**4. Conclusions**

Mineral structures (pebble, stones, and blocks) or macrophytes are useful habitats but in a given environment they would be more frequent in hard substrates as typically rip-rap embankments [79–81]. Few other native species, such as the European bullhead (*Cottus gobio*), the freshwater blenny (*Salaria fluviatilis*) or the ruffe (*Gymnocephalus cernua*), have these characteristics [43, 82]. *Neogobius melanostomus* inhabits a wide range of temperate freshwater and brackish-water ecosystems [72]. It has also demonstrated its capacity to adapt to local conditions in terms of prey availability [72]. This species exhibits a wide thermal tolerance, ranging from −1 to 30°C, but its energetic optimum temperature is estimated to be 26°C [72]. They would also be fairly little sensitive to pollutions. The distinguishing ecological features of the round goby by comparison with native ones make them singular in the range of bio/ecological

There are multiple potential and non-exclusive hypotheses to explain the gobie's success from the hydrosystem point of view. Among these, we emphasize the ideas (1) that the environments are not saturated in species and (2) that the rivers were man-modified in a way favoring

The environments invaded by exotic species are not saturated in species for two main reasons. First, they correspond to hydrosystems that were largely defaunated during the Würm glaciation (80,000–10,000 BP). At the end of this period, the Rhine basin was recolonized by fish species from refuge areas that were outside ice range extension [83]. This recolonization by natural process takes a long time in the Rhine River considering the isolation of this basin and its geographical orientation with the downstream part to the North. The process was artificially accelerated these last centuries by human-aided introduction and the opening of the hydrographic basin with canals. Nowadays, the Upper Rhine is the main navigating way in Europe with two-third of goods transported on that fluvial road (330 millions of tons per year). Man activities allowed species from refuge area during the last glaciation, in particular the Ponto-Caspian area, to reach this unsaturated ecosystem. The Rhine has hence become the main entrance point for the dispersal of many invasive aquatic animal species in France over recent decades [7]. Another reason why ecosystems are not saturated in species is that pollution and human activities have profoundly modified natural communities, leaving vacant ecological niches within the hydrosystem. The decline of the Atlantic salmon (*Salmo salar*) or the European eel (*Anguilla anguilla*) in French inland waters are for example well documented. In conclusion, the ecosystems are not saturated because the post-glaciation process of recolonization is not achieved and several native species in

Second, the river stretches that served as entrance point in French hydrosystems are highly modified in terms of structure, quality, and functioning. The alteration of their habitats has placed the native species in a situation of anachronism: they are un-adapted to their own natural environment. The changes of habitats were too fast since the nineteenth century to allow a real adaptive response from species in place. Most of them disappeared, and the others

profiles of species in place, a distinctiveness that could promote its success [25].

**3.2. Hydrosystems prone to invasions**

the installation of exotic species.

84 Biological Resources of Water

place have already declined.

The main goal of the present chapter is to give an update picture of alien fish species in France and their fate in the past decades. We dissected how the fish introduction history in France switched from voluntary introduction in the nineteenth century to unintentional but humanaided introductions (aquarium trade and global ship transport). The 28 alien fish established represent one-third of the fish species in France and >25% of the European exotic fish. Four species of our list are included among the 100 worst invasive species of Europe (DAISIE) and three others among the 100 worst invasive species of the world (IUCN). The information gathered will allow discussing the possible reasons explaining whether an alien species is able or not to establish sustainable populations in France and thereafter became invasive, such as gobies. Now and in the near future, natural resource managers have no other choice than to deal with them because no invasive fish have spontaneously collapsed up to a local extinction in France.
