Oxidative Stress Biomarkers in <em>Carassius gibelio </em>from Lakes of Varying Ecological Quality

The Water Framework Directive 2000/60/EC requires the assessment of the ecological quality in all surface waters using biological indices, yet the effective application of these indices often demands extensive and long-term monitoring data. Oxidative stress biomarkers offer a promising complementary approach, as they can detect early biochemical responses of organisms to environmental degradation. In this study, we evaluated the suitability of two oxidative stress biomarkers - malondialdehyde (MDA) levels and DNA damage - in the gonads of a freshwater fish species, the Prussian carp Carassius gibelio (Bloch, 1782) as indicators of ecological condition in lakes of differing environmental quality. Fish were sampled from four lakes representing a gradient of physicochemical and ecological status. Both MDA concentrations and DNA damage showed significant (p &lt; 0.05) differences among lakes. However, only DNA damage in the gonads was significantly (p &lt; 0.05) associated with lake ecological quality as determined by the Greek Lake Fish Index (GLFI), with higher biomarker responses observed in lakes of poorer status. These findings demonstrate that oxidative stress biomarkers in C. gibelio reflect variations in lake ecological quality and may serve as sensitive, early-warning tools for biomonitoring and pollution assessment in freshwater ecosystems.