Atlantic salmon cope in submerged cages when given access to an air dome that enables fish to maintain neutral buoyancy

Abstract Salmon lice, Lepeophtheirus salmonis are a financial and fish welfare issue for the salmon aquaculture industry. Avoiding the planktonic, mostly surface-dwelling infective stage of lice by holding salmon deep in submerged sea-cages could reduce host-parasite encounter rates, reducing the lice problem. However, submergence, or long-term lack of surface access (weeks to months) is not feasible for the Atlantic salmon, which have a physostomous swim bladder that refill by gulping air. Without the capacity to refill their swim bladder, salmon become negatively buoyant and their behaviour and growth is negatively affected. Here, we tested if an underwater air dome in a submerged cage allowed salmon to refill their swim bladders and maintain normal swimming behaviours and physical condition using SWIM (Salmon Welfare Index Model) welfare parameters. We used the recommended size air dome (3 m diameter) for commercial sea cage farming, submerged at 10 m deep, on three cohorts of 10,000 farmed salmon (initial weights 0.5–0.8 kg) submerged for 5–7 wk. The 3 m diameter air dome enabled fish up to 1.5 kg to maintain neutral buoyancy while submerged with no negative effects on fish behaviour and condition factor K. In addition, effects observed on fish welfare SWIM scores for fin and skin condition were negligible. Submerged cage farming with access to a 3 m diameter octagonal underwater air dome could be a viable method to create a vertical spatial barrier between salmon held at depth in a submerged cage and salmon lice that are predominately in surface waters. Future replicated trials at industrial scale should test the feasibility of submerged sea cages instrumented with air domes and their specific effects on reducing salmon lice infestation over an entire grow-out production cycle.