Fish excrete digested but excessive dietary phosphorus (P) as dissolved inorganic orthophosphate (PO4-P) . As PO4-P is generally considered harmless to on-growing fish, r ecirculating aquaculture systems (RAS) typically do not apply P removal units within the recirculating loop , allowing PO4-P to accumulate until balanced by water renewal levels (make-up water). However, if no end-of-pipe treatment is applied, high concentrations of PO4-P in the discharge can contribute to the eutrophication of recipient water bodies . Tailoring dietary P levels to exactly match fish requirements may be a means to reduce PO4-P discharges from RAS.
To explore this , three levels of PO4-P concentrations in RAS were examined and achieved by using a low-P diet, a high-P diet , and the high- P diet plus the addition of Na3PO4 . Each treatment was tested in quadruplicated pilot-scale RAS with rainbow trout (Oncorhynchus mykiss) for eight weeks, applying a fixed daily feed loading , fixed about of make-up water, and obtaining weekly water samples from the sumps.
The concentrations of PO4-P in all three treatment groups quickly diverged and stabiliz ed at three significantly different levels from week six and onwards, reflecting the loading. However, the low-P diet did not lead to near-zero PO4-P accumulation in RAS due to compromised feed conversion ratios. Total phosphorus concentrations were likewise significantly different , while there were no effects on other physicochemical water quality parameters including nitrogen compounds, water clarity, and organic matter.
Altogether, the study shows that manipulating dietary P levels to match fish requirements can be an efficient tool to reduce the discharge of dissolved phosphorus from RAS , reducing the need for end-of-pipe treatment and rendering the production more sustainable.