We examine the evolving concept of what constitutes a nonnative (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 nonnative 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 nocturnalfeeding activity. No significant diet overlap was found between racer goby and native percids, i.e. Eurasian perch Perca fluviatilis and ruffe Gymnocephalus cernuus.

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Summary When an invasive predator encounters native and invasive prey, two scenarios are possible: the predator may benefit from the presence of naïve native prey or choose prey from its region of origin, reflecting their common evolutionary history. To determine interactions between an invasive predator and native and invasive prey, we used the Ponto‐Caspian racer goby Babka gymnotrachelus as predator and gammarids as prey: native Gammarus fossarum and Ponto‐Caspian Dikerogammarus villosus and Pontogammarus robustoides. We hypothesised that prey origin would affect fish preferences and growth rate and conducted a series of laboratory experiments on fish predation and growth and estimated profitability of prey of different origin. The goby preferred native prey to the Ponto‐Caspian gammarids, irrespective of prey motility, the presence of shelters or waterborne chemical cues. Moreover, fish grew better when fed native prey. Thus, we suggest that fish selectivity was based on the assessment of prey quality during direct contact with gammarids. A diet consisting of Ponto‐Caspian gammarids did not facilitate an invader originating from the same region, which benefited more from the presence of a local prey species. Ponto‐Caspian gammarids and gobies are successful invaders in inland waters, usually main rivers. The gobies, in contrast to the invasive gammarids, enter smaller tributaries that serve as refugia for native gammarids. We show that the gobies may benefit from the presence of native prey species in such locations.

Racer goby is one of several PontoCaspian 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.