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Euthanasia in Fish

By: Dr. Craig Harms

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Euthanasia is the bringing about of a gentle and easy death, without pain or distress, usually for an animal suffering from a terminal disease. The word literally means an "easy and painless death." An appropriate euthanasia technique should provide rapid unconsciousness followed by cardiac or respiratory arrest and ultimate loss of brain function.

In a culture where fish often meet their demise after a brief struggle at the end of a hook and line followed by slow suffocation out of water, euthanasia is not always a prime consideration for ending a fish's life, until the fish in question is an accepted family pet. The subject of fish euthanasia has received greater attention in laboratory animal settings than in veterinary private practice.

Euthanasia can be accomplished by an overdose of any anesthetic agent used for fish. These cause death by direct depression of the brain and vital centers. Larger fish that cannot easily be transferred to a bath treatment may have the anesthesia solution poured directly over the gills.

In cold-blooded animals, including fish, the heart may continue to beat for long periods after brain function has ceased, allowing for partial and somewhat distressing recoveries later if the fish is kept moist and cool. To be certain the euthanasia is complete, once the fish is deeply anesthetized and insensible, cranial concussion (a sharp blow to the head), decapitation (removal of the head), or exsanguination (bleeding out) may be performed to ensure death if heart function cannot be monitored.

Accepted Methods of Euthanasia

The American Veterinary Medical Association (AVMA) Panel on Euthanasia lists three acceptable methods of euthanasia for fish, and two conditionally acceptable methods. Methods listed as acceptable are overdoses with the anesthetics tricaine methanesulfonate (MS-222), benzocaine (related to MS-222 but less soluble in water) and barbiturates. Methods listed as conditionally acceptable are stunning (by a blow to the head) and decapitation in combination, or decapitation alone. Of the three methods listed as acceptable, tricaine and benzocaine are immersion anesthetics, while barbiturates are administered by intravenous injection. Overdoses of other anesthetics in water, such as eugenol (active ingredient of clove oil) and isoflurane (a volatile anesthetic used for gas anesthesia of terrestrial animals) will produce similar effects.

Hypothermia (chilling, freezing) is not considered to be humane when used as the sole method of euthanasia because the animal is not rendered rapidly insensible to pain or distress. However, freezing is an effective method of ensuring death of a fish and may be employed after the fish is insensible from an anesthetic overdose.

Euthanasia of Fish at Home

Bringing a fish to a veterinarian for euthanasia with standard anesthetics or euthanasia agents may not be practical in all circumstances. In these instances it is useful to have humane, rapid and effective alternatives to the toilet bowl. While flushing into the sewer system is almost invariably an effective means of killing a fish by exposure to intolerable water quality conditions, it is not necessarily rapid, and could pose the risk of introducing fish diseases to the local watershed. As mentioned above, stunning and decapitation together, skillfully applied, is considered a conditionally acceptable method of euthanasia for fish, in that it results in rapid and direct depression of the brain due to hypoxia. However, it is not necessarily an acceptable means when applied to one's own dying pet.

Clove oil, active ingredient eugenol, is being used increasingly in fish anesthesia, and is available from specialty food outlets and many pharmacies. Although it is not completely water soluble without first dissolving in ethanol (grain alcohol), particularly in cold water, enough dissolves to result in anesthesia and subsequent death at doses greater than 0.25 ml per liter of water (about 1 ml per gallon, or 1/4 teaspoon per gallon). Two anesthetic compounds which have been used in fish, but which are not recommended for clinical use due to difficulty in controlling anesthetic level and metabolic derangements they induce, are carbon dioxide and ethanol. Both of these compounds are often available in households when other potential fish anesthetic/euthanasia agents are not.

Carbon dioxide gas is listed as an acceptable method of euthanasia for terrestrial animals, causing death by direct depression of the brain and vital centers, but is not generally considered for aquatic applications. Carbon dioxide is released when Alka-Seltzer® tablets dissolve in water, and euthanasia can be induced with 2 or more tablets per liter (8 tablets per gallon). Ethanol (not rubbing alcohol, which contains a high percentage of isopropyl alcohol) can be used for euthanasia at a dose of 30 ml pure grain alcohol per liter of water (about 1/2 cup per gallon; note that ethanol content in beverages for human consumption is listed as "proof," with 100 proof being 50 percent ethanol, 200 proof being 100 percent ethanol, and adjust dosage accordingly). Be aware that fish undergoing immersion anesthesia may experience a brief excitement phase as inhibitory neurons are depressed prior to achieving complete anesthesia. After the fish is insensible, these methods may be followed by decapitation or freezing to ensure death.

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