22 FISH HATCHERY MANAGEMENT 



(which could clog the filter). It is important that the water be pathogen- 

 free, because an antibiotic or other drug that has to be used in the 

 hatchery can kill the nitrifying bacteria as well. 



Settling chambers and clarifiers can extend the life of biofilters and reduce 

 clogging by removing particulate matter. Filter bed material with large void 

 spaces also can reduce clogging, and foam fractionation will remove dissolved 

 organic substances that accumulate. These foaming devices are also called 

 "protein skimmers," which refers to their ability to remove dissolved organic 

 substances from the water. The foam is wasted through the top of the device 

 and carries with it the organic material. In a small system, air stones can be 

 used to create the foam. The air produces numerous small bubbles that col- 

 lect the organic material onto their surface. Because foam fractionation does 

 not readily remove all particulate organic material, it should follow the set- 

 tling or clarifying unit in a reconditioning system. 



Nitrite (NO7 ) is an intermediate product of nitrification, and a poorly 

 operating biofilter may release dangerous amounts of this toxic ion to the 

 water. A more rapid growth rate of Nitrosomonas in the biological system 

 can lead to accumulation of nitrite, which is highly toxic to freshwater 

 fishes. Nitrite oxidizes blood hemoglobin to methemoglobin, a form which 

 is incapable of carrying oxygen to the tissues. Methemoglobin is 

 chocolate-brown in color, and can be easily seen in the fish's gills. 



Yearling trout are stressed by 0.15 part per million and killed by 0.55 

 part per million nitrite. Channel catfish are more resistant to nitrite, but 29 

 parts per million can kill up to 50'!'() of them in 48 hours. Nitrite toxicity 

 decreases slightly as the hardness and chloride content of water increases. 



ION EXCHANGE REMOVAL OF AMMONIA 



Ion exchange for removal of ammonia from hatchery water can be accom- 

 plished by passing the water through a column of natural zeolite. Zeolites 

 are a class of silicate minerals that have ion exchange capacities (they are 

 used in home water softeners). Among these, clinoptilolite has a partic- 

 ularly good affinity for ammonium ions. It is increasingly being used in 

 hatcheries, where it effects 90 97"o reductions in ammonia (Figure 9). 



Clinoptilolite does not adsorb nitrate or nitrite, nor does it affect water 

 hardness appreciably. It can be regenerated by passing a salt solution 

 through the bed. The ammonia is released from the salt solution as a gas 

 and the solution can be reused. Any ion exchange unit can develop into a 

 biofilter if nitrifying bacteria become established in it. This may lower 

 exchange efficiencies and cause production of nitrite, so periodic disinfec- 

 tion may be necessary. 



