HATCHERY REQUIREMENTS 21 



both are lumped as "total ammonia." Figure 8 shows the reaction that 

 occurs when ammonia is excreted into water by fish. The fraction of total 

 ammonia that is toxic ammonia (NH J varies with salinity oxygen concen- 

 tration and temperature, but is determined primarily by the pH of the 

 solution. For example, an increase of one pH unit from 8.0 to 9.0 increases 

 the amount of un-ionized ammonia approximately 10-fold. These propor- 

 tions have been calculated for a range of temperatures and pH and are 

 given in Appendix B. Note that the amount of NH^ increases as tempera- 

 ture and pH increase. From Appendix B and a measurement of total 

 ammonia (parts per million: ppm), pH, and temperature, the concentration 

 of un-ionized ammonia can be determined: Ppm un-ionized 

 ammonia=(ppm total ammonia x percent un-ionized ammonia) ^ 100. 



When un-ionized ammonia levels exceed 0.0125 part per million, a 

 decline in trout quality may be evidenced by reduction in growth rate and 

 damage to gill, kidney, and liver tissues. Reduced growth and gill damage 

 occur in channel catfish exposed to 0.12 part per million or greater un- 

 ionized ammonia. 



Ammonia rapidly limits fish production in a water-recycling system un- 

 less it is removed efficiently. Biological filtration and ion exchange are the 

 best current means of removing ammonia from large volumes of hatchery 

 water. 



BIOLOGICAL REMOVAL OF AMMONIA 



Biological removal of ammonia is accomplished with cultures of nitrifying 

 bacteria that convert ammonia to harmless nitrate ions (NOj"). These bac- 

 teria, chiefly species of Nitrosomonas and Nitrobacter can be grown on almost 

 any coarse medium, such as rocks or plastic chips. The best culture mate 

 rial contains calcium carbonate, which contributes to the chemical reac- 

 tions and buffers pH changes; oyster shells often are used for this purpose. 



By the time water reaches the biological filter, it should be already well- 

 aerated (oxygen is needed for the process) and free of particulate matter 



FISH "-NHg + HO " Z^ NH^OH ." „ NH^^ OH 



\ 



UNIONIZED pH IONIZED 



TOXIC FORM DEPENDENT NONTOXIC FORM 



Figure 8. Reaction of ammonia excreted into water by fish. 



