Running title: Nutritional background and stress challenges AbstractJuveniles of gilthead sea bream were fed with plant protein-based diets with fish oil (FO diet) or vegetable oils (66VO diet) as dietary lipid sources. No differences in growth performance were found between both groups, and fish with an average body mass of 65-70 g were crowded (90-100 kg/m 3 ) to assess the stress response within the 72 h after the onset of stressor. The rise in plasma cortisol and glucose levels was higher in stressed fish of group 66VO (66VO-S) than in FO group (FO-S), but the former stressed group regained more quickly the cortisol resting values of the corresponding non-stressed diet group. The cell-tissue repair response represented by derlin-1, 75 kDa glucose-regulated protein and 170 kDa glucose-regulated protein was triggered at a lower level in 66VO-S than in FO-S fish. This occurred in concert with a longlasting up-regulation of glucocorticoid receptors, antioxidant enzymes, enzyme subunits of the mitochondrial respiratory chain, and enzymes involved in tissue fatty acid uptake and β-oxidation. This gene expression pattern allows a metabolic phenotype that is prone to "high 2 power" mitochondria, which would support the replacement of fish oil with vegetables oils when theoretical requirements in essential fatty acids for normal growth are met by diet. IntroductionThe continuous growth of global fish production necessitates less reliance on marine fish meal and fish oil as feed ingredients (Tacon and Metian, 2008; Watanabe, 2002). Table 1). Materials and methods Experimental setupFour hundred and eighty juveniles of gilthead sea bream (Sparus aurata, initial body mass 17 g)were randomly allocated in eight 500-L tanks (60 fish per tank) and reared from May to June (10 weeks) under the natural photoperiod and temperature conditions at our latitude (40°5´N; 0°10´E). Two groups (4 replicates each) were established by feeding fish to visual satiety with plant protein-based diets with either fish oil (FO diet) or a blend of vegetable oils at the 66% of replacement of fish oil (66VO diet) as the most important source of dietary lipids (for details of diet composition see Benedito-Palos et al., 2007). As expected, no effect of diet composition 4 was found on growth or nutrient utilisation and both dietary groups (65-70 g final body mass) had high growth rates (SGR = 1.8-1.9) and good feed:gain ratios (1-1.1).Feeding was stopped one day before crowding challenge start. At that point, three replicates from each dietary group (9-10 kg/m 3 ) remained undisturbed (control fish), whereas the remaining replicate was used as a donor tank of stressed fish, according to the protocol conducted in previous studies (Calduch-Giner et al., 2010;Saera-Vila et al., 2009b) and summarized in Fig. 1. Briefly, for each diet group, seven batches of seven fish from the donor tank were used as stressed fish by transferring all of them at the same time to seven cylinder net baskets of 5-L volume (90-100 kg/m 3 ), each one suspended in 90-L tanks w...