Thymol as a new anesthetic in common carp (Cyprinus carpio): Efficacy and physiological effects in comparison with eugenol

The aim of the present study was to investigate thymol anesthetic efficiency and biochemical effects in common carp in comparison to eugenol. In the first experiment, time of induction of and recovery from anesthesia were recorded in the fish anesthetized with either eugenol or thymol at the concentrations of 6.25, 12.5, 25, 50, 75, 100, 125, 150 and 200 mg L−1 (ten fish per concentration). In the second experiment, stress responses, oxidative stress and biochemical effects of eugenol and thymol were investigated in the fish after short-term (5 min; 43 mg L−1 eugenol or 52 mg L−1 thymol) anesthesia and long-term (3 h; 10 or 20 mg L−1 of either of the anesthetics) anesthetic exposure. Eugenol anesthetized the fish within 600-90 s (with recovery time of 190-380 s) at the concentrations of 25-150 mg L−1. Thymol induced anesthesia within 850-60 s (with recovery time of 210-1200 s) at the concentrations of 25-200 mg L−1. Eugenol resulted in a significant increase in plasma aspartate transaminase (AST), lactate dehydrogenase (LDH), malondialdehyde (MDA) and phosphate levels compared to thymol after the short-term anesthesia. After the long-term exposure, eugenol led to significant elevations in cortisol, glucose, lactate, AST, LDH, total antioxidant capacity (TAC), MDA, superoxide dismutase (SOD) and phosphate and decrease in catalase (CAT) compared to thymol. In conclusion, thymol is an efficient anesthetic in carp allowing for rapid sampling as well as surgery. In addition, thymol has fewer side effects compared to eugenol in the case of induction of stress, oxidative stress and tissue damage, thus is recommended for carp anesthesia.