Seasonal Dominance over Morphometric Effects in Regulating Antioxidant Defense in Two Freshwater Capoeta Species

Antioxidant defense systems in fish are highly sensitive to environmental variability and provide valuable indicators of physiological stress in aquatic ecosystems. This study evaluated the combined effects of seasonal variation, morphometric parameters (total length, total weight, and condition factor), sex, and species identity on oxidative stress markers and antioxidant defense responses in Capoeta umbla and Capoeta trutta collected from the Karasu River (Türkiye). Fish were seasonally sampled between April 2023 and March 2024, and malondialdehyde (MDA) levels and the activities of key antioxidant enzymes, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR), were analyzed in edible muscle tissues. Length–weight relationships and Fulton’s condition factor were calculated to describe growth patterns and physiological status. The results indicated that seasonal variation was the primary factor influencing oxidative stress responses and antioxidant defense patterns in both species. Higher oxidative stress levels and increased antioxidant enzyme activities were generally observed during the summer period, reflecting physiological responses to elevated environmental temperatures. Morphometric parameters and species identity showed comparatively weaker associations with antioxidant variability. Overall, the findings highlight the dominant role of seasonal environmental dynamics in regulating oxidative balance in freshwater Capoeta species and support the use of antioxidant biomarkers as effective tools for assessing ecosystem health under changing environmental conditions. © 2026 by the authors.