The use of tranquillizers has an important role in the artificial propagation of finfish. They are used for tranquillizing brood fish both in the hatchery and during transportation.
Tranquillizing brood fish has the following advantages:
the fishes are quietened and, hence, do not jump about; this considerably reduces their chances of injury;
the workers will find it comfortable and easy to handle very large brood fish; and
when stripped, the chances of their scattering the eggs are minimized.
The same brood fish may have to be tranquillized several times during the course of propagation; i.e., when they are sorted out, weighed or marked, during investigation of their maturity stage, at the time of administration of preparatory and decisive injections, and during stripping.
Two chemicals, Quinaldine and MS 222, are in common use as fish tranquillizers.
(a) Quinaldin (e) or Chinaldin (2–4 methylchinolin) This is a toxic liquid and must, therefore, be handled with extreme care. The fish are usually treated with it when they are held in a large volume of water, such as a large concrete tank or in the corner of a pond where the fish are herded together and kept in a net. The dilution rate is 1 part to 40 000, which would mean 25 cm3 of quinaldine in one m3 of water. If the treatment results in the irregular movement of the opercula of the fish, they should be immediately transferred to well oxygenated water without tranquillizer to save their lives.
(b) MS 222 (Sandoz, Basel, Switzerland) This is a very mild tranquillizer and fish easily recover from its effects even after a long stupor. For treating the fish with this chemical, it is mixed with water in a ratio of 1:10 000 (i.e., 10 g MS 222 in 100 1 water) and the fish are kept in it for about half an hour at a temperature of 20°–25°C. Since MS 222 is rather an expensive chemical, it must be used sparingly.
One hundred litres of this solution are generally prepared for one day's use, assuming 10 brood fish (5–8 kg each) are to be tranquillized at a time. The same solution can be used for tranquillizing a series of fish, provided it does not become diluted by additional water in the process. Sometimes it is necessary to aerate the solution. The effectiveness of the solution remains even after 24 hours, but it becomes malodorous after extended use because of the putrifying slime of the treated fish.
MS 222 is also used in tranquillizing common carp while suturing its genital opening. This is done by putting a ball of cotton dipped in a strong solution of the chemical inside the mouth of the fish.
Phenoxy-ethanol is another chemical that has recently come into use as a fish tranquillizer. It is milder and less effective than MS 222, but is far cheaper; 30–40 cm3 of phenoxy-ethanol is mixed with 100 l of water for this treatment.
Tranquillizing brood fish during transport It is not an easy task to transport brood fish for several hours over a long distance. It is especially difficult to transport big fishes in tropical and sub-tropical areas, where the water temperature is high and it is difficult to obtain cool, oxygen-saturated water. The fish generally dash against the wall of the container and get injured, sometimes so seriously that they cannot be used for propagation during that season.
In practice, the fish are first tranquillized with the normal dose and put into the transportation tank or cistern, where the original concentration is diluted by 50 percent by adding the same amount of fresh water. The brood fish will remain tranquillized well in that diluted solution.
It is advisable to find out the right dose for the fish in question through experimentation. Sensitivity resistance and endurance vary from fish to fish. Even the near related species may differ very much in this respect.
