Fish farm effluents are known to affect water quality and freshwater ecosystems, potentially harming non-target organisms and ecosystem processes. We studied the effect of fish farm effluents at different concentrations (3.125 to 100% v/v) on the catalase (CAT) and glutathione S- transferase (GST) activity of Oncorhynchus mykiss fry over 24 to 120 hours. There organisms were exposed to the effluent in a static tank system with daily water exchange and CAT and GST activities were quantified at 24o and 340 nm, respectively. Additionally, the effluent was characterized in terms of physicochemical parameters and the presence of antibiotics. We found that fish farm effluent had higher conductivity, nitrate, nitrite, and total dissolved solids concentrations downstream compared to upstream and no antibiotics were detected. CAT activity significantly increased in the liver at concentrations of 12.5, 50, and 100% of the effluent after 72 hours. In the gills, a significant increase was observed at concentrations ranging from 6.25% to 100% of the effluent after both 24 and 72 hours. GST activity increased significantly in the liver at a concentration of 100% of the effluent after 72 hours and in the gills at concentrations of 25, 50, and 100% after 24 hours, with a decrease noted at higher concentrations. The results demonstrate that fish farm effluents can induce oxidative stress. Our findings emphasize the potential ecological risks posed by fish farm effluents to aquatic organisms.