The effects ofthe digestible lysine level were assessed in feeds for Nile tilapia fingerlings(Oreochromis niloticus), based on the ideal protein concept. Four hundredand thirty two reverted fingerlings were used, average weight 1.12 ±0.02 g, in a randomized complete design, consisting of 6 feeds, six replicationsand twelve fish per experimental unit. The diets consisted of a basal diet with29.12% crude protein and 3,000 kcal/kg digestible energy, supplemented withsynthetic amino acids, resulting in six diets with 0.95; 1.10; 1.25; 1.40; 1.55and 1.70% digestible lysine and minimum ratios between methionine plus cystine,threonine, thryptophan, isoleucine, arginine with the lysine (66, 77, 23, 64and 85%, respectively), based on digestible values. The fish were maintainedin 130 liter aquaria equipped with individual water and controlled temperatureand aeration. The fish were fed to apparent satiation, six times a day, for30 days. Growth performance, body composition, body protein and fat depositionand nitrogen retention efficiency of the fish were evaluated. The increase inthe dietary digestible lysine did not affect the survival rate and body fatlevel of the fishes. However, there was linear improvement in all the otherparameters assessed, except lysine use efficiency and the body humidity, thatshowed quadratic and lineardecline, respectively. The levels of 1.80 (0.600%Mcal of DE) total lysine and 1.70% (0.567% Mcal of DE) digestible lysine, respectively,result in the best performance and carcass characteristics of Nile tilapia fingerlings,when the ideal protein concept is used to formulate the experimental diets.
