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u-sfion 't d O d J. Fish Biol. (1976), 9, 457-462 Use of electroanaesthesia with freshwater teleosts: some physiological consequences in the rainbow trout, Salmo gairdneri Richardson JANE A. MADDEN* Departamento de Fisiologia a Farmacologia, Universidade Federal de Pernambuco, Recife, Pernambuco, Brasil AND ARTHUR H. HOUSTON Department of Biological Sciences, Brock University, St. Catherines, Ontario, Canada L2S 3A1 (Accepted 15 June 1975) Electroanaesthesia prompts both immediate and longer term alterations in the physio- logical status of the rainbow trout. These are not, by comparison with the related brook trout, more extensive than those associated with tricaine methane sulphonate (NIS 222) anaesthesia. Changes in haemotological indices are minimized, while the spontaneous resumption of cardiovascular-respiratory activity, and the character of variations in plasma electrolyte levels suggest that blood gas tensions and acid-base status may be less affected by electroanaesthesia than by chemical anaesthesia. Moreover, recovery, in the control levels proceeded more rapidly in several instances. It is concluded that electroanaesthesia offers a useful alternative to chemical methods of anaesthesia. 1. INTRODUCTION Anaesthetization of freshwater teleosts for experimental purposes has been based principally upon the use of chemical anaesthetics; most notably tricaine methane sulphonate (MS 222).t These agents are not without deleterious effects, however, and it is now known that the overall anaesthetization process leads to significant changes in the physiological state. Houston et al. (1971a) have observed MS 222 prompts immediate alterations in haematological characteristics and in a number of aspects of water-electrolyte status, as well as an immediate depressive effect upon ventilatory and cardiac activity. Branchial perfusion is considered to be necessary during procedures of any substantial duration and to maintain surgical anaesthesia, it is necessary to perfuse the gills with the anaesthetic solution. Blood anaesthetic concentrations increase sharply (Houston & Woods, 1972) and further cardiovascular- respiratory depression occurs. Because of this, vascular hypoxia coupled with hypercapnia and consequent alterations in acid-base balance are to be anticipated, and have been demonstrated (Garey & Rahn, 1970). Recovery is associated with transitory increases in ventilation and cardiac rates (Houston, Czerwinski & Woods, 1973), presumably in response to blood gas tension changes (Taylor, Houston & *Neuroscience Research, V.A. Hospital Wood, Wisconsin 53193, U.S.A. tSandoz Ltd., Basle, Switzerland. 457