ABSTRACT The present study aimed to estimate digestible protein (DP) requirements for maintenance and growth and assess the efficiency of protein utilization in pacu (Piaractus mesopotamicus) juveniles. A complete randomized design consisted of seven treatments, and five replicates (tanks) per treatment was used. After a one-week acclimation period, 350 pacu juveniles of 27.48 ± 1.8 g (initial average body weight - BW) were equally distributed among 35 tanks (10 fish per tank) of 450 L. Fish were fed with seven graded digestible protein levels (64.5, 111.8, 164.6, 217.1, 264.4, 316.2 and 369.5 g kg−1) three times a day until apparent satiation for 120 days. The monomolecular model parameters including nitrogen maintenance requirement (NMR) (37.76 mg BW0.67 kg−1 d−1), daily nitrogen deposition (ND) (194.8 mg BW0.67 kg−1 d−1), and protein intakes (based on metabolic BW) of 1638, 1902, and 2241 mg kg−1BW0.67 d−1 for 75, 80 and 85% maximum theoretical nitrogen retention (NRmaxT), respectively and the efficiency of protein utilization (529×10−6) were estimated through the relationship between nitrogen intake (NI) and ND. The digestible protein requirements for 75, 80, and 85% of NRmaxT were estimated as 187, 217, and 256 g kg−1 respectively by the monomolecular model based on the daily feed intake of 2% of live BW of pacu juveniles.

Abstract Molecular oxygen is a necessary compound for all aerobic organisms, although oxygen is a potent oxidant, which can cause oxidative stress (OS). OS occurs when there is an imbalance between the production of oxidant and antioxidants components, are result of normal cell metabolism, and many of these compounds play a fundamental role in several metabolic pathways. The organism produces several reactive oxygen species (ROS), but they are balanced by an antioxidant defense system that maintains the levels of these oxidizing compounds at an acceptable level. Many of these components are essential in the organism defense and their byproducts are considered potent bactericides that actively act in the destruction of invading pathogens. Fish immune system is composed of innate and acquired mechanisms of defense. Phagocytosis is an innate process of defense, which interconnects these two systems, since the pathogens processing by professional phagocytes is a fundamental stage for antibodies production. During phagocytosis there is production of ROS and consequent production of free radicals (FR), these compounds lead to the formation of potent bactericides to combat microorganisms. However, it is known that OS limits the immune response, with an impairment in defense compounds in an attempt to decrease the ROS production. Studies of fish FR production are preliminary and should be executed to evaluate the effects of ROS on fish, including their beneficial action against pathogens and its deleterious action on the oxidation of cellular components.