Stability of some commonly measured blood‐chemistry variables in juvenile salmonids exposed to a lethal dose of the anaesthetic MS‐222

Fish culturists and ¢sh biologists make use of bloodchemistry indices forevaluationof ¢sh stress responses, nutritional condition, reproductive state, tissue damage due to handling procedures and health status (McDonald & Milligan 1992; Wagner & Congleton 2004; Congleton & Wagner 2006). Stress-related changes in blood chemistry occur within seconds or minutes after ¢share disturbed (MazeaudM Gingerich & Drottar 1989), so that precautions must be taken to ensure that blood-sampling procedures do not alter the indices of interest. Serial sampling of ¢sh from the same rearing unit may cause progressive changes in some indices (Pickering, Pottinger & Christie1982; Laidley & Leatherland1988; Bj rnsson,Young, Lin, Deftos & Bern1989); therefore, all ¢sh needed for a sample are often captured at the same time and immediately anaesthetized (batch sampling). Relatively high concentrations (150^ 200mg L ) of tricaine methanesulphonate (MS222), which bring about an irreversible deep plane of anaesthesia within a few minutes, have been reported to be eiective in preventing changes in plasma cortisol, a primary stress^response indicator (Davis, Parker & Suttle 1982; Barton, Schreck, Ewing, Hemmingsen & Patino 1985), and are recommended for prevention of stress-related changes in blood chemistry (Wedemeyer, Barton & McLeay 1990). Lower, sedating or immobilizing doses of anaesthetic may elicit rather than prevent stress responses (Strange & Schreck1978; Smit, Hattingh & Burger 1979; Iwama, McGeer & Pawluk1989;Thomas & Robertson1991). Although a number of studies have described the physiological responses of ¢sh to sedating and immobilizing doses of MS-222, only a few studies have reported on responses to higher, lethal concentrations of MS-222 or other anaesthetics. The few studies using higher concentrations of MS-222 (125mg L , Laidley & Leatherland 1988; 150mg L , Holloway, Keene, Noakes & Moccia 2004) have evaluated changes in blood chemistry at the time of induction of deep anaesthesia, 2^3min after the initiation of exposure. No previous studies have determined the response of commonly measured blood-chemistry indices to longer (over10min) periods of exposure to higher ( 150mg L ) concentrations of MS-222. When ¢sh are batch sampled, typically in groups of 10^20, collection of blood samples (and often of other tissues) and recording of length and mass data may require 20min or longer, even with the participation of several trained personnel. Therefore, information on blood-chemistry responses to a frequently used and recommended sampling protocol ^ batch anaesthesia in 200mg L MS-222 ^ is lacking. The objective of the present study was to determine the stability of 13 commonly employed blood-chemistry indices when juvenile Chinook salmonOncorhynchus tshawytscha were held in a 200mg L 1 solution of buiered MS-222 for up to 35min. Underyearling spring-run Chinook salmon were obtained from Clearwater National Fish Hatchery, Ahsahka, ID, USA, and reared for 9 months in a 600 L circular tank supplied with 10^12 Lmin 1 of Aquaculture Research, 2006, 37, 1146^1149 doi:10.1111/j.1365-2109.2006. 01528.x