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International audience; Species introductions often coincide with loss of genetic diversity and natural enemies. Anthropogenic translocation of the North-American pumpkinseed Lepomis gibbosus (L., 1758) (Centrarchidae) and its further spread have resulted in recent species establishment in most European countries. This study determines genetic differentiation of non-native European pumpkinseed populations and identifies how their genetic structure relates to the distribution and abundance of parasite species. Microsatellite analysis indicated presence of three genetic lineages, which were well supported by discriminant analysis based on parasite abundance data. The first lineage clustered pumpkinseed populations from northern and southern France and showed high allelic richness, heterozygosity and parasite richness. The second included populations along the ‘‘Southern invasion corridor’’ connecting the rivers Rhine, Main and Danube. The fish exhibited low to high genetic and parasite diversity and generally high parasite abundance. The third lineage clustered populations with low genetic and parasite diversity, located in Portuguese reservoirs and water bodies along the upper Elbe. Parasite species richness was significantly associated with host microsatellite heterozygosity and allelic richness, a trend partially affected by richness of North-American parasites. Furthermore, our results indicate that parasite community composition may serve as a useful biological tool to discriminate non-native fish populations and their inter-relationships.

The western tubenose goby Proterorhinus semilunaris, an invasive Ponto-Caspian fish species, has established populations in a wide range of habitat types in the Dyje/Morava river basin (Danube basin; Czech Republic). In this study, we assessed tubenose goby tolerance to environmental variables potentially contributing to its spread and performance in new habitats. Of the seven aquatic habitats examined (lowland rivers, brooks, backwaters, oxbow lakes, borrow pits, reservoirs and carp aquaculture ponds), all except small brooks proved suitable for survival and reproduction, with habitat size the only limiting factor. Diet analysis indicated chironomid larvae as preferred prey, though tubenose gobies were able to switch to other food items under specific conditions (e.g. high macrozoobenthos density), suggesting feeding plasticity. Fish condition was positively correlated with individual diet range, but not with parasite burden. Eighteen metazoan parasite taxa were identified, greatly exceeding the known parasite fauna from the species’ native range. Parasite species richness decreased significantly with fish host dominance. Foraging plasticity, the ability to occupy different habitats and cope with parasitism observed in this study all may have important implications for tubenose goby dispersal success and invasiveness.

As invaders of European and North American aquatic systems, Ponto–Caspian gobiids are believed to represent a significant negative threat to native fish assemblages and cottid species in particular. To date, relatively few studies have tried to document actual impacts, most being short-term and/or laboratory based. Here, we examine 8 years (2008–2015) of electrofishing data from a 1200-m stretch of rip-rap along the Austrian Danube, initiated following establishment of four non-native gobiids: tubenose goby Proterorhinus semilunaris, bighead goby Ponticola kessleri, racer goby Babka gymnotrachelus and round goby Neogobius melanostomus. While we registered 26 fish species in total, most native species were caught along the rip-rap only occasionally. Only native bullhead Cottus gobio and the four non-native gobiids were caught regularly. Although cottids are presently believed to be most vulnerable to gobiid invasion, we observed no negative trend in bullhead abundance over the 8-year dataset, the population remaining stable and at similar abundances to gobiids. While we observed no significant trend in round, racer or tubenose goby abundance, bighead goby showed a continuous decline. Our data contradict previous reports of drastic impacts on cottid abundance by gobiids (mainly round goby), suggesting potential region-specific effects following invasion.

One of the main assumed impacts of invasive gobies is predation on benthic macroinvertebrates. Despite numerous dietary studies, however, quantitative evaluations of impact in European river systems are scarce. Here, we investigate the impact of tubenose (Proterorhinus semilunaris, Heckel 1837) and round (Neogobius melanostomus, Pallas 1814) gobies on macroinvertebrates in a lowland river (River Dyje, Czech Republic) by allowing and preventing gobiid access to rip-rap substrate naturally colonised by invertebrates at two sites (Site 1—tubenose goby only, Site 2—tubenose and round gobies). Gobies had a negative impact on invertebrates at both sites, with overall invertebrate density reduced by 15% (ca. 17.9 g m−2 per year) at Site 1 and 36% (ca. 23.6 g m−2 per year) at Site 2. Both species showed increased impact in summer and ingested larger invertebrates preferentially, resulting in an overall reduction in invertebrate body size. Tubenose gobies had a significant impact on Annelida, Gastropoda, Crustacea and Ephemeroptera nymphs, while tubenose and round goby together impacted Annelida, Bivalvia (Dreissena), Gastropoda, Crustacea, Ephemeroptera nymphs, Odonata nymphs and Chironomidae larvae. Our results confirm that round and tubenose gobies can have a significant negative impact on aquatic invertebrate density and community composition.

Introduced host species are often characterised by reduced parasite numbers compared to their native populations. Any such advantage gained from parasite release following introduction into a new area may often diminish over a short period as the new host gradually acquires local parasites. In this study, the metazoan parasite communities of four goby species (Proterorhinus semilunaris, Ponticola kessleri, Neogobius melanostomus, and Neogobius fluviatilis) recently introduced into the lower River Rhine were investigated. Mean parasite abundance and infracommunity richness were positively associated with time since host introduction, both parasite variables being the highest in P. semilunaris. In Ponticola and Neogobius species, parasite species richness and the dominance of larval parasites in the Lower Rhine were similar to that for non-native populations in the middle Danube. Sporadic local parasite acquisition and infection, predominantly by species commonly found in the native range, led to a relatively high qualitative similarity in parasite communities between hosts. The relationship between parasite abundance and fish size reflected size-dependant food selectivity and/or parasite accumulation throughout the host’s life. Data from this study emphasise the importance of duration of co-occurrence, host habitat and foraging preference, as well as the co-introduction of suitable intermediate hosts, for parasite community composition in related species.

The EU’s Water Framework Directive (WFD) in particular makes good use of riverine biota—including fish—as ecological indicators of river integrity. However, it is also important to examine when our indicators fail to serve the purpose for which they were originally intended: to consistently and reliably describe the quality of the environment. We argue that in the Czech Republic (and probably several other European countries), the use of adult fish as ecological indicators often fails to fulfill these purposes for several reasons, especially when fish are stocked in natural waters, for practical considerations during field sampling, and problems with public relations. The ecological quality of running waters sensu WFD is usually assessed on the basis of the adult fish assemblage. The aim of this study is to describe disadvantages of monitoring river integrity using adult fish in Czech Republic, which has regular and widespread stocking of more than one-third of fish species present in running waters and to suggest the alternative strategy of young-of-the-year (YOY) fish monitoring for assessing river ecological quality with the ability to provide a sensitive response to water quality and habitat structure regardless of the effect of stocking or river size. We conclude that even though YOY sampling is often a more logical and practical approach to water quality monitoring under specific conditions, no method is perfect and caution should be taken. For example, YOY can have high annual variability in density and higher sensitivity to summer flood events than adults. The suitable time for sampling YOY fishes in late summer is much shorter than for the adult fish community, and therefore must be considered when designing a successful monitoring strategy. Monitoring of YOY fishes does not provide direct information about age structure of fish populations, which is required by WFD. YOY sampling is a useful—albeit imperfect—method for providing a sensitive response to ecological status of rivers, especially under specific stocking conditions.

European rivers are highly degraded and restoration efforts are becoming more frequent. However, only few restoration projects have been rigorously evaluated so far. We investigated the response of fish assemblages to hydromorphological restoration measures including river widening, creation of instream structures, flow enhancement, remeandering and side-channel reconnection. We sampled 15 rivers with pairs of degraded and restored sites and calculated the effect sizes (i.e., restored–degraded) for species richness, species diversity, fish density and habitat traits. We analysed the following factors potentially affecting restoration success: (1) length of the restored river stretch, (2) time after restoration and (3) hydromorphological quality of restoration. While species diversity and density did not respond to restoration, proportion of small rheophilic fish increased and eurytopic decreased. Short-term ( 12 years) of restoration measures have a stronger effect on fish assemblages than mid-term effects. Furthermore, the hydromorphological quality and the length of the restored section are relevant for the restoration effects on the fish community. Future restoration projects should focus on more dynamic, self-sustaining habitat improvements extending over several kilometres and should be coupled with other measures such as restoring the river continuity and species reintroductions.