How submerged cages affect salmon welfare and behaviour
Submerged cages are an exciting new method being tested by the salmon aquaculture industry to avoid salmon lice infestations within farms. However, salmon do not cope with long-term submergence as they need to refill their swim-bladders regularly to maintain buoyancy. Assoc. Prof. Tim Dempster together with researchers from the Institute of Marine Research, Norway tested how salmon coped in submerged cages that were lifted to the surface weekly to allow surface access. Three submerged cages were positioned at 10 m depth were deployed for 8 weeks, and measures of welfare and behaviour were compared against three standard cages. They found that submerged fish swam 1.4 to 3.4 times faster, schooled tighter, and their swim bladder volumes declined gradually from the beginning to the end of each week. When cages were lifted and the surface became available, negatively buoyant fish immediately exhibited jumping and rolling behaviour. However, they found no evidence of acute buoyancy problems during submergence, and growth rates and welfare scores were similar to standard cages. Their research suggests farming salmon in submerged cages with weekly surface access is a viable method to prevent lice infestations.