In connection with fish farms on the territorial sea, when salmon and trout are breed, salmon lice are present, which, if they occur in larger amounts, may cause very large losses to the fish farm. Combating salmon lice must, according to regulatory requirements, most places take place through biological control, with parasite eating fish. Such parasite eaters, however, are very shy and seek shelter when digesting pickled lice, such shells are found naturally on the bottom seabed or the bottom of the territorial sea where the fish farm is located. In addition, many of these useful parasite eaters are lost when the salmon is pumped into transport vessels, resulting in financial loss for the fish farm. A device for improving the efficiency of the control of salmon lice with parasite eaters in fish farms (2), in particular fish farms (2) on the territorial sea comprising at least one mobile hide is provided in the form of a enclosure (6), the sides (16, 18) of which have openings (24, 26), which openings are blockable by releasable slide dampers (28, 106, 106) or dampers through which openings the parasite eaters can pass out and into the enclosure (6), where the enclosure comprises suspension facilities (22) cooperating with side boundaries (8) on the fish farm (2), or with float rings (4) cooperating with the side boundaries (8) of the fish farm, where the enclosure (6) by the suspension facilities (22) is maintained in a pre-selected depth below the water surface. The enclosure (6) may be made up of a plurality of composite tubular members (112) assembled in one or more blocks (114), the walls of which have an open structure (104) in a condition that prevents the passage of parasite eaters, except through the openings (24, 26), or be constituted of a material of closed structure and comprise a bottom (14), where the openings (24, 26) are located at a distance above the bottom (14) of the enclosure (6) wherein the bottom is provided with a layer of artificial seabed.
