Illustration 4). These cages should be anchored to the sea bed in such a position so that there is a good flow of clean sea water through the nets. However, if they are sited in too exposed a position, they can be damaged by storms. The fish farmer usually has to find a compromise on these two requirements and the great majority of units are moored in sheltered lochs.
A recently designed cage unit is capable of withstanding hurricane-force winds and these can be sited further away from the coast. There are considerable benefits for the marine environment from this innovation.
In the initial stages of production, the newly hatched fry are kept in indoor tanks through which flows high-purity water from a nearby stream. When they have grown to the parr stage they are moved to larger tanks, either indoors or outside, where they are given a special feed which is high in protein and fat to speed growth. The feed also contains other minor but important additives such as vitamins, fatty acids, amino acids, phosphorus, zinc, manganese and a pigment. The overflow from the tanks contains some excess food and fish faeces and can pollute the stream into which it empties.
When the fish are transferred to the sea cages to grow to their final weight of about 2-5 kg, many thousands are kept together in the cage units and they may spend between one and three winters at sea. It is during this stage of their lives that problems can develop, the main ones being disease, predation and pollution of the surrounding sea area.
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