire tributary) rivers. These results strongly support that grayling in the Upper Vienne drainage represent native genetic stocks with no signs of non-native introgression. The only evidence potentially supporting a stocking contribution is the co-occurrence of a haplotype in the Allier catchment and Vienne River, although we cannot exclude that this haplotype is also native to both sites. These results suggest that five decades of stocking have provided no added reproductive value to the Vienne populations. We thus hypothesize that long-term survival of stocked fish has either been low or non-existent.