18 FISH HATCHERY MANAGEMENT 



Figure 6. Micro-screen filters consist of a rotating drum covered with woven 

 fabric of steel or synthetic material with various size openings. The raw water 

 enters the center of the drum and passes through the fabric as filtered water. As 

 the fabric becomes clogged, the drum rotates and a high- pressure water spray 

 (arrow) removes the filtered material from the screen into a waste trough. 

 Micro-screen fabric is available with openings as small as 5 microns. (FWS 

 photo.) 



Filtration followed by ultraviolet radiation is a proven method for steri- 

 lizing hatchery water. For example, 125 gallons per minute of river water 

 containing large numbers of fish pathogens can be sterilized by passage 

 through two 30-inch diameter sand filters, then through an 18-lamp ultra- 

 violet radiation unit. The sand filter removes particles as small as 8-15 

 microns and the ultraviolet radiation kills organisms smaller than 15 

 microns. It is important that pathogens be exposed to an adequate amount 

 of ultraviolet intensity for the required effective contact time. Treated 

 water must be clear to permit efficient ultraviolet light penetration. 



Maintenance of sand filters includes frequent backflushing and ultra- 

 violet equipment requires periodic cleaning of the quartz glass shields and 

 lamp replacement. Commercially available microscreen filters can be used 

 as an alternative to sand filters (Figure 6). 



Chlorine gas or hypochlorite can be used as sterilants, but they are toxic 

 to fish and must be neutralized. Ozone is a more powerful oxidizing agent 



