The purpose of this study was to determine the e¡ect of clove oil (4.0 mg L À 1 ) sedation, compared with nonsedation, on the primary (plasma cortisol), secondary (osmoregulation) and tertiary (mortality) stress responses in Atlantic salmon smolts during transport and transfer to sea. Clove oil sedation during on-and o¡-loading su⁄ciently reduced the primary stress response to lower mortality (2.1%) during transfer to sea compared with unsedated ¢sh, which experienced a mortality rate above 12.2%. The unsedated ¢sh experienced an acute mortality that stabilized only 6 days after the transport. None of the secondary stress responses measured in this experiment could contribute towards explaining this phenomenon, with the possible exception of plasma magnesium (Mg 21 ). Plasma Mg 21 di¡ered between the groups; while plasma Mg 21 in the clove oil sedated group returned to pre-stress levels 72 h after transport, the unsedated group showed no such recovery even 1 week after transport, which may indicate a disturbance in the hydromineral balance, and provides a plausible explanation for the delayed mortality in this group. Eugenol-based anaesthetics appear to be promising as a stress-reducing sedative for Atlantic salmon smolts, and, if used properly, this chemical could improve animal welfare and survivability during and after common aquaculture-related incidents. 233 Signi¢cant di¡erences were established at 0.05 levels.Results are given as means AE standard deviation (SD).
Results
Behavioural observationDuring on-loading and o¡-loading, the clove oilsedated ¢sh were calm and easy to load compared Aquaculture Research, 2009, 40, 233^241 Bene¢t of clove oil sedation on salmon smolt M Iversen et al.