Growout of Freshwater Prawn Macrobrachium rosenbergii in Fertilized Ponds with Reduced Levels of Formulated Feed

Formulated feed for freshwater prawn farming under semi-intensive conditions is expensive, presenting a significant portion of the farm's operating costs. Production and availability of natural food in earthen ponds may reduce the needs for formulated feed. This paper evaluates growth of freshwater prawn Macrobrachiurn rosenbergii under three feeding regimes, where supplemental feed was reduced and natural productivity of the ponds was increased by a fertilization program that included the addition of lime, phosphorus and nitrogen. Prawns (4.35 g) were stocked into 12 50-m² earthen ponds (6 juveniles/m²) and grown for 84 d feeding on 25% crude protein ration. Chemical and physical water parameters were monitored routinely. Growth and production were similar at 100% formulated feed supplementation with no fertilization and at 50% formulated feed with fertilization, as determined by weight gain, specific growth rate, and net yield ( P >0.05). However, further reduction in feed supplementation (0, 25%, and 50% of feeding rate, respectively, for each subsequent 28-d period) with fertilization, yielded shrimp smaller than market size. Feed conversion ratios were significantly higher ( P 0.05) for the 100% versus the 50% feed supplementation. Survival rate averaged 86% with no significant differences among treatments. Reducing up to 50% of the supplemental feed with added fertilization for increased natural productivity does not reduce yield of M. rosenbergii raised under semi-intensive conditions in northeastern Brazil. Under the local conditions of semi-arid and warm water temperatures (24.0–33.5 C), increasing natural productivity of ponds significantly improves feed conversion ratio, which is reflected in feed cost savings and potential savings of the farm operating cost.