Learning objectives: Learn which veterinary anesthetics can be used with fish, how are they administered and at what dosages, and how to monitor depth of anesthesia by assessing the fish’s heart rate, pulse and respiration. 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. 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 the 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 be 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 of the anesthetic solution. An aquarium air pump with and air stone should be placed into the water to circulate it to maintain adequate oxygen level, especially with a large fish such as koi. The water should be tested to ensure all the water quality parameters are in the correct range for the fish species.
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 in large fish by attaching the monitor clips to hypodermic needles placed into the muscles on either side of the body by the pectoral fins. This will create a 2-lead ECG that will show the heart rate of the fish. It is important to get a baseline heart rate and monitor for slowing, rather than to see if the heart stops, as the heart in fish can continue to beat long after the fish is dead! 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.
Cardiac arrest - death. Recuperation after anesthesia is accomplished by transferring the fish into a container of fresh, well-aerated water without any anesthetic. It is helpful to move the air pump and air stone to the recovery container to continue to aerate the water. 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.
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