There are occasions when it becomes necessary to anaesthetize a fish so that it can be either examined or handled safely. A common situation is when there is the suspicion of a health problem and it becomes necessary to carry out a thorough examination.

While it is possible to check the general condition of a fish in a bowl or plastic bag, it is usually necessary to anaesthetize the fish to thoroughly examine any skin, gill or fin problems.

Anaesthesia is also usually required to carry out procedures such as a gill biopsy or scrape, injections and topical dressing of lesions.  It really isn't possible to properly clean and dress a lesion on a fish that is flapping around. Although it is possible to carry out a skin scrape on an conscious fish, it is often better to lightly anaesthetize larger fish such as koi so that the scrape can be carried out properly and safely.

It can be dangerous

Anaesthesia and indeed many of the examination and treatment procedures described in these pages go beyond what would be considered as "normal" fish keeping practices.  When it comes to anaesthetics it is important that we have a thorough understanding of their use, basic fish physiology and the potential problems.  It is important to stress that all anaesthetics carry a real degree of risk and if we have any doubts about our ability to carry out this procedure responsibly and safely, then it is best to find someone to assist.

As with other techniques, if you do intend to try and learn fish anaesthesia it is best to try it a few times on healthy fish when all you have to worry about is "putting them under" and then bringing them around again, rather than waiting until you need to do it for real.  This way you can become confident in using anaesthetics.

Planes or stages of anaesthesia

There are several stages of anaesthesia varying from light sedation  during which the fish appears normal but there is a slight loss of reactivity - up to surgical anaesthesia during which there is a total loss of reactivity and the heart rate and respiratory rate slows.  The final stage is medullary collapse, during which there is a total loss of gill movement followed by cardiac arrest!!!

The plan of anaesthesia reached depends on several factors such as dosage rate, period of immersion in anaesthetic solution, temperature, fish species and the condition of the fish.  For most practical procedures we are looking to achieve a deep narcosis during which the fish has total loss of equilibrium, loss of muscle tone (keeps still), but can be easily revived.

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Expanded Content by Dr. Erik Johnson, DrJohnson.com and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved