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Possibilities for controlling environmental impact
It will become increasingly important to control the environmental impacts of aquaculture, mainly the discharge of nutrients, organic substances and micro-organisms, as well as escape of fish. The possibilities for control will depend very much on the design and construction of the production unit. For closed units, control is normally possible by collecting and treating the outlet water from the unit. Closed production units may also float in the sea; here treatment of the outlet water is rather more difficult; it is best to have a small height difference between the production unit and the treatment plant. Floating production units may also be damaged by waves, so shallow sites must be chosen for such installations.
For open production units like sea cages the possibilities for controlling the environmental impact are limited. It is therefore important to use sites that tolerate discharge of nutrients and organic sub-stances without local accumulation.
In open production units in the sea there will always be possibilities for fish to escape as a result of construction failure. The weather is unpredictable and large waves can result in breakage of the production unit. Necessary precautions must therefore be taken, including optimal design of the unit and units adapted to the site. A typical open production unit is the sea cage for which correct mooring is important. In addition, it is important that the net bags are strong enough for the site and are regularly inspected. Double net bags have also been used; as have nets around the farm area, all to prevent fish escaping.
Fish escape from closed production units may also occur, especially at the fry stage, even if this theoretically can be avoided. The fry follow the outlet pipes out of the unit. An absolutely secure screen ought therefore to be set in the outlet system, its sole purpose being to prevent escape of fish.
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