Mechanistic Biomarker Integration Reveals Oxidative and Osmoregulatory Disruption by S‐Metolachlor in Rainbow Trout

The widespread use of the chloroacetanilide herbicide S-metolachlor has raised concerns about its ecotoxicological impact on aquatic organisms, yet tissue-specific effects in fish remain poorly understood. To address this, juvenile rainbow trout (Oncorhynchus mykiss) were exposed to environmentally relevant concentrations (0.0123, 0.0615, and 0.123 mg/L) for 7, 14, and 21 days, and biomarkers were assessed in the liver, kidney, gill, muscle, and brain. Antioxidant defenses were generally stimulated, particularly in the muscle and gill, while oxidative damage markers increased across tissues, accompanied by protein depletion. These alterations were mechanistically linked to suppression of ATPase activity in the gill and kidney, indicating a cascade from oxidative stress to impaired osmoregulation. Brain tissue showed only modest responses, and acetylcholinesterase activity was not significantly affected. The integrated biomarker response (IBR) index synthesized these outcomes, highlighting gill and muscle as the most vulnerable tissues. Disturbances at this level serve as sensitive early-warning indicators that, if sustained, can compromise individual performance traits such as growth, survival, and reproductive capacity. At the population level, these impairments may lower resilience to additional environmental stressors, while at the ecosystem scale, they have the potential to alter community dynamics and energy flow. Collectively, the findings establish a mechanistic basis for connecting sublethal biomarker responses to ecologically relevant adverse outcomes, thereby enhancing the predictive utility of biomarker-based approaches in environmental risk assessment.