Dietary Phosphorus Requirement for Tambaqui, Colossoma macropomum, in the Grow-Out Phase.

Phosphorus is essential for fish growth as it is crucial in skeletal development and metabolic reactions. The dietary requirement for this mineral varies among fish species and growth stages. Therefore, the objective of this study was to determine the available phosphorus (AP) requirement for tambaqui during the grow-out phase (± 400 to 1000 g) using growth parameters, whole-body macronutrient composition, whole-body, scales, and bone mineral content, biochemical blood parameters, and activity of antioxidant enzymes as response parameters. A total of 128 tambaqui (395 g ± 20) were distributed across 15 tanks (1000 L each) connected to a recirculating water system, following a completely randomized design with five dietary treatments (4.1, 5.8, 8.0, 9.1, and 10.3 g/kg of AP) and three replicates per treatment. The fish were fed with experimental diets to apparent satiation for 180 days. The apparent digestibility coefficients (ADC) of the experimental diets were also determined to report the requirement on an available nutrient basis. No mortality or apparent signs of P deficiency were observed during the growth trial. The ADC of the diets decreased with increasing total phosphorus levels. AP supplementation in the diet did not affect growth performance parameters except phosphorus utilization, which decreased linearly with increasing AP supplementation. Bone mineralization increased with dietary AP supplementation, while the scale and whole-body mineralization were unaffected. Dietary AP levels, except for serum phosphorus and triglycerides, significantly affected blood biochemical parameters. The highest superoxide dismutase (SOD) activity was observed in fish fed 8 g/kg AP in the diet. The estimated requirement for the highest serum immunoglobulin concentration was 6.17 g/kg of AP. Our findings suggest that tambaqui in the grow-out stage can develop adequately without inorganic phosphorus supplementation to plant-based diets; 4.1 g/kg AP (or 25.2 mg AP/kg BW^0.8/day) seems to be sufficient to maintain growth parameters. However, this minimal level is associated with increased adiposity. To maximize bone mineralization, 10.3 g/kg AP (or 63.5 mg AP/kg BW^0.8/day) is required. Intermediate dietary levels (around 6.17 g/kg AP or 37.9 mg AP/kg BW^0.8/day) were associated with higher total serum protein, lysozyme, and immunoglobulin concentrations, suggesting possible physiological benefits.