A fish cage for use in water comprising a main part (1) made of a watertight, substantially rigid material and has an outlet for effluent (12). The main part (1) is substantially hemispherical, and may slip unhindered through the water along a spherical path without influencing the body of water inside the cage substantially. Because the forces acting on the main part (1) from the surrounding water act mainly along the shell as opposed to perpendicular to the shell, the risk for tearing and the risk for swashing are reduced. Embodiments having flow and lighting conditions adaptable to different species are also disclosed. The cage can be localized in areas with currents and waves, and it can easily be adapted for different species.