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.
Using Atlantic cod, Gadus morhua, as a model for the stress response in gadoid ¢sh, the changes in the expression of some stress and immune genes as well as the pro¢les of plasma cortisol were examined. Adult ¢sh were kept at a density of ca.100 kg m À 3 by lowering the water level in the rearing tank for 1h and this short-term crowding stress was repeated thrice over a 12-h interval period. Blood samples were collected before exposure and at 2, 24 and 72 h post crowding. Plasma cortisol level signi¢cantly increased at 2 h post crowding but returned to pre-crowding levels 24 h after exposure. The relative expression of the stress response genes, glucose transporter-3 and a putative heat shock protein 70 signi¢cantly increased at 2 and 24 h post crowding respectively. Signi¢cant upregulation of the pro-in£ammatory cytokines, interleukin (IL)-1b and IL-8, as well as anti-bacterial genes, g-type lysozyme and bactericidal permeability-increasing protein/lipopolysaccharide-binding protein (BPI/LBP) was also observed at 2 h and the levels were maintained until 72 h post exposure, except for BPI/LBP which had maximum up-regulation at 24 h. The present observations have implications with respect to ¢sh welfare and assessment of the health status of the farmed ¢sh.
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