17. Salmon hatcheries usually use water from clean burns or rivers. Where necessary this may be filtered or treated with ultra violet light to ensure that it is of suitable quality for the fish. The water is continually flowing to provide oxygen and to clear waste. Farms sometimes require back-up systems, with alarms, to ensure a constant supply of water.
18. The initial production stages of fertilisation, hatching and fry production are mainly conducted indoors. Eggs are placed in incubators which range from troughs and trays that hold a monolayer of eggs to stacks of trays that use space more efficiently and to specially designed containers supplied with upwelling water that can hold tens of thousands of eggs. The newly hatched alevins drop to the bottom of the trough or tray, often onto an artificial substratum, to absorb the yolk sac. After this stage they start to take food and are termed fry.
19. Fry develop in tanks indoors until, on reaching the parr stage, the fish are transferred to larger tanks, usually outside, or to cages in fresh water. Outdoor tanks are netted or covered to prevent predation and keep out debris. Indoor tanks may also have net covers or lids to prevent escape. Some parr are kept indoors in tanks which may be heated and subjected to lighting regimes simulating accelerated seasons so as to become smolts after six months of development.
20. A high level of hygiene is necessary in the hatchery. Before eggs are brought in, all equipment is washed with detergents and left to soak in solutions of iodophore-based or hypochlorite disinfectants before being thoroughly rinsed. The interior surfaces of the hatchery and all furnishings should at least be washed down and preferably sprayed with disinfectants. High pressure hosing and, better still, steam hosing are effective methods of cleaning. All water supply lines and drains should be thoroughly flushed and, where necessary, disinfected. Ideally, the eggs should be disinfected before they are transferred to the hatchery and, once there, frequently inspected to enable the prompt removal of dead or unfertilised eggs. Equipment used to remove dead or inert material from hatching tanks should be disinfected after each session and whenever possible should be dedicated to one tank only. Tanks for growing fry should be cleaned similarly to those for hatching. A bigger volume of water and larger surface area necessitates the use of hand nets to remove dead fish. Water circulation in the tanks should carry inert material to the outfall and the screens over the outfall should be cleaned daily.