Measured oxygen levels in Norwegian waters and implications for future offshore Atlantic salmon aquaculture.

Offshore salmon farms are being planned on the Norwegian Shelf. However, there are concerns of low oxygen levels within sea cages during periods of low water exchange. Hypoxia will compromise fish welfare and production performance, and it is therefore important to characterize oxygen dynamics at new offshore aquaculture sites. Here, we investigated oxygen variability in Norwegian waters by utilizing data from repeated sections and fisheries surveys collected between 2010 and 2023. In total more than 5000 profiles were analysed. Hypoxia was regularly observed in the inshore and fjord areas. However, no evidence for severe hypoxic conditions were seen on the shelf regions despite nutrients input being rather high. This lack of hypoxia at depth suggests that the cross-shelf exchange supplying oceanic water dominates the oxygen balance at the shelf relative to biological consumption. Meanwhile, seasonal cycles in oxygen, apparent oxygen utilization, temperature, and stratification were observed. Notably, oxygen saturations of 70-80% occasionally occurred offshore below the mixed layer during summer and fall. Such moderate hypoxia represents the initial conditions to be considered when planning cage sizes and allowable biomass. Since oxygen will not always be fully saturated, challenges with hypoxia within sea cages will likely prevail when moving offshore.