Evaluation of strategies to conserve and restore intraspecific biodiversity of brown trout: outcomes from genetic monitoring in the French Alps

Introduction of nonnative cultured fish is one of the most important threats to native salmonid populations. In brown trout, more than a century of stocking practices has led to a large hybridization between initially geographically isolated lineages, threatening native populations and thereby intraspecific diversity. In the French region of Haute-Savoie, managers and scientists implemented together three management strategies (genetic refuge, direct translocation of wild spawners and stocking with native fry) on 19 test sites for more than 15 years, in the aim to recover pure or nearly pure native populations. Here we propose an assessment of the different management strategies based on a synthetic analysis of the evolution of the introgression rate. While none of the implemented strategies completely achieves the initial objective to restore pure native populations, they differ in their efficiency: introgression rates tend to decrease quickly when direct translocation of native spawners of stocking with native fry strategies are used. The genetic refuge strategy shows slower and more heterogeneous changes of introgression rates. In general, pure nonnative fish are efficiently removed but at the cost of an increased presence of hybrids. Our results imply that intraspecific dynamics react quickly to management practices and that these changes are probably fueled by evolutionary feedbacks that are not yet well understood.