SRAC Publication No. 3900 VI November 2004 PR Anesthetics in Aquaculture Shawn D. Coyle, Robert M. Durborow and James H. Tidwell* Fish are easily stressed by han- (Table 1). The stage achieved usu- The condition of the animals must dling and transport and stress can ally depends on the dose and the be visually monitored during this result in immuno-suppression, length of exposure. When an anes- maintenance period. A change in physical injury, or even death. In thetic is first administered (induc- breathing rate is the most obvious aquaculture, anesthetics are used tion) fish may become hyperactive indicator of over-exposure. If this during transportation to prevent for a few seconds. occurs, animals must be moved or physical injury and reduce metab- the systems flushed immediately. olism (DO consumption and excre- Maintenance tion). They are also used to immo- Recovery bilize fish so they can be handled Once the desired degree of anes- more easily during harvesting, thesia is reached, it may be desir- During the recovery stage the anes- sampling and spawning proce- able to maintain fish in that state thetic is withdrawn and fish return dures. for some time. Because drug dose to a normal state. To reduce recov- and exposure time are often ery time, induction should be rapid An ideal anesthetic should induce cumulative, it is difficult to main- and handling time should be mini- anesthesia rapidly with minimum tain a uniform depth of anesthe- mal. Initial recovery may take from hyperactivity or stress. It should sia. One reason for this is that lev- a few seconds to several minutes, be easy to administer and should els of anesthetic may continue to depending on the anesthetic admin- maintain the animal in the chosen accumulate in the brain and mus- istered. Typically, the animal will state. When the animal is removed cle even after blood levels have attempt to right itself and will begin from the anesthetic, recovery attained equilibrium. A desired to respond to noise and other sen- should be rapid. The anesthetic level of anesthesia can usually be sory stimuli. Full recovery can take should be effective at low doses maintained by reducing the minutes to hours, depending on the and the toxic dose should greatly dosage. species and drug used. exceed the effective dose so that there is a wide margin of safety. Table 1. Stages of anesthesia in fish. Stages of anesthesia Stage Condition Behavior/Response Induction I Sedation Motion & breathing reduced Most anesthetics can produce sev- II Anesthesia Partial loss of equilibrium eral levels or stages of anesthesia. Reactive to touch stimuli Stages include sedation, anesthe- sia, surgical anesthesia and death III Surgical anesthesia Total loss of equilibrium No reaction to touch stimuli IV Death Breathing & heart beat stop *Kentucky State University Aquaculture Overdose - eventual death Research Center. Great care should be taken during Factors affecting Anesthesia of fish the recovery stage to minimize anesthesia stress and prevent mortality. If an Fish are usually anesthetized by animal fails to recover, increasing Many factors affect the efficacy of immersing them in an anesthetic the flow of anesthetic-free water anesthetics in fish. These can be bath containing a suitable concen- over the gills will often accelerate divided into biological and envi- tration of drug so that the drug is and normalize the heart beat. ronmental factors. absorbed through the gills and rapidly enters the blood stream. Move the fish backwards and for- Often, the rate at which anesthetic wards in the recovery bath or gen- The simplest procedure is to pre- drugs become effective is related to pare the required drug concentra- tly pass water over the gills with a the gill area to body weight ratio, hose. This increases gill blood tion in an aerated container and which can vary considerably quickly but gently transfer the fish flow and eliminates the drug more among fish species. Aquatic species rapidly. to the container. The anesthetic also have different metabolic rates bath and recovery tank should use Legal aspects that affect the rate at which chemi- water (at a similar temperature cals are absorbed and anesthesia is and chemistry) from which the Many chemical anesthetics have induced. For example, cold-water animals originated. Water quality been used on fish over the years. species seem to respond to lower needs to be carefully controlled, Most have now been discarded or concentrations of anesthetic than especially where large numbers of are not widely used. The U.S. Food warm-water species. animals are being handled and and Drug Administration (FDA) There are also factors that can baths are being reused. Main con- regulates which chemicals can be affect anesthesia within a particu- cerns involve maintaining proper used on food fish. When fish are lar species. Larger individuals temperature, adequate dissolved exposed to an anesthetic, residues generally require a greater concen- oxygen, low ammonia and a mini- or metabolites of the substance tration of anesthetic than smaller mum amount of fecal matter. remain in the flesh for a period of individuals. In contrast, it has also Applying an anesthetic solution to time until they are excreted or been reported that the larger, the gills with a spray bottle can be metabolized. Therefore, FDA may more active fish in a group are useful with large animals or if require a specific withdrawal time anesthetized faster than smaller immersion is impractical. A 100- before the animal can be used for ones. Many drugs such as MS-222 to 200-mg/L solution of MS-222 is food or released into the environ- and benzocaine are fat-soluble; reported to be effective when ment where it might be captured therefore, in larger fish or gravid applied to the gills of salmonid for food. females, anesthesia may last broodstock. This method allows Anesthetics are licensed for use in longer and recovery may be slow- the fish to be handled without food animals only after complet- er as the drug is removed from immersion, and it has no effect on ing a full drug development pro- the lipid reserves. Also, diseased subsequent egg hatching success. gram designed to protect the cul- or weakened animals are much tured animals, human users, the more susceptible to anesthetic Anesthesia of treatment. food chain, and the environment. aquatic invertebrates The program requires a wide Environmental factors can also range of inputs from the drug profoundly affect the efficacy of Less is known about anesthetizing company, research scientists, certain anesthetics. Aquatic inver- invertebrates because it is not national agencies, and the farming tebrates and fish are ectotherms; done as often. Most operations in and feed industries. Licensing a their body temperature closely fol- crustacean culture can be conduct- new drug is time consuming and lows that of their environment. As ed without anesthesia, although costly. Aquaculture is an impor- a result, physicochemical passage the rapid movement of shrimp tant industry worldwide, but it is of the drug into the fish is also can present handling problems still relatively small compared to temperature related. At lower and their cannibalistic nature can other animal production indus- water temperatures, higher doses be a problem during holding and tries and the human medical or longer exposure times are transporting. Consequently, there industry. For this reason, drug required with MS-222, benzocaine has been some interest in investi- companies have not been able to and 2-phenoxyethanol, presum- gating crustacean anesthetics, par- justify the costs of licensing new ably because the absorption rate ticularly for transport. Crustaceans drugs because the expected finan- decreases at lower temperatures. respond differently to anesthesia cial return is low. The only anes- The pH of an anesthetic solution than finfish, possibly because thetic drug currently approved by also can influence its efficacy, pos- their synaptic receptor sites are the FDA for use on food fish is tri- sibly by affecting the ratio of not affected by certain anesthetics. caine methanesulfonate (MS-222). charged to uncharged molecules. For example, MS-222 is not effec- This is most pronounced with tive on many crustaceans. It seems quinaldine, which loses its efficacy that much higher concentrations in solutions with low pH. are required to anesthetize crus- taceans than fish. Aqui-S has 100 mg/L is required for some hardness or pH. As with MS-222, it been reported to be effective on species, including tilapia. is fat-soluble and recovery times freshwater prawns (Macrobrachium Generally, concentrations greater can be prolonged in older fish or rosenbergii), but only at concentra- than 100 mg/L should not be gravid females. Benzocaine is not tions 5 to 10 times higher (100 to used for salmonids, and levels approved by FDA for use on food 200 mg/L) than those used on fin- higher than 250 mg/L should not fish in the U. S. fish (20 mg/L). Carbon dioxide is be used for warm-water fish. Quinaldine an effective anesthetic for most Recovery is usually rapid and crustaceans. It is most frequently equilibrium can be expected to Quinaldine is a yellowish, oily liq- dispensed as a mixture of baking return after only a few minutes. A uid with limited water solubility soda and acetic acid. Cooling is recovery time longer than 10 min- that must be dissolved in acetone also an effective way to immobi- utes suggests that too much anes- or alcohol before it is mixed with lize crustaceans, but one must be thetic is being used or that the water. While it is an effective careful because cooling can kill the exposure time is too long. MS-222 anesthetic, it is an irritant to fish, animals. has a good safety margin in fish.