tidal action. In lakes and ponds the swimming and feeding activity 

 of fish cultured in the cages, and of wild fish around the cages, 

 can generate sufficient water movement to permit water exchange in 

 cages. Wind-generated waves also help to circulate and exchange 

 water around cages and pens placed in ponds and lakes with little 

 or no natural current. 



Cages placed in rivers or industrial effluents take advantage of 

 the flowing water to remove wastes. It is usually not necessary 

 for water quality to be further manipulated; however, cages, rafts, 

 and other off-bottom systems may be moved from one site to another 

 to take advantage of better environmental conditions. For example, 

 caged fish cultured in the intake canal of a power plant during the 

 spring and summer may be moved to the discharge canal to take 

 advantage of the thermal effluent during the fall and winter. 



Factors which may seasonally alter water quality — for example, 

 nitrogen gas supersaturation in thermal effluents, agricultural 

 run-off, industrial discharges, and reservoir drawdown for 

 irrigation or flood control — must be understood by the 

 aquaculturist prior to placing cages or pens in a body of water. 



Other Systems 



Silos or deep cylindrical tanks are similar in operation to 

 horizontal raceways. Water is exchanged at a rapid rate to 

 maintain adequate oxygen levels and to remove waste products. 

 Water quality in silos is usually no better than that of the 

 incoming water; however, dissolved oxygen may be increased by 

 aeration and agitation, or in some units by the addition of gaseous 

 or liquid oxygen. Silos may be constructed with sediment basins to 

 remove solid waste before water is discharged. 



In polyculture systems the waste produced from one cultured species 

 may be used as a food source by another species. Aquaculture 

 systems may be further integrated with both aquatic and terrestrial 

 animals and plants. For example, wastes discharged from 

 aquaculture production units or sediment basins may be applied to 

 agricultural cropland, or used to support the hydroponic culture of 

 high-value terrestrial plants. Wastes from agricultural operations 

 may serve as food supplements in certain aquaculture systems. 



Hatchery systems for the production of fry and larvae or other 

 seedstock may use elaborate processes including aeration, 

 filtration, treatments with ultraviolet light and ozone to maintain 

 or adjust water quality. Discharges from these systems are 

 frequently of higher quality than the receiving waters. Water 

 quality can be more easily and economically manipulated in small 

 systems used for fry production than in larger systems used for 

 production of food fish. Equipment failure and system 

 mismanagement are two of the greater risks encountered in intensive 

 culture and water reuse systems. 



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