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Summary The aim of this study was to test young-of-the-year (YOY) assemblage applicability for the ecological quality assessment on the basis of samples from 55 sites scattered across the entire Czech Republic. European Fish Index (EFI) values estimated with YOY and adults were positively correlated, indicating that YOY and adult sampling is comparable. YOY bias the EFI toward significantly lower values, however, adults also lead to underestimated EFI values on some occasions. Relative differences between EFI YOY and EFI adults show almost normal distributions and indicate that there is no detectable systematic bias caused by environmental conditions or intensity of sampling. As shown in the Elbe River example, the reason for lower EFI YOY values could be found in a longer period of detection of evident changes in adult fish assemblages, reflecting longevity of adult fishes or regular stocking. YOY sampling was suggested as a useful method for assessing river ecological quality, with the ability to provide a sensitive response to the habitat structure regardless of the effect of stocking or river size.

Summary This study aimed to characterise the shoreline distribution and metazoan parasite community in black-striped pipefish Syngnathus abaster along the freshwater section of the River Danube (Bulgaria). An extensive survey of the shoreline zone showed the regular presence of S. abaster along the entire stretch from Vetren to Vrav (395–836 river km). The preference of shoreline habitat was analysed using abundance data and the proportion of this species in the fish community within particular habitat types. Highest frequencies of occurrence and density were found in sites with a muddy substrate, as compared to gravel and sand. A subsample of S. abaster from the upper section of the Bulgarian stretch was examined for metazoan parasites. The parasite fauna comprised eight metazoan species, representing half the parasite species richness known from its original range. Only three parasite species previously reported from S. abaster were found in the Danubian range of expansion: the adult trematode Orientocreadium siluri, metacercariae of Diplostomum sp., and an accidental finding of the trematode Nicolla skrjabini. The other five species appear to have been acquired in the new area, although these were recorded at a very low prevalence and/or abundance. Syngnathus abaster, therefore, is not likely to represent an important component in native parasite life cycles. Fish condition was not affected by either total parasite abundance or abundance of core species. Regular occurrence and dominance of Syngnathus abaster indicates that this species now represents a significant component of the Danubian shoreline fish community.

Summary Immediate mortality induced by electrofishing and handling of young-of the-year (YOY) fish of five cyprinid species (bitterling Rhodeus amarus, chub Leuciscus cephalus, bleak Alburnus alburnus, barbel Barbus barbus, roach Rutilus rutilus) was quantified during routine YOY fish monitoring (point abundance sampling with backpack electrofisher, in situ identification and measurement). Sampled, identified and measured fish were held in screened containers immersed in the river for 5 h after electrofishing. Bleak exhibited significantly higher mortality (17%) than all other species (1–3%). Mortality decreased significantly with fish length; fish

Parasite fauna of round goby Neogobius melanostomus in the Danube River was investigated in both the native range (two sites in Bulgarian stretch of the Danube) and the non-native range of its distribution (Croatian, Slovak and Austrian stretches) during 2005 and 2006. The aim was to identify possible changes in parasite community associated with the introduction of a host into the new environment. A total of 29 metazoan parasite species were found to parasitize round goby in the Danube River. Out of them, twelve parasite species occurred in both the native and non-native range of round goby distribution. Introduction of a novel parasite species to the non-native range via round goby was not found. Eight parasite species occurred only in the native range and nine species occurred only in the non-native range of the round goby distribution. Loss of native parasite species in non-native round goby populations and/or acquiring of novel parasite species in new environment were not significant. Thirteen parasite taxa were recorded in round gobies for the first time. Three parasite taxa (Diplostomum spp., Pomphorhynchus laevis and Raphidascaris acus) occurred at each sampling site in both the native and non-native range in high prevalence and abundance. Parasite species diversity was assessed for each sampling site and season using three diversity indices (Shannon, Simpson and Equitability index) with the highest values found in non-native site in Slovakia (1.38, 0.69 and 0.60, respectively) and the lowest values found in one of the native sites in Bulgaria (0.28, 0.12 and 0.14, respectively, for the same season). Species diversity was higher in both non-native round goby populations (Slovak and Austrian) compared to native Bulgarian populations. However, diversity indices values varied among almost all sampling sites.

Summary The Czech Republic has experienced a number of inter- and intra-continental introductions (42 introduced species total), with 29 species historically confirmed and 20 species still present in the wild. Fourteen species have become established, representing 27% of the current fish fauna. Pond aquaculture and recreational fisheries are the two major pathways of introduction and spread, and they are responsible for 80% and 36% of all introduced fish, respectively. Over the last decade, there has been a significant increase in the management practice of stocking recreational fisheries with non-native fish, particularly alarming in salmonid waters. Pond aquaculture is associated with high risk in terms of the numbers of unintentional fish introductions; all species introduced as contaminants of carp stocks have become established (Lepomis gibbosus, Ameiurus melas) and then widespread (Pseudorasbora parva, Carassius gibelio) and have been evaluated as invasive. Here, the historical chronology and facts regarding fish introductions in the Czech Republic are given, in addition to the current distribution of established populations. Present issues and future perspectives concerning the control of non-native fish and are briefly discussed.

Summary Metazoan parasites were investigated in three non-native fishes (monkey goby Neogobius fluviatilis, bighead goby Neogobius kessleri and round goby Neogobius melanostomus) collected from the former and current main channel of the River Danube and from the River Hron, Slovakia, in November 2003. Thirteen parasite species were identified: Triaenophorus crassus, Diplostomum sp., Tylodelphys clavata, Metorchis xanthosomus, Nicolla skrjabini, Gyrodactylus proterorhini, Pomphorhynchus laevis, Contracoecum sp., Raphidascaris acus, Anguillicola crassus, Unio tumidus, Anodonta anatina and Pseudoanodonta complanata. The maximum parasite diversity was found in N. fluviatilis. Total parasite abundance was significantly higher in N. kessleri, but no significant differences among sampling sites were observed. Pomphorhynchus laevis and glochidia of Anodonta anatina reached 100% prevalence in N. kessleri in the new channel of the Danube and, in general, these species were also the most prevalent parasites in all three goby species. For endoparasites, gobies served mostly as intermediate (digenean, cestodes and nematodes) or paratenic (acantocephalan and nematodes) hosts. All parasite species found are common parasites in the Middle Danube basin. No parasites specific to Neogobius, known from their native populations, were observed.