This study investigated the impact of lethal (4.5 mg/L) and sublethal (0.45 mg/L) concentrations of Isoproturon on oxidative stress responses in the freshwater fish Cyprinus carpio. Experimental fish were exposed to lethal concentrations for short durations (1–4 days) and to sublethal concentrations for extended periods (1, 5, 10, and 15 days) under controlled laboratory conditions. The study mainly focused on evaluating oxidative stress biomarkers and antioxidant defense mechanisms in the gill, liver, and kidney tissues. Exposure to Isoproturon caused significant biochemical and physiological alterations in the treated fish compared to the control group. Activities of antioxidant-related enzymes such as catalase and protease showed noticeable elevation, reflecting the activation of cellular defense mechanisms against herbicide-induced oxidative stress. Similarly, the levels of hydrogen peroxide, malondialdehyde, protein carbonyls, and free amino acids increased considerably in all examined tissues, indicating enhanced lipid peroxidation, oxidative protein damage, and metabolic imbalance. Conversely, total protein content decreased significantly, possibly due to increased protein degradation and higher energy demands under toxic stress conditions.

The observed responses were dependent on both concentration and exposure duration, with more severe effects recorded at higher concentrations and longer exposure periods. Among the tissues analyzed, the liver displayed the highest degree of oxidative damage, likely because of its central role in detoxification and metabolic activities. In addition to biochemical changes, exposed fish exhibited behavioral disturbances such as abnormal swimming patterns, reduced feeding behavior, excessive mucus secretion, and loss of equilibrium, which further confirmed the toxic influence of Isoproturon. The overall findings suggest that Isoproturon induces pronounced oxidative stress and impairs antioxidant defense systems in Cyprinus carpio. Therefore, oxidative stress biomarkers may be effectively used as reliable indicators for assessing herbicide toxicity and monitoring environmental contamination in freshwater ecosystems.