When a condition is untreatable
As with all other animals, there are situations when the fish, for a variety of reasons, either fails to respond to treatment or has a condition that is untreatable. It is my belief that fish are capable of suffering and therefore the most humane act in such a situation is to euthanase the fish.
Getting up on my soapbox – euthanasia should be a last resort and not simply a way to save on treatment.
Any form of euthanasia should cause death with the minimum of discomfort or suffering to the animal. Typical methods used are:
Anaesthetic overdose: Use 3-4 times the recommended dosage of any of the commonly used anaesthesia agents. Because of the risk of recovery the fish should be left in the solution for several hours.
Ideally, after losing consciousness in the anaesthetic solution the fish should have an injection of sodium pentobarbital so that death is instantaneous – but this is a restricted drug and not available to members of the public. An alternative is to cut all the gill filaments on both sides once the fish is unconscious and allow the fish to bleed to death. Clearly this can be distressing, but the fish feels no pain.
Decapitation: This is only suitable for smaller fish. It is assumed to cause instant unconsciousness.
Cranial concussion: A sharp, single blow to the head with enough force to damage the central nervous system. This may kill small fish outright, but larger fish may only be stunned – so it should be followed by a method such as bleeding, to cause death.
Sodium bicarbonate (baking powder or Alka Seltzer®): This is not an accepted form of euthanasia and will cause a degree of discomfort and stress, but it is readily available and can be used in the home if there are no other alternatives available. Again bleeding may be required once the fish has lost consciousness.
Poisoning: There are many chemicals that will effectively kill fish such as Rotenone. None are considered humane and will all cause considerable distress unless the fish is euthanased first
Freezing: Place the fish in a comfortable container which will fit into your freezer. Make sure there is enough water for the fish to rest in. Place the container into the freezer. The fish is a poikilotherm, which means its entire metabolism, neurological function, etc. are all conducted at ambient temperatures. As the temperatures decline and finally achieve freezing levels, the fish will lose consciousness and pass away. It has been assumed that the fish “feel” ice biting into its tissues however since its neurological system (ability to realize anything) is tied to temperature, by the time the fish has plunged to ‘freezing’ it has fully shut down neurologically. It realizes nothing.