Learn which veterinary anesthetics can be used with fish, how are they administered and at what dosages, how fish recover from anesthesia, and what anesthetics are used for euthanasia in fish. Learn methods to monitor fish under anesthesia to determine their heart rate, pulse and depth of anesthesia.
Sedation aids in handling fish during physical examination, for biopsy sampling, or for purposes such as egg stripping during artificial spawning. Anesthesia and analgesia are required for surgical or invasive procedures. Surgery can be performed on anesthetized fish to repair wounds, remove skin and fin tumors, or to remove abdominal masses [See Surgery in Fish]. And sometimes euthanasia is needed to end the suffering of a sick or injured animal, or for research or other purposes. Each of these techniques can be accomplished with fish by adding anesthetic medications to the water, and sometimes by injection or oral administration of anesthetics. Food fish have specific limitations to medications that can be used with them, and withdrawal times for approved medications must be observed. This paper will focus on anesthetics used for ornamental and pet fish.
Many chemicals have been used to induce tranquilization or anesthesia in fish. All have some element of risk, but, when used carefully, they have successfully induced sedation or anesthesia. Anesthetic agents used in lower doses produce tranquilization, and at higher doses they are used for anesthesia purposes. Care must be taken not to overdose the fish, or leave them anesthetized too deeply for too long of time. It is recommended to start with a lower dose and add more as needed if using a new drug or working with an unfamiliar species of fish. Monitor heart rate, blood oxygen concentration, and operculum (gill cover) motion during anesthesia to ensure fish is not too deeply anesthetized.
Most fish anesthetics are added to clean, well-oxygenated water in a suitable glass or plastic container. The water is thoroughly mixed to ensure all the chemical is dissolved and dispersed evenly. The anesthetic solution should have the same temperature and pH as the aquarium or pond water. Use a thermometer to monitor the water temperature during surgery, and if an oxygen meter is available, also monitor the dissolved oxygen concentration. A pulse oximeter can be clipped onto the caudal fin of large fish such as koi, near the tail base, in order to monitor the pulse and blood oxygen concentration. ECG monitors can also be used by attaching the monitor clips to hypodermic needles placed into the muscles on either side of the body by the pectoral fin.
Commonly Utilized Chemicals for Anesthesia and Tranquilization1-5
Benzocaine (Ethyl p-Aminobenzoate)
Dose at 12.5 mg/L of water for a shipping sedative, 25–500 mg/L for anesthesia (may need to dissolve in ethanol first).
Induction time in 1–3 minutes, recovery in fresh water in 3–15 minutes.
The longer the fish is under anesthesia, the longer it usually takes to recover.
Carbon Dioxide (CO2)
A dose of 100–400 mg/L bubbled through the water will cause unconsciousness, high exposure will cause death. Use with caution, under constant observation!
Use in ventilated area. Avoid breathing CO2 released from the water.
Induction is in 1–2 minutes and recovery in 5–10 minutes in fresh water.
Effective to use for euthanasia in absence of other anesthetic agents.
A sedative and muscle relaxant, used as a preanesthetic agent.
Can be injected intramuscularly at 0.1–0.5 mg/kg, or given orally at 1–4 mg/kg.
Ethanol (Ethyl Alcohol)
1% added to the water will produce sedation, 3% or more will result in euthanasia.
20 ml of 100 proof (50%) grain alcohol in 1 liter of water will produce a 1% solution.
Ether (Dimethyl Ether)
Dose at 10–15 ml/L water.
Induction occurs in 2–3 minutes, recovery in clean water in 2–3 minutes.
Highly explosive! Do not use near flames or sparks!
Eugenol/Isoeugenol (Clove Oil)
Eugenol: 1 drop = 0.029 ml = 28.6 mg
Use 30–60 mg/L (1–2 drops/L water). Mix vigorously. Induction occurs in 2–3 minutes. Excellent for short-duration physical examinations.
A dose of 4 drops per liter (114 mg/L) induces euthanasia in 10–60 minutes.
Isoflurane (1-Chloro-2,2,2-Trifluoroethyldifluoromethyl Ether)
Dose at 0.5–1 ml/L water for anesthesia. Euthanasia dose 4 ml/L.
Spray the required dose through a 25-gauge needle under the water while mixing.
Induction in 2–8 minutes, recovery in clean water in 3–30 minutes.
Dose at 1 g/L water, or 66-100 mg/kg injected intramuscularly.
Provides sedation and immobilization for handling or transportation.
Anesthesia induction dose is 1.5–2.5 mg/kg intravenously.
Induction time is 5 minutes, recovery in 60–75 minutes. Can be used as a sedative at 1 mg/L in the water.
Quinaldine Sulfate (2-Methylquinoline Sulfate)
Dose at 5–10 mg/L for sedation, 25–200 mg/L for anesthesia.
Induction in 2–6 minutes, recovery in fresh water in 5–20 minutes.
Acidifies low alkaline water, use sodium bicarbonate buffer in water as necessary.
Tricaine Methane Sulfonate, MS-222 (3-Aminobenzoic Acid Ethyl Ester)
Dose at 10–40 mg/L for sedation (handling/shipping).
Dose at 50–400 mg/L for anesthesia induction, 50-100 mg/L for maintenance.
Induction in 1–5 minutes, recovery in 3–15 minutes in clean water.
Acidifies water - buffer with equal volume of sodium bicarbonate or use hard water.
When placed into the container with the anesthetic in the water, the fish will gradually begin to lie on its side and the respiratory rate will slow as the chemical induces anesthesia. In some cases, there may be an excitatory stage, so the anesthetic chamber may need to be covered to prevent fish from jumping out. After the fish is anesthetized in the anesthetic bath, it can be removed from the water for short-term examination or diagnostic procedures. If the fish is removed for longer procedures, anesthetic solution can be dripped across the gills through an IV bag and drip line, by hand with a syringe, or with a recirculating water pump or aquarium filter powerhead. Have oxygenated fresh water on hand to syringe across the gills if the plane of anesthesia becomes too deep. Keep the body moist if out of the water for examination or surgery. Use ophthalmic ointment on the eyes to keep them from drying. Monitor the respiration rate (operculum movements) to assess the depth of anesthesia.
Recuperation after anesthesia is accomplished by transferring the fish into a container of fresh, well-aerated water without any anesthetic. Never leave a fish unattended while it is under anesthesia. Some large fish have a tendency to jump during induction or recovery from anesthesia. Moving the fish gently in a forward direction will aid the flow of fresh water across the gills, hastening anesthesia release from the gills. Do not slosh the fish back and forth in the water. Once there are steady operculum movements let the fish rest and gradually recover in a quiet, dim environment. The longer a fish is under anesthesia, the longer it will take to recover from the anesthetic. Monitor the fish until it has regained its equilibrium and is swimming normally and can be transferred back into the aquarium or pond.
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. The aim is to accomplish death for these animals rapidly with the minimum amount of pain and distress practicable.4
Euthanasia in pet fish is similar to anesthesia, but typically uses a higher dosage of anesthetic agent, and the fish is left in the solution for a longer time. We want to produce anesthesia with loss of consciousness, to avoid pain, followed by respiratory depression that will result in humane death if the fish is not removed from the anesthetic solution after unconsciousness and cessation of respiration has occurred. Leaving the fish in the euthanasia solution for an hour after respiration ceases is a practical criterion for ensuring death.
1. Loh R. Fish Vetting Medicines: Formulary of Fish Treatments. Perth, Australia: Richmond Loh Publishing; 2012.
2. Saint-Erne N. Advanced Koi Care. 2nd ed. Glendale, AZ: Erne Enterprises; 2010.
3. Bowser PR. Anesthetic options for fish. In: Gleed RD, Ludders JW, eds. Recent Advances in Veterinary Anesthesia and Analgesia: Companion Animals. Ithaca, NY: International Veterinary Information Service; 2001. www.ivis.org/advances/Anesthesia_Gleed/bowser/chapter_frm.asp?LA=1. (VIN editor: link access requires IVIS membership)
4. Members of the Panel on Euthanasia, AVMA Staff Consultants. AVMA Guidelines for the Euthanasia of Animals: 2013 Edition. Schaumburg, IL: American Veterinary Medical Association; 2013. www.avma.org/kb/policies/documents/euthanasia.pdf
5. Neiffer DL, Stamper MA. Fish sedation, anesthesia, analgesia, and euthanasia: considerations, methods, and types of drugs. ILAR J. 2009;50(4):343–360. http://vpresearch.louisiana.edu/sites/research/files/NeifferFishSedation.pdf