352 FISH HATCHERY MANAGEMENT 



(1) "V"-shaped, elliptical, or partially round tanks promote better mix- 

 ing and circulation of water as the size of the tank increases. 



(2) Polyurethane insulation, which has the best insulating qualities, lends 

 itself ideally to a round or oval tank. It can be injected easily within the 

 walls of the tank. 



(3) These tanks can be constructed with few structural members and 

 without sharp corners that might injure fish. 



(4) Rapid ejection of fish is facilitated by an elliptical tank. 



(5) Lowering the water level in these tanks reduces surface area and sim- 

 plifies the removal of fish with dip nets. The rounded bottom also contri- 

 butes in this respect. 



(6) As this shape of tank is widely used by bulk liquid transport com- 

 panies, they are mass-produced and readily available. 



(7) This shape conforms to a truck chassis and holds the center of grav- 

 ity towards the area of greatest strength. 



(8) Construction weight is less than that of rectangular tanks of the same 

 capacity. 



Circulation 



Circulation systems are of various sizes and designs; all have plumbing ad- 

 ded for the pickup and discharge of water. Suction lines to the pumps lie 

 on the bottom of the tank and are covered by perforated screens. Water is 

 carried to the pumps and then forced through overhead spray heads 

 mounted above the waterline. In most systems, oxygen is introduced in one 

 of the suction lines just ahead of the pump. This usually is controlled by a 

 medical gas-flow meter; because of the danger involved in handling and 

 transporting bottled oxygen, care must be taken to follow all prescribed 

 safety procedures. 



Self- priming pumps powered by gasoline engines are used to circulate 

 water in many distribution units. Pumps may be close-coupled or flexibly 

 coupled. Although the former type is more compact, it tends to transfer 

 heat to the water. Depending upon ambient air temperature, close-coupled 

 pumps may increase the temperature of 400 gallons of water by about 7°F 

 an hour, whereas flexible coupling will reduce heat transfer to approxi- 

 mately 3°F per hour. 



Pipes used in conjunction with pumps usually are black or galvanized 

 steel. Although steel is durable, threads may rust, and replacement or 

 modification following installation may prove difficult. Aluminum pipe also 

 has been used in systems of this type, but its advantages and disadvantages 

 are reportedly similar to those of steel except aluminum pipe does not rust. 

 Because of the ease of installation, plastic pipe should be considered for 



