A submersible rotatable cage for fish farming comprises a central axle, a buoyant structure positioned about the central axle, and a netting attached to the buoyant structure. An actuator can be placed in operable communication with at least part of the buoyant structure to facilitate rotation of the buoyant structure about the central axle while the cage is in a submerged position. The cage can form part of a system, which includes a net cleaning apparatus, a tethering mechanism and sweep net, that forms the basis for underwater fish farming.