In Mediterranean aquaculture little research has examined the interaction between rearing density and dietary composition on main key performance indicators, physiological processes and gut bacterial community. A study was undertaken, therefore to assess growth response, digestive enzyme activity, humoral immunity on skin mucus, plasma biochemistry and gut microbiota of gilthead sea bream (Sparus aurata, L. 1758) reared at high (HD) and low (LD) final stocking densities and fed high (FM30FO15, 30 % fishmeal FM, 15 % fish oil, FO) and low (FM10FO3; 10 % FM and 3 % FO) FM and FO levels. Isonitrogenous and isolipidic extruded diets were fed to triplicate fish groups (initial weight: 96.2 g) to overfeeding over 98 days. The densities tested had no major effects on overall growth and feed efficiency of sea bream reared at high or low FM and FO dietary level. Results of digestive enzyme activity indicated a comparable digestive efficiency among rearing densities and within each dietary treatment. Plasma parameters related to nutritional and physiological conditions were not affected by rearing densities under both nutritional conditions a similar observation was also achieved through the study of lysozyme, protease, antiprotease and total protein determination in skin mucus, For the first time on this species, the effect of rearing density on gut bacterial community was studied. Different response in relation to dietary treatment under HD and LD were detected. Low FM-FO diet maintained steady the biodiversity of the gut bacterial community between LD and HD conditions while fish fed high FM-FO level showed a reduced biodiversity at HD. According to the results, it seems feasible to rear gilthead sea bream at the on-growing phase at a density up to 36-44 kg m3 with low or high FM-FO diet without negatively affecting growth, feed efficiency, welfare condition and gut bacterial community.
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