Apparatus (1) for rendering an external parasite on a fish harmless, the apparatus (1) comprising a submersible hull (10) provided with an energy system (20), a propulsion system (30) and a navigation system (50), the apparatus (1) further comprising means of identifying aquatic organisms, a control system (90) arranged to move the apparatus (1) to a vicinity of the fish or the parasite, and a tool (82) arranged to render the parasite harmless.