Non-native gobies facilitate the transmission of Bucephalus polymorphus (Trematoda).

BACKGROUND: Introduced species can modify local host-parasite dynamics by amplifying parasite infection which can 'spill-back' to the native fauna, whether they are competent hosts for local parasites, or by acting as parasite sinks with 'dilution' of infection decreasing the parasite burden of native hosts. Recently infection by the trematode Bucephalus polymorphus has increased in several European rivers, being attributed to the introduction of intermediate host species from the Ponto-Caspian region. Using a combination of field and experimental data, we evaluated the competence of non-native and native fish as intermediate hosts for B. polymorphus and its role for parasite development in a definitive host. METHODS: The density of 0+ juvenile fish (the second intermediate hosts for B. polymorphus) was measured in the River Morava, Czech Republic and fish were screened for natural metacercariae infection. The stomach contents of predatory fish that are definitive hosts of B. polymorphus were examined to assess the importance of non-native gobies for parasite transmission. In semi-natural conditions, parasite establishment, initial survival, and maturity rates in experimentally infected definitive hosts pikeperch Sander lucioperca were measured in flukes recovered from native white bream Abramis bjoerkna and non-native tubenose goby Proterorhinus semilunaris and round goby Neogobius melanostomus. Adult fluke size and egg production was also measured to evaluate the potential effect of intermediate host species on parasite fitness. RESULTS: We detected high natural infection parameters of B. polymorphus in native cyprinids and non-native gobies compared to data from the period prior to goby establishment. Both fish groups are consumed by predatory fish and represent a major component of the littoral fish community. Parasite establishment and adult size in definitive hosts was equivalent among the second intermediate host species, despite a lower size of metacercariae recovered from round gobies. However, development in the definitive host of flukes recovered from gobies was reduced, showing higher mortality, delayed maturity and lower egg production, in comparison with parasites from native hosts. CONCLUSIONS: Substantial 'spill-back' of B. polymorphus due to higher transmission rates after establishment of non-native gobies was partially buffered by decreased fitness of B. polymorphus that underwent development in gobies.

Consequent effects of the great cormorant (Phalacrocorax carbo sinensis) predation on parasite infection and body condition of common carp (Cyprinus carpio).

Lesions ranging from surface wounds to deep tissue wounds caused by cormorant predation were observed on several species of the farmed fish in Pohorelice, Czech Republic. Two-year-old stocked common carp Cyprinus carpio harvested in late March were examined for ectoparasites and endoparasites, injuries extent, and lysozyme concentration in skin mucus. Additionally, three body condition indices were measured. Endoparasite infection occurred only scarcely. Wounded fish were more susceptible to the ectoparasites Gyrodactylus spp. and Dactylogyrus spp. (Monogenea), and Ichthyophthirius multifiliis (Ciliophora). The intensity of infection of other ectoparasites Eudiplozoon nipponicum (Monogenea), Argulus spp. (Branchiura) and trichodinids (Ciliophora) did not significantly differ between wounded and control groups of fish. Lysozyme concentration in fish mucus was significantly higher in wounded fish and was positively associated with both the extent of damaged epithelium and Gyrodactylus spp. abundance. There were no differences in Fulton's condition factor and lipid content in muscle and liver tissues between wounded and non-wounded fish. Higher values of spleen-somatic index in wounded fish corresponded to increased intensity of parasite infection, most likely reflecting changes in immune system of infected fish. Although our results did not show any significant effect of cormorant attacks on fish condition, the wounded fish had significantly higher parasite numbers which could impact the growth or survival of the fish throughout the production season.