Fish are easily stressed by handling and transport and stress can result in immuno-suppression, physical injury, or even death. In aquaculture, anesthetics are used during transportation to prevent physical injury and reduce metabolism (DO consumption and excretion).
They are also used to immobilize fish so they can be handled more easily during harvesting, sampling and spawning procedures. An ideal anesthetic should induce anesthesia rapidly with minimum hyperactivity or stress.
It should be easy to administer and should maintain the animal in the chosen state. When the animal is removed from the anesthetic, recovery should be rapid. The anesthetic should be effective at low doses and the toxic dose should greatly exceed the effective dose so that there is a wide margin of safety.
Stages of anesthesia:
Induction:
Most anesthetics can produce several levels or stages of anesthesia. Stages include sedation, anesthesia, surgical anesthesia and death (Table 1). The stage achieved usually depends on the dose and the length of exposure. When an anesthetic is first administered (induction) fish may become hyperactive for a few seconds.
Maintenance:
Once the desired degree of anesthesia is reached, it may be desirable to maintain fish in that state for some time. Because drug dose and exposure time are often cumulative, it is difficult to maintain a uniform depth of anesthesia.
One reason for this is that levels of anesthetic may continue to accumulate in the brain and muscle even after blood levels have attained equilibrium. A desired level of anesthesia can usually be maintained by reducing the dosage.
The condition of the animals must be visually monitored during this maintenance period. A change in breathing rate is the most obvious indicator of over-exposure. If this occurs, animals must be moved or the systems flushed immediately.
Recovery:
During the recovery stage the anesthetic is withdrawn and fish return to a normal state. To reduce recovery time, induction should be rapid and handling time should be minimal. Initial recovery may take from a few seconds to several minutes, depending on the anesthetic administered.
Typically, the animal will attempt to right itself and will begin to respond to noise and other sensory stimuli. Full recovery can take minutes to hours, depending on the species and drug used.
Great care should be taken during the recovery stage to minimize stress and prevent mortality. If an animal fails to recover, increasing the flow of anesthetic-free water over the gills will often accelerate and normalize the heart beat.
Move the fish backwards and forwards in the recovery bath or gently pass water over the gills with a hose. This increases gill blood flow and eliminates the drug more rapidly.
Legal aspects:
Many chemical anesthetics have been used on fish over the years. Most have now been discarded or are not widely used. The U.S. Food and Drug Administration (FDA) regulates which chemicals can be used on food fish.
When fish are exposed to an anesthetic, residues or metabolites of the substance remain in the flesh for a period of time until they are excreted or metabolized. Therefore, FDA may require a specific withdrawal time before the animal can be used for food or released into the environment where it might be captured for food.
Anesthetics are licensed for use in food animals only after completing a full drug development program designed to protect the cultured animals, human users, the food chain, and the environment.
The program requires a wide range of inputs from the drug company, research scientists, national agencies, and the farming and feed industries. Licensing a new drug is time consuming and costly.
Aquaculture is an important industry worldwide, but it is still relatively small compared to other animal production industries and the human medical industry. For this reason, drug companies have not been able to justify the costs of licensing new drugs because the expected financial return is low.
The only anesthetic drug currently approved by the FDA for use on food fish is tricaine methanesulfonate (MS-222).
Authors:
Shawn D. Coyle, Robert M. Durborow and James H. Tidwell
