Anaesthesia And Anaesthetics For The Controlled Management Of Fresh Water Fishes Aquatic Fish Database est. 1991

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Anaesthesia And Anaesthetics For The Controlled Management Of Fresh Water Fishes

Sajan Sajeevan

Research Scholar, Faculty of Science, Mahatma Gandhi University, Kerala, 686560, India. Correspondence:


Anesthesia is a state caused by an applied external agent resulting in a loss of sensation through depression of the nervous system. The use of anesthetics in fisheries and aquaculture research greatly facilitates in procedures including induction of breeding, handling during stripping and transport of broodstock. Anesthesia and sedation is usually essential to minimize stress and physical damage in handling the fish for routine operations (Summerfelt and Smith, 1990; Iwama et al, 1997; Ross et al, 1999). Although the use of anesthetics is primarily for the purpose of holding fish immobile while the animal is being handled for sampling, anesthetics are also used to lower the level of stress associated with such procedures.

When choosing an anesthetic a number of considerations are important such as its efficacy, cost, availability, ease of use and side effects on fish, humans and the environment (Marking and Meyer, 1985). Overdosing of an anesthetic or retaining the fish in an anesthetic bath for too long leads to the fading of ventilation, hypoxia, and finally, respiratory and cardiac collapse (Tytler and Hawkins, 1981). A wide variety of compounds have been utilized to anesthetize fish during artificial propagation techniques. The use of anesthetics in fish has spanned more than the last five decades and many chemicals (MS-222, benzocaine, quinaldine, chlorobutanol, phenoxyethanol, metomidate etc.) have been employed in fresh water fishes. Stages of anesthetization included induction, maintenance and recovery.

Induction of anesthesia

During anaesthetization anaesthetic agents are inhaled through the gills and rapidly enter the blood. From there they are transported to the central nervous system and excreted via the gills upon the fish's return to fresh water. They work by inducing a calming effect followed by a successive loss of equilibrium, mobility, consciousness, and reflex action. Respiratory and cardiac failure follows overdose or exposure. The induction time is the period from the time when an experimental fish is placed in the anesthetic tank to the time it does not respond to external stimuli. The lowest effective concentration is the concentration that produces general anaesthesia (Stage IV of Anesthesia) within 3 min and allows the recovery within 10 min (Gilderhus, 1990; Weyl et al, 1996). The stages achieved usually depend on the dose and the length of exposure.

For the anaesthetic procedure fishes were netted from the holding tank and placed in experimental tank mixed with the anesthetic solution. When a fish become anaesthetized, it was immediately taken out and then put into a recovery tank with fresh aerated water. Each fish was used only once and then subjected to monitoring for any adverse effect for another 24 hours. There is a considerable variation in the response to anaesthetics which can result from individual metabolic differences, temperatures and physical condition. By careful observation you will minimized the possibility of an overdose. It is important to observe the fish at all times and if necessary to terminate the induction. It is important that anaesthetic and recovery tanks were prepared ahead of time

During anesthesia a number of guidelines were followed such as; Stopped feeding 24-hour before the experiment, properly aerated anesthetic baths, same temperature was maintained in bath as well as in holding tanks and thoroughly aerated recovery bath. It is critical to monitor water quality in order to reduce anesthetic mortality; Assuming aeration, DO, pH, and temperature are appropriate, the important one is ammonia concentrations.

Measures of Anaesthesia

Stages of anaesthetization include induction, maintenance and recovery. A maximum duration from initial anaesthetic exposure to induction (stage IV) and the induction stage achieved usually depends on the dose and the length of exposure. Generally, an ideal anaesthetic should produce anesthesia rapidly (e.g., less than 3 or 5 min), allow a speedy recovery, not be toxic to fish and users, leave low tissue residues, and be inexpensive (Marking and Meyer, 1985; Gilderhus and Marking, 1987). The anaesthetic induction time is the period from the time when an experimental fish is placed in the anaesthetic tank until the time it does not respond to external stimuli. The lowest effective concentration is the concentration that produces general anaesthesia within 3 min and allows the recovery within 10 min (Gilderhus, 1990; Weyl et al, 1996).

Stages of anesthesia and recovery (Iwama et al., 1989)

Stages of Anesthesia





Loss of equilibrium

Loss of gross body movements but with continued opercular movements

As in Stage II with cessation of opercular movements

Stages of Recovery





Body immobilized but opercular movements just starting

Regular opercular movements and gross body movements beginning

Equilibrium regained and pre anaesthetic appearance

Recovery of anesthesia

Recovery occurs when the fish is placed in anesthetic-free water after induction process. In most cases the fish will be treated near to the tank or pond and can be returned to its home for recovery. If this is not the case a well-aerated recovery tank should be prepared in advance. The recovery time is the period from the time when an anesthetized fish is placed in a recovery tank to the time it recovers from anesthetization with full equilibrium motion. Initial recovery took a few seconds to minutes, depending on the concentration of anesthetic administered.

Most fish fully recover from inhalant anesthetics within 5 minutes; recoveries extending more than 10 minutes indicate an excessive anesthetic dosage or a compromised animal (Ross, 2001). As the fish recovers, respiration increases, muscle tone returns, fin movements resume and the fish swims progressively until it regains full equilibrium. If there is no sign of recovery within a minute or so, it will be necessary to try and get water moving over the gill filaments. Alternatively the fish can be held in the water stream which will force oxygenated water over the gills. The fish should be monitored for 24-hours for any adverse effects.

Commonly used anaesthetics

  1. Tricaine methanesulfonate

TMS (MS-222), [3-aminobenzoic acidethyl ester methanesulfonate] is the most widely used fish anesthetic, and it is extremely effective for rapid induction of deep anesthesia. It is sold as a powder which readily mixes with water. It may be prudent to check the pH of the water after addition of MS-222 to ascertain adequate buffering has been achieved. It is a white crystalline powder that is easily dissolved in water, with a solubility of 1.25 g/mL water, at 20 oC. Dose is related to species, size and density of the fish, as well as water temperature and hardness, but in general, anesthetic doses are usually between 25 to 100 mg/L. TMS is also known as MS-222, TM18Finquel, Tricaine, tricaine methanesulfonate and Metacaine.

  1. Benzocaine

Benzocaine [p-aminobenzoic acid ethyl ester] has two forms: a crystalline salt with water solubility of 0.4 g/L, or a freebase fo The efficacy of benzocaine has been shown to be affected by the size of the fish, where the smallest fish require the lowest dose, as well as by the temperature of the water (Gilderhus, 1989). Reported doses range from 25 B 100 mg/L with doses for salmonids falling in the range between 25 B 45 mg/L (Gilderhus, 1989). Benzocaine must be dissolved in ethyl alcohol first at 0.2 g/mL. Benzocaine is also known as TM1Anesthesin, TM14Anesthone, TM2Americaine, ethyl aminobenzoate, Orthesin and Parathesin.

  1. 2-Phenoxyethanol

2-Phenoxyethanol (2-PE) [1-hydroxy-2-phenoxyethane] is a colourless, oily, aromatic liquid with a burning taste, and has solubility in water of 27 g/L at 20 oC (Merck and Company, 1989). The efficacy of 2-PE varies with the size of the fish and with the temperature of the water (Sehdev et al., 1963). While the effective dosage for salmonids is in the range of 200 B 300 μL/L, the lethal dose is as low as 500 μL/L, which leaves little margin for safety. 2-phenoxyethanol is also known by the names phenyl cellosolve, phenoxethol, phenoxetol, ethylene glycol monophenyl ether, and beta-hydroxyethyl phenyl ether.

  1. Isoflurane

Isoflurane may be dissolved in water by injecting the solution through a fine gauge needle underwater and mixing. Isoflurane is utilized for human anesthesia in a vaporized form. It is very safe and has been utilized for extensive surgical procedures in koi. It is a controlled substance and expensive. While safe and effective, more economical alternatives are available to immobilize koi for venipuncture

  1. Clove oil

Clove oil, as commonly sold, varies from lot to lot in the strength and composition of its components. The major active ingredients are 85-95% eugenol with the balance isoeugenol and methyleugenol. It is inexpensive and readily available without a prescription. A starting dose might be 0.5 ml per gallon of water. As clove oil does not mix readily with water, steps must be taken to ensure the oil is evenly mixed with the water in the anesthesia water. Some put a measured amount of clove oil in a vinyl bag, add water and shake vigorously. This is then mixed with the rest of the water in the anesthesia tub. Another approach is to mix the clove oil with ethanol then add it to the water in the anesthesia container.

  1. Propoxate

Propoxate [propyl-DL-1-(phenylethyl) imidazole-5-carboxylate hydrochloride] is a crystalline powder which resembles metomidate and etomidate structurally, and is freely soluble in both fresh water and salt water. It is stable in solution for long periods and is 100 times more soluble than TMS (Thienpont & Niemegeers, 1965). Propoxate is 10 times more potent than TMS. Effective concentrations range from 0.5 mg/L to 10 mg/L (Summerfelt & Smith, 1990). A level of 0.25 mg/L is safe for anesthesia of lengths up to 16 hours. Ross & Ross (1984) recommend a dose of between 1 and 4 mg/L to anaesthetize fish resulting in induction times ranging from 30 seconds for higher doses.

  1. Metomidate and Etomidate

Metomidate [1-(1-phenylethyl)-1H-imidazole-5-carboxylic acid methyl ester] is a watersoluble powder which has the properties of a hypnotic, or sleep-inducing, drug. Etomidate [1-(1-phenylethyl)-1H-imidazole-5-carboxylic acid ethyl ester] is a colourless, odourless crystalline analogue of metomidate and propoxate (Merck and Company, 1989). It has been used on humans as a hypnotic drug, but it is very expensive and difficult to obtain (Bell, 1987). Efficient dosages range from 1 B 10 mg/L (Olsen et al., 1995).

  1. Lidocaine

Lidocaine [2-(diethylamino)-N-(2,6-dimethylphenyl) acetimide], in freebase form, is insoluble in water, but freely soluble in acetone or alcohol. It is generally used in the hydrochloride salt form which is freely soluble in water (Merck and Company, 1989). Lidocaine has been used in combination with sodium bicarbonate to anaesthetize carp (Cyprinus carpio), tilapia (Oreochromis/Tilapia mossambica) and catfish (Ictalurus punctatus). The addition of sodium bicarbonate, at 1 g/L, has been demonstrated to enhance the anaesthetic effects of lidocaine. Without the addition of bicarbonate, there are huge variations in required doses. For example, tilapia required in excess of 800% more lidocaine than carp when it was administered in the absence of sodium bicarbonate.


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