ABSTRACT. The objective of this study was to evaluate the effect of stocking densities on productive performance of the freshwater prawn Macrobrachium rosenbergii in biofloc system. Experimental tanks (microcosms) with 0.20 m² area were used as experimental units. The tanks were connected to two 300 L matrix tanks (macrocosm) with biofloc technology, used as recirculating units. M. rosenbergii juveniles, with an initial weight of 0.315 ± 0.06 g and initial length of 33.34 ± 2.26 mm, was randomly distributed in the experimental tanks at different stocking densities (50, 100, 150, 200 and 250 ind m -²) and reared during 60 days. The total biomass at the end of the experiment was significant higher (P < 0.05) with the use of higher stocking density (250 ind m -2 ). However, prawns stocked at the density of 50 m -2 showed significant higher (P < 0.05) survival (73%) and significantly lower values (P < 0.05) for feed conversion rate (1.28). The different stocking densities evaluated did not affect the weight and length of prawns. The recommended density for growing M. rosenbergii in the biofloc system is 50 ind m -2 .

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Experiments were carried out to study the effects of ammonia and nitrite on food consumption of Macrobrachium amazonicum postlarvae. Postlarvae (average weight of 0.0625 g) were exposed during 21 days to four concentrations of total ammonia (0, 1.05, 2.1 and 4.2 mg. NH3+NH4+ L-1) and four concentrations of nitrite (0, 0.075, 0.15 and 0.30 mg N-NO2- L-1). After the exposure period, six prawns per treatment were maintained individually in 250 mL experimental units to analyze the food consumption as a function of the amount of food offered and the leftovers during a 24 h period. The food consumption presented significant alterations for prawns exposed to all nitrite concentrations and at concentrations of twice the recommended safety level for total ammonia. According to the results obtained, high concentrations of total ammonia and nitrite affect the food consumption of M. amazonicum adversely, influencing the species performance in culture systems.