Implementing the concept of sustainable development implies the availability of assessment methods that require the adoption of global and multi-criteria approaches to take account of economic, environmental and social components.  These methods include Life Cycle Assessment, a standardized method (ISO 14040-14043) which comprises a series of environmental impact indicators that provide data on all the resources consumed and "pollutant" substances emitted during the genesis and use of a product, its elimination, recycling and waste.  Some of the impact indicators available have been deemed relevant to aquaculture.  These include the potentials for eutrophication, acidification and climate change, utilisation of the primary product and energy use. 
The scientists have applied the LCA method to the farming of turbot in a re-circulating water system.  The indicators chosen were expressed with reference to the production of a tonne of fish, taken as a functional unit.  This production system included different systems of mechanical and bacterial filters to purify the water, thus limiting its withdrawal  from the natural environment.  Fluxes of the nutrient elements produced were estimated by modelling for three parameters (nitrogen, phosphorus, suspended matter) and by chemical analysis of the circulating water and stored sludge.  The different LCA impact indicators were compiled using data collected from the production system, using two methods to evaluate nutrient fluxes (based on nitrogen or on phosphorus).

Evaluations of nutrient fluxes by modelling or direct measurements produced comparable results for the emission of phosphorus and suspended matter. 
However, the nitrogen emissions measured were half those estimated by modelling, and it is possible that there was an emission of nitrogen gas (in the form of N₂, NO, N₂O or NH₃) into the atmosphere from the bacterial filters and during sludge storage.  These hypotheses led to a review of the potential levels of eutrophication (related to nitrogen emissions into water) and climate change (linked to N₂O emissions into the air) obtained by LCA for this type of production system.
Furthermore, by comparing these results with those obtained from an open-circuit rainbow trout production system characterised by high water consumption,  the turbot re-circulating production system differed in terms of higher energy consumption, with potential consequences regarding acidification of the environment and climate change.

This study made it possible to establish an environmental profile for this type of fish production system.  Nitrogen gas emissions (the forms of which still need to be clarified) and high energy consumption are the two areas upon which the search for technical innovations should focus in order to reduce the environmental impact of aquacultural production systems that use recycled water.

References:
« Characterisation of the environmental impact of a turbot (Scophthalmus maximus) re-circulating production system using Life Cycle Assessment »
Aubin J., Papatryphon E., Van der Werf H.M.G., Petit J., Morvan Y.M., 2006, Aquaculture, 261, 1259-1268