During their life cycle as farmed animals, there are several situations in which fish are subjected to handling and confinement. Netting, weighing, sorting, vaccination, transport and, at the end, slaughter are frequent events under farming conditions. As research subjects, fish may also undergo surgical procedures that range from tagging, sampling and small incisions to invasive procedures. In these situations, treatment with anaesthetic agents may be necessary in order to ensure the welfare of the fish. The main objective of this paper is to review our knowledge of the effects of anaesthetic agents in farmed fish and their possible implications for welfare. As wide variations in response to anaesthesia have been observed both between and within species, special attention has been paid to the importance of secondary factors such as body weight, water temperature and acute stress. In this review, we have limited ourselves to the anaesthetic agents such as benzocaine, metacaine (MS-222), metomidate hydrochloride, isoeugenol, 2-phenoxyethanol and quinaldine. Anaesthetic protocols of fish usually refer to one single agent, whereas protocols of human and veterinary medicine cover combinations of several drugs, each contributing to the effects needed in the anaesthesia. As stress prior to anaesthesia may result in abnormal reactions, pre-anaesthetic sedation is regularly used in order to reduce or avoid stress and is an integral part of the veterinary protocols of higher vertebrates. Furthermore, the anaesthetic agents that are used in order to obtain general anaesthesia are combined with analgesic agents that target nociception. The increased use of such combinations in fish is therefore included as a special section. Anaesthetic agents are widely used to avoid stress during various farming procedures. While several studies report that anaesthetics are effective in reducing the stress associated with confinement and handling, there are indications that anaesthesia may in itself induce a stress response, measured by elevated levels of cortisol. MS-222 has been reported to elicit high cortisol release rates immediately following exposure, while benzocaine causes a bimodal response. Metomidate has an inhibitory effect on cortisol in fish and seems to induce the lowest release of cortisol of the agents reported in the literature. Compared to what is observed following severe stressors such as handling and confinement, the amount of cortisol released in response to anaesthesia appears to be low but may represent an extra load under otherwise stressful circumstances. Furthermore, anaesthetics may cause secondary adverse reactions such as acidosis and osmotic stress due to respiratory arrest and insufficient exchange of gas and ions between the blood and the water. All in all, anaesthetics may reduce stress and thereby improve welfare but can also have unwanted side effects that reduce the welfare of the fish and should therefore always be used with caution. Finally, on the basis of the data reported in the literature and o...
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, 2phenoxyethanol, 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, vi 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. T...
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