The two species of Atlantic sea sturgeon on either shore of the North Atlantic, Acipenser sturio in Europe and A. oxyrinchus in North America, probably diverged with the closure of the Tethys Sea and the onset of the North Atlantic Gyre 15–20 million years ago, and contact between them was then presumably precluded by geographic distance. Here we present genetic, morphological and archaeological evidence indicating that the North American sturgeon colonized the Baltic during the Middle Ages and replaced the native sturgeon there, before recently becoming extinct itself in Europe as a result of human activities. In addition to representing a unique transatlantic colonization event by a fish that swims upriver to spawn, our findings have important implications for projects aimed at restocking Baltic waters with the European sturgeon.

1. Institute for Zoo and Wildlife Research, Department of Evolutionary Genetics, 10252 Berlin, Germany
2. Institute of Zoology, University of Rostock, 18051 Rostock, Germany
3. Department of Environmental Medicine, New York University School of Medicine, Tuxedo, New York 10987, USA
4. German Archaeological Institute, Eurasian Division, 14195 Berlin, Germany
5. Institute for Animal Breeding and Genetics, University of Göttingen, 37075 Göttingen, Germany
6. Cemagref, Inland Living Aquatic Resources Research Unit, 33612 Cestas cedex, France
7. Hudson River Foundation for Science and Environmental Research, New York, New York 10011, USA

Correspondence to: Arne Ludwig¹ e-mail: Email: ludwig@izw-berlin.de