Phylogeography of European grayling, Thymallus thymallus (Actinopterygii, Salmonidae), within the Northern Adriatic basin: evidence for native and exotic mitochondrial DNA lineages

Phylogeographic information on European grayling, Thymallus thymallus, is still fragmentary for the Northern Adriatic basin. Here, we provide complete mitochondrial DNA control region sequence data of 456 grayling specimens from 21 sampling sites across distinct river drainages. Thirty-seven haplotypes were resolved and clustered into Danubian, Atlantic and Adriatic lineages. The latter clade, composed of 16 new haplotypes, was identified in 12 out of 17 Adriatic sampling sites and reached frequencies of 0.97 within single water courses of the Adige and the Po drainages. However, native Adriatic haplotypes were accompanied by Danubian and/or Atlantic variants in all cases. A positive correlation between hatchery haplotype frequency and annual stocking input pointed to a direct effect of stocking intensity on the genetic architecture of wild populations, although natural trans-basin colonisation may have additionally complicated the situation. However, both the extent and patterns of introgression between native and foreign strains, as well as microgeographic population structure within the Adriatic lineage will be clarified by future molecular surveys, based on nuclear genetic markers. Until then, conservation management must include an immediate cessation of stocking of commercial grayling stocks, as well as the prohibition of grayling translocation, even at the intra-drainage level.