Identification of a suitable eugenol concentration range for anesthesia in spotted sea bass (Lateolabrax maculatus) based on acute physiological responses

Spotted sea bass ( Lateolabrax maculatus ) is an important farmed marine fish in China that is highly sensitive to handling stress; therefore, anesthesia is commonly used during aquaculture operations. This study evaluated the acute physiological responses of spotted sea bass to graded eugenol concentrations to identify the most suitable anesthetic concentration range among those tested. As eugenol concentration increased, induction time decreased, whereas recovery time increased. Fish exposed to 30–80 mg/L eugenol were anesthetized within 3 min and recovered within 5 min. After deep anesthesia with 50 mg/L eugenol, fish exposed to air for 2–20 min showed 100% recovery and 100% 24 h survival. Compared with the control, serum cortisol (COR) and glucose (GLU) levels and aspartate aminotransferase (AST), alanine aminotransferase (ALT), and lactate dehydrogenase (LDH) activities increased significantly in all eugenol-treated groups, but were relatively lower at 50–70 mg/L than in the other eugenol-treated groups ( P < 0.05). The oxidative stress indices of gill and liver tissues, including superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GSH-Px), and malondialdehyde (MDA), exhibited varying degrees of elevation, but the 50–70 mg/L groups showed more balanced antioxidant responses, suggesting relatively milder oxidative disturbance. Gill and liver histopathological alterations were observed in all anesthetized groups but were less pronounced at 50–70 mg/L. Overall, 50–70 mg/L eugenol was identified as the most suitable anesthetic concentration range among those tested for spotted sea bass. These findings provide useful reference information for the short-term use of eugenol anesthesia in spotted sea bass under the tested conditions.