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. Larvae need live feed for first feeding but once these develop into fry they are fed on fabricated diets, up to market size.

The final on-growing is accomplished in land-based concrete or  earth tanks, raceways or ponds, usually supplied with high volumes  of pumped water, which is always released outside the farm after its  use (for this reason these systems are also called 'flow-through  systems'). A combination of good water quality, appropriate temperature and oxygenation can allow stocking densities of more than 50 kg/m² (5% fish in water) without compromising the health or welfare of the fish.

(b) Recirculation systems

Indoor systems where the culture water is treated and recirculated are a particular development of the land-based intensive culture system. The development started in the 1950s in Japan and experimentally in Europe in the 1970s. Commercial utilisation started in northern Europe, mainly in the Netherlands, Denmark and Germany in the early 1980s. Instead of being released, the waste water is recycled after mechanical and biological filtration and oxygen injection. A combination of good water quality, appropriate temperature and oxygenation can allow stocking densities of more than 50 kg/m² (5% fish in water) without compromising the health or welfare of the fish.

These units have shown modest economic viability so far, but there are clear signs that the increasing pressure for a more effective utilisation of the water resource and more stringent environmental legislation will lead to the development of this technique in the future. The best results have been obtained in the weaning of glass eels, but also the economic performance of the eel and catfish on-growing farms has recently increased. However, substantial improvements in terms of system performance are required to stay competitive with other production systems which also presently undergo optimisation and cost-efficiency improvements. This can be achieved namely in  optimisation of water treatment, increasing of filter efficiency and  proficient management of the farms. However, success in this area will require appropriate education of operators and managers.

(c) Cage farming

The intensive on-growing of marine and  freshwater fishes, mainly salmon and, more recently, of trout, sea  bass and sea bream is often carried out using marine, floating  enclosures such as net cages. 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.