Growth and welfare of submerged Atlantic salmon under continuous lighting

Although surface-based cages dominate the marine finfish aquaculture industry, production issues that arise at the surface such as poor environmental conditions and the presence of parasites has spurred interest in submerging cages. However, submerged culture is not without its own issues; for example, the adverse effects on fish buoyancy levels can alter swimming speeds and cause tilted swimming at night time, leading to reduced growth rates and vertebral deformities. The use of continuous artificial lighting is common practice in surface-based salmon farming to inhibit maturation. Its implementation can also increase swimming speeds at night, and, if used in submerged cages, may reduce the incidence of tilted swimming. Here we compared submerged (below 10 m) and surface culture of Atlantic salmon Salmo salar for 42 d under continuous lighting. The use of continuous lightning during submergence of large (3.4 kg) Atlantic salmon increased swimming speeds, reduced tilted swimming, and spinal deformities did not arise. Submerged culture also decreased infestation by attached sea lice stages by 72 %, from 4.4 to 1.2 lice per fish. However, specific growth rates of submerged fish were 30% lower than those of surface-reared fish. Developments in engineering and technologies that allow salmon to refill their swim bladders during submergence show promise in eliminating welfare and growth problems. Robust scientific experiments at full commercial scale of cages and operating systems that consider both production and welfare outcomes are critical to the successful development of submerged farming.