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Simple Summary: The impacts of pollutants on the aquatic environment have become an increasingly important subject of study over the past few decades. Pollutants, including pharmaceuticals, can have direct and/or indirect effects on biota, affecting individual trophic levels in the food chain, the composition of populations, or even the degree of parasitism, a secondary stressor to the host. In this study, we assessed uptake of pharmaceutical compounds in tissues of common carp (Cyprinus carpio L.) and parasite community response to the change in environmental conditions six months after relocation from a control to a treatment pond loaded by organic pollution from a sewage treatment plant outlet using partial cross-over experimental design. By comparing fish from control and treatment ponds, we observed higher pollutant uptake and the concentration of pharmacological compounds in fish tissues restocked to the treatment pond, along with changes in fish biometric parameters and parasite load. Fish from polluted environment exhibited decreased parasite diversity and higher ectoparasite abundance; however, the major differences were observed between families within taxonomic groups. Our results, therefore, highlight the need for more detailed taxonomic analyses in studies using parasites as potential environmental bioindicators. The response of parasite communities to aquatic contamination has been shown to vary with both type of pollutant and parasite lifestyle. In this semi-experimental study, we examined uptake of pharmaceutical compounds in common carp (Cyprinus carpio L.) restocked from a control pond to a treatment pond fed with organic pollution from a sewage treatment plant and assessed changes in parasite community composition and fish biometric parameters. The parasite community of restocked fish changed over the six-month exposure period, and the composition of pharmaceutical compounds in the liver and brain was almost the same as that in fish living in the treatment pond their whole life. While fish size and weight were significantly higher in both treatment groups compared to the control, condition indices, including condition factor, hepatosomatic index, and splenosomatic index, were significantly higher in control fish. Parasite diversity and species richness decreased at the polluted site, alongside a significant increase in the abundance of a single parasite species, Gyrodactylus sprostonae. Oviparous monogeneans of the Dactylogyridae and Diplozoidae families and parasitic crustaceans responded to pollution with a significant decrease in abundance, the reduction in numbers most likely related to the sensitivity of their free-living stages to pollution. [ABSTRACT FROM AUTHOR]

Increasing levels of persistent pollutants in the aquatic environment are likely to have serious impacts on host–parasite relationships. In this study, common carp (Cyprinus carpio) of the same age, origin and source were randomly stocked in two pond localities, one fed by treated water from a municipal sewage treatment plant and the other fed by a non‐polluted stream. Fish biometric parameters and health status were evaluated after 2 years of exposure through the determination of somatic condition indices, parasite load and concentration of pharmaceuticals. Municipal pollutants were found at both sites, although at significantly higher levels at the polluted site, with antidepressants, antibiotics, analgesics and beta‐blockers dominant in each case. Owing to a higher availability of natural food, carp from the polluted locality displayed significantly better body condition than those from the control site, while parasite abundance and species richness were higher at the control site. Our results suggest that, compared with fish from control locality exhibiting diverse and abundant natural parasite fauna, fish from the polluted locality were better able to cope with parasites, due to their better condition status. Fish from this study site may have profited from municipal waste water by avoidance of high levels of parasite infections. [ABSTRACT FROM AUTHOR]