Growth performance, deformity rate, body composition and digestive organ morphology of juvenile common carp fed dry diet enriched with hydrochloric, citric or acetic acid

A 60-day experiment on juvenile common carp ( Cyprinus carpio ) evaluated the effects of different acidic dietary additives on growth performance and various welfare indices of the fish. Four feeding groups were established: CON (control group, fed with an unmodified commercial feed), HCL (the same feed supplemented with 1.5 % hydrochloric acid), CIT (+1.5 % of citric acid), and ACE (+1.5 % of acetic acid). In result, only group HCL achieved a significantly higher body weight (25.98 g; p = 0.0109) and total length (11.94 cm; p = 0.0044) than CON (23.51 g, 11.51 cm), and the skeletal deformity rate in all three experimental groups was evenly reduced (from 16.7 % in CON to 2.2 %; p < 0.0003). Group CIT had a significantly higher whole-body protein content (153.6 g kg –1 ; p = 0.0077) than CON (141.7 g kg –1 ). Histological measurements revealed predominantly favorable alterations of hepatocytes in groups HCL and CIT, and also of intestinal mucosa in HCL. TEM analysis showed an elongation of enterocyte microvilli in HCL (1.503 µm), CIT (1.579 µm) and ACE (1.520 µm), in comparison to CON (1.363 µm; all p < 0.0001). Beneficially, groups CIT and ACE showed increased ( p = 0.0134 and 0.0009, respectively) hepatocyte proliferation (60.52 and 64.28 %) than CON (44.72 %), as well as lowered ( p = 0.0031 and 0.0024) hepatic activity of superoxide dismutase (SOD; 12.84 and 12.89 g –1 , as compared to 21.37 g –1 ). In summary, hydrochloric acid yielded the best results in terms of fish growth and histological markers of enteric homeostasis, while citric and acetic acids notably improved several physiological markers of the liver. Hence, these cheap ingredients could be tested on a broader scale, to optimize their dosage for common carp of various size classes, as well as to assess their effects in combination with different dry diets.