Better water quality for salmon farms

European aquaculture produces more than 300 million salmon smolts (young fish) every year to stock sea-based salmon farms. Aquaculture uses flow-through systems that are replaced by recirculating aquaculture systems (RASs). More precise management of high-quality water and nutrients is thereby required.

The aim of the EU-funded FEED&TREAT (Optimizing water quality and treatment efficiency in recirculating aquaculture systems for salmon smolt production through better adjustment of fish feed and water treatment devices) initiative was to optimise water quality in RASs by improving the quality of fish feed and water treatment devices. RASs have several advantages over conventional flow-through systems: lower fresh water requirements, lower energy consumption, better control of water quality and better disease prevention.

Scientists tested four diet variables: raw material, digestible protein-digestible energy ratio, grinding of raw materials and the use of binders. From 33 diets, researchers identified improved treatability of fish excrements caused by an increase in mechanical strength linked to raw materials like soy protein and the inclusion of binders.

FEED&TREAT chose two diets for further testing in RASs. To achieve this, researchers built six RASs and created a model of consumption and excretion based on carbon-nitrogen ratios. Finally, the optimal selected diet for salmon smolts was tested on commercial scale on a smolt farm in Norway.

Consortium members developed bioreactors for denitrification of waste and validated them for both salmon and pig slurry.

The proposed modifications to the fish feed and treatment of excrements will result in improved water quality and lower mortality in RASs. This means fewer eggs required for the same production level, lower production costs and reduced initial costs, thereby enabling fish farms to be more competitive.