We examine the evolving concept of what constitutes a non-native (or alien) freshwater fish. In an attempt to distinguish between biogeographical and socio-political perspectives, we review the patterns in the introduction and dispersal of non-native fishes in Europe and North America, and especially the recent expansion of Ponto-Caspian gobies in Europe. We assess patterns in the development of national policy and legislation in response to the perceived threat of non-native fish introductions to native species and ecosystems. We review, and provide a glossary of, the terms and definitions associated with non-native species. Finally, we discuss perspectives as regards the future treatment of naturalized species.

The first recording of the Ponto-Caspian racer goby in Poland was during 1995 in the River Bug (River Vistula system). Within 5 years, the species had spread to the downstream section of the Vistula. One of the potential impacts of invasive species on native fauna is competition for food. Therefore, the diel patterns in diet composition and gut fullness coefficient (FC) of racer goby were examined at one study site in the Włocławski Reservoir (lower River Vistula), during May 2003. An average of 20 individuals were examined each 4 h over one 24-h period (125 fish in total). The proportion of main food items and diet width did not differ among three size-groups (43–59, 60–79 and 80–97 mm total length), and the relative biomass ratio of main food categories did not differ over the diel cycle. Amphipods constituted 11–70% of total gut content biomass and were found on average in 84% of analysed alimentary tracts. The second prey types were chironomid larvae (16–63% of total food biomass; frequency occurrence: 61–91%), and to a lesser extent chironomid pupa, ceratopogonid larvae, oligochaets, dipteran imagines and copepods, with fish larvae found in the gut of eight gobies. Gut fullness coefficient (FC) differed significantly over the 24-h period, with the highest value at night and in early morning. In conclusion, racer goby forages mainly on benthos and has a nocturnal-feeding activity. No significant diet overlap was found between racer goby and native percids, i.e. Eurasian perch Perca fluviatilis and ruffe Gymnocephalus cernuus.

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 negati...

Racer goby is one of several Ponto–Caspian gobiids spreading throughout European rivers and concurrent with recent declines in threatened populations of a native species of similar biology, the European bullhead. Although suggestive of competitive interactions, evidence thereof is scarce, so we examined behavioural interactions between racer goby and bullhead (single specimens of each species together, also pairs of each species) under experimental conditions (shared space with two shelters) to determine whether the invader displaces the native species when food resources are limited. Food (live chironomids) was added to a single feeder at rates below satiation levels twice over 24 h (once in light and once in darkness), with fish behaviour (aggressive interactions: attacks and threatening) and feeding activity (time spent near or inside the feeder) recorded using video cameras and infrared illumination. Racer goby exhibited aggressive behaviour towards bullhead (mean = 2.5 aggressive events h⁻¹), but rarely the inverse (threatening only, mean = 0.05 events h⁻¹), significantly limiting bullhead foraging time (by 62 %) and being faster to reach food in the feeding time in 76 % of cases. Gobies were more aggressive during daylight (77 % of all aggressive events occurring in light), and both species spent more time on feeding activities in darkness (88 and 66 % of all time spent in the feeder by bullheads and gobies, respectively). However, the adverse impact of goby on bullhead was independent of light conditions. Our results suggest that under natural conditions, racer goby are likely to displace bullhead during feeding, with potential consequences for foraging efficiency.

The most successful and widespread non-indigenous invasive fish species occurring in the Central European bioregion are heterogeneous with respect to their origins, taxonomy, mode of arrival, vectors and pathways of dispersal. We surveyed whether these non-native species have common life-history that might facilitate their invasion and differentiate them from native species. Ten life-history traits of 59 freshwater fish species were examined: maximum body length, longevity, age and length at maturation, maximum absolute fecundity, egg size, larval size at hatching, spawning duration and type, parental investment. Principal Component Analysis revealed that the traits that accounted for the first two principle components (86.4 % of total variation) were maximum body length, age and size at maturation, longevity, fecundity, egg size and larval length at hatching. Five groups of species were distinguished by cluster analysis, those comprising native fish species being characterized by medium-to-large size, elevated longevity, late maturation, high fecundity, and no parental care. In contrast, groups of non-native invasive fish species were characterized by small-to-medium body length, short longevity, early maturation, relatively low fecundity, relatively large eggs, multiple spawning, extended reproductive seasons and some form of parental care. These life-history traits, along with broad diet breath and environmental tolerance, appear to facilitate the invasion of freshwater fish in the Central European bioregion.