Citric acid as a functional supplement in diets for juvenile turbot, Scophthalmus maximus L.: Effects on phosphorus discharge, growth performance, and intestinal health


The present study was conducted to investigate the suitability of citric acid as a feed additive for juvenile turbot (Scophthalmus maximus) to reduce phosphorus (P) excretion. A practical diet with similar profile as commercial diet was used as the positive control (C-P). C-P diet had no inorganic P supplementations, which was used as the negative control (C-0). Two citric acid supplemented diets, C-1.5 and C-3.0, were formulated by incorporating 1.5% and 3.0% citric acid into the C-0 diet, respectively. Each diet was fed to triplicate groups of 30 fish. The feeding trial lasted for 12 weeks. Turbot fed different diets showed similar growth performance, feed utilization efficiency and whole-body composition. Compared with group C-0, the pH of stomach contents in group C-3.0 was significantly lower while the pepsin activity and apparent digestibility coefficient of P in this group were notably higher. Diet C-3.0 resulted in the highest P retention, which was significantly higher compared to diet C-P but not to diet C-0. Compared to both control diets, citric acid supplemented diets significantly increased the activity of pepsin. No diet-related difference was observed in the blood biochemistry, nor in the histology and bacterial community profile of distal intestine. In summary, dietary inclusion of 3% citric acid markedly improved the bioavailability of P, without compromising intestinal function and health of fish. Citric acid seemed to be a promising feed additive for aqua-feeds to reduce P discharge into environment.