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BackgroundLysine is an essential amino acid and, generally, the first limiting in diets for tambaqui (Colossoma macropomum), main native fish produced in continental aquaculture from South America. However, there is a lack of information on their amino acids requirements, especially for maintenance and efficiency of utilization of lysine.ResultsDiets with increasing levels of digestible lysine significantly increased final weight and the weight gain of tambaqui, for all studied ages. Feed intake decreased significantly only for fish weighing 121 g (p < 0.05), and was not affected for other ages. However, digestible lysine consumption significantly increased in all body weights groups evaluated (p < 0.05). In addition, digestible lysine consumption provided sufficient intervals to obtain of values negative, near zero and positive of retention body protein and lysine, allowing estimation of maintenance requirements without extrapolation. The maintenance lysine requirement was determined at the intersection of the point to maintain body protein retention equal to zero, and the efficiency of lysine utilization was the slope of the line between the digestible lysine consumption and its retention, for different tambaqui body weights (121, 235 and 596 g). Linear responses (p < 0.05) were observed between protein retention and body lysine in function on the consumption of digestible lysine for all evaluated body weights, and the parallelism test showed influences of body weight on the values of the maintenance requirement and efficiency of utilization for lysine, with an increase in the maintenance requirement as the fish grow. However, the efficiency of using lysine decreases with the increase in fish body weight.ConclusionsThe requirement of tambaqui for maintenance lysine and efficiency utilization are directly related to the body weight. The requirement for digestible lysine for tambaqui maintenance was 82.03 mg kg-0.7 day-1 for body weights 121 to 235 g and utilization efficiency of 55%. For weight of 596 g, the requirement of maintenance was 106.85 mg kg-0.7 day-1 and 40% of utilization efficiency of lysine.