Anaesthesia of farmed fish with special empasis on atlantic cod (Gadus morhua) and atlantic habibut (Hippoglossus hippoglossus)

Abstract

During the life cycle as farmed animals there are numerous situations where fish are subjected to handling and confinement. Netting, weighing, sorting, vaccination, transport and, at the end, slaughter are common events under farming conditions. As research animals fish may also undergo surgical procedures, ranging from tagging, sampling, and small incisions, to larger operations. Under these varying situations treatment with anaesthetic agents might be necessary in order to ensure the welfare of the fish. Anaesthetic protocols for new species that are introduced to research or cultivation are generally based on protocols developed for the more established species. In Norway the anaesthetic protocols for Atlantic cod (Gadus morhua) and Atlantic halibut (Hippoglossus hippoglossus), which were introduced to fish farming in the 1980’s, have thus been based on protocols used for salmonid species. Atlantic salmon (Salmo salar) has been farmed since the 1960’s and is the most important species in Norwegian fish farming. The main objective of the current investigation was to gain more knowledge regarding the effect of anaesthetic agents in farmed fish, with special emphasis on Atlantic cod and Atlantic halibut. Large variations in response to anaesthetic agents exist both between and within fish species. Factors such as body weight, water temperature and acute stress may be important for the response and were studied in the current investigation. The anaesthetic agents benzocaine, metacaine (MS-222), metomidate hydrochloride, isoeugenol, 2- phenoxyethanol, and quinaldine were used. In Atlantic cod and Atlantic halibut the agents were studied with regards to efficacy, assessed as induction and recovery times as well as reaction to handling under anaesthesia. In Atlantic salmon pharmacokinetic properties of the agents were examined. Both in Atlantic cod and Atlantic halibut large differences in efficacy between the different anaesthetic treatments were found. Induction and recovery times varied both in relation to body weight and in relation to water temperature. The main trend observed in Atlantic cod at high water temperature was shorter induction and recovery times for all weight groups and treatments. Whereas in Atlantic halibut higher water temperature resulted in shorter induction times, longer recovery times, and increased responsiveness to handling. Atlantic halibut of large body size displayed longer induction times, shorter recovery times, and reduced responsiveness to handling in comparison with fish of smaller body size. However, in Atlantic cod no uniform trend was found in the relationship between the size of the fish and anaesthetic efficacy. In Atlantic cod induction and recovery times were found to increase with increasing body weight for benzocaine and MS-222. For metomidate the recovery time increased with increasing weight whereas there were no weight related differences in induction time. No differences in either induction or recovery times associated to body weight were found for 2-phenoxyethanol. The pharmacokinetic study in Atlantic salmon showed that the anaesthetics were rapidly eliminated and that elimination was related to the water soluble characteristics of the agents. The recovery times were shorter in fish that were given artificial gill ventilation. In the assessment of the importance of acute stress prior to anaesthesia of Atlantic cod it was found that the stress resulted in significantly shorter induction time and prolonged recovery time, as well as deeper anaesthetised fish. The anaesthetic dosage had to be reduced in order to avoid mortality in fish anaesthetised subsequent to acute stress. Anaesthetic protocols for fish have generally comprised one single agent, whereas protocols of human and veterinary medicine comprise combinations of several drugs, each one contributing with effects needed in the anaesthesia. Stress prior to anaesthesia may result in abnormal reactions, as seen in Atlantic cod in the current study, and may require dosage adjustments of drugs both for induction and maintenance. Pre-anaesthetic sedation is therefore commonly used in order to avoid stress and is an integrated part of the veterinary protocols. In the current study, protocols comprising combinations of two anaesthetic agents, one agent to induce sedation followed by one agent to induce anaesthesia, were tested in Atlantic cod and Atlantic halibut. In both species combination anaesthesia allowed a reduction of the dosages used for inducing anaesthesia. In Atlantic cod combination anaesthesia resulted in markedly reduced recovery times compared to agents administered individually. In Atlantic halibut combination anaesthesia had no effect on the induction times in small fish in comparison with individual agents, but resulted in significantly shorter recovery times and reduced responsiveness to handling. In Atlantic halibut of large body size combination anaesthesia gave rise to shorter induction times than individual administered agents whereas no uniform trend was observed in recovery times and no differences in responsiveness to handling were noticed. Anaesthetic agents are commonly used in fish farming to avoid stress during various farming practices. While several studies report that anaesthetic agents are effective in reducing the stress associated with confinement and handling, there are also reports indicating that the exposure to anaesthetics may in itself induce a stress response, measured by increased levels of cortisol. In order to examine stress induced by exposure to anaesthetic agents the release of cortisol to water following anaesthetic exposure was examined in Atlantic cod, Atlantic halibut and Atlantic salmon. In this examination the fish were not subjected to any concomitant handling in connection to anaesthesia or sampling. The plasma cortisol concentration during anaesthesia was examined in Atlantic salmon, however this examination included some degree of handling. All of the anaesthetics tested induced a release of cortisol to water in all three species, with maximum release rates measured 0.5-1 hour post exposure. This also complied with the plasma cortisol levels measured in Atlantic salmon. MS-222 elicited the highest cortisol release rates while benzocaine caused a bimodal response where the initial peak in cortisol release rate was followed by a second and smaller peak. Metomidate induced the lowest release of cortisol of the agents tested in both Atlantic halibut and Atlantic cod, but resulted in a bimodal response in Atlantic salmon where the initial increase in cortisol release was followed by an even larger increase. The stress induced in Atlantic salmon by isoeugenol resembled that of MS-222, but did not reach the same elevated level. Over all the cortisol release was most profound in Atlantic salmon followed by Atlantic halibut and Atlantic cod. Based on the findings in the current study it is recommended that anaesthetic protocols should always be tested on a few fish under prevailing conditions in order to ensure an adequate level of depth while avoiding overdosing. This recommendation applies whether one single agent or a combination of agents are used although it was found here that protocols comprising combinations of agents provide wider margins of safety. While exposure to anaesthetic agents was found to elicit a stress response, displayed as increased levels of cortisol, the amount of cortisol released in response to anaesthesia was low compared to what is reported following strong stressors such as handling and confinement. Stress caused by anaesthetic agents may however represent an extra load during otherwise stressful circumstances.