A buoyancy system (1) for a fish pen (2), the fish pen (2) comprising an enclosure (21) for keeping fish confined in the fish pen (2), the enclosure (21) being secured to an inner flexible buoyancy body (3) floating on a water surface (9), the buoyancy sys- tern (1) comprising an outer buoyancy body (4) on the outside of the inner buoyancy body (3)； and the inner buoyancy body (3) being attached to the outer buoyancy body (4) with a mooring (47) so that the outer buoyancy body (4) and the inner buoyancy body (3) move independently of each other relative to waves.
