21. Almost all final salmon production takes place in cages in sea water. There are many designs the two most popular of which are polyethylene circles and galvanised steel square frames supported by flotation collars and from which cages made of synthetic fibre netting are suspended. Circular cages are more suitable for exposed locations but are usually reliant on boats to service them, while the steel framed cages tend to be parts of a small floating island with steel walkways, some also having storerooms and offices. A tensioned cage observed at one site had a framework of wires under tension and girders around it so that it was kept taut. The mesh was made of heavy gauge material. The intention was to exclude predators such as seals (see paragraphs 222-229). Cage sizes generally were in the region of 15m x 15m x 10m deep, although depth was variable and dependent on the depth of the water under the cages. The requirements of salmon for environmental stimulation are not fully understood and the relationships between these requirements and the welfare of fish could usefully be investigated.
22. Injuries to the snout and fins of salmon are relatively common and readily noticed in cages. Nets have been developed to minimise abrasion and a small mesh size helps to limit skin or fin injuries but at the cost of reduced water flow. Fouling organisms can build up on nets and present an abrasive surface so it is important to keep the nets clean. It is also possible for injuries to arise from collision or aggression between fish. Lesions are potential sites for infection.
23. Cages and sites are often left fallow for disease control and environmental reasons (see paragraph 179). The length of the fallow period may depend on a number of factors including geography, local management agreements and economics. Maintenance of cages is normally carried out during a period of fallowing when nets should be removed, washed and disinfected; cage nets should be lifted, air-dried and then shaken free of debris and the framework of the cage should be cleaned. Nets may be cleaned in situ using remotely operated vehicles (ROVs), water jets or divers. An alternative is to change the net when fish are graded.
24. Dead fish are removed from cages by a variety of methods. Some farms we visited used contract divers on a weekly basis, which we do not consider to be adequate, and a few used ROVs, controlled remotely from the surface, on a daily basis. Another popular method is a 'dead sock'. This is an elongated tube of netting attached to the base of the mesh cage into which dead and dying fish fall. The sock may be closed and raised to the surface for dead fish to be removed. There is also a method for the removal of dead fish which uses an air lift. Such methods are good because dead fish can be removed daily. Whichever method is used, care should be taken to minimise disturbance to the remaining fish (see paragraph 55). FAWC was told that in very large cages the dead sock method is difficult to use effectively. Large cages should not be used unless an effective and satisfactory method for daily removal of dead and moribund fish is available.
25. Salmon in sea cages are fed either by hand or, more commonly in recent years, by automatic feeding equipment which scatters feed over the surface of the water. There are some feeders which pump feed suspended in water into the cage. Automatic equipment must be checked frequently and properly maintained.