The dual objective of sustainable aquaculture, i.e., to produce food while sustaining natural resources is achieved only when production systems with a minimum ecological impact are used. Recirculating aquaculture systems (RASs) provide opportunities to reduce water usage and to improve waste management and nutrient recycling. RAS makes intensive fish production compatible with environmental sustainability. This review aims to summarize the most recent developments within RAS that have contributed to the environmental sustainability of the European aquaculture sector. The review first shows the ongoing expansion of RAS production by species and country in Europe. Life cycle analysis showed that feed, fish production and waste and energy are the principal components explaining the ecological impact of RAS. Ongoing developments in RAS show two trends focusing on: (1) technical improvements within the recirculation loop and (2) recycling of nutrients through integrated farming. Both trends contributed to improvements in the environmental sustainability of RAS. Developments within the recirculation loop that are reviewed are the introduction of denitrification reactors, sludge thickening technologies and the use of ozone. New approached towards integrated systems include the incorporation of wetlands and algal controlled systems in RAS. Finally, the review identifies the key research priorities that will contribute to the future reduction of the ecological impact of RAS. Possible future breakthroughs in the fields of waste production and removal might further enhance the sustainabilty of fish production in RAS.

This study explores the effect of rearing environment on water bacterial communities (BC) and the association with those present in the gut of Nile tilapia larvae (Oreochromis niloticus, Linnaeus) grown in either recirculating or active suspension systems. 454 pyrosequencing of PCR-amplified 16S rRNA gene fragments was applied to characterize the composition of water, feed and gut bacteria communities. Observed changes in water BC over time and differences in water BCs between systems were highly correlated with corresponding water physico-chemical properties. Differences in gut bacterial communities during larval development were correlated with differences in water communities between systems. The correlation of feed BC with those in the gut was minor compared to that between gut and water, reflected by the fact that 4 to 43 times more OTUs were shared between water and gut than between gut and feed BC. Shared OTUs between water and gut suggest a successful transfer of microorganisms from water into the gut, and give insight about the niche and ecological adaptability of water microorganisms inside the gut. These findings suggest that steering of gut microbial communities could be possible through water microbial management derived by the design and functionality of the rearing system.

Recirculation aquaculture systems (RASs) are increasingly considered as production systems of the future with a minimum ecological impact for the production of aquatic food. To maintain a good water quality and to produce quality and healthy fishery products, the systems depend on a diverse microbial community involved in different processes of water purification but also in unwanted effects. The article reviews the present knowledge on microorganisms in RAS, their relative distribution within the system compartments and their role in system performance. The system possesses different microniches in which microorganisms retrieve their preferential conditions for oxygen and nutrients. Ammonia and nitrite are oxidized under aerobic conditions into less toxic compounds. Anaerobic ammonium oxidizers in the biofilm oxidize anaerobically both ammonia and nitrites into less harmful dinitrogen gas. Heterotrophic bacteria mineralize organic matter derived from uneaten feeds, dead bodies and excreta of fish. Under conditions of high organic load and high C/N content, nitrifiers are overgrown by heterotrophs with negative effects to the nitrification process. For not yet understood reasons, the presence of off‐flavour‐producing microorganisms occurs also in RAS. Microbial management and management of inputs to the systems to prevent the proliferation of pathogens are discussed and possible management techniques of off‐flavours are also presented. Research orientations are given to explore further the potential of heterotrophic bacteria in microbial management and intensive aquaculture production in systems other than RAS.

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