Variations in mucous cell numbers in gills of Atlantic salmon (Salmo salar) presmolt in commercial freshwater farms in Norway

Fish gills are heavily exposed to the external milieu and may react against irritants with different cellular responses. We describe variations in mucous cell counts in gills from healthy Atlantic salmon (Salmo salar) presmolts in five recirculating aquaculture system (RAS) farms and one flow-through farm. Based on certain criteria, mucous cells were histologically quantified in a defined lamellar region of the gills and the counts were analysed. Immunohistochemistry (IHC) was used to investigate epithelial responses. The median number of total mucous cells in the defined region was 59 per fish. Between the farms, the medians varied from 31 to 101 with the lowest in the flow-through farm. A regression model was fitted with total mucous cells as the dependent variable and with fish length and fish farm as independent variables. The proportion of variation in mucous cell counts explained by the model was twice as high when fish farm was included compared to only fish length. IHC revealed proliferative responses in coherence with high mucous cell numbers. Conclusively, the variation in mucous cell counts depends on combined farm-related factors. Establishing a baseline for mucous cell counts is fundamental in the development of high-throughput monitoring programmes of gill health in farmed fish.

Automated summary

The study investigates variation in mucous cell counts in gills of healthy Atlantic salmon presmolts from different aquaculture farms, revealing significant differences between farms and identifying proliferative responses in coherence with high mucous cell numbers through immunohistochemistry. The findings suggest that farm-related factors play a major role in determining mucous cell counts and emphasize the importance of establishing baseline values for monitoring gill health in farmed fish.