Fish Hatchery Management - fisheries & aquaculture

352 FISH HATCHERY NlANAC; EMEN.'f(1) "V"-shaped, elliptical, or partially round tanks promote better mixingand circulation of water as the size of the tank increases.(2) Polyurethane insulation, which has the best insulating qualities, lendsitself ideally to a round or oval tank. It can be injected easily within thewalls of the tank.(3) These tanks can be constructed with few structural members andwithout sharp corners that might injure fish.(4J Rapid ejection of fish is facilitated by an elliptical tank.(5) Lowering the water level in these tanks reduces surface area and simplifiesthe removal of fish with dip nets. The rounded bottom also contributesin this respect.(6) As this shape of tank is widely used by bulk liquid transport companies,thev are mass-produced and readily available.(Z) f nis shape conforms to a truck chassis and holds the center of gravitytowards the area oI greatest strength.(8) Construction weight is iess than that of rectangular tanks of the samecapacity.CirculationCirculation systems are of various sizes and designs; all have plumbing addedfor the pickup and discharge of water. Suction lines to the pumps lieon the bottom of the tank and are covered by perforated screens. Water iscarried to the pumps and then forced through overhead spray headsmounted above the waterline. In most systems, oxygen is introduced in oneof the suction lines just ahead of the pump. This usually is controlled by amedical gas-flow meter; because of the danger involved in handling andtransporting bottled oxygen, care must be taken to follow all prescribedsafety procedures.Self-priming pumps powered by gasoline engines are used to circulatewater in many distribution units. Pumps may be close-coupled or flexiblycoupled. Althoughthe former type is more compact, it tends to transferheat to the water. Depending upon ambient air temperature, close-coupledpumps may increase the temperature of 400 gallons of water by about 7"Fan hour, whereas flexible coupling will reduce heat transfer to approximately3'F per hour.Pipes used in conjunction with pumps usually are black or galvanizedsteel. Althoughsteel is durable, threads may rust, and replacement ormodification following installation may prove difficult. Aluminum pipe alsohas been used in systems of this type, but its advantages and disadvantagesare reportedly similar to those of steel except aluminum pipe does not rust.Because of the ease of installation, plastic pipe should be considered for