From fish excretions to high-protein dietary ingredient: Feeding intensively cultured barramundi (Lates calcarifer) a diet containing microbial biomass (biofloc) from effluent of an aquaculture system

Aquaculture is the fastest growing agricultural sector in the world, and the annual aquaculture production reached 82 million tonnes in 2018. Fish feed is the most expensive component of aquaculture. The partial replacement of commercial fish feed with biofloc was evaluated as a dietary ingredient in feed for barramundi (Lates calcarifer), a high-value carnivorous fish. Biofloc was produced from the effluent of a recirculating aquaculture system under microaerophilic conditions, as part of its water treatment process, which was based on nutrient assimilation by microbial biomass. The biofloc was harvested and used in fish feed. The experimental feed was prepared by replacing 20% (w/w) of commercial barramundi feed with dry biofloc powder. Six 100 l plastic tanks were stocked with barramundi (average weight 27 ± 0.9 g) at a density of 20 kg/m³. The fish in three tanks were fed a commercial feed and the others with the experimental diet for 8 weeks, at 2% bodyweight per day. The fish were weighed weekly. At the end of the trial, the proximate body compositions and immune functions of the fish were analyzed. Fifteen fish from each treatment were challenged with Vibrio harveyi to examine their disease resistance. Weight gain was 14% higher (p < 0.05) in the control group compared to biofloc -fed group, but fish fed biofloc showed significantly higher survival rates after bacterial challenge (89% survival) compared to control (58% survival, p = 0.01). Sera of the biofloc -fed fish had higher levels of lysozyme and myeloperoxidase activity (p < 0.001). Similarly, the expression of immune-related genes in the kidneys was significantly higher in the biofloc -fed fish than in the control (p < 0.05). These results, together with an economic evaluation, demonstrate the potential practical application of biofloc as a feed ingredient in barramundi aquaculture. Further optimization of this practice is required.