An offshore fish cage for farming fish comprises a polygonal frame composed of a multiplicity of frame elements. An upper net is hung down from the polygonal frame and is slackened enough so that it has a length of slack sufficient to cover up and down movement of the polygonal frame caused by waves. A lower net composed of a side net fixed to the upper net and a bottom portion fixed to a bottom end of the side net is provided. The lower net has an opening at its top end. Underwater floats are fitted to the side net, and mooring wires are provided to moor the bottom end of the lower net to the bottom of the sea. The polygonal frame is composed of a multiplicity of frame elements which are joined by flexible joints.
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