Remediation of chronic ammonia effects in common carp by dietary thymol supplementation

Abstract

Chronic ammonia toxicity represents a major challenge in intensive aquaculture, often leading to impaired growth and physiological distress. This study evaluated the efficacy of dietary thymol supplementation in mitigating the deleterious effects of chronic ammonia exposure in common carp, Cyprinus carpio . Fish (initial weight: 10.41 ± 0.23 g) were assigned to one of four treatments in triplicate: (1) Control (clean water, basal diet), (2) AMM (0.06 mg/L unionized ammonia, basal diet), (3) AMM + T1 (0.06 mg/L unionized ammonia, 100 mg/kg thymol), and (4) AMM + T2 (0.06 mg/L unionized ammonia, 400 mg/kg thymol). Each replicate was stocked with 15 fish. After an eight‐week trial, ammonia exposure significantly elicited a marked stress response (elevated plasma ammonia, cortisol, and glucose; p  < 0.05), while dietary thymol significantly attenuated these alterations ( p  < 0.05). Ammonia also impaired liver and metabolic function, as reflected by elevated hepatic enzymes (alanine aminotransferase, aspartate aminotransferase, lactate dehydrogenase), urea, and creatinine alongside reduced thyroid hormones (T 3 , T 4 ); thymol restored most of these parameters toward control values. Growth performance and feed efficiency were significantly suppressed under ammonia stress ( p  < 0.05) but improved with both thymol doses. Furthermore, ammonia exposure compromised innate immunity, reducing lysozyme, alternative complement, and bactericidal activities in plasma and tissues, whereas thymol supplementation restored these immune parameters. Finally, ammonia induced significant oxidative damage, evidenced by elevated malondialdehyde and protein carbonyl, while thymol enhanced antioxidant enzyme activities (superoxide dismutase, catalase, glutathione peroxidase) and radical‐scavenging capacities ( p  < 0.05). No significant differences were observed between the 100 mg/kg and 400 mg/kg thymol treatments for most variables ( p  > 0.05), suggesting that 100 mg/kg is an optimal and economical dose (compared to 400 mg/kg) for dietary inclusion. These findings demonstrate that thymol acts as a potent anti‐stress and antioxidant feed additive, offering a viable strategy to safeguard common carp health in high‐density or low‐water‐exchange aquaculture systems prone to ammonia accumulation.