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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