HATCHERY REQUIREMENTS 19 



than hypochlorite, and has been used experimentally with some success. It 

 is unstable and has to be produced on site (from oxygen, with electrical or 

 ultraviolet energy). Ozonated water must be reaerated before fish can live 

 in it. Although very effective against microorganisms, ozone is extremely 

 corrosive and can be a human health hazard. 



Treatment of Water for Reuse 



Often it is feasible to reuse water in a hatchery; some operations run the 

 same water through a series of raceways or ponds as many as ten times. 

 Any of several reasons can make it worthwhile to bear the added cost of 

 reconditioning the water. The quantity of source water may be low; the 

 cost of pollution control of hatchery effluent may be high. The price of en- 

 ergy to continuously heat large volumes of fresh source water may limit 

 production of fish; continuous quality control and sterilization may be 

 expensive. 



A hatchery that uses water only once through the facility is called a 

 "single- pass" system. Hatcheries that recycle water for additional passes by 

 pumping and reconditioning it are termed "reuse-reconditioning" systems. 

 In either system, water that passes through two or more rearing units is 

 termed "reused." Most practical water-reconditioning systems recycle 

 90-95% of the water, the supplement of make-up water coming from the 

 source supply. To be practical, the system must operate for long periods 

 without problems and carry out several important functions (Figure 7). 



As water passes through or within a hatchery, fish remove oxygen, give 

 off carbon dioxide, urea, and ammonia, and deposit feces. Uneaten food 

 accumulates and water temperatures may change. This decline in water 

 quality will lower growth and increase mortality of fish if the water is recy- 

 cled but not purified. A water-reconditioning system must restore original 

 temperatures and oxygen concentrations, filter out suspended solids, and 

 remove accumulated carbon dioxide and ammonia. Urea is not a problem 

 for fish at the concentrations encountered in hatcheries. 



Temperatures are controlled, and suspended solids filtered, in ways out- 

 lined above for incoming water. Oxygen is added and excess carbon diox- 

 ide removed by mechanical aeration. The removal of ammonia is more 

 involved, and represents one of the major costs of recycling systems. 



The advantage of manipulating rearing environments in a recycle system 

 has been demonstrated in the rearing of striped bass fry and fingerlings. 

 They have been reared to fingerling size with increased success when the 

 salinity of the recycled water was raised to 47 parts per thousand during 

 the rearing period. Channel catfish also have been successfully reared in 

 recycled-water systems. 



