The invention relates to a semi-submersible spar-type offshore fish farm (1) for cultivating fish at open sea, comprising an elongated center column (2) that comprises a first end part (3), and a ballast system (4) that is arranged at the first end part and that comprises at least one buoyancy tank (5) and at least one clump weight (6) that are movably interconnected, wherein in use of said fish farm the at least one buoyancy tank and the at least one clump weight are arranged at a distance, D, with respect to each other.
