16. Whilst hatcheries and smolt production units are based on-shore, for their main growth period farmed salmon are in sea cages, although small numbers are farmed in fresh water. Sites at sea need to be carefully selected, for example to avoid excessive exposure to rough sea conditions which could cause cage damage and put staff at risk, but the location must still provide an adequate flux of clean, uncontaminated water. Most sea cages are in sheltered inlets, such as fjords, along the Atlantic coast. Some sites are being established offshore in more exposed positions. The design of cages and equipment should take into account the potential effects of exposure to adverse weather conditions. Increased cage depth can to some extent compensate for the impact of severe weather but there should be contingency plans for the care of the fish under these conditions. Precautions may also be needed against natural hazards, such as predatory mammals and birds, stinging jellyfish, and potentially toxic or oxygen-depleting plankton blooms, as well as man-made hazards such as oil spills and shipping. It is also important to ensure that the farms themselves do not have adverse effects upon the environment or other users of the coastal seas.