Methomyl-Induced Oxidative Stress Disrupts Antioxidant Defense in the Nile Tilapia (Oreochromis niloticus)

Methomyl contamination in freshwater systems poses a risk to non-target aquatic organisms, particularly fish cultured in agricultural regions. Thus, main objective of this study was to evaluate the effects of acute methomyl exposure on antioxidant defense in the Nile tilapia (Oreochromis niloticus) using a 96 h dose–response approach. Fish were exposed to eight concentrations (0.0, 1.35, 1.8, 2.4, 3.2, 4.2, 6.5, and 8.7 ppm), and changes in superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and malondialdehyde (MDA) were analyzed. Antioxidant enzyme activities remained stable at lower concentrations (≤1.8 ppm), indicating maintained redox balance. A significant decline (P < 0.05) in SOD, CAT, and GPx was observed at 2.4 ppm, accompanied by a concurrent increase in MDA, indicating the onset of oxidative stress. At higher concentrations (≥4.2 ppm), enzyme activities remained significantly suppressed without further significant decreases (P > 0.05), while MDA continued to increase. Increased variability at elevated concentrations indicated greater inter-individual variation in physiological response. The overall pattern indicates a non-linear response characterized by an initial tolerance phase, a threshold shift, and a saturation phase of antioxidant impairment. These findings identify a critical transition point in methomyl toxicity and provide insight into the limits of antioxidant regulation in O. niloticus under acute pesticide exposure.