A pen system (1) for farming aquatic organisms, the pen system (1) comprising an outer surrounding rigid structure (2) and an internal, closed enclosure (3); the outer surrounding rigid structure (2) forming an upper portion (20) and a lower portion (29), the pen system (1) further including : - an internal surrounding connecting element (5; 500), the closed enclosure (3) being attached to the connecting element (5; 500); - a heave-compensation system (4) comprising a plurality of fluid-filled cylinders (41; 410, 490), each fluid-filled cylinder (41; 410, 490) being hingedly attached, in a first end portion (49, 411, 491), to the connecting element (5; 500), and being hingedly attached, in a second end portion (40; 419, 499), to the inside of the rigid structure (2).
