The invention relates to submersible cages for use in fish farms. A framework (16, 18) of tubular struts supports netting panels (20) to form a cage to retain the fish captive. Permanent ballast may be provided by the use of water-filled struts and the buoyancy of the cage may be altered by supplying compressed air to selected hollow tubular struts (24) to control the level at which the cage is held. The cage may be held at any level from semi-submerged at water-level to deeply submerged to escape adverse weather conditions. In an example of the invention, a cage or cages may be towed to achieve a substantial flow path of water through each cage.