Water salinity in salmon smolt operations using recirculating aquaculture systems (RAS) often ranges from 2 to 5 ppt to minimise disease and water quality issues. However, the sodium content in effluent can limit its reuse in aquaponics due to negative impacts on plant growth. This experiment tested mineral supplementation strategies to balance nutrients, mitigate sodium toxicity, and enhance plant performance in a decoupled aquaponics setting.
Four treatments (three replicates each) were applied: control, 0 ppt using hydroponics (HYD); ~5 ppt salmon effluent without supplementation (SAL); ~5 ppt salmon effluent + commercial liquid fertiliser blends (SAL+C); and ~5 ppt salmon effluent + precision supplementation with individual agriculture-grade fertilisers (SAL+P). Nutrient film technique (NFT) units (1.6 m², 100 L reservoir) were stocked with 26 lettuce seedlings at 16.5 plants/m². Salmon effluent was sourced twice from RAS smolt production and used in two consecutive crops of 22 days each, totalling 44 days. Water quality and nutrient profiles were monitored.
Both supplementation strategies at 5 ppt (SAL+C and SAL+P) significantly improved lettuce wet weight, length, and productivity compared with unsupplemented salmon effluent (SAL) (P<0.05). Plant greenness (SPAD) and visual quality also increased with supplementation. The 0 ppt hydroponic control (HYD) outperformed all treatments in most parameters, except SPAD and plant quality. These results demonstrate that, with appropriate mineral supplementation, brackish RAS effluent from smolt production can be effectively upcycled to produce high-value, high-quality plants in a decoupled aquaponics system.