Alien invasive fish species in Polish waters: an overview.
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.