The effect of protein sparing and amino acid application in African catfish (Clarias gariepinus) diet on growth rate and water pollution by nitrogen

A 10-week feeding experiment in aquaria with African catfish (initial weight 21.1 +/- 4.2 g) was performed with the aim to reduce the loading of water environment with nitrogen by both sparing the crude protein (fish meal) content in the diet, and fortification of energy content (soya oil) and essential amino acids (lysine, methionine, threonine). Seven isocaloric (16.5 kJ.g(-1) gross energy) feed mixtures were formulated with graded crude protein levels and amino acid (A) supplementation - 45% (control group), 40%, 40%+A, 35%, 35%+A, 30% and 30%+A. The amino acid application was manifested positively in all investigated parameters and groups. A survival rate amounted to 100% in all groups during the whole experiment. Growth rare, weight gain, SGR and feed conversion ratio were not affected by reduction of crude protein to 40%, 40%+A a 35%+A (P > 0.05) in comparison to control (45%). Carcass crude protein and ash content did not differ significantly in all experimental groups (P > 0.05), higher fat deposition in comparison to control was recorded in groups with low crude protein content (35%, 35%+A, 30%, 30%+A). The highest crude protein retention was found in variant 35%+A (51.3%), whilst the lowest in the group with 35% crude protein in diet (44.7%). Nitrogen loading of water environment per unit of weight gain was decreased in all groups (except 35%) with reduced crude protein content. This effect was more pronounced in all groups with amino acid supplementation.