Influence of dietary phosphorus on orthophosphate accumulation in recirculating aquaculture systems with rainbow trout (Oncorhynchus mykiss)

Dietary phosphorus (P) levels affect the excretion of orthophosphate (PO₄-P) from fish. Since most recirculating aquaculture systems (RAS) do not specifically remove PO₄-P within the recirculating loop, dietary P levels may influence PO₄-P concentrations in RAS water. A feeding trial was performed in freshwater RAS, aiming to determine the relationship between dietary P levels and PO₄-P accumulation in RAS water, and to examine potential effects of dietary P levels on microbial activity and abundance in RAS water given that P is an essential mineral for all microorganisms. Three diets with 1.49% (HiP), 1.20% (MeP), and 0.89% (LoP) total P, respectively, were each fed to triplicate pilot-scale RAS with juvenile rainbow trout (Oncorhynchus mykiss) for five weeks, applying a daily feed loading of 1.67 kg feed/m³ make-up water. Digestible dietary P levels correlated significantly with the PO₄-P concentrations in RAS water at the end of the trial, while no similar correlations with microbial abundance or activity were observed. Nitrate-N concentrations measured by the end of the trial matched predicted equilibrium concentrations. In contrast, measured PO₄-P concentrations were approximately 40 – 50% lower than predicted regardless of diets, suggesting that PO₄-P was likely utilized by RAS microbes. We therefore speculate whether lowering dietary P levels could become an effective tool for microbial management in RAS.