The widespread application of antibiotics in aquaculture raises concerns about their potential physiological impacts on cultured species and associated ecological risks. This study evaluated the physiological responses of C . carpio, a globally distributed freshwater fish of ecological and economic importance, when exposed to Oxytetracycline (OTC), a tetracycline-class antibiotic widely used in aquaculture. Juvenile C. carpio (5.3 ± 0.7 g) were fed for 45 days with experimental diets (35% crude protein) supplemented with graded concentrations of OTC (25, 50, 75, and 100 mg/kg diet) at 5% of body biomass.
A reduction in oxygen uptake was recorded in all concentrations of OTC with highest decline in 75 mg OTC/kg, relative to control, indicating respiratory stress. The highest final body weight and most efficient feed conversion ratio (FCR) were obtained at 50 mg OTC/kg diet (Fig. 1), whereas fish fed 75 mg/kg diet exhibited the lowest body weight and poorest FCR (Fig. 2) , likely due to diversion of metabolic energy toward detoxification processes. Reduced growth, decrease in ammonia excretion and the increases in oxygen-to-nitrogen ratio at 75 mg/kg, suggests a metabolic shift toward lipid and carbohydrate utilization. Variations in oxygen consumption and excretory patterns may be attributed to gill damage, stress-induced metabolic suppression, and altered enzymatic activity caused by OTC accumulation.
These findings indicate that dietary OTC disrupts normal physiological functions in C. carpio. The observed alterations in respiratory activity, growth performance, and excretory patterns highlight the potential use of these responses as sensitive biomarkers for assessing environmental risks associated with antibiotic exposure.