40 FISH HAICIIKKY MANAGEMENT 



temperature is practical with today's equipment, which requires relatively 

 less water flow than older methods of egg incubation. Various types of egg 

 incubation are described in detail in Chapter 3. 



Rearing Facilities 



Rearing units for intensive fish culture include starting tanks or troughs for 

 swim- up fry, intermediate rearing tanks for fingerlings, and large outdoor 

 rearing ponds or raceways. 



Rearing units should be constructed so they can be drained separately 

 and quickly. They should be adequate not only for the normal operating 

 flow in the hatchery but also for increased volumes of water needed during 

 draining and cleaning of the facilities. 



Much personal opinion and preference is involved in the selection of a 

 rearing unit. Fish can be raised successfully in almost all types of rearing 

 units, although some designs have distinct advantages in certain applica- 

 tions. Adequate water flow with good circulation to provide oxygen and 

 flush metabolic waste products are of paramount importance in the selec- 

 tion of any facility. Ease of cleaning also must be considered. 



CIRCULAR REARING UNITS 



Limited water supplies make semiclosed water recycling systems highly 

 desirable. The most efficient involve circular units and pressurized water 

 systems. By common acceptance, circular "tanks" refer to portable or semi- 

 portable units up to 12 feet in diameter, while "pools" refer to permanently 

 installed units up to 40 feet in diameter. 



There are basic criteria for construction and design of circular tanks and 

 pools that are essential for their satisfactory operation. Double-walled or in- 

 sulated tanks reduce external condensation and eliminate dripping water. 

 Adequate reinforcement must be incorporated in the bottom of the tank to 

 support the filled units. There is no need for a sloping bottom except to 

 dry out the tank. Flat-bottomed tanks will self-clean well if proper water 

 velocities are established. The walls should be smooth for easy cleaning. In 

 the case of portable tanks, the preferred material is fiber glass, but good 

 tanks can also be constructed of wood or metal. Large circular pools are 

 usually constructed of masonry. 



Without proper equipment, removal of fish from larger circular tanks is 

 difficult. Crowding screens facilitate the removal of fish (Figures 16 and 

 17). Some types of pools have inside collection wells for the accumulation 

 of waste and removal of fish. 



