Among the foreign species of animal in Lake Constance are the zebra mussel ("Dreissena polymorpha") which, since the 18th century, has spread from the Black Sea region across most of Europe and was carried into Lake Constance between 1960 and 1965. After a huge increase in numbers during the 1980s in the Rhine and large lakes, this species is now in retreat today. The zebra mussel causes problems because, among other things, it blocks water extraction pipes. In addition, the species can be a disaster for domestic shellfish, because it competes for their food. Today, according to the Institute for Lake Research ("Institut für Seenforschung", ISF), the zebra mussel is also an important food for overwintering waterfowl. In fact, the number of overwinterers has more than doubled in around 30 years.