Evaluating anaesthetic impacts on rainbow trout mucus biomarkers: towards sustainable aquatic animal welfare

Anaesthesia is a necessary step during fish manipulation. Tricaine methane-sulfonate (MS-222) is the most commonly used anaesthetic in experimental trials; however, its use in food fish production is strictly regulated. This study aimed to evaluate clove oil, less persistent and authorized in European countries, as an alternative to MS-222 for skin mucus analyses in aquaculture settings. For this purpose, rainbow trout individuals from a commercial factory were sedated with clove oil or MS-222. The concentration of protein, glucose, lactate, and cortisol and the antibacterial activity were measured in both skin mucus and plasma. Additionally, FRAP, total protease activity, and lysozyme activity were also analysed for mucus samples. Recovery times differed significantly between treatments, being approximately four times longer for clove oil (at the farm's standard dose) than for MS-222 (at the standard laboratory dose). However, none of the stress-related biomarkers in mucus were affected by the anaesthetic treatment, whereas plasma from clove oil-treated fish showed increased lactate and reduced cortisol levels. The in vitro bacterial growth inhibition assay using mucus and plasma provides a reliable and rapid method for assessing fish innate immunity. No significant differences were observed between treatments against any of the bacterial strains tested. Overall, the findings strongly endorse the use of skin mucus as an effective method for studying and monitoring trout in aquaculture settings.