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 characterized by genetically distinct populations including (a) the Norwegian fjords, (b) the Baltic Sea, (c) an oceanic component ranging from the North Sea, Kattegat-Skagerrak, Celtic Sea and Bay of Biscay, as well as the southern groups such as (d) the Mediterranean (Adriatic) and (e) the Black Sea. Additionally, a self-recruiting population established 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. Comparison with existing reference data clearly identified the oceanic component as the likely source of the Icelandic sprat, and three potential scenarios can be considered to explain the colonization process. Because adult sprat generally exhibit limited long-distance movement and are largely sedentary, a scenario involving adult migration into Iceland can be reasonably ruled out. Natural dispersal of eggs or larvae from the Faroe Islands cannot be entirely dismissed under specific conditions, given the complex oceanographic circulation southeast of Iceland. However, an anthropogenically mediated vector-most notably transport via ballast water-remains a more plausible mechanism.