An outlet basin (4) for a fish pen (1) provided with at least one outlet hose (91 99) extending from the outlet basin (4) to a surface (17) the outlet basin (4) including : a cylinder shaped housing (41) which is provided at its upper portion (410) with a mounting collar (43) arranged to be attached to a bottom portion (179) of the fish pen (1) and which is provided at its lower portion (419) with a through opening (413) a cylinder shaped outlet channel (45) which in a portion is housed in the housing (41) the outlet channel (45) being provided at its upper portion (450) with a strainer (453) covering the cross section of the outlet channel (45) an elongated annular space (46) between the housing (41) and the outlet channel (45) a slanted grating (47) covering the entire annular space (46) the lowermost portion (479) of the slanted grating (47) being positioned just below the opening (413) of the housing (41) a slanted bottom plate (49) sealingly covering the entire annular space (46) and a lower through opening (493) at the lowermost portion (499) of the bottom plate (49).
