Buoyancy matters: Establishing the maximum neutral buoyancy depth of Atlantic salmon

Buoyancy regulation is a fundamental process by which pelagic fish maintain their position in the water column in an energy efficient way. Buoyancy has been largely overlooked as an important factor in Atlantic salmon production as salmon have a physostomous swim bladder that they fill by gulping air from the surface. The speed and ease at which the swim bladder can be filled and emptied by salmon has made understanding how much it contributes to buoyancy difficult to calculate. Here, we used an increased excess mass WA to investigate the maximum neutral buoyancy depth in seawater of farmed post-smolt Atlantic salmon across a range of sizes. The WA involved adding small weights to salmon over time to make them heavier. The salmon compensate for the extra weight via gulping air at the water's surface to fill their swim bladders and maintain neutral buoyancy. We monitored the swimming behaviour of salmon to determine the weight at which they could not maintain neutral buoyancy and used this to calculate the maximum neutral buoyancy depth (MNBD). For postsmolt salmon of 175-2400 g, the MNBD in seawater ranged from 21 to 24 m, with some variation with fish size: the average MNBD of 175 g salmon was 3.2 m shallower than 2400 g salmon. Fish body density also influenced MNBD. The individual with the lowest recorded body density (1.043 g/cm(3)) could achieve neutral buoyancy 11.2 m deeper than the individual with highest recorded body density (1.065 g/cm(3)). We found that salmon weigh approximately 2.65% of their weight in air in seawater. Depth-modified cages are becoming increasingly popular in Atlantic salmon aquaculture; farming fish deeper requires knowledge of the basic limits of salmon buoyancy and our results will inform guidelines that ensure optimal welfare in these new cage types.