HATCHERY OPERATIONS 91 



Screens 



Various materials have been used to construct pond or raceway screens. 

 Door screening and galvanized hardware cloth can be used, but clog easily. 

 Wire screening fatigues and breaks after much brushing and must be re- 

 placed periodically. Perforated sheet aluminum screens are used commonly 

 in many fish hatcheries today. They can be mounted on wood or metal an- 

 gle frames. Redwood frames are easier than metal ones to fit to irregular 

 concrete slots in raceway walls. 



Perforated aluminum sheets generally can be obtained from any sheet 

 metal company. Some suggested sheet thicknesses are 16 gauge for large 

 screens (ponds, raceways: 30x96 inches) and 18-20 gauge for small 

 screens (troughs: 7 x 13 inches). Round holes and oblong slots are available 

 in a number of sizes (Figure 36). Horizontal oblong slots are preferred by 

 some fish culturists who feel they are easier to clean and do not clog as 

 readily as round holes. They can be used with the following fish sizes: 



Slot size Fish size 



_\_ 



111 



j_ 



s 



\_ 



4 



J_ 

 2 



Perforated aluminum center screens can also be used in circular rearing 

 tanks, but only the bottom 2-3 inches of the cylinder should be perforated. 

 These provide some self-cleaning action for the tank and prevent short- 

 circuiting of water flows by drawing waste water off the bottom of the 

 tank. 



Pond Management 



PRESEASON PREPARATION 



Proper management of earthen ponds begins before water is introduced 

 into them. During the winter it is advisable to dry and disk ponds to pro- 

 mote aerobic breakdown of the nutrient-rich sediments. Although some nu- 

 trients are desirable for fingerling culture, because they promote algal 

 growth on which zooplankton graze, an overabundance tends to produce 

 more undesirable blue-green and filamentous algae. Relatively new ponds 

 with little buildup of organic material, or those with sandy, permeable bot- 

 toms that allow nutrients to escape to the groundwater, are less likely than 

 older or more impermeable ponds to require drying and disking. They may 



