A method and apparatus 20 for temporarily reducing the volume of water within a fish farm enclosure comprises the provision of a membrane 28 within the interior of the enclosure (e.g. by suspension from floats within the interior of the enclosure) to create a temporary inner enclosure for enclosing a volume of water. One or more escape hatches 30 are located in the membrane as well as a means for selectively gathering the membrane inwards to form a reduced dimension enclosure with a smaller volume of water contained therein. In use, the method involves progressively gathering in the membrane (e.g. in stages as indicated by dashed lines (28A-D)) to form the reduced dimension inner enclosure. As the membrane is gathered-in, water and/or fish enclosed therein may escape through the escape hatches. Fish contained within the smaller enclosure may be treated for removal of parasites (e.g. sea-louse).
