Over the past 150 years, the waters of the Czech Republic were experimentally stocked or invaded by a total of 41 alien (non-native) fish species. The following species have become fully naturalized and produced self-sustained populations: Carassius gibelio, Pseudorasbora parva, Ameiurus nebulosus and Gasterosteus aculeatus, which produced stable populations in several spatially limited localities. In some cases Oncorhynchus mykiss, Salvelinus fontinalis and Coregonus maraena will produce instable temporary populations based on released material obtained from fish farms and ponds. The occurrence of the remaining acclimatized alien species (Coregonus peled, Ctenopharyngodon idella, Hypophthalmichthys molitrix, Aristichthys nobilis) in natural ecosystems and fishponds depends on stocking fish obtained from artificial spawning and cultures. The documented annual average production of alien species fit for human consumption amounts to around 2 000 tonnes, i.e. 8.2 % of the annual average production of marketable fish cultures in the Czech Republic. A significant negative impact of the introduced species on native ichthyofauna has been ascertained as regards its ecological, biological properties, biodiversity and health. Considered a typical invasive alien species, Carassius gibelio heavily depressed the occurrence and numbers of indigenous Carassius carassius populations and also contributed to the decreased numbers of Tinca tinca, Leucaspius delineatus and other native cyprinid fish. P. parva and A. nebulosus show a much weaker and limited impact. The introduction of C. idella was accompanied by the introduction of the tapeworm species, Bothriocephalus gowkongensis, which subsequently caused heavy losses in cultures of Cyprinus carpio. In 2008, Neogobius melanostomus was recorded for the first time in this country at the confluence of the Morava and Dyje rivers.
The Amur sleeper, Perccottus glenii Dybowski, 1877, is indigenous in eastern Asia. During the second half of the 20th century, with the aid of man, it spread over the eastern part of Europe as well as in central Asia. In the course of 50 years of its dispersal in the western direction the species already reached the Vistula drainage area (the Baltic Sea basin) and the Danube drainage area (the Black Sea basin). In the latter basin, its occurrence was ascertained in the drainage area of the Tisza river in Hungary in 1997. In eastern Slovakia, the Amur sleeper was first recorded in 1998 in the Latorica drainage area. In the course of subsequent years it has become a common species in the streams in the basins of the Latorica, Bodrog and Tisza rivers. In shallow lentic waters densely grown with aquatic plants the species becomes a superdominant or even exclusive species in the local fish communities. It has no marketable value but presents a serious threat to the existence of native fish species with similar identical microhabitat requirements.