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).
