Investigations into the requirements of the tilapia Sarotherodon mossambicus for dietary methionine, lysine and arginine in semi-synthetic diets

Abstract

Male Sarotherodon mossambicus (mean weight 1.7 g) were fed complete diets containing 20% protein (N × 6.25) from a mixture of groundnut, soya and fish meal and the equivalent of 20% protein in the form of synthetic L-amino acids. The amino acid for which the requirement was being determined was supplemented at varying levels producing dietary inclusion levels for methionine from 0.53 to 0.86% (cystine level 0.74%), for lysine from 1.42 to 2.44% and arginine from 1.59% to 3.18%. In addition two diets were tested containing no synthetic amino acids, a basal diet of 20% protein and a positive control with 40% protein. The results demonstrated that the tilapia could utilise free amino acids for growth. The minimum dietary level of methionine at the cystine level employed producing satisfactory growth was less than 0.53%. Growth rates appeared to deteriorate with increasing levels indicating an inhibitory effect of excess free methionine and a need for further investigations on methionine/cystine interactions. The minimum lysine level promoting good growth was 1.62% while for arginine there was no significant difference between any of the levels tested indicating a minimum dietary level of less than 1.59%. These values are lower than has been reported for most other fish.