When people think of aquaculture, the first things that come to mind are ponds or net cages, like those used in salmon farming in Norway. Although this is not incorrect, it is only one of the many options for farming fish and seafood today. More and more products are produced in land-based facilities that operate independently of the sea or open inland waters. They get their water supply from rivers and small streams, or springs and subterranean water that is pumped from the earth. The water flows into basins or tanks, which then become “mini water bodies” in which fish live and grow. These basins make land-based farms largely independent of the local aquatic conditions.
Basins and tanks are used not only here, however, but in all of the different aquacultural processes. They serve as storage tanks for adult fish and as hatchery tanks for young fish. They are used for the production of live feed and as grow-out tanks, as transport and quarantine containers, and for holding, presentation, or short-term fish storage. The shapes of the containers are no less varied than their applications. They can have rectangular or round bases; they can be cylindrical or cone shaped and stackable in one another. The tank floor can be flat or slope conically towards the centre. There are also “hanging” funnel-shaped tanks, which can stand either on supports or on the ground. Or they can be partly sunk in the ground, which sometimes makes work easier.
The decision for or against any of these solutions depends on the fish species as well as on specific production regulations, climatic conditions, concrete environmental requirements, and of course on the financial scope of the investor. Anyone planning to use salt water in their facility should note that the materials for tank construction are not the same as those required for freshwater tanks. Local energy costs can also influence tank construction. Farmers are often forced to find an acceptable balance between what they would like and what is actually possible.
When choosing tank materials, however, there are some areas where compromises are not possible, because the material has to be watertight, rustproof, non-toxic, and should keep its shape when filled. The surface of the material should not cause skin abrasion if fish rub against it. In practice, plastics are used, such as polyethylene, fibreglass, or polyresin, a sturdy resin compound made of sand or ground stone mixed with synthetic resin. This category also includes tanks made of PVC or PE film that are suspended on stable frames. Because plastics lack natural stability, however, they are best used for smaller tanks measuring up to ca. 8 m in diameter. If sturdier materials are needed, farmers should choose steel or concrete (coated or non-coated).
Basins and tanks are available in all sizes, in a wide range of designs and for various applications. However, irrespective of the design (which is often the result of compromise), nearly all farming tanks can be divided into two basic types: channels, also known as raceways or flow-through systems, and round tanks. The two types differ not only in their design but also in the realisable water-exchange rates. The higher the water-exchange rate, the more often the water in the tank can be renewed. This depends, for example, on how fast sediments settle in the tank and whether or not these can be completely washed away. The water-exchange rate also influences the oxygen and ammonia concentrations in the water and thus ultimately the possible fish densities in the tank.
Raceways have a strong self-cleaning effect
If fast and effective water exchange is important, farmers will often choose raceways. The freshwater that flows into the channel from one side pushes the old water out the other side, thereby replacing it almost completely. This leads to a continuous current, which carries any waste materials (fish faeces and feed residue) out of the system. However, this only works if the raceway dimensions are correct and the fish species and density fit the chosen design. The ratio of width to depth should be between 2:1 and 4:1. The length of a raceway depends on the fish species that it contains and the species’ requirements for water quality. For example, although the oxygen content in the water decreases between the supply end and the exit, ammonia content increases simultaneously. Along the length of the raceway, the conditions become poorer with every metre. They are best directly at the water inlet and poorest at the outlet. This “inhomogeneity” of living conditions is one of the serious drawbacks of raceways.
On the positive side however, raceways save space because they consist of several long channels, often a dozen or more, which are installed next to each other. The construction of such facilities does not demand any special know-how and can as a rule be done by non-specialist building firms, if certain basic rules are followed during planning. For example, the channels should have a slope of 1–2% from the water inlet to the outlet so that gravity will ensure constant current within the raceway. At the same time, the current is the prerequisite for optimal self-cleaning of the channel, because dirt particles are regularly rinsed out of the system with the water current. If too much dirt settles on the bottom, it is often sufficient to simply reduce the water level. This will lead to an increase in current speed, which in turn, intensifies the cleaning effect.
As the water level falls, fish density in the channel rises proportionately. As the fish move, more dirt is churned up from the bottom, picked up by the current, and removed from the system. Small bends or slight changes in direction within the raceway can create zones without any current at all, where dirt particles will settle. Sometimes steps or sharp edges are built into the floor contour to create turbulence that causes more dirt to remain suspended and prevents it from settling. Channels or raceways are among the traditional techniques used in freshwater aquaculture. Clearly, raceways are particularly suited to current-loving species, particularly salmonids. Typical trout raceways are ca. 30 m long, 2–3 m wide, and 1–2 m deep.
Other fish species, for example tilapia, sturgeon, some catfish, or individual cyprinid species, are often farmed in raceways. As a rule, the water volume in the raceways is renewed completely once an hour. Still, for species requiring a large amount of oxygen, such as young salmon or trout, this is often inadequate to ensure sufficient oxygen. For this reason, many raceway operators also use additional technical aids, for example, paddle wheels, injectors, or air blowers, or they even add pure oxygen to the water.
Raceways are flow-through systems requiring a lot of water. Therefore, they are, above all, suitable to locations with sufficient water available in a suitable quality. Because the raceways are constantly flushed through, the concentration of dirt particles in the outgoing water is relatively low, and cleaning the water is rather complicated and expensive. For this reason, sedimentation tanks or plate separators, in which the outgoing water remains for some time, are often used, allowing dirt particles to settle. Sedimentation tanks should not be too small if they are to achieve measurable cleaning effects. Although they require a lot of space in the farming facility, they usually repay the investment quickly because operational costs are low, apart from the regular removal of the sludge that settles there. Occasionally, however, other technologies are used to clean the water, for example, swirl separators, air flotation, foam fractionation, or chemical flocculation.
Elongated tanks like those used for young fish in hatcheries and in other areas of aquaculture are only variations on the traditional raceway described above. Many models and types are available. Mostly, they are 2–4 m long, are made of durable plastics, and have numerous additional options, including outlet pipes and faeces pits in which dirt particles can collect; floor screens and separator screens for separating the different batches of young fish; and hanging nets for the hatching eggs or for larvae.
Round tanks offer homogeneous living conditions
The second basic type of fish-farming tank is the round tank of which there is also a variety of models. Round tanks also have a natural self-cleaning potential, although it is not as effective as that of raceways, because the stronger mixing of old and new water in round tanks reduces the water exchange rate considerably. Round tanks require ca. 10 times the tank’s water volume to fully renew its contents once. If 10 m³ of water are added to a full 10 m³ round tank, only 60% of the water content is renewed, because part of the incoming freshwater flows immediately out of the tank with the old water. Even if the volume of freshwater added is 9 times the tank volume, it will only replace 98% of the original contents of the tank!
That said, the prerequisites for self-cleaning in round tanks are actually very favourable. The circulation of the water in the tank causes the dirt particles to drift to the middle of the tank for effective removal. This centripetal effect is often supported by a slight sloping of the tank floor (on average ca. 2%) to the centre. The self-cleaning principle works best in round tanks with a diameter of less than 6–8 m. In larger tanks, as dirt particles move towards the tank centre, they are churned up repeatedly by the fish and often dissolve before leaving the tank.
As unfavourable as the mixing of old and new water might be for the cleaning of the tank, it is nevertheless advantageous to the homogeneity of the living conditions for the fish stock. For example, this can be seen in the oxygen within the tank, which spreads out more evenly in round tanks, creating more uniform conditions. Further, round tanks have a more favourable ratio of surfaces (side walls and bottom) to the contained water volume than raceways. This means that the fish contact these surfaces slightly less often than in raceways, so reducing the risk of injury. The circular current in round tanks also suits the natural behaviour of many species that swim against it and develop more muscle.
The disadvantage of round tanks is that they use more of the farm’s floor space. Whereas raceways are often arranged directly next to each other, round tanks should be positioned at a greater distance from each other, because they are relatively difficult to manage, which becomes apparent, for example, when sorting or harvesting the fish. For this reason, many round tanks are not higher than 1.5–2 m.
Other tanks with different dimensions are based on the typical round tank. Oval or polyhedral tanks, for example six- or eight-sided tanks, are easily recognisable as developments of the round tank. Rectangular and cone-shaped tanks can also be included in this category. This increases the spectrum of possibilities in this sector, offering tanks of different shapes and sizes for many different purposes. The choice ranges from cylindrical containers with a cone-shaped base in which the height of the tank is greater than its diameter, to classic round tanks with diameters of 1–10 m and more.
The so-called D-ended tanks are a compromise between raceways and round tanks. They are mostly channel-shaped or elongated-oval designs, and they have a central dividing panel along the longitudinal axis around which the water circulates. This tank shape also has certain advantages and disadvantages with regard to its self-cleaning capacity and the homogeneity of the living conditions. D-ended tanks are often used where space is restricted and only a limited amount of water is available.
Various plastics, steel, and concrete
The building materials and installation options are as numerous as the shapes and sizes available. As a rule, tanks for aquaculture are made of plastics such as PE, PVC, and fiberglass-reinforced resins, steel, or concrete. Plastics do not corrode easily, do not rust, and do not release toxic substances into the water. However, tank size is limited by the material’s relatively low stability.
Steel presents no problems in this regard, of course, but it is susceptible to rust, particularly when in contact with salt water. That is why plastic coated, zinc coated, or enamelled steel is often used, although this is relatively expensive. For this reason, in some constructions a tight-fitting PE film is simply suspended in the tank to prevent direct contact between the water and the steel shell. For particularly large steel tanks, corrugated plates are often used as cladding, which is then stabilised by the water pressure.
Although concrete is a sturdy building material that allows various tank shapes and sizes, its rough surface is often a drawback because the fish injure their skin on it. But there are some solutions to this problem as well, for example, a protective coating or sealant that sticks well, is watertight, and ensures smooth surfaces. Some of these products can be brushed on like paint and then, once they are dry, seal the underlying surface like a flexible film. Particularly popular are coating materials made from polyurethane, liquid neoprene, and rubber- and enamel-based products.
An original construction and insulation tank variant consists of components formed from PVC, which can be assembled in the desired shape, like double wall click laminate, and then filled with concrete. This makes the tanks stable and long lasting. The wall surfaces are smooth inside and out, and do not require maintenance, so incur no additional costs. A fish-farming tank can be constructed in many different ways. Some tanks are assembled on the their actual site, and then the individual pieces are assembled according to a strict building plan. They can also be purchased ready-made or as individual prefabricated components to be assembled by the buyer.
To add to existing facilities, it is recommended that the owner assemble the extension, using prefabricated segments that fit exactly and can be easily transported through narrow doors or large rooms. Once at their destination, the segments are assembled to produce the desired result, and then screwed, welded, or fixed together. Such system solutions are available in concrete. Drössler Ultralith, a high-performance, extremely resilient heavy-duty concrete is often used. It has smooth, leak-proof surfaces, a high resistance to pressure, and a high bending tensile strength. Thanks to its 3D modular construction, it is suitable for the construction of both round tanks and raceways. The prefabricated segments are fixed together on site, using a special technology that shortens construction time.
For some applications, mobile-farming tank solutions are available that can be built quickly. Here too, the spectrum of possibilities is broad, ranging from foldable tanks made of PE film that only have to be unfolded, to “instant tanks” in roll containers that are transported by truck. Before use, a film bag is inserted that is cut to the exact interior measurements of the container.
With all of these options, it should always be possible to find a suitable tank type for any aquacultural application.