Supplementation of dietary L-threonine in rohu (Labeo rohita): Effects of stocking density on nutritional physiology, immune response, antioxidant system and disease resistance against Aeromonas hydrophila

A 60-day feeding trial was carried out to examine the ameliorative effects of graded levels of dietary L-threonine [T1 = 7.3 (un-supplemented), T2 = 10.6, T3 = 13.5 and T4 = 16.4 g/kg feed] on immune response, nutritional physiology and disease resistance in rohu (Labeo rohita) fingerlings against different stocking densities and Aeromonas hydrophila. A total of 504 numbers of uniform-sized rohu fingerlings (3.0 +/- 0.03 g) were stocked at two stocking densities, that is, in high stocking density (30 numbers/tank) and low stocking density (12 numbers/tank) in triplicate groups and hand-fed to apparent satiation twice daily. The findings indicated significantly (p < 0.05) improved growth and feed utilization (specific growth rate, absolute weight gain, food conversion ratio, protein efficiency ratio) in a fish-fed diet incorporated with 13.5 g/kg of L-threonine at low stocking compared with the higher stocking. Haematological (haemoglobin, red blood cells, platelets, white blood cells) and serum stress indices (aspartate aminotransferase, alanine aminotransferase, cholesterol, triglycerides, glucose) showed significantly (p < 0.05) improved performance in the same diet group. Likewise, immune response (lysozyme activity, myeloperoxidase, nitro blue tetrazolium, total immunoglobulin) and antioxidant activities (glutathione peroxides, catalase, superoxide dismutase, glutathione S-transferase, malondialdehyde) were significantly (p < 0.05) better in the same diet group at low stocking density. The low stocking group showed significantly (p < 0.05) lower cumulative mortality challenged with A. hydrophila. This investigation has proved the efficacy of supplementation of 13.5 g/kg of dietary L-threonine by amelioration of immunity, nutritional physiology, antioxidant activity and disease resistance in rohu by reducing crowding stress as well.