Effects of new plant based anesthetics Origanum sp. and Eucalyptus sp. oils on stress and welfare parameters in Dicentrarchus labrax and their comparison with clove oil

The effects of two new anesthetics (essential oils) extracted from oregano (OO) and eucalyptus (EO) plants were studied in European sea bass (Dicentrarchus labrax) stress response and welfare parameters in comparison to the effects of clove oil (CO) that is one of the most used anesthetics in aquaculture. A time-course study on evolution of plasma cortisol after anesthesia was conducted. Relative expression of stress-related genes, including glucocorticoid receptor (gr), heat shock protein 70 and 90 (hsp70, hsp90) steroidogenic acute regulatory protein (star), cytochrome 11B (cyp11b1) and hypoxia inducible factor (hif) was determined in liver, gills and head kidney after anesthesia. After anesthetic use, there was a time effect on plasma cortisol concentration, with highest values registered after 2 h for all the experimental groups. EO induced a secondary increase of plasma cortisol 24 h after the use of this anesthetic. Hsps expression level in gills was high at 0 h in EO fish group, whereas hif expression increased after 2 h of exposure in CO fish. In hepatic tissue, gr expression increased after 24 h in the EO group. In head kidney tissue, expression of steroidogenesis-related genes star and cyp11b1 increased in EO fish group, whereas in the OO group, the expression of the same genes decreased at 2 h and 24 h of exposure. Although the three anesthetics generally showed similar patterns of variation of all analyzed parameters, results indicate that EO could be deleterious for welfare, whereas OO affects fish welfare to a lesser extent than CO and EO. Overall, these results show that OO could be a good anesthetic for fish based on greater effectiveness, lower optimum concentration and less impact on fish stress.