Fish sometimes require intensive veterinary examination and procedures, including surgery, and that means they need anesthesia. The process reduces the ability to feel pain and provides effective restraint when procedures are performed out of the water, a condition objectionable to most fish.
Anesthesia for fish is usually delivered in the water and is essentially done by inhalation, as the anesthetic agent is absorbed across the gills. Injected anesthetics commonly used in mammals are less effective in most fish species.
How It's Done
Fish are often anesthetized for just a few minutes for diagnostic procedures like skin scrapings and gill biopsies. In these cases, veterinaries have traditionally used what is known as an inexact addition of anesthetics "to effect." In this approach, the anesthetic is sprinkled or poured into the water until the fish loses balance and becomes non-responsive. The needed procedure is then performed as quickly as possible and the fish is returned to anesthetic-free water to recover.
While this approach can work for minor procedures, such imprecision is not advised for longer, more intricate surgery. Continuous delivery to the gills of known concentrations of anesthetic in water is required for longer procedures. A fish is put under anesthesia by immersion in a tank with a known concentration of anesthetic. The fish may go through an excitement phase as inhibitory neurons are depressed and before anesthesia is achieved, so it is a good idea to have a cover on the tank. When the fish is anesthetized, it is removed to a surgery platform. There, water containing anesthetic is pumped from the tank and over the gills. The anesthesia water needs to be well aerated, because under anesthesia breathing (gilling) will be reduced. Fish have been maintained on anesthesia for procedures lasting over two hours with such systems.
Cold Is Not An Anesthetic
Chilling a fish in the refrigerator is one way to slow it down and reduce responsiveness. While the fish is then easier to handle, cold does not eliminate the sensation of pain. Furthermore, chilling the fish can impair its immune system and may make it more susceptible to infections following the procedure. Chilling is NOT recommended as an alternative to anesthesia for restraint.
Preparing for Anesthesia
Prior to anesthesia, your veterinarian will request that you not feed your fish for one feeding cycle. A fish with a full stomach may regurgitate under anesthesia and clog its gills partially and foul the water. Minimize stress on the fish, as an excited fish will not experience a smooth anesthesia event. Dim lights help reduce stimulation of the fish. The fish should be handled carefully (and minimally) to avoid abrasions and loss of protective mucus.
For recovery from anesthesia the fish is placed in anesthetic-free water. If it is not respiring (gilling) well on its own, water is directed over the gills by pulling the fish forward through the water or by using a syringe or pump. Respirations will gradually strengthen, normal upright attitude will return, and the fish will resist restraint. Even once the fish is recovered and appears outwardly normal, it may remain hypoxic (low blood oxygen levels) for some time, so maintaining adequate aeration of the water is important.