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.

Summary Beginning in the early 1990s, species of the genus Neogobius rapidly expanded their native range from the Black Sea and the lower reaches of the River Danube upstream to include the middle and upper Danube. In 2002, 83 rip‐rap sites at 25 locations along the Austrian section of the River Danube were sampled by catch per unit effort (CPUE) with electrofishing to assess goby distribution. Bighead goby N. kessleri was found in all locations and at 76% of these sites, it was the third‐most abundant species (17% of CPUE). Densities of Neogobius spp. were highest in industrial harbours (IH), whereas the native species European bullhead Cottus gobio was found primarily in the main channel and tubenose goby Proterorhinus marmoratus in non‐industrial harbours. The history of records and the present distribution of Neogobius spp. suggest that IHs are the main dispersal points.

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This study examines the invasion history of alien fish species based on exhaustive national data sets on fish invasions of two contiguous central European countries (Germany and Austria). Fifteen alien fish species are currently established in both countries, constituting 14 and 17% of the total freshwater fish fauna of Germany and Austria, respectively. In both countries, six alien species are present, but not established. The status of five alien species in Germany and three species in Austria remains unknown. Accumulation rates of alien fish species have increased in recent decades with >50% of them reported after 1971. North America and Asia were the primary sources of alien fish species in Germany and Austria up to the 1980s, whereas European species of Ponto-Caspian origin dominate now. Fisheries (including aquaculture) and the animal trade were responsible for most earlier introductions, whereas waterways were the main pathway for recent invaders. The extent of the spatial distribution of alien species was positively correlated with residence time, i.e. the time elapsed since the first national record. Different thermal preferences of early invaders (mostly coldwater species) and new invaders (typically warmwater adapted) may benefit the latter in the face of climate change. It is concluded that new challenges for alien fish management arise and that ecosystem-based approaches as endorsed by the E.U. Water Framework Directive (maintaining or restoring good ecological status of rivers and streams) should become the centrepiece of river management in Europe.

Romanogobio skywalkeri, new species, is described from the upper Mur River in the Austrian Danube drainage. It is related to R. banarescui from the Mediterranean basin. Romanogobio skywalkeri is distinguished from R. banarescui by lacking epithelial crests on the predorsal back, having 12-14 total pectoral-fin rays (vs. 10-11) and usually 8½ branched dorsal-fin rays (vs. 7½). It is distinguished from other Romanogobio species in the Danube drainage by having a very slender body; a moderately long barbel, extending slightly beyond the posterior eye margin; and no epithelial crests on the predorsal back. Romanogobio skywalkeri is distinguished by a minimum net divergence of 6.3% (uncorrected p-distance against R. banarescui) in the COI barcoding region from other European Romanogobio species. A key to the Romanogobio species of the Danube drainage is provided. Romanogobio banarescui from the Vardar drainage and R. carpathorossicus from the Danube drainage are treated as valid species.