HATCHERY REQUIREMENTS 47 



vertical turning vanes at each pond corner. The water generally flows 

 parallel to the outside walls of the unit, gradually moves toward the center 

 wall, and leaves the pond through the perforated plates in the pond bottom 

 at opposite ends of the center wall. 



The rectangular pond operates well at a water depth of either 30 or 36 

 inches, depth being controlled by a removable standpipe in the waste line. 

 An advantage of the rectangular circulation pond is that fish are well dis- 

 tributed through the pond and the water current carries food to the fish. 

 This reduces concentrations of fish at feeding time. It is relatively self- 

 cleaning due to the water path created by the turning vanes at inflows of 

 400 gallons per minute or greater. The water flow and turbulence along the 

 center wall carry debris and waste material to the outlet. 



Pond dimensions and water flows are very specific, and any change in 

 the design criteria of this rearing unit may drastically alter the hydraulic 

 performance. This can prove a distinct disadvantage when flexibility of fish 

 loads and water flows is desired. 



EARTHEN PONDS 



There is general agreement that concrete raceways are cheaper to maintain 

 and operate than earthen ponds. Many fish culturists contend, however, 

 that fish reared in dirt raceways and ponds are healthier and more colorful, 

 have better appearing fins, and are a better product. 



Rectangular earth ponds usually are more convenient and efficient, and 

 may range in size from | acre to 3 acres or more. Large ponds of irregular 

 shapes are more difficult to clean, and it is harder to feed and harvest fish 

 and to control disease in them. 



It is doubtful that fish production will become as intensive in large 

 earthen ponds as in smaller types of rearing units that have more water 

 changeovers. Earth ponds do have relatively low water requirements and 

 produce some natural food. Successful culturing of trout and salmon have 

 been accomplished in this type of facility and use of supplemental aeration 

 has increased catfish production dramatically in recent years. 



Harvest methods must be considered in the design of an earthen pond. 

 Ponds must be drainable and contain a basin or collection area for harvest- 

 ing the fish (Figures 21 and 22), although many of the fish can be seined 

 from the pond before it is drained. The bottom of the pond should slope 

 gradually toward the outlet from all sides. Pond banks should be built with 

 as steep a slope as possible to avoid shallow-water areas along the edge of 

 the ponds. Shallow areas collect waste material and allow dense growths of 

 vegetation to develop. 



Topography for construction of earthen ponds should be gently sloping 

 and should have only moderate relief that can be economically removed. 



