Effects of water salinity in an aquaponic system with rainbow trout (Oncorhynchus mykiss), black bullhead catfish (Ameiurus melas), Swiss chard (Beta vulgaris), and cherry tomato (Solanum lycopersicum)

This study evaluated the role of increasing salinity in brackish-water aquaponics, also defined as haloponics, during an 8-month cycle characterized by two fish monoculture phases in autumn and spring and a polyculture phase in winter. The effects of three water salinity levels (low: 0.5‰; medium: 3.0‰; high: 6.0‰) were assessed on the health, growth performance and carcass traits of black bullhead catfish (Ameiurus melas) and rainbow trout (Oncorhynchus mykiss), and the concurrent production of Swiss chard (Beta vulgaris, ssp. vulgaris) and cherry tomato (Solanum lycopersicum). During 268 days, from September to June, a total of 261 catfish (initial weight 147 ± 22 g) were distributed in the nine units of an experimental aquaponic system (three units per treatment; initial stocking density 8.50 kg/m3). In December, 150 trout (initial weight 153 ± 22 g) were added to the system (initial stocking density 5.06 kg/m3) and reared for 103 days with catfish. During the trial, two growing cycles of Swiss chard and one of cherry tomato were carried out. Water microbiota communities were dominated by the phyla of Proteobacteria (60%) and Bacteroidota (19%), followed by Actinobacteriota, Cyanobacteria, Fusobacteriota, Patescibacteria, and Firmicutes. The Shannon entropy index for alpha-diversity decreased when the water salinity increased, where low and high salinities groups significantly differed. As for the 20 most abundant genera, differences according to water salinities were found on their relative abundance. Water salinity did not affect the final weight (348 g, on average), eviscerated carcass yield (87.5%) and fillet (52.0%) yields of trout, as well as the final weight (193 g), eviscerated carcass yield (84.1%) and fillet yield (48.3%) of catfish. Water salinity did not affect the fillet chemical composition in both species. At low salinity, the fillet fatty acid profile of catfish showed lower (-6.5%; P < 0.05) MUFA and higher PUFA (+6.8%; P < 0.05) and n-6 (+5.9%; P < 0.05) proportions compared with high salinity. At high and medium salinity, Swiss chard showed higher total yield (+61%; P < 0.01) in the first production cycle and higher total (+28%; P < 0.01) and marketable (+32%; P < 0.001) yield in the second production cycle compared with low salinity, whereas cherry tomato truss weight was lower (-32%: P < 0.001) at high compared with medium and low salinity. The use of brackish water until 6‰ proved to be a viable alternative to reduce freshwater consumption, without impairing fish and leafy vegetable growth or the overall balance of the aquaponic system.