In a method for removing blood released during filleting from the backbone and the indentations therein in the region of the vertebrae of fish that are conveyed in a row with the head forwards and the belly downwards, the removal of blood is carried out at the same time and in the same place as the cutting of the backbone. The control module includes a programmed processor and a storage memory. The light guide plate includes a light incident surface, an emission surface adjacent to the light incident surface, a bottom surface opposite to the emission surface, a plurality of side surfaces between the emission surface and the bottom surface, and a plurality of diffusion units formed on the bottom surface.