Welfare, behaviour and feasibility of farming Atlantic salmon in submerged cages with weekly surface access to refill their swim bladders

Abstract Farming Atlantic salmon ( Salmo salar L) in submerged cages may be a way for the industry to avoid surface related problems such as salmon lice and make new farming sites available. However, salmon do not cope with long term submergence and regularly need to access the surface to refill their swim-bladders and maintain buoyancy. We tested how salmon coped with submerged cages that were lifted to the surface once each week to allow surface access. Three submerged cages were stationed with a roof depth at 10 m for a period of 8 weeks and measures of welfare and behaviour were compared against three standard cages. All cages held approximately 1700 newly sea-transferred smolts. Submerged fish swam 1.4× to 3.4× times faster, schooled tighter and swim bladder volumes declined gradually from the start to the end of each week. When cages were lifted and the surface became available, an immediate burst of jump and rolling behaviour showed that the salmon had been negatively buoyant. Nevertheless, no evidence of acute buoyancy problems occurred during submergence and similar growth rates and welfare scores were recorded. Our results indicate that farming salmon in submerged cages with weekly surface access is viable and could assist in solving key production problems.