Isotopic turnover of carbon δ13C and nitrogen δ15N in the muscle tissue of tambaqui Colossoma macropomum juveniles fed diets containing different levels of digestible protein

The technique that uses natural stable isotopes rises as a safe alternative to evaluate the deposition of potential food constituents’ elements. Diets with distinct isotopic signatures can be used to measure turnover rates in all animal tissues, improving knowledge of metabolic processes in fish nutrition. The aim of the study was to evaluate the growth and turnover of stable isotopes of carbon and nitrogen in the muscle tissue of tambaqui juveniles fed diets containing different levels of digestible protein. A total of 1750 tambaquis were used (weight 6.53 ± 0.43 g and length 7.58 ± 0.18 cm) and distributed in 35 tanks (450 L), with a density of 50 fish per tank. Seven isoenergetic (3378.4 cal g−1) experimental diets were formulated with digestible protein (DP) levels of 14%, 17%, 20%, 23%, 26%, 29% and 32%. During 120 days of experiment, the animals grew over time in all treatments (P < 0.05); and in the evaluated periods, the protein levels of 29% and 32% stood out from the first biometric evaluations, at 30 days, with similar weight averages (P > 0.05) and higher than the averages in the other treatments (P < 0.05). The speed of isotopic changes in muscle tissue was influenced by the characteristics of the diets, and the rates of carbon and nitrogen turnover were more accelerated the higher the protein level. The trial period of 120 days was sufficient for the fish to reflect the isotopic composition of the diets in all treatments (P < 0.05). However, the isotopic contribution of the diet to the tissue as a function of time was different, with indices that reached about 96–97% in 49–63 days of feeding for the diets of 29% and 32% DP and indices of 81–82% for diets of 14% and 17% DP in the same period. The isotopic data analyzed by exponential equations and exploratory analysis showed the formation of three groups with homogeneous characteristics in response to the effects of protein levels: a grouping of diets with 14%, 17% and 20% DP (P > 0.05); another with intermediate levels (23% and 26%); and a third with the highest levels (29% and 32%). Thus, the diet containing 29% of digestible protein proved to be the most attractive for the productive performance of tambaqui juveniles in the growth phase studied.