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 amounting collar (43) arranged to be attached to a bottom portion (179) of thefish 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 thehousing (41),the outlet channel (45) being provided, at its upper portion (450), with astrainer (453)covering the cross section of the outlet channel (45)；- an elongated annular space (46) between the housing (41) and the outletchannel (45)；- a slanted grating (47) covering the entire annular space (46), the lowermostportion (479)of the slanted grating (47) being positioned just below the opening (413) ofthe 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 bottomplate (49).
