36. Hatcheries are usually indoors because eggs and fry require a great deal of attention and much of the work is done in the winter months. Whilst the water supply for a farm producing fish for sale usually comes from surface sources such as rivers or lakes, for the hatchery pure water is needed. A bore hole, a spring supply or sterilised river water is advisable. Without these relatively pure water sources, the high densities employed for eggs and small fry would be impossible because of the high risk of parasite infestations or disease. Water is continually flowing through the tanks to provide oxygen and to clear waste. Hatcheries should have a back up system with alarms to ensure a constant supply of water. A further benefit of ground water is its uniformity of temperature in the UK at approximately 10C throughout the year.
37. Within the hatchery the eggs are incubated in monolayers in trays or packed into funnels of about 10 litres capacity with upflowing water. On emergence, and after completing yolk sac absorption, the fry are held indoors in shallow trays or circular tanks where they stay until they reach a weight of about 5g. Feeding is by hand or by automatic feeder which introduces a small amount of food into the water at regular intervals or continuously.
38. Hygiene is as important in trout hatcheries as it is in those rearing salmon (see paragraph 20). The same types of precautions should be taken prior to new batches of eggs and fish being introduced into the system.