European aquaculture takes a variety of forms: extensive or intensive, in natural settings or tanks, in fresh water or sea water, in flow-through or recirculation systems, traditional or modern, classic or organic, sheltered or exposed, and so on.
Extensive pond farming
Traditional extensive freshwater fish farming is practiced across the whole Europe, and is particularly common in Central and Eastern Europe. This long-established farming method consists of maintaining ponds (natural or artificial) in such a way that they foster the development of aquatic fauna. Every winter, the ponds and lagoons are cleaned and fertilised to stimulate aquatic vegetation and consequently intensify the presence of micro-organisms, small molluscs and crustaceans, larvae and worms, which form the base of the aquatic food pyramid. This encourages the development of ‘marketable’ animals at a higher yield than that of the natural ecosystem. Production in extensive farms is generally low (less than 1 t/ha/y).
The species produced vary according to regions: whitefish (Coregonidae), zander, pike and different species of carp, catfish, crayfish and frog.
Semi-intensive freshwater aquaculture In a semi-intensive system, the production of the pond is increased beyond the level of extensive aquaculture by adding supplementary feed, usually in the form of dry pellets, to integrate the feed naturally available in the pond, allowing for higher stocking density and production per hectare.
Intensive freshwater fish farming
Recirculation has been used for a long time in aquariums and hatcheries. Its use for on-growing is more recent, but is attracting growing interest. In fresh water, this system is mainly used for rainbow trout, catfish and eel, but it is suited to all species, including marine species like turbot.
Extensive brackish water farming
Traditional extensive fish farming in lagoons and coastal ponds is one of the most ancient aquaculture methods, and is is still practiced across Europe. It consists of maintaining lagoons in such a way that they foster the development of aquatic fauna. Every winter, the lagoons are cleaned and fertilised to stimulate aquatic vegetation and consequently intensify the presence of micro-organisms, small molluscs and crustaceans, larvae and worms, which form the base of the aquatic food pyramid. This encourages the development of ‘marketable’ animals at a higher yield than that of the natural ecosystem. Production in extensive farms is generally low (less than 1 t/ha/y).
Depending on their geographical situation, lagoons and coastal ponds provide seabass, eels and different species of seabream, mullets, sturgeons, crayfishes and shellfish. In Italian valliculture in the Po and Adige deltas, lagoons are seeded with seabass and seabream fry to make up for the increased scarcity of these species in the wild and to compensate for the disappearance of eels. In Spanish (esteros) and in Portugal, this practice has led to testing with new species, including turbot, common sole and Senegalese sole.
Intensive sea farming
Sea cages hold fish captive in a large pocket-shaped net anchored to the bottom and maintained on the surface by a rectangular or circular floating framework. They are widely used for rearing finfish, such as salmon, sea bass and sea bream, and to a lesser extent trout, in coastal and open waters, in areas sheltered from excessive wave action, with sufficiently deep water and relatively low current speeds. Several cages are typically grouped together in rafts, often housing moorings and walkways for boat access, feed storage and feeding equipment. As the water flows freely to the cages, the openness of the system makes it vulnerable to external influences (i.e. pollution events or physical impact) as well as exposing the adjacent environment to the stock, and the fish farm effluents.
Shellfish farming is based primarily on specimens born in the wild and on nutriments provided by the environment, without any type of input since those animals feed on plankton filtered through their gills.
Different techniques can be used, including bottom farming which is often practised in shallow coastal or estuarine areas, up to ten metres deep, inter-tidal shellfish farming where areas between high and low tide are used, and floating systems such as rafts and longlines, which can be used in open sea or estuarine environments. Rafts are solid floating platforms supporting the farmed shellfish, while longlines are floating lines anchored at both ends, on which shellfish are suspended (either directly or on dropper lines). These systems allow the farming of shellfish in deeper waters, taking advantage of spat fall locations as well as areas of good water quality.
The main species farmed in Europe are oyster and mussel. Other farmed species include clams and scallops.
Seaweed can be cultivated on big ropes or nets in coastal area, protected from the winds and strong currents where they can be constantly immerged underwater. In the EU, the main seaweed species cultivated is Laminaria digitata.
Phototrophic organisms such as algae or cyanobacteria have high potential as energy source (biofuels) and are also often used as feed for other aquaculture systems, especially to feed larval stages of fish or crustaceans. It could be said that the whole aquaculture sector depends on those microscopic organism also called phytoplankton.
The first controlled production of phototrophic organisms was done in open systems, i.e. natural or artificial ponds. These have a huge potential as carbon (CO2) sinks but their productivity is quite low and high amounts of water have to be processed to harvest the organisms.
In the 1950s research started to develop closed systems called photobioreactors (PBR) in an attempt to lower the water demand. Because these systems are closed, the farmer must provide all nutrients, including CO2 to have them functioning.
Many different systems with various materials (plastic, glass, PVC,…) and shapes (vertical, horizontal, Christmas tree,…) have been tested in recent years to find a balance between a thin layer of culture suspension, optimized light application, low pumping energy consumption, capital expenditure and microbial purity. New approaches test methods to produce ultra-thin layers for maximal growth and hybrid systems combining open ponds and PBRs.
Innovative systems have been developed to increase the productivity and reduce the environmental impacts of aquaculture by combining different types of production.
Integrated multi-trophic aquaculture (IMTA) includes organisms from different trophic levels of an ecosystem (e.g. fish, shellfish, algae), so that the byproducts of one become the inputs of another.
Such systems can be used to recycle waste nutrients from higher trophic-level species into the production of lower trophic-level crops of commercial value. IMTA may reduce the environmental impacts directly through the uptake of dissolved nutrients by primary producers (e.g. macroalgae) and of particulate nutrients by suspension feeders (e.g. mussels), and through removing the nutrients from the location.
Aquaponics refers to any system that combines aquaculture (usually in aquarium-like structures) with hydroponics (cultivating plants in water). The water from an aquaculture system is fed to a hydroponic system where the by-products are broken down by nitrification bacteria into nitrates and nitrites, which are utilized by the plants as nutrients, and the water is then recirculated back to the aquaculture system.
Aquaponics is a highly efficient way of producing food but is yet not widely spread onto the market, and is the subject of several research projects at the EU and international level.