144 FISH HATCHERY MANAGEMENT 



When golden shiners are seined from a pond, the seine should be of cot- 

 ton or very soft material because the scales are very loose on this species. 

 Harvesting at water temperatures below 75°F will reduce stress. 



TILAPIA 



Fish of the genus Tilapia are native to Africa, the Near East, and the 

 Indo-Pacific, but are presently widely distributed through the world. Tila- 

 pia are cichlids, and most species are mouth brooders; females incubate 

 eggs and newly hatched fry in their mouth for 10-14 days. When the fry 

 are free- swimming they begin feeding on algae and plankton. 



Tilapia tolerate temperatures in excess of 100°F, but do not survive 

 below 50-55°F. Consequently, their culture as forage fish is restricted to 

 the southern United States. Even there, broodfish usually need to be 

 overwintered in water warmer than 55°F. Most tilapia are very durable and 

 tolerant, able to survive low oxygen and high ammonia concentrations. 



Tilapia are excellent forage species in areas where culture is possible: 

 easy to propagate; prolific; rapid growing; disease-resistant; and hardy for 

 transferring in hot weather. Rearing ponds should be prepared and fertil- 

 ized to produce an abundance of phytoplankton. If 200-250 adults are 

 stocked in a pond after the water temperature is 75°F or above, they will 

 produce 100,000 juveniles of 1-3 inches in 2-3 months. The adults will 

 spawn and rear a new brood every 10-14 days throughout the summer. 

 Tilapia accept dry food, and supplemental feeding will increase the growth 

 rate. 



Improvement of Broodstocks 



Fish stocks may be improved by several methods, some of which are: selec- 

 tive breeding, the choosing of individuals of a single strain and species; hy- 

 bridization, the crossing of different species; and crossbreeding, the mating 

 of unrelated strains of the same species to avoid inbreeding. 



SELECTIVE BREEDING 



Selective breeding is artificial selection, as opposed to natural selection. It 

 involves selected mating of fish with a resulting reduction in genetic varia- 

 bility in the population. 



Criteria that often influence broodfish selection for selective breeding 

 include size, color, shape, growth, feed conversion, time of spawning, age at 

 maturity, reproductive capacity, and past survival rates. These may vary 

 with conditions at different hatcheries. No matter what type of selection 



