Where are you from? Finding the origin of the recently observed sprat in Iceland using a panel of SNPs

The European sprat is a small pelagic fish characterised by genetically distinct populations including the Norwegian fjords, the Baltic Sea, the oceanic component ranging from the North Sea, Kattegat–Skagerrak, Celtic Sea and Bay of Biscay, as well as the southern groups such as the Mediterranean (Adriatic) and Black Seas. Additionally, a self-recruiting population evolved in Landvikvannet, a lake on the Norwegian coast of Skagerrak that turned brackish following artificial connection to the sea. Sprat was first reported in Icelandic waters in 2017, and in subsequent years it has become increasingly frequent and has spread along the south and west cost of the country. As the population of origin of this introduction was unknown, we used a panel of 91 SNP loci that display high genetic resolution in this species to characterize the genetic background of 64 sprat individuals collected in Icelandic waters in 2021. Analysis of Icelandic sprat, compared with existing reference data, clearly identified the oceanic component as the likely source of the introduction. While this aligns with expected colonisation routes along the Greenland-Scotland Ridge associated with range-expansion, it does not preclude an anthropogenically-driven vector, such as transport via ballast water.