Abstract: The invention relates to a submerged observation unit (6) for a fish tank, wherein the observation unit (6) is suspended in a node cable (5) from a hoist winch (3) of a surface buoy (2). The node cable (5) is torsion-resistant, wherein the node cable (5) is at its upper end wound on the hoist winch (3) having a horizontal drum axis. The observation unit (6) is arranged to be motorized and to azimuthally rotate about a vertical axis. The surface buoy (2) is arranged to be motorized and to move along a main span (10) of a surface cable (1), wherein the main span (10) is arranged to be spanned across a float ring (9) of the fish tank.

Abstract: Device for destroying parasites on fish, such as salmon lice on salmon in fish farms, comprising a camera (4) communicating with a controlling unit (51) which in turn communicates with a light source (5) adapted to fire pulses of point shaped light which is harmful for the parasite in question. The controlling unit (51) controls a system for optical recognition within a defined coordinate system and is arranged to detect points and to update in real time coordinates that exhibits contrast differences typical for parasites on a fish surface and to trigger a light pulse from the light source (5) when the coordinates for at detected point coincides with the coordinates for the aiming point of the light source.