34 JUNE 2013 • WORLD AQUACULTURE • WWW.WAS.ORG Ecological Aspects A flooded rice field can be considered a multi-trophic and multi-spatial environment. In the experiment, prawns foraged on periphyton attached to rice stems and benthos on the bottom of rice fields. Thus, rice fields provided shelter against some predators, such as birds, and natural food to prawns. At low stocking densities of Amazon river prawn, natural food was sufficient and commercial feed was not necessary. In intensive aquaculture systems, where formulated feeds are required, effluent may pollute receiving waters. In our study, the quality of outlet water was sometimes better than the inlet water. Perhaps, this may be explained by the recycling of nutrients by prawns and rice, because they can indirectly take advantage of dissolved nutrients in rice fields. In the rice-prawn system, the water depth in rice fields is about 40 cm. In this shallow aquatic environment, the daily temperature often oscillates widely, reducing growth and survival of prawns. In the region where this study was conducted, this problem restricts the rearing of prawns to only one short cycle between October and April. In other regions where temperatures are milder, it may be possible to extend the production cycle to increase the size of animals or get two rearing cycles of prawns. Productivity and Economic Aspects Prawn harvest occurred 137 days after stocking (Figs. 8, 9 and 10) and rice was harvested 120 days after seedlings were transplanted (Fig. 11). Prawns did not affect rice productivity and survival was better at lower density (Table 1), favoring the sale of prawns as bait when they were sold per unit. A relatively low investment was necessary for rice farmers to implement this system in rice fields (~US$1900/ha). The main cost was construction of trenches to collect prawns, representing 54 percent of the total investment. The acquisition of prawn postlarvae and extra labor for daily cleaning of nets and harvesting prawns were the most expensive operational costs. The best economic indicators were obtained using the lowest density of animals (5/m2), because survival was greater and the cost of prawn PLs was less. In this treatment, the benefit-to-cost ratio was US$2.52 per US$ invested; the internal rate of return was 15.2 percent per year and the payback period was 6.3 years. TABLE 1. Weight, length, survival and productivity (mean ± SD) of M. amazonicum reared in rice-prawn (RP) system and rice productivity in RP and rice monoculture (RM) systems. Different letters superscript to the means in the same row, means statistical significant differences (P<0.05). Variable Treatment 5RP 10RP 20RP RM Weight (g) 2.9 ± 0.4 3.3 ± 0.7 2.6 ± 0.3 - Length (cm) 7.4 ± 0.3 7.7 ± 0.5 7.2 ± 0.3 - Survival (percent) 38 ± 13a 16 ± 14ab 8 ± 10b - Productivity (prawns/ha * cycle) 18,800 ± 13,000 15,900 ± 13,800 16,400 ± 19,500 - Rice productivity (kg/ha * cycle) 917 ± 570 893 ± 563 657 ± 353 950 ± 513 FIGURE 9. Harvest of prawns from trenches using dip nets. FIGURE 10. Amazon River prawn harvested from the rice-prawn system.
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