What is the most humane way to euthanize aquarium fish?
Sometimes fish are just too far gone, and people want to humanely kill them. There is nothing wrong with that but there are many PETA folks on social media who will blast anyone considering humanely euthanizing a fish. That is just the way social media is. I find it best just to ignore the PETA folks.
There are five acceptable ways to humanely euthanize a fish per the American Veterinary Medical Association (AVMA Guidelines for the Euthanasia of Animals: 2020 Edition). Cranial concussion, decapitation, vodka, clove oil and iced very cold saltwater are all acceptable methods of euthanizing a tropical fish.
Euthanasia in more Detail
Paraphrasing the recommendations of the American Veterinary Medical Association:
1, Cranial concussion followed by decapitation, exsanguination or pithing can be performed by the client at home. Catching the fish quickly and calmly will limit any stress. The fish can then be restrained (tea towel or similar is helpful) and struck hard over the head with a heavy blunt object such as a hammer or a mallet. When performed correctly cranial concussion will cause sufficient damage to the central nervous system that pithing or decapitation should not be required – nonetheless, it is often recommended when a less experienced person is performing the euthanasia procedure.
2, Decapitation with a sharp knife followed by pithing the brain is an acceptable euthanizing method. Rapid severance of the head and brain from the spinal cord, followed by pithing of the brain, will cause rapid death and unconsciousness. Decapitation alone is not considered a humane approach to euthanasia, especially for species that may be particularly tolerant of low O2 concentrations. Pithing helps ensure rapid loss of brain function and death for those species.
3, Vodka (or Gin, Scotch, Bourbon, etc.) is an acceptable alternative method for fish euthanasia at concentrations from one to three tablespoons 90 proof vodka per quart of water (20 to 60 milliliter of 90 proof vodka per liter). At this dose, alcohol induces anesthesia, and prolonged immersion produces death via respiratory depression causing anoxia. After ten minutes add more vodka to ensure the death of the fish while anesthetized. This is not equivalent to immersing fish directly into vodka, which is not acceptable as a euthanasia method.
4, Clove oil is a sedative which can be used to humanely euthanize fish. Around 400 mg (half a gram or about 1/8th of a teaspoon) of clove oil per quart (or liter) of aquarium water is sufficient to cause death in exposed fish. The clove oil should be mixed with a little hot water first before adding it to the water and fish slowly. Do not add all at once as fish appear to become very pained (they thrash around a lot). Add the clove oil mix over a 5-minute period. Leave the fish in the solution at least half an hour. When exposed to clove oil at this concentration fish lose consciousness, stop breathing and die from hypoxia.
5, Iced very cold saltwater is an acceptable euthanizing method. Rapid chilling (below 38° F, 4° C) of tropical fish resulted in cessation of vital signs (10.6 ± 3.28 seconds), 20 times as fast as in the case of clove oil overdose (216.3 ± 62 seconds). Mix crushed ice or ice cubes in saltwater to create a very cold saltwater. Put the fish into the cold saltwater for a minimum of 30 minutes. Do not simply put a fish in the freezer or refrigerator.
Never flush a fish down the toilet. This will result in a very painful slow death in most cases. In some cases the fish lives and creates a problem in the local water ways.
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