Abstract: A fish pen (100) includes an elongate spar buoy (110) and a reserve buoyancy buoy (120) disposed over a top end of the spar buoy. Lower and upper rim assemblies (130, 140) are attached to the spar buoy and each other with tension members (150, 152, 154). An adjustable upper connection plate (114) on the spar buoy provides a means for tensioning the tension members. A docking station (190) and portal (185) provide easy access to the fish pen. A deployable panel system (180, 182) allows for crowding fish into a smaller portion of the pen. The panel system may be nonporous and include additional panels (184) such that a portion of the pen can be substantially isolated.