Condition Factors Do Not Reflect Parasite Loads: A Case Study on Juvenile Cyprinus carpio (Cypriniformes, Cyprinidae) from the Lower Danube River

The present study aimed to evaluate whether commonly used condition indices reflect parasite load and bacterial colonization in juvenile Cyprinus carpio under natural environmental conditions in the Lower Danube River. A total of 260 specimens were examined for parasitological, microbiological, and biometric parameters, including 20 individuals analyzed for bacterial communities. Twenty-three parasite taxa belonging to eight major taxonomic groups were identified. Ectoparasites were found on the gills, skin, and fins, with monogeneans and ciliates, notably Dactylogyrus ssp. and Trichodina ssp., representing the dominant groups. Infection intensity was generally low to moderate, and histopathological examination revealed only mild epithelial alterations, including focal hemorrhage and mucus hypersecretion in more heavily infected individuals. Microbiological analysis identified six bacterial taxa associated with the skin, with Aeromonas hydrophila being the most frequently detected species. Correlation analyses showed no significant relationships between parasite abundance and condition indices (Fulton’s K, Le Cren’s Kn, scaled mass index, and BMI), although a slight reduction in Fulton’s K was observed in infected individuals. These findings indicate a stable host–parasite–microbiota equilibrium under natural environmental conditions. The results provide baseline ecological data that contribute to understanding fish health dynamics in the Lower Danube River and may support future monitoring and management strategies.