FISH HEALTH MANAGEMENT 271 



seconds, that depends on the type of chemical, its concentration, and the 

 species of fish being treated. Metal containers should not be used to hold 

 the treatment solution because some chemicals can react with the metal 

 and form toxic compounds, particularly if the water is acid. 



This method of treatment is dangerous because the difference between 

 an effective dose and a killing dose often is very small. However, if done 

 properly, it is very effective for treating small numbers of fish. Other disad- 

 vantages to this method include its high labor costs and stress on the fish 

 due to handling. 



PROLONGED BATH 



For prolonged- bath treatments, the inflowing water is cut off and the correct 

 amount of chemical is added directly to the unit being treated (Appendix 

 G). After a specified time, the chemical is flushed out quickly with fresh 

 water. This treatment can be used in any unit that has an adequate supply 

 of fresh water and can be flushed out within 5 to 10 minutes. 



Several precautions must be observed with this method to prevent seri- 

 ous losses: (l) Because the water flow is turned off, the oxygen concentra- 

 tion of the water may be reduced to the point that the fish are stressed and 

 losses occur. The more fish per unit volume of water, the more likely this is 

 to occur. Aerators of some type must be installed in the unit being treated 

 to insure an adequate oxygen supply or must be available if needed. (2) 

 Regardless of the treatment time that is recommended, the fish always 

 should be observed throughout the treatment and, at the first sign of dis- 

 tress, fresh water must be added quickly. (3) The chemical must be uni- 

 formly distributed throughout the unit to prevent the occurrence of "hot 

 spots" of the chemical. Fish being treated may be killed or severely 

 injured by overdoses if they swim through hot spots. Conversely, fish that 

 avoid these hot spots may not be exposed to a concentration high enough 

 to be effective. The method used for distributing the chemical throughout 

 the unit will depend on the kind of chemical being used, type and size of 

 unit being treated, and equipment and labor available. Common sense 

 must be used as it is impossible to lay down hard and fast guidelines that 

 will cover every situation. 



INDEFINITE BATH 



Indefinite baths usually are used to treat ponds or hauling tanks. A low 

 concentration of a chemical is applied and left to dissipate naturally. This 

 generally is one of the safest methods of treatment. One major drawback, 

 however, is that the large quantities of chemicals required can be expen- 

 sive to the point of being prohibitive. Another drawback relates to the 



