Available phosphorus (AP) requirement for tambaqui (Colossoma macropomum) was evaluated using different response parameters, and CT scanning was used to determine the adequacy of dietary P along with bone mineralization data. A total of 192 tambaqui juveniles (17 ± 0.85 g) were stocked into 24 70 L-tanks using randomized block design (different floors) with six treatments (1.3, 2.4, 4.8, 6.3, 7.8 and 8.8 g kg −1 AP) and four replicates. Fish were fed ad libitum with six semi-purified diets containing graded levels of AP for 90 days. The dietary AP requirement was estimated using non-linear regression models. No mortality or apparent signs of P deficiency were observed. Increasing dietary AP levels improved all performance variables. Dietary P levels significantly affected bone density. The estimated P requirement for tambaqui was 6.3, 3.68, 5.0 and 6.91 g kg −1 diet for improved weight gain, bone density, vertebrae mineralization and carcass mineral deposition, respectively. CT scanning allowed the visualization of bone structures affected by P deficiency and could predict the P requirement based on vertebrae mineralization. Additionally, blood chemistry parameters were significantly affected by the dietary P level, except for serum calcium. Thus, the use of 6.3 g kg −1 AP in tambaqui diets can maintain normal growth and bone mineralization of juveniles from 17-150 g.

27Phosphorus (P) is an essential mineral for fish growth, as it plays pivotal roles in skeletal 28 development and energy transfer reactions. However, the dietary requirement of this 29 mineral is variable among fish species and the growth stages. Thus, this study aimed to 30 65 However, the P requirement in animals diverge depending on the response 66 parameters evaluated and the models used to estimate the requirement [6]. For instance, 67 P requirement based on weight gain are lower than those estimated based on bone 68 mineralization [7-10]. The use of P requirement estimates based on WG have led to 69 several bone deformities in salmonids imposing a threat to the welfare of fish and the 70 quality of the final product [10][11][12]. This has been used as role for most of the 4 71 aquacultured fish species, at least for Nile tilapia and carp aquaculture [8, 13]. However, 72 limited data on other fish species are available and further extrapolations should be made 73 with care. 74 Tambaqui (Colossoma macropomum) is the second most farmed species in Brazil 75and possess an important role on food security for Latin American countries [14]. 76 Additionally, the high P discharge in tambaqui aquaculture might be a serious problem 77 for the Amazonian aquatic ecosystem where most of the tambaqui farms are located. 78Therefore, the precise P formulation in tambaqui diets is of utmost importance for 79 protecting this ecosystem. 80Although the importance of this species for aquaculture, information on its 81 nutrient requirements are limited and, to the best of our knowledge, just one study have 82 reported the phosphorus requirement for tambaqui in the grow-out stage (~ 150g) [7]. 83 Therefore, we designed a trial to determine the P requirement for tambaqui juveniles 84 (from 15 to 150g) using different response parameters. Additionally, for the first time in 85 a requirement study, we provide evidence that the P requirement for whole-body 86 mineralization might be similar to the estimate based on weight gain while the 87 requirement for bone mineralization are markedly low in fish with dense mineralized 88 bones. 89 5 90 2. Materials and methods 91 All the procedures involving animals in this study was approved by the Animal 92 Ethics Committee of the Universidade Federal de Goiás (protocol # 063/15). 93 94 2.1. Experimental diets 95A basal semi-purified diet was formulated to meet the protein (360 g kg -1 ) and 96 energy (14.02 Mj kg -1 diet) requirements for tambaqui and be deficient in phosphorus (1.5 97 g kg -1 ). Then, incremental levels of dietary digestible phosphorus were obtained by 98 supplementing the basal diet with potassium phosphate to achieve the following levels: 99 3.0, 4.5, 6.0, 7.5 and 9.0 g kg -1 (Table 1). Digestible phosphorus content of potassium 100 phosphate was calculated according to a previous study of our group [15]. 101 102 6 103 104 1 The supplement provided the following: MOS (mín.) 120.0000 mg/kg; Vitamin A (mín.) 16.000 UI/kg; 105 Vitamin D3 (mín.) 4.500 UI/kg; Vitamin E (mín...