Nanofiltration of intake water in RAS: Effect on fish performance, health and welfare

The experiment tested the effect of nanomembrane filtration in RAS on the salmon's physiology, welfare and growth. A specially designed membrane was used to remove sulphate from seawater and to adjust salinity from 32 to 12 ‰. A RAS where salinity was adjusted to 12 ‰ by mixing raw seawater and freshwater was used as control. The fish were kept in the two systems for 10 weeks before being transferred to tanks with seawater flow through for 4 weeks. Histology and measurements of gene expression in the gills, intestine, and skin were done and blood samples were taken to measure the content of cortisol and blood plasma ions after transfer to seawater. Post smolt in RAS with nanofiltered water grew slightly better compared to traditional RAS and the same tendency was maintained after transfer to seawater. There were no differences between systems in welfare indicators or skin morphology. In RAS with nanofiltration, some individuals showed an increase in the area of the gill lamella covered by mucus-producing cells and a lower number of chloride cells on the secondary lamellae in the gills. The effect of nanofiltration on gene expression in skin and gills was small, with only two genes that were differentially expressed in skin and 28 genes in gills