Metomidate anaesthesia in Atlantic salmon, Salmo salar, prevents plasma cortisol increase during stress

Atlantic salmon parr (58 g) in fresh water at 5.0 °C and adult salmon (1130g) in sea water at 7.7 °C were exposed to water containing different concentrations of metomidate in the range 1 to 10 mg 1 −1 . Metomidate was efficacious in inducing anaesthesia (hypnosis), and efficacy increased with concentration over the interval tested. The anaesthetic was more potent in the adult salmon acclimated to sea water than in freshwater parr. Metomidate at 3 mg 1 −1 or higher completely prevented any plasma cortisol increase after a handling stressor when stressor and anaesthetic were applied concomitantly. The lack of a cortisol response seemed to be due to a blockage at the level of the interrenal cell, since exogenous ACTH injected intraperitoneally did not produce a cortisol increase in metomidate-anaesthetized fish but did in those anaesthetized with MS-222. Blood lactate levels and haematocrit increased in fish during metomidate anaesthesia.