An open-sea aquaculture system comprising: a semisubmersible platform having storage and maintenance facilities for supporting aquaculture, the semisubmersible platform comprising and being buoyant by a plurality of vertical columns which are partly below an ocean surface, and at least one motor unit, a rigid assembly of aquaculture cages (120A, 120B,...), the rigid assembly comprising a plurality of vertical cavities having forms configured to receive corresponding vertical columns of the semisubmersible platform, a mechanical position control mechanism comprising a plurality of tension applying members configured to connect, mechanically, the rigid assembly to the at least one motor unit on said semisubmersible platform and to control a relative position therebetween by releasing and collecting the tension applying members, to provide at least two operational positions: a raised position in which the rigid assembly encloses the corresponding vertical columns in the vertical cavities, and a lowered position in which the rigid assembly is below the corresponding vertical columns, a control unit configured to control the mechanical position control mechanism and the at least one motor unit to move the rigid assembly from the raised to the lowered position (101, 102) upon occurrence of specified conditions, and to move the rigid assembly from the lowered to the raised position (102, 101) upon specified conditions, and a guiding construction, configured to guide top openings of the vertical cavities of the rigid assembly toward bottoms of the corresponding vertical columns of the semisubmersible platform, upon raising the rigid assembly from the lowered to the raised position (102, 101). a semisubmersible platform having storage and maintenance facilities for supporting aquaculture, the semisubmersible platform comprising and being buoyant by a plurality of vertical columns which are partly below an ocean surface, and at least one motor unit, a rigid assembly of aquaculture cages (120A, 120B,...), the rigid assembly comprising a plurality of vertical cavities having forms configured to receive corresponding vertical columns of the semisubmersible platform, a mechanical position control mechanism comprising a plurality of tension applying members configured to connect, mechanically, the rigid assembly to the at least one motor unit on said semisubmersible platform and to control a relative position therebetween by releasing and collecting the tension applying members, to provide at least two operational positions: a raised position in which the rigid assembly encloses the corresponding vertical columns in the vertical cavities, and a lowered position in which the rigid assembly is below the corresponding vertical columns, a raised position in which the rigid assembly encloses the corresponding vertical columns in the vertical cavities, and a lowered position in which the rigid assembly is below the corresponding vertical columns, a control unit configured to control the mechanical position control mechanism and the at least one motor unit to move the rigid assembly from the raised to the lowered position (101, 102) upon occurrence of specified conditions, and to move the rigid assembly from the lowered to the raised position (102, 101) upon specified conditions, and a guiding construction, configured to guide top openings of the vertical cavities of the rigid assembly toward bottoms of the corresponding vertical columns of the semisubmersible platform, upon raising the rigid assembly from the lowered to the raised position (102, 101).