Abstract: Fish-breeding apparatus (10), which comprises a frame, supporting one or a plurality of breeding cages (11) and provided with hollow buoyancy elements (17), and at least one device for controlling the buoyancy of the apparatus by filling, totally or partially, the buoyancy elements with water and emptying them of the water, totally or partially, whenever desired. The buoyancy elements are emptied of water by replacing the water with air. The buoyancy elements are such that when they are empty of water to a predetermined extent, the entire apparatus floats, even when it is fully charged. The frame has depending legs (17), so that even in its lowermost position the bottoms of the breeding cage will be spaced from the sea bed.