In Europe, water recirculation systems for fish farming have been mainly developed to:

1. ensure a reliable production of high quality marine fish fingerlings;
2. respect environmental and regulatory constraints for fresh or marine water fish ongrowing
3. make it feasible to produce e.g. warm-water species all-year round.

During their early life stages, fish need a high level of water quality. Another key issue is the biosecurity of the production system. From a practical viewpoint, the concept of biosecurity means a combination of

• the sanitary control of breeders and
• an optimal control of the rearing water quality and of the effluents.

These two requirements can be met using recirculation systems.

During the last fifteen years, fish ongrowing systems with water recirculation have gained much interest within the profession. Full commercial-scale systems are operated with either fresh or marine water and have proved their efficiency for several fish species, notably for catfish, eel and sea bass. However, growth retardation has been observed for most farmed fish species and is a major restraint on the production of some of them, mainly when needing to attain the large individual size that is necessary (e.g. turbot).

This is believed to be related to an accumulation of growth inhibiting factors that are produced by bacteria and/or by the fish. Improvements in the use and application of feed nutrients, the control and valorisation of soluble and particulate waste materials and the optimisation of energy use will also greatly contribute to improving the economic efficiency of recirculating aquaculture systems.

Recently, recirculation technologies have been introduced into traditional fish farming systems. Again, reduced environmental impact and potential control and optimisation of production parameters are concomitant benefits, whereas the financial investments needed and the increased need for system surveillance are seen as the major disadvantages of installation and operation.