The invention is a fish rearing tank comprising an egg-shaped shell (1) with a vertical long axis and gradually narrowing shape towards a tip portion (4). The egg-shaped shell holds a water volume in a major, wider, lower portion and enclosing air in said minor, upper, tip portion (4), and forming a closed, generally rigid tank and with water inlets (11) and outlets (16, 29), and a ring-shaped buoyancy collar (3, 24). The shell (1) has a fixed ballast (7) in a lower end of said wider, lower portion. A ring-shaped buoyancy collar (3, 24) is arranged on the egg-shaped shell (1). An axially oriented central tube (2) extends from the tip portion (4) of the egg-shaped shell (1) to the lower, wider end of the shell (1). A vertical mooring line (26) extends from a hydraulic cylinder or winch (28) arranged near said upper tip portion (4), and downwardly through the axial oriented central tube (2) and through the lower end of the central tube (2) to an anchor at the seabed below the egg-shaped shell (1). The hydraulic cylinder or winch (28) is arranged for holding the vertical mooring line (26) taut and such that the vertical mooring line constitutes a single point mooring for the fish rearing tank.
