The different farming methods include extensive farming, semi-intensive farming and intensive farming.
Both carp and catfish are easy-to-farm warm water species which are usually raised in earth ponds. Production is achieved essentially from the natural and renewable resources of the pond. Water quality and flow requirements of these species are modest; juvenile production is usually carried out in the same ponds, where the adult spawning process occurs naturally. The rearing technique is based on the optimisation of the pond's trophic chain in order to achieve the maximum fish yield.
Also mussel- and oyster rearing are examples of extensive systems which rely mainly on natural feed resources
Semi-intensive farming: This form of aquaculture is characterised by resorting to egg incubation, hatching and larval rearing in controlled facilities to produce juvenile fish that are stocked in ponds, and the provision of some food supply, allowing higher stocking density in the ponds. These improvements of extensive techniques are also widely used for the production of several freshwater fish species like tench, roach, pike and perch, mostly sold for restocking sport fishing ponds or rivers.
Intensive farming: the most common way
a) Land-based intensive flow-through farming
This technique aims to produce high value fish at high stocking density. The most important species concerned are trout, salmon, sea bass, sea bream, eel and turbot.
Apart from eel, artificial reproduction of all these species is possible. Specialised commercial hatcheries exist, equipped for breeding hatching and the first stages of larval and juvenile development. Year-round supplies of fry are assured by the control of the maturation and breeding processes of the broodstock, using techniques like the manipulation of light and/or temperature regimes. Marine larvae need live feed for first feeding but once these develop into fry they are fed on fabricated diets, up to market size. More info on hatchery production of seabass and seabream can be found in the attached FAO manuals.
In traditional flow-through aquaculture systems, water passes through the culture system only once and is then discharged back to the aquatic environment. The flow of water through the culture system supplies oxygen to the fish and carries dissolved and suspended wastes out of the system. Water is taken from the river, circulated through the farm and treated before being released downstream. All water in the farm is renewed at least once per day. The most widely-practiced form of flow-through aquaculture in Europe is trout farming. Most of the EU member states have trout farms near to rivers, and use concrete basins or ponds.
b) Recirculation Aquaculture systems (RAS)
These are land-based systems in which water is re-used after mechanical and biological treatment. These systems present several advantages, such as: water saving, a rigorous control of water quality, high biosecurity levels and an easier control of waste production as compared to other production systems. They have however high capital and high operational costs. RAS is still a small fraction of Europe’s aquaculture production and is most significant in the Netherlands and Denmark. The main freshwater species produced in RAS are eel, trout and catfish but other species are already being produced using this type of technology.
(c) Cage farming
Other aquaculture systems practiced in the sea held fish captive in a large pocket-shaped net anchored to the bottom and maintained on the surface by a rectangular or circular floating framework. These sea cages 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.
Originally, cages tended to be placed at well-protected, largely enclosed and insufficiently flushed sites. The scarcity of protected sites and certain environmental set-backs have contributed to the development of programme and strategies that determine carrying capacity of areas in accordance with their geophysical, oceanographic, hydrographic and bio-ecological characteristics. In most cases, these strategies require the operation of cage systems in somewhat more exposed sites where stronger and more resistant equipment is required to withstand heavy weather conditions, allowing for the exploitation of sea areas more distant from the coasts.
Other developments of cage farming concern the construction of submersible cages (which, so far, have not provided the same economic performances as floating cages) and automation of feeding and control by the use of computers. Some cage farming is also practised in freshwater lakes, mostly for the growing of (juvenile) salmonids and coregonids.