Adriatic grayling within the Adige River Basin: a myth or still reality?

Freshwater fish biodiversity is facing a severe extinction crisis. This is especially true for most salmonid species, threatened by water pollution, environmental degradation and, finally, intensive stocking of non-native conspecifics. European grayling, Thymallus thymallus, is known to consist of distinct mitochondrial genetic lineages, with grayling native to the Adige River supposed to be fixed for the Adriatic mtDNA grouping. However, intensive stocking of Danubian and/or Atlantic hatchery stocks, carried out at least since 1975 to mitigate dramatic demographic population declines of local grayling stocks, is thought to generally have replaced native grayling in the Northern Adriatic drainage system. Nevertheless, up to now no extensive molecular genetic data examining the autochthony of grayling within the Adige basin is available. Consequently, in the framework of the newly established research project ABaTe we firstly sequenced the entire control region (1104 bp) of grayling samples from several upper and middle reaches of the Adige drainage. Preliminary results indicate the existence of highly divergent mtDNA haplotypes belonging to both native Adriatic and potentially introduced clades of Danubian or Atlantic origin. Presumably introduced mtDNA haplotypes were found in all localities, pointing to a considerable impact stocking measures had on local grayling stocks. On the other hand, in some sampling sites of the Adige River frequencies of Adriatic clade haplotypes reached up to 0.9. These first results could indicate limited gene flow from foreign to native stocks and, hence, point to the survival of relic (sub)populations of Adriatic grayling in the area of interest. The additional analysis of nuclear markers, e.g. highly variable microsatellite loci, and Bayesian assignment statistics will reveal intrapopulation structure, the degree of introgression between foreign and native grayling and, potentially, serve as a valuable selection tool for future rehabilitation measures