The Zalew Wiślany is a relatively young body of water as which it was created by a natural separation of the Baltic Bay of Gdańsk by the Vistula sand reef. It is a basin for several larger rivers and a number of streamlets which makes it a quite large resource of freshwater. However, the fresh water mixes with salty waters from of the open Baltic. Salt concentration fluctuates depending on the location and season. It is reflected in the species distribution of the fish occuring here. In the Polish part of the Zalew Wiślany, there are nine sea species. Some of them are common - the herring, flounder, while others species occur scarcely. Absence of the sprat and cod in fish yields has been symptomatic in recent years.