Refilling behaviour of Atlantic salmon (Salmo salar) with different air-dome heights

The increased salmon lice (Lepeophtheirus salmonis), issue causes welfare concerns in salmon aquaculture industry. The resistance of lice to different medicinal treatments has led research to increasingly focus on preventive measures rather than lice removal techniques. One preventive measure is to submerge sea cages and force the salmon to stay deeper in the water column, with the goal of mismatching the distribution of farmed salmon from the surface-searching infective salmon lice copepodids. Submergence, however, faces some challenges for the salmon, who have a physostomous swim bladder that requires them to access the surface to take in air to refill their swim bladder. To compensate for this need, an underwater air-dome installed in the center of the cage can ensure air access for the salmon. Different sizes of the dome have been tested, and this study aim to find a preferred height of the dome where the salmon can refill swim bladder, execute normal behaviour and maintain good welfare. In this study we tested three different heights of a surface based dome to test potential differences in surface behaviour and welfare indicators (using SWIM, Salmon Welfare Index Model) between the different heights. All domes were 1 m diameter and mounted in the center of a 3 m diameter cylindric indoor tank. The different experimental heights of the dome were 2 cm, 10 cm and 95 cm with three replicate tanks of each heights, totalling of nine tanks. 3 600 salmon were distributed between the nine tanks (400 in each tank). After an acclimation period with domes, salmon lice were introduced in all tanks as a stressor. Behavioural observations and SWIM assessments were conducted regularly during the whole experimental period. Results indicate that both welfare and behaviour were not negatively affected by dome height, suggesting that 2 cm dome height is sufficient for swim bladder refilling and conducting natural behaviour. Results, however, revealed increased snout damage in 66 % of tanks, a condition that has been observed in previous submergence trials.