For the last 800 years, 35 alien fish species have been introduced, mainly intentionally, in Polish inland waters. The paper reviews the present state of alien fish fauna in Poland, with special attention paid to those considered to be invasive. Till now 26 species have been reported as naturalized, acclimatized or casual and it means that 34% of fish fauna are non-indigenous species. The majority came from North America, Eastern Asia and Siberia or different regions of Europe. More than 65 % of all introductions took place in the last 60 years. After the World War II the rapid expansion was noted specially for brown bullhead, Ameiurus nebulosus and gibel, Carassius gibelio. In the recent decade similar explosive spread has been observed for three Neogobius species (round goby, N. melanostomus, racer goby, N. gymnotrachelus, monkey goby, N. fluviatilis), Amur sleeper, Perccottus glenii and topmouth gudgeon, Pseudorasbora parva. The occurrence of introduced fish species resulted in several negative changes in aquatic environments. Some of them are as follows: hybridisation with native species, destruction of spawning grounds and habitats for many freshwater organisms, decrease of native fish reproduction success due to predation on eggs and offspring and finally the aliens might be vectors for parasites and diseases.
Observations of a chamber fish-pass at the lowest situated dam and power station on the Odra River were conducted between March 2002 and February 2003. Twenty two fish species were recorded during 21 catches at the pass, with dominant species Alburnus alburnus (90.8% of individuals), Rutilus rutilus(4.9%) and Gobio gobio (1%). The most intense upstream migrations were observed in spring and early summer during the spawning period of most species occurring there. The migration intensity of most analysed species was positively correlated with water temperature. No potamodromous fishes were observed in the pass at water temperatures below 8 ºC. The only diadromous species recorded were Anguilla anguilla, Vimba vimba and Salmo trutta trutta, all represented by very few individuals.