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Chapter: Aquaculture Engineering : Sea Cages

Site selection - Aquaculture Sea Cages

Selecting a good site is of major importance for the future economic viability of the cage farm. A suit-able site for cage farming must fulfill a number of requirements.

Site selection

Selecting a good site is of major importance for the future economic viability of the cage farm. A suit-able site for cage farming must fulfill a number of requirements. It is normally difficult to fulfill all of these, and they will depend also on the cage technology used; for example, the extent of wave tolerance. There are a number of ways to classify the factors that must be evaluated when selecting a site. Several of the factors also affect each other directly.

The main factor is, of course, the water quality. This must be satisfactory for the cultured species, including temperature, salinity and oxygen content. A continuous supply of oxygen requires a current to exchange the water. This is also required to remove metabolic products from the cage area. A good water exchange will occur with a water velocity above 0.1 m/s. This is normally sufficient to supply enough oxygen and to remove fish excrement. However, the water currents ought to be below 1 m/s because velocities above this result in very large forces on the cage structures and mooring system; in these situations specially de-signed systems must be used. Fjords with a sill are not recommended because the water current and water exchange are normally too low to trans-port the faeces and eventual feed loss away so this will collect on the bottom below the cages and decomposition under anaerobic conditions may occur. Hydrogen sulphide (H2S) gas which is toxic for the fish may then be released from the bottom sediments (Fig. 15.2). Areas where the water can be polluted with toxic substances must also be avoided; this can, for instance, be near various industries. Some areas are also more exposed to algal blooms and some sites are particularly exposed to fouling; this must be taken into consideration when selecting a site.

Shelter from the weather is also important. Wave height is normally the most critical parameter. It is usual to avoid areas with high waves, even if it is theoretically possible to build farms and mooring systems that can tolerate very large waves. However, these farms are difficult and expensive to operate when the waves are large and operational access is reduced. In addition, large expensive boats have to be used to operate such farms. If the wave height is below 2 m the cage is easy to operate, and many available cages are constructed to tolerate such wave heights. Several sup-pliers deliver cages that may tolerate 4–5 m wave height. Ocean cages can tolerate up to 7–8 m, but special routines for operation of such farms must be taken into consideration before selecting such sites.

Another factor included in the geographical conditions on site involves water depth; a distance of more than 5 m from the bottom of the net to the sea bottom is recommended, but this depends on the current conditions. Depths above 100 m will greatly increase the costs of the mooring system because long mooring lines will be needed. Areas with frequently shipping traffic should be avoided because of disturbance to the fish and creation of waves. When selecting a site, good infrastructure, for example, proximity to roads, available electricity, will also be of benefit.

The legal requirements for fish farming in an area must be satisfied. There may be areas publicly designated for other purposes, or where cage farming is unwanted from an environmental point of view. For example, it will be difficult to establish a cage farm for salmonid production just outside an important salmon river because of the risk of escape. The legal requirements for access to land for an on-shore base and on-shore mooring are also included here.

Before choosing a site, the environmental conditions must be clearly known. This information may be obtained from government maritime departments or by the use of special oceanographic buoys that automatically monitor environmental conditions on the sites. Talking with people living in the area and local fishermen could also give valuable supplementary information.

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