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Fractional factorial design is a practical approach for studying multiple factors, with a minimum of experimental units. The objective of this work was to study the simultaneous effects of nutritional and environmental factors on both growth and flesh quality of common perch (Perca fluviatilis), a new inland aquaculture species. This study sought answering the two following questions: (i) which combinations of factors allow improving growth, food efficiency, and technological, sensorial and nutritional qualities? (ii) is it possible to simultaneously improve growth performances and flesh quality? In a first experiment, twelve factors (7 nutritional and 5 environmental factors) were each tested at 2 levels in a fractional factorial design in 24 independent recirculating 500 L tanks. The best 4 combinations identified in this first experiment were then validated in a second experiment. The first phase of the multifactorial approach used here allowed revealing emergent information: i) there is a combination of factors that allows reducing both the heterogeneity of the production and the losses of nitrates and phosphates, while preserving good characteristics of growth and quality of fillets; ii) it is possible to improve the quality of the aquatic production system, without decreasing significantly growth efficiency; iii) the effect of a given factor, even such an important one like diet, temperature or target biomass, depends on the levels of the other rearing factor levels, thus the usual reference optimum used for a given factor has no meaning theoretically and can be questioned according to the levels of the other factors which act altogether on the functioning of the rearing system. The input factor combinations resulting in a significant enhancement of single output variables or several output variables were identified (e.g. improvement of feed efficiency, and/or fillet docosahexanoic acid content). Our results clearly demonstrate a strong interdependence of input factors into the animal rearing system, particularly between

A wide range of factors can be attributed to the syndrome of fatty liver observed in some cultured fish species. The objective of the study was therefore to quantify different hepatocyte ultrastructural features as potentially influenced by twelve nutritional and husbandry factors, in order to discriminate the most influent factors in Eurasian perch (Perca fluviatilis), a typical carnivorous temperate fish species. Twenty-four groups of juveniles (initial weight 57·6 (SD 14·4) g) were intensively reared for 116 d and fed sixteen different isoproteic diets. The distribution of the experimental treatments was based on a multivariate fractional factorial design (L 24 2 12 ) with either high (þ 1) or low (21) level of each of the following factors: diet (lipid and protein sources, lipid content, astaxanthin enrichment), feeding level, daily and weekly distribution frequency, fish density, initial weight heterogeneity, temperature, photoperiod, and light spectrum. Liver lipid droplets, glycogen, mitochondria and rough endoplasmic reticulum (RER) were semi-quantified and analysed by a soft imaging system using transmission electronic microscopy photographs. Important variability of hepatocyte ultrastructural features was observed. The present study confirms that the rearing temperature, through its influences in the general metabolic activity, seems to be the main factor modifying mainly lipid droplet accumulation and RER development. However, factors that could be pooled under the designation of factors leading to food accessibility and lipid and protein quality intensify or compensate the effect of temperature. Perca fluviatilis: Ultrastructural features of hepatocytes: Nutritional and husbandry factors: Fractional factorial designDuring the last few decades there has been a trend in commercial fish feed formulations to increase dietary lipid levels in order to mainly enhance the protein-sparing effect and to improve growth performances and production. More recently, because of the limitation of worldwide fish oil supply, the partial or total replacement of fish oil by vegetable oils is increasingly widespread in fish feed production. However, the use of high-energy diets is known to have a direct influence on the fat deposition in fish, depending on species and age (1,2) . Eurasian perch (Perca fluviatilis L.) is a carnivorous species classified as a lean fish, and wild specimens are known to store energy supply mainly as perivisceral fat while muscles contain very little fat, ranging between 1 and 2 % (3 -5) . However, in culture conditions, percid species are fed diets formulated for salmonids (6,7) or European sea bass (8) . Pale and fatty livers as well as unexplained mortality in juvenile production when fed such high-energy diets have been reported (9 -12) .Numerous causes are listed in the literature to explain the accumulation of lipids, particularly in the liver, which may eventually cause hepatic steatosis. This condition is described as the presence of lipid microvesicles inside the nucleus and in the ...