Carbon dioxide anaesthesia in rainbow trout: effects of hypercapnic level and stress on induction and recovery from anaesthetic treatment

The physiological and anaesthetic effects of three different levels of air-saturated and buffered CO2 anaesthesia, PwCO2 = 37, 78, or 125 mmHg, were examined in cannulated rainbow trout Oncorhynchus mykiss. Complete anaesthesia (no opercular movements) was not achieved by these hypercapnic levels after 20 min of CO2 exposure. Although increasing PwCO2 reduced the induction times to the early stages of anaesthesia, it also resulted in increasing hyperventilatory, hypoxaemic, and acid-base disturbances. After a 10-min recovery period, while the respiratory acidosis component of the acid-base disturbance was corrected, there was a significant metabolic acidosis, Recovery time was longest in the high PwCO2 treatment where 33% of the fish died. Two additional groups (PwCO2 = 37 and 78 mmHg) were exposed to an acute stress prior to the anaesthetic treatment. Stress reduced the hypoventilatory effects of the low PwCO2 treatment, increased the recruitment of anaerobic metabolism and prolonged recovery lime. Although the increase in plasma catecholamines elicited by the stress was small relative to the response obtained with the anaesthetic, stress prior to CO2 anaesthesia impaired the efficiency of the treatment. Overall, our results suggest that PwCO2 levels above 37 mmHg and/or stress prior to the anaesthesia impair the efficiency of air-saturated and buffered CO2 anaesthesia by exacerbating the hypoxaemic effects of the hypercapnic treatment. (C) 1998 The Fisheries Society of thm British Isles.