The most important forms of aquaculture

The controlled breeding of organisms living in water has been feeding people in Africa, Europe and Asia for thousands of years. Even if modern aquaculture is one of the fastest growing segments of agriculture, a large proportion of the world's reared fish is produced in family-run fish farms in Asia.

Flow-through systems
Species: trout, tilapia, bass

A flow-through system consists of a series of either artificial or natural basins and channels, through which fresh water flows continuously. They are particularly suitable for fish species whose natural habitat is in flowing water or that have enhanced demands regarding the oxygen content of the water.
Water flowing through basins may be pumped from neighbouring ponds or it may be diverted from rivers. Similarly, the basins are drained by either channelling or pumping water from the outlet of the system back into the original water body. The continuous water exchange ensures a constantly high oxygen content and, in turn, good living conditions, enabling more fish to be kept in the same space than would be possible with pond farming.

To avoid polluting the original water with feed residues and excrement, the waste water is treated before it is returned to the source that it was taken from.

Net enclosures
Species: gilt-head bream, cod, salmon, tilapia, bass

Pen or cage systems are employed in all types of natural water bodies, ranging from ponds and rivers to bays and the open sea. The use of net enclosures originates from the short term keeping (cultivation) of caught fish for subsequent sale. The use of pens enables the fish to be farmed in natural environments while in constant exchange with the surrounding water. This simplifies the processes of feeding, controlling and harvesting. However, with this form of breeding, products of the fishes' metabolic processes enter the surrounding water directly, as do feed residues and any medications administered. Higher controls of these inputs shall be in place in order to avoid unnecessary negative effect to the local ecosystem.

Cage and pen farming are very common throughout the world, although the systems used vary greatly in terms of their size, shape and materials.

Many farms in Asia are small, family-run operations with cages made of bamboo that contain between 10 and 150 cubic metres of water. In contrast, modern salmon farms in Norway or South America employ rectangular steel or circular plastic constructions with net depths of 10 to 40 metres and a volume of 3,000 to 40,000 cubic metres. Even hexagonal and octagonal cages with diameters of between 12 and 50 metres are in use, for instance for farming bass and gilt-head bream in Turkey.

Closed recirculating systems
Species: tilapia, bass, sturgeon, shrimp

Closed recirculating aquaculture systems are largely environmentally self-sufficient and comprise a combination of tanks and filter systems, in which the waste water is continuously treated and returned to the breeding tank. Being independent from natural water sources means that they have minimal interaction with the external environment, provided their construction does not affect the surroundings.

As the water is pumped through these recirculation systems, oxygen is added at a constant rate. After the water leaves and before it re-enters the system, it first passes through a mechanical filter to remove feed residues and excrement. The remaining metabolic products are then removed by biofilters and protein skimmers.

Due to their higher technical complexity, closed circulating systems are more cost-intensive than other systems, but they enable ecological breeding of sea creatures at just about any location, with minimal impact on the environment.

Aquaponics is a special form of aquaculture in recirculating systems. It is a combination of aquaculture and hydroponics (soilless plant cultivation), and in addition to the closed water recirculation system, it also has a closed nutrient circulation system. Waste water from the fish breeding tanks is channelled to the plant beds, where it serves as a nutrient source for crop plants such as tomatoes, basil and aubergines. These stand in an inorganic plant substrate, such as gravel or clay granules. The plants and substrate filter the water sufficiently effectively that it can be returned to the fish tank without first needing to undergo any treatment.

Species: mussels, oysters

Species: mussels, oysters

Shellfish are farmed in the sea, i.e. in marine aquaculture. Like their wild relatives, these shellfish live from the naturally occurring plankton in the water, which means that the use of additional feedstuffs is not necessary.

The most common methods of shellfish farming are bed cultivation, cultivation on artificial collectors (such as longline systems) or farming in wire baskets or net sacks. In longline culture systems, plastic cords are used as adhesion materials for spats. These cords run vertically through the water and are attached at the top and bottom surfaces. With both bed culture and breeding using wire baskets or net sacks. Spats is the name used for the youngest stage of shellfish species. The spats adhere to the artificial collectors, where they remain the whole time as a fixed position and grow until harvest size.

Pond farming
Predominant species: carp, trout, zander, pangasius, shrimps/prawns

Pond farming is not only the oldest but to this day also the most widespread form of aquaculture. It has a broad spectrum, ranging from natural water bodies to partially or even fully artificial ponds created for the purpose of cultivating fish or crustaceans. Such pond systems exist in a wide range of sizes, from small family-run fish farms to large-scale commercial operations, covering all levels of intensive and extensive farming.

In the case of European pond farming, commercial fish breeding is largely determined by artificial ponds or tanks. Traditionally,  the majority of fish bred here are those that have adapted to living in standing water, such as carp, pike, zander or catfish. With a little more sophistication, it is also possible to farm trout in ponds, as a transition to flow-through systems.

It is possible to control the water inflows and outflows of traditional ponds; however, due to the low stocking densities, they require low  water refreshment. In Germany too, such ponds are generally stocked so extensively that the dietary requirements of the fish are largely covered by naturally occurring nutrients.

In intensive farming (i.e. many fish in a small space), the need for fresh water is considerably higher. The increase in oxygen consumption and in the incidence of excrement and feed residues resulting from the added nutrient materials can strongly affect water quality,  needing regular water replacement or artificial aeration in order to avoid the risk of disease and the use of antibiotics and other medical treatments.

Freshwater fish, such as trout, Arctic char or grayling, that require flowing water with a high oxygen content are traditionally kept in elongated ponds divided into areas with different currents (cf. through-flow systems). The fish are frequently fed artificially – generally with industrially manufactured, ready-made feed pellets. The fats and proteins they contain for meat-eating fish are frequently extracted from fish oil and meal. Both originate from wild catches or from edible fish residues, such as those used in ecological trout farming.