The aim of this study was to determine the requirement for lysine in the diet of juvenile tambaqui (Colossoma macropomum). In total, 750 juvenile fish (0.34 ± 0.02g) were distributed in tanks arranged in a randomized block design with six treatments, five replications in two blocks, and 25 fish per unit, for 50 d. Six levels of digestible lysine were tested (1.30, 1.48, 1.66, 1.84, 2.02, and 2.20%), in formulated diets based on the ideal protein concept. The performance, feed efficiency, daily deposition of protein and body fat, and nitrogen retention efficiency of the fish were evaluated. The consumption of rations and protein, specific growth rate, body composition, and deposition of body fat were not influenced by the lysine levels tested. The consumption of digestible lysine increased linearly and the efficiency of digestible lysine for weight gain decreased linearly with the increase in lysine levels. The levels of digestible lysine that optimized weight gain and body protein deposition were estimated at 1.73 and 1.78%, respectively. The feed conversion and nitrogen retention efficiency were most improved at 1.66% and 1.84%, respectively. The recommended level of dietary lysine for providing better weight gain and body protein deposition in juvenile tambaqui is 1.78%, equivalent to 2.00% total lysine.

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Cross breeding of native fish species is a technique to produce hybrids that can express higher weight gain and feed efficiency compared to the parental species. The digestible threonine requirement in diets for tambatinga fingerlings (Colossoma macropomum ♀ x Piaractus brachypomus ♂) was determined in this study. For this, 700 fingerlings with an average initial weight of 2.39 ±0.02 g and average final weight of 35.96 ±2.03 g were distributed in a completely randomized design consisting of six treatments (0.600, 0.800, 1.000, 1.200, 1.400, and 1.600% digestible threonine) and five replicates per treatment, with 20 fish per experimental unit. Diets were formulated by the “diet dilution” technique using the ideal protein concept. Performance, feed efficiency, body depositions of protein, ash and fat, and nitrogen retention efficiency of the fish were evaluated. The digestible threonine levels that optimized weight gain, specific growth rate, and feed conversion ratio were 1.40, 1.27 and 1.10%, respectively. Body deposition of fat was reduced in a quadratic manner (p<0.01), and the body depositions of protein and ash, together with the efficiency of nitrogen retention, were optimized by the digestible threonine level of 1.20; 1.33, and 0.82%, respectively. The recommended digestible threonine level in the diet for tambatinga fingerlings is 1.20 to 1.40% (0.40 and 0.47% Mcal DE-1) to obtain higher body deposition of protein and weight gain, respectively.

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