A device for the treatment, e.g. delousing, of fish in net cages comprises a tarpaulin having a dimension which is enclosing the net cage in a completely covering manner beneath the sea level. To either raise or lower the tarpaulin up into a position surrounding the net cage beneath the sea surface or down to a level beneath its lower portion, respectively, the tarpaulin is provided with at or close to the outer circumference thereof more buoyancy elements each individually adapted to be filled with or drained for a fluid such as air or liquid as to raise or lower the tarpaulin up to the sea surface or down beneath a lower portion of the net cage, respectively,
