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.

The effect of the type of non-protein energy (NPE) on energy utilisation in Nile tilapia was studied, focusing on digestible energy utilisation for growth (k gDE ). Furthermore, literature data on k gDE across fish species were analysed in order to evaluate the effect of dietary macronutrient composition. A total of twelve groups of fish were assigned in a 2 £ 2 factorial design: two diets ('fat' v. 'starch') and two feeding levels ('low' v. 'high'). In the 'fat'-diet, 125 g fish oil and in the 'starch'-diet 300 g maize starch were added to 875 g of an identical basal mixture. Fish were fed restrictively one of two ration levels ('low' or 'high') for estimating k gDE . Nutrient digestibility, N and energy balances were measured. For estimating k gDE , data of the present study were combined with previous data of Nile tilapia fed similar diets to satiation. The type of NPE affected k gDE (0·561 and 0·663 with the 'starch' and 'fat'-diets, respectively; P,0·001). Across fish species, literature values of k gDE range from 0·31 to 0·82. Variability in k gDE was related to dietary macronutrient composition, the trophic level of the fish species and the composition of growth (fat:protein gain ratio). The across-species comparison suggested that the relationships of k gDE with trophic level and with growth composition were predominantly induced by dietary macronutrient composition. Reported k gDE values increased linearly with increasing dietary fat content and decreasing dietary carbohydrate content. In contrast, k gDE related curvilinearly to dietary crude protein content. In conclusion, energy utilisation for growth is influenced by dietary macronutrient composition.Key words: Nile tilapia: Digestible energy: Dietary nutrient composition: Energy metabolism Fish feeds will have to further diversify ingredient composition due to the limited availability of wild fishery-derived fishmeal and fish oil as dietary protein and lipid sources (1) . This diversification is already reflected by the substantial amount of ingredients from oilseeds, pulses and cereals in fish diets (2 -5) , which increases the variability in dietary nutrient composition. Inclusion of plant ingredients as a protein source inevitably increases the dietary carbohydrate content. Digestible carbohydrates (i.e. starch) constitute a partial alternative for the digestible energy (DE) supplied by fish oil or vegetable oils in diets, especially for species such as tilapia.An adequate formulation of animal feed in terms of energy supply requires information on (1) nutrient digestibility of ingredients, (2) energy requirements for maintenance and (3) utilisation efficiency of DE or metabolisable energy (ME) for growth (respectively, k gDE and k gME ). Most energy evaluation systems for farm animals, which have a net energy approach, take into account dietary macronutrient composition-induced differences in energy utilisation efficiency (k gDE or k gME ) as seen in pigs (6,7) . For instance, in the net energy evaluation system for the Dutch pig, the u...

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Currently, energy evaluation of fish feeds is performed on a digestible energy basis. In contrast to net energy (NE) evaluation systems, digestible energy evaluation systems do not differentiate between the different types of digested nutrients regarding their potential for growth. The aim was to develop an NE evaluation for fish by estimating the energy efficiency of digestible nutrients (protein, fat and carbohydrates) and to assess whether these efficiencies differed between Nile tilapia and rainbow trout. Two data sets were constructed. The tilapia and rainbow data set contained, respectively, eight and nine experiments in which the digestibility of protein, fat and energy and the complete energy balances for twenty-three and forty-five diets was measured. The digestible protein (dCP), digestible fat (dFat) and digestible carbohydrate intakes (dCarb) were calculated. By multiple regression analysis, retained energy (RE) was related to dCP, dFat and dCarb. In tilapia, all digestible nutrients were linearly related to RE (P<0·001). In trout, RE was quadratically related to dCarb (P<0·01) and linearly to dCP and dFat (P<0·001). The NE formula was NE=11·5×dCP+35·8×dFAT+11·3×dCarb for tilapia and NE=13·5×dCP+33·0×dFAT+34·0×dCarb-3·64×(dCarb)2 for trout (NE in kJ/(kg0·8×d); dCP, dFat and dCarb in g/(kg0·8×d)). In tilapia, the energetic efficiency of dCP, dFat and dCarb was 49, 91 and 66 %, respectively, showing large similarity with pigs. Tilapia and trout had similar energy efficiencies of dCP (49 v. 57 %) and dFat (91 v. 84 %), but differed regarding dCarb.

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