Evaluation of nitrogen uptake and excretion by tilapia in bio floc tanks, using 15N tracing

Uptake and excretion of 15N by tilapia was evaluated in simulated bio floc technology (BFT) tanks. Bio floc suspension (suspended solids in the range of 200 mg l− 1) was enriched in respect to 15N by adding tagged ammonium salt and starch, to ensure a complete immobilization of ammonium in the bio flocs. Fish were held for 14 days in tanks, when the only feed source was the flocs. Nitrogen species in the water, as well as 15N enrichment in the suspended matter and in fish muscle were monitored at frequent intervals during this period.The system under study underwent a series of processes, including the uptake by fish of suspended organic nitrogen, excretion of tagged and non tagged nitrogen by fish, microbial degradation of the bio flocs and the opposing process of new bio flocs production using the excreted nitrogen.The daily net nitrogen uptake (i.e. nitrogen retained by the fish) was found to be 240 mg N kg fish− 1. This is equivalent to a daily uptake of about 1.6 g protein per kg fish, or about 25% of protein normally added with feed. Nitrogen excretion was found to be about twice as high as the net uptake, i.e. gross daily N uptake was about 700 mg N kg fish− 1. Bio floc residence time was estimated to be about 8 h, indicative of the high dynamics of the bio floc system. The results of this experiment were used to evaluate experimental and computational approaches to determining nitrogen fluxes.