An installation for cultivating biomass is formed as a rectangular frame made of rope or other flexible, elongated structures. The frame is equipped with buoys or floats arranged at predetermined locations about its periphery. The frame is anchored to the seabed or shore at a location selected as suitable for the species to be cultivated. The frame may be arranged to float at the surface, or may alternately be arranged to sink at a desired depth below the surface of the water. Along at least two sides of the frame are arranged curved suspension lines. The suspensions lines are arranged with an essentially catenary shape, and under tension hold the frame in a rectangular shape. Juveniles of the biomass species being cultivated are attached to substrate lines, which are arranged between the sides of the rectangular frame, and the species allowed to grow.
