Euthanasia (What is the most humane way to put a dying fish out of its misery?)

Posted in Health and Disease

The term euthanasia is derived from the Greek terms "eu" meaning good and "thanatos" meaning death. A “good death” would be one that occurs with minimal pain and distress.

Euthanasia is the act of inducing humane death in an animal. It is our responsibility to ensure that if an animal’s life is to be taken, it is done with the highest degree of respect, and with an emphasis on making the death as painless and distress free as possible.

Euthanasia techniques should result in rapid loss of consciousness followed by cardiac or respiratory arrest and the ultimate loss of brain function. In addition, the technique should minimize distress and anxiety experienced by the animal prior to loss of consciousness.

Euthanatizing agents cause death by three basic mechanisms:

  1. Hypoxia - direct or indirect
  2. Direct depression of neurons necessary for life function
  3. Physical disruption of brain activity and destruction of neurons necessary for life.


Agents that induce death by direct or indirect hypoxia, which is defined as an insufficient amount of oxygen reaching tissues despite the presence of oxygenated blood, can act at various sites and can cause loss of consciousness at different rates. For death to be painless and distress-free, loss of consciousness should precede loss of motor activity (muscle movement). Loss of motor activity, however, cannot be equated with loss of consciousness and absence of distress. Thus, agents that induce muscle paralysis without loss of consciousness are not acceptable as sole agents for euthanasia (eg, depolarizing and nondepolarizing muscle relaxants, strychnine, nicotine, and magnesium salts). With other techniques that induce hypoxia, some animals may have motor activity following loss of consciousness, but this is reflex activity and is not perceived by the animal.


A second group of euthanatizing agents depress nerve cells of the brain, inducing loss of consciousness followed by death. Some of these agents release inhibition of motor activity during the first stage of anesthesia, resulting in a so-called excitement or delirium phase, during which there may be some muscle contraction. These responses do not appear to be purposeful. Death follows loss of consciousness, and is attributable to cardiac arrest and/or hypoxemia following direct depression of respiratory centers.


Physical disruption of brain activity, caused by concussion, direct destruction of the brain, or electrical depolarization of neurons, induces rapid loss of consciousness. Death occurs because of destruction of midbrain centers controlling cardiac and respiratory activity or as a result of adjunctive methods (e.g., exsanguinations or “loss of blood”) used to kill the animal. Exaggerated muscular activity can follow loss of consciousness and, although this may disturb some observers, the animal is not experiencing pain or distress.

Classification of Methods

  • Acceptable - (Methods that consistently produce a humane death when used as the sole means of euthanasia)
  • Conditionally acceptable - Those methods that by the nature of the technique or because of greater potential for operator error or safety hazards might not consistently produce humane death or the methods are not well documented in the scientific literature.
  • Unacceptable - those methods deemed inhumane under any conditions or that pose a substantial risk to the human applying the technique.
  • Adjunctive methods of euthanasia - Methods that cannot be used as the sole means of euthanasia but may be used in conjunction with other acceptable methods to insure death.



  • This chemical may be placed in water for euthanasia of fish and amphibians.
  • A concentration of at least 250 mg/L is recommended. The animal should be left in the solution for at least 10 minutes following cessation of respiratory movements.
  • MS 222 is acidic and in concentrations of 500 mg/L or higher should be buffered with sodium bicarbonate to saturation (pH 7.0 - 7.5).
  • There is a 21-day withdrawal time for MS 222 in the US. It should not be used for animals intended for food.


Sodium pentobarbital (60-100 mg/kg) can be administered intravenously, intra-abdominally or intra-pleuroperitoneally in most ectothermic animals.

  • Time to effect is variable (up to 30 minutes).
  • Inhalant anesthetics
  • Many reptiles and amphibians, including chelonians, can hold their breath and survive long periods without oxygen. Because of this, the use of inhalant anesthetics for euthanasia is not recommended for these species.
  • Lizards, snakes and fish do not hold their breath and can be euthanized using inhalant agents.


  • Loss of consciousness develops rapidly, but exposure times required for euthanasia are prolonged.
  • CO2 is more effective in active species and those with fewer tendencies to hold their breath.
  • For fish, CO2 can be introduced via Alka-Seltzer tablets. The number of tablets depends of the fish. For large fish, such as Oscars, as much as 1/2 a box of Alka-Seltzer (per 10 gallons of water) may be required, being careful to drop the tablets (one at a time) so that the fish does not inhale the fizz.

BENZOCAINE HCL (via immersion)

  • This chemical may be placed in water for euthanasia of fish or amphibians.
  • A concentration of at least 250 mg/L should be used. Animals should be left in the solution for at least 10 minutes following cessation of respiratory movement.
  • Benzocaine (not the hydrochloride) is not water-soluble and is not recommended.

Conditionally Acceptable

  1. Decapitation
  2. Pithing (puncturing the brain with a sharp object such as an ice pick)
  3. Stunning



There is no evidence that whole body cooling reduces pain or is clinically efficacious when used as an adjunct to physical methods of euthanasia in ectothermic animals. Freezing of un-anesthetized animals is not acceptable as a method of euthanasia. Rapid freezing (in liquid nitrogen) of deeply anesthetized animals is acceptable.


Air embolism may be accompanied by convulsions, opisthotonos, and vocalization. If used, it should be done only in anesthetized animals.


Unacceptable practice for most species.


Chemical or thermal burning of an animal is not an acceptable method of euthanasia.


Chloroform is a known hepatotoxin and suspected carcinogen and, therefore, is extremely hazardous to personnel.


Cyanide poses an extreme danger to personnel and the manner of death is aesthetically objectionable.


Decompression is unacceptable for euthanasia because of numerous disadvantages. Many chambers are designed to produce decompression at a rate 15 to 60 times faster than that recommended as optimum for animals, resulting in pain and distress attributable to expanding gases trapped in body cavities. Immature animals are tolerant of hypoxia, and longer periods of decompression are required before respiration ceases. Accidental recompression, with recovery of injured animals, can occur. Bleeding, vomiting, convulsions, urination, and defecation, which are aesthetically unpleasant, may develop in unconscious animals.


Drowning is not a means of euthanasia and is inhumane.


Because of the anxiety associated with extreme hypovolemia, exsanguinations - should be done only in sedated, stunned, or anesthetized animals.


Direct immersion of an animal into formalin, as a means of euthanasia, is inhumane.


Household products and solvents Acetone, quaternary compounds (including CCl4), laxatives, clove oil, dimethylketone, and quaternary ammonium products*, antacids, and other commercial and household products or solvents are not acceptable agents for euthanasia.


Hypothermia is not an appropriate method of euthanasia.


When used alone, these drugs all cause respiratory arrest before loss of consciousness, so the animal may perceive pain and distress after it is immobilized. (Nicotine, magnesium sulfate, potassium chloride, all curariform agents)


Rapid freezing as a sole means of euthanasia is not considered to be humane. If used, animals should be anesthetized prior to freezing.


Strychnine causes violent convulsions and painful muscle contractions.


Stunning may render an animal unconscious, but it is not a method of euthanasia



Stunning and pithing, when properly done, induce loss of consciousness but do not ensure death. Therefore, these methods must be used only in conjunction with other procedures, such as pharmacologic agents, Exsanguination, or decapitation to euthanatize the animal.


Exsanguination can be used to ensure death subsequent to stunning, or in otherwise unconscious animals. Because anxiety is associated with extreme hypovolemia, exsanguination must not be used as a sole means of euthanasia. Animals may be exsanguinated to obtain blood products, but only when they are sedated, stunned, or anesthetized.


Animals may be stunned by a blow to the head, by use of a non-penetrating captive bolt, or by use of electric current. Stunning must be followed immediately by a method that ensures death. With stunning, evaluating loss of consciousness is difficult, but it is usually associated with a loss of the menace or blink response, pupillary dilatation, and a loss of coordinated movements.


Stunning by a blow to the head is used primarily in small laboratory animals with thin craniums.9,173-175 A single sharp blow must be delivered to the central skull bones with sufficient force to produce immediate depression of the central nervous system. When properly done, consciousness is lost rapidly.


Alternating electrical current has been used for stunning species such as dogs, cattle, sheep, goats, hogs, fish and chickens. Experiments with dogs have identified a need to direct the electrical current through the brain to induce rapid loss of consciousness. In dogs, when electricity passes only between fore- and hind limbs or neck and feet, it causes the heart to fibrillate but does not induce sudden loss of consciousness.139 For electrical stunning of any animal, an apparatus that applies electrodes to opposite sides of the head, or in another way directs electrical current immediately through the brain, is necessary to induce rapid loss of consciousness. Attachment of electrodes and animal restraint can pose problems with this form of stunning. Signs of effective electrical stunning are extension of the limbs, opisthotonos, downward rotation of the eyeballs, and tonic spasm changing to clonic spasm, with eventual muscle flaccidity. Electrical stunning should be followed promptly by electrically induced cardiac fibrillation, exsanguination, or other appropriate methods to ensure death.


In general, pithing is used as an adjunctive procedure to ensure death in an animal that has been rendered unconscious by other means. For some species, such as frogs, with anatomic features that facilitate easy access to the central nervous system, pithing may be used as a sole means of euthanasia, but an anesthetic overdose is a more suitable method.