Types of closed production units
The flow pattern of the water in the unit can be used to classify closed production units (Fig. 13.2):
· Production units with a circulating water flow
· Production units with a one-way water flow.
Production units with a circulating water flow may again be separated into tanks with a circular flow pattern (as is most common), or oval tanks that have an oval flow pattern, of which there are several types; for example, Foster Lucas tanks and Burrow tanks, as well as different ovals and pipe connections. Among the traditional tanks with circular water flow, one type may be defined as a farming silo. This is a circular tank of greater height than diameter, i.e. a tower. It is normally difficult to achieve satisfactory water exchange and self-cleaning in silos.
Earth ponds belong to the group of production units with one-way water flow and represent the oldest type of closed production units used for fish production. They are mainly used for extensive fish farming; i.e. there is a low production per unit farming volume.
According to the requirements for closed production units, the main interest in using ponds is the low initial cost per unit farming volume. In addition, normally a natural ecosystem that can be utilized is created inside the ponds. In some countries such as Norway, no new permits for earth ponds are given unless they are dried once a year. This is because it is quite difficult to control disease as pathogenic micro-organisms may survive in the earth. If the ponds are dried during the winter season these micro-organisms will probably be killed. A layer of lime which increases the pH may also be used as a disinfectant. If the ponds are covered with a plastic tarpaulin (polyvinyl chloride (PVC) or polyethylene (PE)) the problems are avoided because such ponds may be cleaned inside, but this is not a normal pond design, and such installations are more like a traditional tank with no ecosystem inside.
A further development of the earth pond is the raceway, which also uses the one-way flow pattern. This is a fixed construction often of concrete, built as a long rectangle. The water is supplied at one end and the outlet is located at the opposite end. Race-ways are quite commonly used for various species throughout the world. However, raceways require quite large amounts of water to have effective hydraulic self-cleaning of their total volume, and even then it is very difficult to get good cleaning results. Normally, some kind of mechanical equip-ment is necessary for additional cleaning of the raceway. It is therefore important to create a good flow pattern inside the unit, with a correctly designed flow inlet and outlet, to ensure uniform water flow through the entire cross-sectional area and length of the raceway to reduce the require-ment for manual cleaning. During the past few years, a specially designed raceway with a very low water level (10–50 cm) has been developed. The unit is specially designed for fish species that need a bottom to lie on and do not utilize the entire water column, for example halibut and wolf fish. The unit is constructed so that it can be installed in tiers, one above the other.
Closed floating cages are also a type of closed production unit. In this case, both circulating water flow and one-way flow systems have been tried in different variants that have been constructed using different materials. In one variant the traditional net bag in a sea cage has been substituted by plastic sheeting. Water is pumped into the bag tangential to the edge and the outlet is placed in the centre of the unit. This creates water circulation inside the bag (see, for example, Ref. 6). The advantage of this type of unit is that it lies on the water surface and there is only a small head to over-come to pump the water into the cage, compared with, for instance, closed production units placed on shore and where seawater is normally pumped several metres.
As shown, a closed productions unit can be built in several ways and can have different water flow patterns. The design of the production units, however, depends on the type of aquatic organism to be grown and its requirements regarding water distribution and bottom conditions.