A device (10) for trapping and killing marine organisms (14) such as animal plankton, salmon lice and other parasites, comprises a body (12； 12′) configured for submersion in water (15). The body (12； 12′) comprises at least one light source (16) configured and controlled for attracting said marine organisms, and an internal cavity (20) having an opening (20′) for fluid communication with at least a portion of the water. The light sources (16) are arranged inside the cavity (20) and arranged and controlled to emit light waves through said opening and into at least a portion of the water. At least one positive electrode (24； 24′； 24″； 32) is arranged in the cavity and electrically connected to a low-voltage power source (26； 27； 30), and at least one negative electrode (15a) is arranged in the water and electrically connected to said power source (26； 27； 30). The invented device generates, by means of electrolysis with the water (15), chlorine gas (21) at or in a region near the at least one positive electrode, and the chlorine gas (21) reacts with water inside the cavity to form hypochloric acid and hydrochloric acid (31).
