Replacing aquaculture feeds based on fisheries-derived resources with plant-based dietscould be a relevant strategy to improve the sustainability of aquaculture. Recent studieson sea bass have shown that the total and early replacement of marine products by plantproducts would have a moderate effect on fish growth and body lipid content. Whether aplant-based diet impacts behavioural and physiological traits possibly linked to fishwelfare, is not known, however. Here, we studied the effect of a totally plant-based dietintroduced at an early stage of sea bass development on self-feeding behaviour, learningability in a T-maze and stress biomarkers. We first compared learning processes inself-feeding conditions, between naive fish fed a plant-based diet (PBF) and fish fed aclassic marine diet (MF). Then, we tested fish individually in a T-maze to compare the twofeed groups for swimming activity, exploration and the ability to learn to discriminatebetween two two-dimensional objects associated with a reward. Blood physiologicalvariables, including stress indicators (cortisol and glucose concentrations), were alsodetermined. We did not find any indications of differences in self-feeding behaviourbetween PBF and MF in the first 30 days. A second experiment showed similar swimmingactivities in both fish categories. The “no-choice” percentage was high in both fishcategories (~60%), but all the fish moved preferentially toward the reward. Theirfirst turns indicated an ability to discriminate between two two-dimensional objects tocomplete a simple task. However, the high percentage of “no-choice” responses in both fishcategories could have rendered the results non significant. The T-maze test procedureinduced the production of high concentrations of cortisol, indicating acute stress in fishof both groups during testing. Plasma cortisol concentration was higher in MF than PBF,suggesting that the plant-based diet may affect the short-term release of cortisol. Thisstudy provides the first insight into the impact of a plant-based diet on sea bassbehavioural traits, and confirms the effect of this diet on cortisol release in responseto stress. Overall, in this first experiment, we did not find any major impact of aplant-based diet on sea bass behavioural traits, which is an interesting point for thedevelopment potential of such a sustainable aquaculture strategy.