Can you cut the following black shape into exactly three pieces, and then rearrange those pieces into a square?
Cut along the red lines and move the pieces as indicated by the yellow arrows.
As is usual with this kind of dissection, it helps if you look at the area to work out the length of the side of the final square. Given the grid lines, you also know the orientation of the square, so you can try to place a square over the original picture at such a location that the pieces reveal themselves.
It is similar to this cross dissection, as this fish shape tiles the plane too, and the final square is a fundamental region of that tiling. This is also why the pieces do not have to be rotated.