The function of the bag is to contain and protect the fish and to provide a marine habitat. The net is normally flexible and made of synthetic netting of nylon or polythene fibres reinforced with polythene ropes, although recently new stronger materials like Spectra or Dynema have appeared. The nets are kept stretched vertically with weights at the bottom of the cage or fastened by rope to the framework depending of the type of cages. Rigid cages made of metal netting (galvanized mesh, copper-nickel mesh or vinyl-coated mesh) mounted on rigid metal frameworks also are used. The flexible net bag is most used due to cost.