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This study evaluated growth performance, hematological parameters, histological liver analysis, and production costs of Nile tilapia fed increasing levels of digestible protein, digestible energy, and choline. Twelve thousand Nile tilapia (148 ± 6.7 g) were randomly distributed into 80 1 m 3 net cages, in a 5 × 2 × 2 factorial design with five digestible protein (DP) levels (24, 26, 28, 30, and 32% DP), two digestible energy (DE) levels (13.4 and 14.65 MJ DE kg −1 diet), and two choline levels (0.0 and 1000 mg kg −1 diet), with four replicates per treatment. Fish fed the higher energy level showed a sparing effect of protein; the higher protein level determined the highest fillet yield. Fish fed diets with 24% DP showed the highest liver lipid, and independently of treatment all analyzed fish showed hepatocyte degeneration. The best benefit cost ratio for whole fish production was achieved with 28% DP/13.4 MJ DE kg −1 , and for fillet production with 30% DP/13.4 MJ DE kg −1. The results of the hematological assay showed alterations in red blood cells, mean corpuscular volume, albumin, Albumin:Globulin ratio, and glucose after size-sorting stress. Overall, these results indicate a lower resistance to stress, mainly for fish fed with no choline and oil supplementation. In this study we determined that the best performance was achieved with DP:DE ratios of 21.45 g MJ −1 (28.74% DP/13.4 MJ DE kg −1) and 18.60 g MJ −1 (27.25% DP/14.65 MJ DE kg −1). The highest fillet yield was obtained with 30% DP, regardless of the dietary energetic level. Sustained homeostasis was observed in this setting, and even though size-sorting stress altered some hematological parameters, they were still within the range recognized as healthy. Choline was not effective in protecting the liver against hepatic steatosis, but was able to buffer some of the negative effects of stress under these rearing conditions. Statement of Relevance: This research has been approved by the Ethics Committee of our Institution. Our team has been working with nutrition and fish health since 2000. According to the NRC (2011), only a few studies have been conducted to estimate the dietary nutrient requirements of farmed fish under intensive culture conditions. Moreover, a number of factors may affect the dietary and nutritional requirements of fish differentially in the laboratory and under intensive culture. For example, climatic conditions may fluctuate widely in the field, directly affecting physiological responses, hence nutritional requirements. Similarly, fish densities are much higher under intensive farming. Establishing the appropriate nutritional requirements in these settings must also consider the trade-off between growth performance and production costs. Based on this, we investigated the effect of different levels of digestible protein and digestible energy on growth performance and hematological responses of the Nile tilapia in a commercial fish farm in Brazil, where fish farmers are known to use diets with excessive levels of crude protein through...

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To determine the dietary β‐glucan and vitamin C levels that would enable the fish to cope with different stress, 252 tilapia (16.86 ± 0.24 g) were distributed in thirty six 250‐L aquaria and fed experimental diets for 60 d. Diets were supplemented with β‐glucan (0.1, 0.2, 0.4, and 0.8%) and vitamin C (400 and 600 mg/kg diet) and distributed in a factorial design that included a control diet (0 β‐glucan/125 mg vitamin C/kg). After growth performance analyses, a group of 108 fish were subjected to cold‐induced stress and another to Aeromonas hydrophila infection. Hematological and immunological parameters were determined before and after the stress. Growth performance was not affected. Red blood cells, hematocrit, mean corpuscular volume, total plasma protein, albumin : globulin ratio and leukocytes, lymphocytes, neutrophils, and monocytes were affected (P < 0.05) by stress and/or diets. The control diet resulted in higher post‐infection cortisol (P < 0.05). Liver vitamin C was directly proportional to dietary vitamin C (P < 0.05). The production of hydrogen peroxide and nitric oxide was also affected by diets and/or bacterial challenge (P < 0.05). In total, 0.1–0.2% β‐glucan and 600 mg/kg vitamin C increased fish resistance to stress and 0.8% β‐glucan resulted in reduced immune responses regardless of the vitamin C supplementation level.