Socio-economic factors related to management of costal zones and biological parameters related to species-specific requirements set limitations for the rearing location of net-pen sea cages. A cage was designed and installed at 45 m depth based on the REFA Tension Length Cage technology at the cage facility of the Hellenic Centre for Marine Research in Crete. The cage-net module consisted of a net-pen contained within a hexagonal rigid frame to maintain the shape of the net. Vertical beams of the frame glider on six tension legs were placed in a circle. The operation of the cage was evaluated using a group of red porgy (Pagrus pagrus) for a rearing period of 4 months whereas a second group reared in a surface net-pen cage served as control. Growth performance was similar between the experimental groups. The rearing method significantly affected fish skin pigmentation. Individuals reared in the submerged cage had a brighter skin colour and lower skin melatonin content compared with the ones reared in the surface cage. The presented submerged cage may provide a feasible alternative for rearing fish in exposed areas and particularly species requiring specific biological conditions, such as narrow temperature ranges and low light intensity.