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.