Nitrogen, phosphorus, and carbon dynamics in biofloc system of Nile tilapia fed with high non-starch polysaccharides diet

Samenvatting

While microbiota need to consume organic carbon to maintain water quality, commercial aquaculture feed produces a post-feeding waste with a carbon to nitrogen ratio that is too low to allow the bacteria to mineralize all the waste. The goal of this study was to investigate the effects of replacing starch-rich ingredients used in a Control-diet with non-starch polysaccharides (NSP) rich ingredients resulting in a High-NSP-diet on the water quality and nutrient budgets in biofloc tanks with Nile tilapia (Oreochromis niloticus). The main NSP-rich ingredients in the High-NSP-diet were palm kernel meal, rice bran and corn gluten feed. Both diets were isonitrogenous, but the High-NSP-diet contained 3 times more fiber than the Control-diet. After 8 weeks of cultivation, the individual fish growth and feed conversion ratio was similar between two dietary treatments, but due to a lower survival in tanks fed the High-NSP-diet the total biomass gain was slightly better in the Control-diet tanks. The contribution of biofloc to fish growth with the High-NSP-diet was 17 %, compared to 14 % when feeding the Control-diet. The water quality was similar between both dietary treatments, with less accumulation of nutrients in the water column in the High-NSP-diet tanks. This was shown by a 4 % lower retention efficiency of carbon (C), 12 % of nitrogen (N) and 7 % of phosphorus (P) in the water column in High-NSP-diet fed tanks than in the Control-diet tanks. The High-NSP-diet did not affect the C, N, and P concentration of the biofloc, with a relatively stable concentration of C and N among weeks and a steady increase in P concentration until week six (W6). The biofloc concentration increased until W6 and decreased in week eight. Correlation analysis revealed strong correlations between the concentrations of total suspended solids (TSS), total organic matter (TOM) and NO₃, and between TSS and Chl-a. Subsequent regression analysis showed that the TSS concentration can be best predicted based on the TOM concentrations in tanks fed the Control-diet and based on the NO₃ concentration in tanks fed the High-NSP-diet. This approach will help modellers or farmers to predict the amount of TSS based on a limited number of other water quality parameters. Economic analysis showed that in spite of a lower production at system level when feeding the High-NSP-diet, due to the lower cost of this diet, the effect on income is negligible, with a higher 0.03 USD profit per kg fish produced compared to the Control-diet.