Abstract

The effects of substitution dietary fish protein by soybean protein on growth, survival, biochemical composition and mineral composition of juvenileHeterobranchus longifilis, Sarotherodon melanotheron and Oreochromis niloticuswere evaluated. Three diets were formulated to be isonitrogenous (35% crude protein) by substituting fish meal for soybean meal at levels of 0 (FM), 25 (SBM25) and 50% (SBM50). Diets were fed to triplicate groups of each species at ratio of 5% body weight. At the end of the study period, final body weight (FBW), final body length (FBL), specific growth rate (SGR) and protein efficiency ratio (PER) decreased with an increasing dietary soybean protein level in H. longifilis. Survival rate (SR) was 1.5 times lower in juvenile catfish fed SBM50 (29.3%) than FM- and SBM25-fed fish (44%). S. melanotheron fed all three diets had no significant difference in FBW, FBL, SGR, SR, FCR, PER or production time. In O. niloticus, SR, FCR and PER were not affected by dietary protein substitution level. However, FBW, FBL and SGR increased with an increasing dietary soybean protein level. The production time generally showed a decline at lower protein substitution levels. The production cost was significantly lower in fish fed SBM50. Proximate composition analysis indicated that the carcass moisture in the three species and carcass protein content in H. longifilis and O. niloticus were not affected by dietary protein source. In S. melanotheron, carcass protein content showed a decline in fish fed SBM50. Carcass lipid, ash and gross energy levels in these species were also significantly affected by dietary protein sources. Carcass calcium and phosphorus concentrations in fish were significantly reduced with a high inclusion of dietary soybean meal. The results of this study indicated that fish meal can be replaced with soybean meal up to 50% level in diets for Tilapia S. melanotheronwithout adverse effects on growth, nutrient utilization or nitrogen balance.

Key words: Fish protein, soya protein, growth, feed efficiency, proximate composition, mineral composition.