Ranching marshes, lakes, swamps and other waters by species which are not naturally distributed in these habitats is unacceptable and strictly prohibited by law. In spite of this, ichthyofauna in our country includes a total of 16 introduced fish species. They were introduced in the watercourses and stagnant waters in the last century and have adapted to their new habitats. These introduced species are considered one of the main reasons of extinction of naturally distributed species. The spread of the introduced species is favoured by disappearance of swamps and loss of other natural habitats, as well as pollution of rivers and lakes. Some of the most famous introduced species in Croatia are Prussian carp, rainbow trout, pumpkinseed, mosquitofish and small catfish called brown bullhead. Two other species appear which are on the list of invasive species and which raise concern in the European Union - Stone Moroko and Amur sleeper.