Parasitism of monkey goby, Neogobius fliviatilis, was assessed at three brackish water localities with different salinity levels (Gulf of Odessa, Khadzibey Estuary, Lake Kytay) and one freshwater site along the Lower Danube River (Vidin). A total of 25 parasite taxa were identified, with minimum parasite richness recorded in the Khadzhibey Estuary (three species) and the maximum along the Lower Danube (11 species). Parasite richness in the mesohaline Gulf of Odessa and oligohaline Lake Kytay was lower, but still relatively high compared to the Khadzibey Estuary. Our study indicates that freshwater populations of monkey goby host richer and more abundant parasite communities than those inhabiting brackish waters with (more or less) stable salinity. Unstable abiotic conditions that probably affected the parasite’s intermediate hosts contributed to the reduction in parasite species in the estuarine zone of the Danube.
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We conducted a snapshot study to check the importance of non-native Ponto-Caspian gobies (monkey and racer goby) in the diet of native obligate (northern pike, pikeperch) and facultative (Eurasian perch) predators in the Vistula River 3-4 years after the appearance of aliens. In total 71 fish with non-empty stomachs, taken randomly from net catches from various parts of the river were analyzed, including 32 pike, 20 pikeperch and 19 perch specimens. We found that gobiids prevailed in the diet of smaller (<30 cm standard length, SL) individuals of pike and pikeperch, as well as larger (>15 cm SL) specimens of perch, although the effect of predator size on the presence of gobiids in the diet was not significant in the case of perch. Our results indicate that gobiids as a prey fish can stimulate considerable changes in local food webs, which require further studies.
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