Atlantic salmon (Salmo salar L.) in a submerged sea-cage adapt rapidly to re-fill their swim bladders in an underwater air filled dome

Abstract The latest literature demonstrates that long-term culture of Atlantic salmon in submerged cages is presently unfeasible. Submergence causes loss of air from the swim bladder, leading to negative buoyancy, modified swimming behaviours, and reduced growth and feed utilisation. We tested a possible solution to resolve negative buoyancy during submergence in a 175 m 3 sea-cage, using an underwater dome containing 120 L of air integrated into the roof netting. We tested whether salmon accessed the dome to refill air into their physostomous swim bladders. 15 adult salmon (mean weight 3.3 kg, length 66 cm, age 2.5 yr) were individually PIT tagged and placed in the submerged cage. The dome was equipped with a PIT antenna which detected individuals that passed within approximately 0.5 m. During periods with no air in the dome, fish approached the dome on average 20 times day −1 (range 0.1–63 times day −1 ), indicating they were searching for air or the surface. When the dome was filled with air, salmon rapidly swallowed air repeatedly from the underwater air-pocket (mean: 5 swallows fish −1 within 24 h of air being present, range: 0–14 fills fish −1 ). After this intense initial bout of swim bladder filling behaviour, the salmon swallowed air from the surface less frequently when air continued to be present in the dome over the following days (mean: 1 swallow fish −1 day −1 : range: 0–9). Swimming speeds of salmon were 1.5–2 times faster when no air was available and quickly returned to normal speeds when air was made available in the dome. Our results demonstrate that salmon rapidly adapted their behaviour to use this new equipment and open the possibility for the use of underwater air domes in sea-cages.