Efficiency of salmon production in submerged cages with air domes matches standard surface cages when environments are similar

Submerged cage technology may enable placement of salmon farms at more exposed sites whilst avoiding the impacts of damaging weather on farm structures. Field trials have demonstrated that Atlantic salmon grow well in submerged cages for a full production cycle if an air dome is provided to enable salmon to refill their swim bladders at depth, but on occasion submerged fish have slower growth and poorer fish welfare. This could be due to submerged fish experiencing less optimal environmental conditions, the physiological effects of holding fish submerged at depth, restricted access to surface air, or an interaction between these factors. Here, we designed an experiment to remove possible environmental and depth-related effects, which enabled a direct test of the effect of restricted surface access to air due to submergence. Atlantic salmon were cultured for 11 months in either standard surface cages or submerged cages with air domes installed with a ceiling at 1-m depth to stop normal surface access. Environmental conditions were similar in surface and submerged cages. Restricted surface access had minimal effect on fish behaviour, growth, condition, welfare and harvest quality of submerged fish relative to fish held in standard surface cages. Backscattering from echo signals showed that the volume of air in the swim bladders of submerged and surface cage fish was similar, indicating submerged fish captured enough air via the air dome to maintain neutral buoyancy throughout the production period. These findings help progress commercial use of air domes and submergence, as they show the feasibility of maintaining optimal growth rates in Atlantic salmon when submerged for long periods if environmental conditions during submergence are equivalent or better than surface conditions.