A feasibility study on using a simplified mass balance to predict potassium in integrated freshwater aquaculture

This study assessed whether simplified mass balance calculations that neglect retention could be used to predict 
the concentration of potassium (hypothesis: yes) and phosphorus (hypothesis: no) in aquaculture systems, 
irrespective of feed ingredient choice. Three feeds with distinct formulations and graded levels of potassium and 
phosphorus were fed to Nile tilapia (Oreochromis niloticus) in recirculating aquaculture systems. All inputs (feed, 
water, caustic) and outputs (water, sludge, fish) were recorded throughout the 91-day experiment. Mass balance 
calculations with and without retention term (fish, sludge) were then used for prediction. The steady state 
concentration based on a simplified mass balance was systematically about 20 % lower than predicted. No unambiguous statement could be made about phosphorus due to unaccounted inputs that led to considerable deviation of observed from predicted concentrations. The results of this study indicate that the simplified mass 
balance might be appropriate to estimate potassium concentrations. Additional studies are, however, necessary 
to validate the outcomes for a wider range of feed ingredients and fish species.