Post-smolt Atlantic salmon can regulate buoyancy in submerged sea-cages by gulping air bubbles

Abstract

Salmon lice (Lepeophtheirus salmonis) are the most severe health challenge facing Atlantic salmon (Salmo salar) aquaculture. Infestations threaten farmed and wild salmonids, drive up production costs, and limit industry growth. Submersible sea-cages can reduce infestation pressure by holding salmon below the surface layers where lice larvae are most abundant. However, submergence leads to loss of buoyancy over time, as salmonids must periodically refill their swim bladders by gulping air. Pockets of air held at depth within ‘air domes’ enable salmon to gulp air, but air domes are large and unwieldy structures. A more intuitive solution may be to deliver a stream of air bubbles at depth, as bubbling is commonly used in sea-cages to promote vertical mixing, and salmon interact with these bubbles. Here, we conducted a pilot study to test whether newly sea-transferred salmon would use bubbles for buoyancy regulation. We conducted 3 trials in sequence, each involving 15–19 days of continuous submergence, and recorded buoyancy-related behaviors in 1 cage with full surface access, 1 cage submerged without air, and 2 cages submerged with air bubbling. Bubbling was provided continuously in the first two trials, before being reduced to 90 min day⁻¹ in the third trial. Salmon submerged with bubbling maintained their swim bladder volume and swimming speed at similar levels to the cage with full surface access throughout all trials, indicating that they successfully refilled using bubbles. The same parameters deteriorated in salmon in the cage submerged without air. Acclimation to bubbles took ∼3 days whether at the surface or after submergence, indicating that several days of bubbling before submergence may be beneficial. Some fish accumulated fluid in the swim bladder during submergence (more severe in fish submerged without bubbling). Together, the results suggest that bubbling can improve outcomes for salmon undergoing prolonged submergence. Moreover, bubbling offers an alternative to air domes that is less technically demanding, provides a greater spatial distribution of air, and supports welfare through behavioral enrichment.