Variations in mucous cell parameters in the gills and skin of farmed Atlantic salmon (Salmo salar) from freshwater flow-through and recirculating aquaculture systems compared to the gills of wild Atlantic salmon juveniles

This study describes mucous cell (MC) parameters in barrier epithelial tissue of Atlantic salmon (Salmo salar) juveniles reared in freshwater flow-through (FT) and recirculating aquaculture systems (RAS). Using Veribarr (TM) technology, we found no significant differences in the mean MC size and abundance in the gill lamella and skin between FT and RAS systems (p > 0.05, lme). Notably, 100% of gill lamella MC parameters in RAS facilities fell within the most common range (mean +/- 0.5 standard deviation, 38% confidence interval) for farmed salmon gill mucosa, whereas MC parameters in only 20% of FT facilities fell in this most typical range. The differences between individual FT facilities resulted in a greater variability in MC parameters compared to RAS facilities, especially in the gills, likely due to a more controlled environment in RAS. MC sizes and abundances in gills at some FT facilities were comparable to those reported in wild Atlantic salmon juveniles. This finding suggests that the water conditions at these facilities may not evoke a response from the gill mucosa, possibly indicating an environment better suited to the natural physiology of Atlantic salmon. Facility-specific conditions, rather than system-specific conditions, play an important role in the health of salmon barrier tissues. The study underscores the potential of using MC parameters as a monitoring tool, paving the way for innovative baseline health promotion as a preventative disease control strategy in aquaculture systems.