Black soldier fly larvae (BSFL) are increasingly being investigated as an ingredient in fish diets, but a by-product is the left over ‘frass’, which is their excrement rich in minerals and amino acids. There is also interest with dietary inclusions, however, the composition of frass can be influence by the initial substrate. In this study, BSFL frass was made using either spoiled feeds (SF) or vegetables/fruits (FV) that were high and low in protein, respectively. The proximate and amino acid composition of these frass types were analysed to formulate isonitrogenous and isolipidic diets containing 5 or 10% SF or FV frass, while a control diet was made with no frass inclusions. These diets were fed to tilapia juveniles (initial weight = 2.94g) for 2 months in a closed recirculating system.
Results showed that 5 and 10% FV frass diets significantly enhanced growth compared to the control or FV frass diets. Contributors to enhanced growth were increased palatability based on a higher feed intake and a better amino acid profile. Liver histology revealed no inflammation or any other adverse response in any of the treatments, indicating dietary frass at the levels used showed no hepatotoxicity. Moreover, the intestinal villi of tilapia fed the SF frass diets were significantly longer than those in the control (Fig. 1).
Fish muscle phosphorus was significantly higher at FV frass treatment while magnesium was significantly higher at 10% FV frass compared to the 5% SF frass treatment. The remaining biochemical analysis (proximate, amino acid, and fatty acid composition) are being analyzed. Preliminary gene expression analysis indicates that dietary frass inclusions and type greatly influences metabolism and detoxification pathways.