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Euthanasia is a necessary aspect of all animal healthcare. Euthanasia is used for two main purposes – to relieve the suffering of severely ill fish and to humanely destroy severely injured or deformed fish, particularly fry.
When to Euthanize fish?
Several fish diseases are so consistently fatal that euthanasia is invariably the best option. These include neon tetra disease, dwarf gourami iridovirus, chronic wasting disease, dropsy and advanced Hexamita infections.
In some cases, euthanasia is also part of the management of fish disease outbreaks. Neon tetra disease, for example, spreads to healthy fish when they nibble on the corpses of sick fish. Removing and euthanizing infected fish promptly will minimize the chances of healthy fish becoming sick.
Fish with poorly developed swim bladders (belly sliders) and deformed spines are particularly common among inbred fish, such as some fancy live bearers. In this case, euthanasia removes bad genes from a particular batch of fish, ensuring each new generation is healthy and conforms to the standards of the type.
Flushing Fish:
Flushing fish down the toilet is never an option. Flushing a sick fish into the sewer is not only inhumane, but it can also allow disease-causing organisms to enter local waters, leading to outbreaks of disease amongst native fish.
How not to Euthanize fish?
Immersion in crushed ice or boiled water are not humane ways to euthanize fish, though ice-cold water may be used under some circumstances. Stunning and decapitating fish is only humane if followed by pithing. Suffocating fish by leaving them out of water is not humane either.
Verifying Death:
Veterinarians and biologists consider a fish to be dead 10 minutes after the last sign of gill movement. It is important not to remove a fish for disposal until death has been established.
Ice-Cold Bath:
Tropical fish less than 2 inches in length can be euthanized by exposing them to freezing cold water. The fish is put into a small container along with some aquarium water at the normal temperature. This container is then placed into a much larger container filled with crushed ice. This will rapidly chill the water in the smaller container, eventually rendering the fish unconscious. When death is verified, the fish can be removed.
Decapitation:
While too grisly for most aquarists, stunning a fish, decapitating it and then pithing it is a humane way to euthanize a fish. Because fish can remain conscious for some time after decapitation, the pithing step is essential.
Clove oil:
Also known as eugenol, clove oil is a sedative at low doses, but at higher doses it has been recommended by some researchers as an inexpensive way to euthanize fish, particularly small fish. In a container, mix aquarium water and clove oil and mix. When exposed to high concentrations of clove oil, fish quickly lose consciousness and stop breathing, both of which reduce pain. Hypoxia eventually causes death, and once verified, the fish can be removed from the water and clove oil mixture. The required dose for euthanasia is 400mg/l or more.
Carbon Dioxide:
Pressurized carbon dioxide vigorously bubbled through water for at least 30 seconds will quickly displace dissolved oxygen. A fish placed in such a container will quickly become unconscious and soon die.
Anesthetic Overdose:
By administering an overdose of an anesthetic, vets can ensure that large fish are humanely euthanized. A variety of anesthetics can be used for this purpose, including 2-phenoxythanol, benzocaine hydrochloride, sodium pentobarbital. It may take more than 30 minutes for death to occur and it is recommended that fish be left in anesthetic baths for at least 2 hours to be sure.
Disposing Dead Fish:
Dead fish should not be flushed. The body can be burned, disposed of the household trash or buried in the garden away from bodies of water.
