Comparative evaluation of Brewer's yeast as a replacement for fishmeal in diets for tilapia (Oreochromis niloticus), reared in clear water or biofloc environments

Brewer's yeast as a replacement for fishmeal in the diet of tilapia (Oreochromis niloticus) was evaluated in a clear water and a biofloc environment. Triplicate groups, each of 20 juvenile tilapia (initial weight of 29 +/- 3.2 g), were randomly assigned to 24 conical 500-L tanks equipped with feed waste traps. Twelve of the tanks were in a recirculating aquaculture system (RAS) with clear water, with mechanical and biological filters and on average 15% daily replacement of water. The other 12 tanks were connected to four serial 10 m3 open biogenerator tanks that were initially fed with Bacillus subtilis and nutrients in order to achieve a C: N ratio of 10, supporting bacterial growth. In this biofloc system, only evaporated water was replaced. Four iso-nitrogenous (35%) and isoenergetic (19 MJ kg-1) diets were formulated to contain graded levels of brewer's yeast where 0%, 30%, 60% and 100% of fishmeal protein was replaced by yeast. Fish were reared for 3 months, with hand-feeding twice a day, achieving at least a fivefold increase in weight. At the end of the experiment, feed intake, protein intake, weight gain, daily weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio, survival rate and body indices of tilapia were determined. In fish reared in clear water there was a lower weight gain, daily weight gain and specific growth rate with replacement of fishmeal, but significant only in the 100% replacement group. Despite nearly identical feed and protein intake in both environments, significantly higher growth was observed in fish kept in the biofloc environment, accompanied by significantly improved feed conversion ratio and protein efficiency and reduced mortality. A significant decrease in entero-somatic and intestinal index with increasing level of dietary brewer's yeast was observed, but only in the clear water environment. No significant effects or trends were noted in any other body index data, either for yeast inclusion rates or water environments. Based on this, we concluded that brewer's yeast represents a possible high-volume substitute for fishmeal in tilapia diets, especially if the fish are reared in a high-density microbial environment, i.e. a so-called biofloc environment.