World Aquaculture March 2019

WWW.WA S.ORG • WORLD AQUACULTURE • MARCH 2019 59 ( C O N T I N U E D O N P A G E 6 0 ) TABLE 1. Production indicators registered in pellet-free system after 134 and 188 days of growing. Survival was not recorded on day 134. Indicator 134 days 188 days Survival (%) - 87.5 Mean weight (g) 14.2 16.4 Productivity (kg/ha) 621 716 Total growth rate (g/d) 0.106 0.087 Growth rate by period (g/d) 0.106 0.040 postlarvae (0.08 ± 0.05 g) at 5 PL/m 2 . To stimulate the production of natural food, the pond was fertilized with about 100-200 g/m 2 of organic materials, primarily vegetables and rotten fruits (mango and guava). To improve natural food production and provide shelters for prawns, six lines with substrates (48 pieces of polyethylene mesh measuring 1 × 0.5 m each; Fig. 2) were vertically oriented throughout the pond, corresponding to a total area of 24 m 2 (Fig. 3). Some PVC pipes (2-4 inches in diameter) were arrayed on the pond bottom to provide extra refuge and shelter for prawns. There was no water exchange but water losses due to percolation and evaporation were replaced. Water temperature (24-28 C), dissolved oxygen concentration (3-5 mg/L) and pH (6.5-7.5) were adequate for good freshwater prawn growth. Throughout the trial, prawns were observed feeding on periph- yton naturally growing on substrates and pond walls and shoals of small fish ( Astyanax altiparanae ) were seen swimming among sub- strates. As Macrobrachium prawns are well adapted to prey upon small fish, these could be an additional source of nutrition. A preliminary evaluation of growth and production was done after 134 days. Then, after 188 days, prawns were harvested, weighed and counted, resulting in a mean weight of 16 ± 8 g with survival of 87.5 percent and a productivity equivalent of 716 kg/ ha (Table 1). All male morphotypes were present, as well as many ovigerous females (Figs. 4 and 5). Mean weight and productivity did not differ greatly (about 15 percent). The growth rate after 134 days (0.106 g/d) was 21 percent greater than the growth rate registered after 188 days (0.087 g/d). After day 134, the amount of natural feed available was insufficient to maintain the same growth rate. The results obtained were quite satisfactory in comparison with conventional growing systems in Southeast Brazil (Table 2). Trial results correspond to about 80 percent of the mean weight and 50-70 percent of the productivity registered in conventional pellet- fed prawn farming. Survival was even better than those obtained in conventional systems. Operating costs were reduced by about 40 FIGURE 3. Six lines of substrates were placed in the pond. Upon draining for harvest, substrates were covered with periphyton. FIGURE 4. Harvested pellet-free prawns. FIGURE 5. Harvested prawns detailing the characteristic presence of ovigerous females and various male morphotypes. TABLE 2. Comparison between the pellet-free test and conventional system in terms of stocking density, time of culture, survival, mean weight and productivity. Data on performance of conventional prawn production system of monoculture in tropical countries from New (2002). Indicator Pellet free Conventional Stocking density (#/m2) 5 10 Time (d) 188 150-180 Survival (%) 87.5 60-70 Mean weight (g) 16.4 16-20 Productivity (kg/ha) 715 1,000-1,400