An outlet basin for a fish pen is provided with at least one outlet hose extending from the outlet basin to a surface. The outlet basin includes a cylinder-shaped housing which is provided, at its upper portion, with a mounting collar arranged to be attached to a bottom portion of the fish pen, and which is provided, at its lower portion, with a through opening. The outlet basin further includes a cylinder-shaped outlet channel which, in a portion, is housed in the housing, the outlet channel is provided, at its upper portion, with a strainer covering the cross section of the outlet channel, an elongated annular space between the housing and the outlet channel, a slanted grating covering the entire annular space, the lowermost portion of the slanted grating being positioned just below the opening of the housing, a slanted bottom plate sealingly covering the entire annular space, and a lower through opening at the lowermost portion of the bottom plate.
