There has been the assessment of current occurrence and situation of non-native fish species in two hydrological regions – the Upper Odra and Morava river basins. The river basin of Upper Odra is situated in the area of River Odra springs featuring mostly minor streams which are not currently accessible for the migration of fishes from the lower river parts and from the Baltic Sea because of the existence of tens of migration barriers in the area of Poland. There has been actually proved the occurrence of eight non-native fish species the presence of which is, with the exclusions of Carassius gibelio and Pseudorasbora parva, the result of the planned introduction related to the fish management activities. The River Morava basin consists of two parts – the system of the River Morava itself and the entire River Dyje basin. Currently, there are 16 non-native fish species. Apart from the eight species existing also in the Odra river basin, there are also eight other species the existence of which relates to the migration accessibility from the confluence of these two rivers for fishes from the River Danube through the lower part of the River Morava. The species Lepomis gibbosus, Neogobius melanostomus and Proterorhinchus semilunaris were the last ones which have arrived there by themselves. Coregonus maraena and Coregonus peled are bred in fishponds.
The basic part of the non-native fishes’ spectrum existing in free waters of the two basins consists of the species which are bred in aquaculture and fishponds (Oncorhyncus mykiss, Salvelinus fontinalis, Ctenopharyngodon idella, Hypophthalmichthys molitrix, and Hypophthalmichthys nobilis). These species, with the exception of Hypophthalmichthys molitrix and Hypophthalmichthys nobilis, introduced also into the fishing grounds and especially Oncorhynchus mykiss and Ctenopharyngodon idella make the important fishes for anglers. Their presence in the waters of the Czech Republic might be considered as beneficial from the economic point of view. From the ecological point of view, the im- portant fact is that they have become very well acclimatised in the local conditions and their natural breeding does not take place and their distribution in water ecosystems, as well as their numbers are fully determined by the management of the Czech Anglers Union. However, Carassius gibelio, Pseudorasbora parva and also Lepomis gibbosus in the River Morava basin have been assessed as distinctly invasive species having more negative impacts on the original ichthyocenosis and biodiversity. Neogobius melanostomus is also potentially dangerous in this basin. In the case of spread of Lepomis gibosus into the River Odra basin, it might become also potentially highly dangerous.
