Simple SummaryThe 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 carpioL.) 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.AbstractThe 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 carpioL.) 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.