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).