Minimizing phosphorus discharge from aquaculture

While there are effective methods for removing nitrogen and organic matter from the effluent of land-based fish farms, there is a lack of cost-effective strategies for removing phosphorus.

Phosphorous is an important potential pollutant to natural water bodies. Any excess dietary phosphorus that fish don’t absorb and incorporate is excreted in their feces and urine, and fish farming might lead to phosphorus eutrophication. The waste production from aquaculture, especially the fraction in dissolved form (orthophosphate), is difficult to remove and withhold in treatment devices. Therefore, minimizing excretion from the fish is a promising way to reduce farm discharge.

To minimize the environmental impact of fish farming in terms of phosphorus, the total dietary phosphorus content must be minimized while ensuring that the available dietary phosphorus matches the fish’s needs. This study tested how much total dietary phosphorus can be lowered without compromising fish performance. This was achieved by carefully selecting raw materials with knowingly high phosphorus availability and adding phytase to improve the uptake of plant-based, phytin-bound phosphorus. Several low-phosphorus diets, tailored to match fish requirements, were produced and fed to rainbow trout (Oncorhynchus mykiss) in a mass-balance setup as well as to rainbow trout in a commercial recirculating aquaculture system (RAS).

Overall, the study showed that it is possible to significantly reduce the excretion of total phosphorus including reducing the excretion of dissolved phosphorus to a minimum without affecting fish performance, thereby reducing the discharge and/or the need for subsequent effluent phosphorus treatment.