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).
