By: Dr Mustapha ABA, Aquaculture Researcher. Morocco
In order to optimise management without stress factors and damage to the physical integrity of the fish representing a negative point in the production process, the use of anaesthetics has become a necessary practice. To a lesser degree it is a simple analgesia that allows to carry out calmly and painlessly the sampling in fish farming, the examination of lesions, the carrying out of tissue biopsies for histological or bacteriological study. It also allows to calm the animals during their transport by reducing the motricity, the consumption of oxygen, the excretion of faeces and finally the stress.
Anaesthetics are generally used during routine procedures in fish farms, when they are administered in two ways: by inhalation and by injection. Anaesthesia, which is often an indispensable procedure in fish farming systems, must be carried out in the most efficient way possible, both biologically and economically. Biologically, the aim of the operation is to anaesthetise the fish without causing growth and reproduction problems. From an economic point of view, the use of the correct dose of anaesthetic is fundamental to avoid wastage of the product or the death of the fish by “overdose”. Here is some basic information on how to anaesthetize fish safely, thus contributing to a correct anaesthetic operation in fish farms in Africa. However, the use and/or application of any of the commercial products cannot be approved or recommended.
Stages of anaesthesia
When immersed in water with an anaesthetic, fish go through several stages of anaesthesia in succession. For the anaesthetic operation to be successful, the operator must observe these different stages in the fish. The stage normally used for biometrics, fish handling and broodstock is deep anaesthesia and this stage should be reached within 1 to 3 minutes. For health assessments and surgical procedures, the desired stage is surgical anaesthesia and this stage should be reached within 3-5 minutes. Recovery of the animals should be rapid and less than 5 minutes is considered appropriate.
Each anaesthetic will require a different concentration to induce the desired stage of anaesthesia within the time considered adequate. This concentration should be safe, should not cause strong reaction of the fish to the anaesthetic and should not cause mortality. Several authors consider that 10 minutes should be the maximum duration of anaesthesia. Fish that have been anaesthetised for longer periods reduce the safety margin of the ideal dose. Therefore, if it is necessary to anaesthetise the fish for more than 10 minutes, it is advisable to reduce the dose of anaesthetic.
The choice of anaesthetic
The choice of an anaesthetic should be based not only on the time required for induction and recovery of the anaesthetic, but also on criteria such as the value of the product, efficacy, availability on the market, ease of obtaining it, safety during use and possible side effects on the fish, the handler and also on the environment.
Factors influencing anaesthesia in fish
The anaesthesia of fish can be affected by biological and environmental factors. Biological factors include differences between species, such as body shape and size of the gill area, and differences between fish of the same species, such as size, variations in metabolic rate and amount of body fat. For this reason, the recommended doses may not apply to your fish, so it is necessary to adjust the dose.
Among the most important environmental factors in anaesthetising freshwater fish are temperature and pH. Temperature determines the metabolic rate of the fish. The higher the metabolic rate, the faster the induction of anaesthesia. The pH of the water is directly related to the effectiveness of certain anaesthetics. To ensure that these environmental factors are not a problem during the anaesthetic operation, the operator must anaesthetise the fish systematically at the same temperature and pH. In this way, the operator can normalise the whole operation, almost always using the same dose of anaesthetic.
Anaesthetics are chemical agents that calm animals and make them gradually lose their mobility, balance, consciousness and finally their reflex action. In the aquaculture sector, anaesthetics are useful to reduce the stress caused by sampling for biometric studies or transport. Many factors can affect the effectiveness of anaesthetic treatments; therefore, experimental dosages should be tested on a small group of fish prior to any large-scale anaesthesia.