Production of Tilapia aurea in simulated sewage lagoons receiving laying hen wastes

Abstract

Experiments were carried out in the summers of 1978 and 1979 in 12 outdoor plastic-lined circular wading pools (about 7 m('2) in area) at the Aquaculture Research Center of the Texas Agricultural Experiment Station near College Station to determine the optimum manuring rate for fresh laying hen wastes in simulated sewage lagoons stocked with all-male Tilapia aurea (50 fish/pool). One and one-half year old laying hens were maintained in cages suspended over the pools and supplied direct, constant manure input. Four experimental manuring rates with three replicate pools of each were used each year. The extrapolated manuring rate treatments for 1978 were 0, 35, 70, and 140 kg dry weight of manure/ha/day and for 1979 were 70, 140, 210, and 280 kg dry weight of manure/ha/day. The results of tilapia growth and survival, and water quality data indicated that the optimum range of manuring rate with laying hen wastes for the summer months lies from 70 to 140 kg dry weight of manure/ha/day, depending on the alkalinity of the water. In the 1979 experiment highly alkaline water was found to be detrimental to proper functioning of the sewage lagoon system and tilapia production in it. The high buffering capacity of the alkaline waters (above about 400 mg/liter as CaCO(,3)) prevented the normal diurnal pH shift which would have allowed direct volatilization of ammonia from the system; a major mechanism of ammonia removal. As a result, ammonia accumulated in the pools at much higher levels than it did at the same manuring rates in the 1978 experiment which had only moderately alkaline water. The tilapia at the 140, 210, and 280 kg dry weight of manure/ha/day treatments in 1979 did not tolerate the high levels of ammonia which may have acted synergistically with low dissolved oxygen levels to kill all the tilapia in these treatments. These results indicated the need for a laboratory study to determine if there are synergistic effects on Tilapia aurea between high ammonia and low dissolved oxygen concentrations and to quantify the effective concentrations. Reduced tilapia growth occurred in 1979 at the 70 kg dry weight of manure/ha/day treatment as a result of poor water quality conditions. By comparison in the moderately alkaline waters used in 1978, tilapia growth increased with increased manuring rate to a leveling off point at 70 to 140 kg dry weight of manure/ha/day.