Many factors affect the efficacy of anesthetics in fish. These can be divided into biological and environmental factors. Often, the rate at which anesthetic drugs become effective is related to the gill area to body weight ratio, which can vary considerably among fish species.
Aquatic species also have different metabolic rates that affect the rate at which chemicals are absorbed and anesthesia is induced. For example, cold-water species seem to respond to lower concentrations of anesthetic than warm-water species.
There are also factors that can affect anesthesia within a particular species. Larger individuals generally require a greater concentration of anesthetic than smaller individuals. In contrast, it has also been reported that the larger, more active fish in a group are anesthetized faster than smaller ones.
Many drugs such as MS-222 and benzocaine are fat-soluble; therefore, in larger fish or gravid females, anesthesia may last longer and recovery may be slower as the drug is removed from the lipid reserves. Also, diseased or weakened animals are much more susceptible to anesthetic treatment.
Environmental factors can also profoundly affect the efficacy of certain anesthetics. Aquatic invertebrates and fish are ectotherms; their body temperature closely follows that of their environment. As a result, physicochemical passage of the drug into the fish is also temperature related.
At lower water temperatures, higher doses or longer exposure times are required with MS-222, benzocaine and 2-phenoxyethanol, presumably because the absorption rate decreases at lower temperatures.
The pH of an anesthetic solution also can influence its efficacy, possibly by affecting the ratio of charged to uncharged molecules. This is most pronounced with quinaldine, which loses its efficacy in solutions with low pH.
Anesthesia of fish:
Fish are usually anesthetized by immersing them in an anesthetic bath containing a suitable concentration of drug so that the drug is absorbed through the gills and rapidly enters the blood stream.
The simplest procedure is to prepare the required drug concentration in an aerated container and quickly but gently transfer the fish to the container. The anesthetic bath and recovery tank should use water (at a similar temperature and chemistry) from which the animals originated.
Water quality needs to be carefully controlled, especially where large numbers of animals are being handled and baths are being reused. Main concerns involve maintaining proper temperature, adequate dissolved oxygen, low ammonia and a minimum amount of fecal matter.
Applying an anesthetic solution to the gills with a spray bottle can be useful with large animals or if immersion is impractical. A 100- to 200-mg/L solution of MS-222 is reported to be effective when applied to the gills of salmonid broodstock. This method allows the fish to be handled without immersion, and it has no effect on subsequent egg hatching success.
Anesthesia of aquatic invertebrates:
Less is known about anesthetizing invertebrates because it is not done as often. Most operations in crustacean culture can be conducted without anesthesia, although the rapid movement of shrimp can present handling problems and their cannibalistic nature can be a problem during holding and transporting.
Consequently, there has been some interest in investigating crustacean anesthetics, particularly for transport. Crustaceans respond differently to anesthesia than finfish, possibly because their synaptic receptor sites are not affected by certain anesthetics.
For example, MS-222 is not effective on many crustaceans. It seems that much higher concentrations are required to anesthetize crustaceans than fish. Aqui-S has been reported to be effective on freshwater prawns (Macrobrachium rosenbergii), but only at concentrations 5 to 10 times higher (100 to 200 mg/L) than those used on finfish (20 mg/L).
Carbon dioxide is an effective anesthetic for most crustaceans. It is most frequently dispensed as a mixture of baking soda and acetic acid. Cooling is also an effective way to immobilize crustaceans, but one must be careful because cooling can kill the animals.
Authors:
Shawn D. Coyle, Robert M. Durborow and James H. Tidwell
