Results and Discussion Tilapia production results are given in Table 1. The fish grew at a higher rate (4.0 g/day) and reached a large size (912 g) in Trial 1 because larger fingerlings were stocked. Therefore initial growth rates were higher. In addition, the stocking rate was higher (25 fish/m3) in Trials 3-4, which can reduce the growth rate of individual fish. The feed conversion ratios ranged from 1.8 in Trial 3 to 2.2 in Trial 1 and were higher than expected, which may have been due in part to low survival rates (78.9% to 86.0% in the first three trials) caused by bird predation. Herons perched on the side of the tank and preyed on the fish during the beginning of each production cycle in the first three trials. Fish that were too large to swallow were found on the ground or floating dead in the water. An electric wire to repel birds was strung along the top of the tank midway through the first trial. This device failed in the second trial, and bird predation was heavy again. The anti-bird orchard netting that was attached vertically above the tank wall in the third trial reduced perching sites, but predation continued. Complete enclosure of the tank by netting prevented bird predation entirely in Trial 4 and resulted in excellent survival of 99.7%. Final biomass density in Trial 4 reached the highest value (18.6 kg/m3) of all four trials. Total production in Trial 4 was 3,720 kg. However, the feed conversion ratio remained high (2.0) due in part to a 2-week period of high nitrite-nitrogen values and reduced feeding (Figures 8 and 10)
Table 1. Results of four tilapia production trials in a 200-m3 biofloc system.
Stocking Rate (#/m3)
Initial Size (g)
Final Size (g)
Culture Period (d)
Growth Rate (g/d)
Final Biomass (kg/m3)
Another factor leading to a high feed conversion ratio appeared to result from the interaction of water quality and feed consumption. The daily feed consumption varied considerably, but there was an upward trend in consumption at the beginning of Trials 1 and 2 (Figures 5 and 6). In Trial 1, feed consumption increased initially but then leveled off during the middle of the trial and declined slightly by the end of the trial. In Trial 2, feed consumption generally increased through most of the trial but decreased near the end. The maximum feeding rate in these trials was approximately 40 kg/day. In Trial 3, feed consumption generally increased throughout the culture period except for sharp decreases near days 121 and 181 (Figure 7). The maximum feeding rate reached 45 kg/day. In Trial 4, feed consumption increased to a peak at day 79 and declined dramatically at day 92 for a 2-week period (Figure 8). Feed was restricted during this period due to high nitrite-nitrogen levels (Figure 10). After nitrite levels declined and unrestricted feeding resumed, feed consumption quickly returned to peak levels of approximately 50 kg/day but decreased moderately near the end of the trial. As fish grow, a continuous increase in the daily feed ration is expected. If the daily ration reaches a limit due to water quality deterioration, gradually a smaller proportion of the daily ration goes to fish growth, which causes a decline in the growth rate and an increase in the feed conversion ratio.