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Chapter: Aquaculture Engineering : Sea Cages

Sea Cages

A cage represents a delineated volume in the body of water where the aquatic organisms can be farmed.

Sea Cages


A cage represents a delineated volume in the body of water where the aquatic organisms can be farmed. Cage aquaculture may date back to as early as the 1200s in some areas of Asia, and is currently a major form of aquaculture in countries including Canada, Chile, Japan, Norway and Scotland, where it has been successfully used, mainly for salmonid farming. However, a large variety of species are grown in cages today and include seawater, fresh-water and diadromous species. Therefore today cages are used worldwide in the sea, in lakes and large rivers.The main differences are in the size and construction for withstanding waves and cur-rents. Trends today are that new more weather-exposed sites are taken into use to ensure continuous growth in the cage farming industry. The number of good sites in less exposed locations is limited.

There are a number of approaches to designing a cage and also classifying the various cage systems. One classification is based on where in the water column the cage floats. Three categories can be used: floating, submerged, or submersible. The last two types consist of a frame that can float on the surface and that maintains its shape when lowered below the water surface.

Another classification is according to the type of net used in cages: rigid or flexible. Rigid nets may be created by using a flexible net attached to a stiff framework to distend it. Instead of using a flexible net a rigid metal net may be used. A rigid net cage will maintain its original shape regardless of the waves.


Instead of using a floating construction, a fixed construction may be used. This can, for example, bepilings driven into the seabed to which the net is then fixed to fence in an area.

Another classification of sea cages divides them into two categories depending on the nature of the bag that makes up the cage; it may be an open bag of net, or a closed bag of plastic, for instance. A closed bag will normally require water to be pumped into it, and there is an outlet pipe from the bag. Actually, a closed production unit has been created.

Open offshore cages can be classified as follows:


·                  Class 1 Gravity cages that rely on buoyancy and weight to hold their shape and volume against environmental forces


·  Class 2 Anchor tension cages that rely on the anchor tension to keep their shape and volume


·  Class 3 Self-supporting cages that rely on a combination of compression in rigid elements and tension in flexible elements to keep the net in position so the shape and volume are maintained


·  Class 4 Rigid self-supporting cages that rely on rigid constructions such as beams and joints to keep their shape and volume.


A traditional open cage comprises the following main parts (Fig. 15.1):


·  Net bag with weights in the bottom to spread the bag

·  A jumping net above the surface fixed to the net bag to prevent fish escaping

·  Cage collar for spreading out the net bag and give buoyancy to keep the bag in the correct position in the water column

·  Mooring system.

When choosing technology and systems for farming in traditionally open sea cages, there are many conditions to be evaluated. This is also the case when the actual type of cage and mooring system is chosen and designed. The following list can be used to help when establishing a new sea cage farm:

·                 Choose a site that is suitable for farming


·                 Describe and calculate the environmental conditions on the site


·                 Choose farming systems, i.e. the cage and mooring system, adapted to site conditions


·                 Design the cages (normally done by the cage manufacture) and mooring system


·                 Set out the cages and mooring system


·                 Establish requirements for operational control of the system.


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