Monitoring anesthesia

The depth of anesthesia in fish can be monitored by observing the behavior of the fish in water.  Several factors can affect the stages of anesthesia such as the size and species of fish, the dose of anesthetic agent, and the quality of the water.

1.       MS 222

v      FDA approved for consumable fish but has a 21-day withdrawal time. 

v      Most small-medium fish require 100mg/l for surgical anesthesia.

v      Precautions:

Toxic with prolonged exposure to sunlight

ACIDIC, must be buffered to a neutral pH (approximately 7.0)

Adequate buffers include: imidazole, sodium hydrogen phosphate, or sodium hydroxide

2.       Benzocaine

v      Parent compound of MS222

v      Insoluble in water.  First dissolve 500 mg benzocaine in 5 ml ethanol or acetone; add 0.5 ml stock solution to one liter of water for 50 mg/l concentration.   

v      Some species (Tilapia) require higher concentrations of 100 ppm (100 mg/l)

v      Protect from sunlight.

v      Recovery may be prolonged

3.       Urethane

v      Wide margin of safety

v      Precautions:

Carcinogenic and leukopenic in people (never documented in fish)

Maintaining fish under general anesthesia

Remove the fish from solution and perform the procedure, or place the fish so that the body is raised out of the solution for the procedure while the gills remain in the anesthetic.  Recirculating systems are excellent ways to maintain general anesthesia. An inexpensive set up is described by Brown, 1987.  In this set up, they did not require additional aeration or temperature control, but build up of ammonia was directly related to age of solution and the number of fish that had been anesthetized.  MS222 was found to be stable and effective for up to 15 days after preparation.

Stage      Plane      Response

I                              Swimming erratically with some loss of equilibrium.  Opercular activity increased.

II                             Obvious loss of equilibrium.  Efforts to maintain an upright position with slow and aimless swimming.  Opercular activity decreased.

III           1              Loss of muscle tone.  Opercular activity increased. 

2              Swimming movements absent.  Opercular activity rapid and shallow.  No response to external stimulation.  Surgical plane of anesthesia.

3                     Opercular movements very shallow.  The fish may be easily revived by moving it through untreated water. 

IV                           Spasmodic over distension of the opercula. Cardiac failure ensues within a few minutes.  Revival is possible up to the point of cardiac failure.

Postanesthetic Care

The fish should be placed in untreated water in a holding tank.  The fish may be moved gently and slowly through the water to enhance opercular movement and to promote movement of untreated, oxygenated water over the gills.  Moving the fish too quickly through the water may damage gills.  The fish should be monitored frequently for at least 24 hours after recovery from anesthesia.  For lengthy anesthetic periods, recovery may be prolonged.

Additional anesthetic regimens and information may be obtained from:

1.       Brown, LA. Anesthesia and Restraint. In: Fish Medicine, ed. MK Stoskopf, WB Saunders, 1993, pp. 79-90. 

2.       Stoskopf, MK. Clinical Examination and Procedures; Surgery. In: Fish Medicine, ed. MK Stoskopf, WB Saunders, 1993, pp. 62-78, 91-97.

3.       Thurmon JC, WJ Tranquilli, and GJ Benson.  Anesthesia of Wild, Exotic, and Laboratory Animals. In: Lumb and Jones’ Veterinary Anesthesia, Third Edition, eds. JC Thurmon, WJ Tranquilli, and GJ Benson, Williams and Wilkins, 1996, pp 686-690. 

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