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Juveniles of the protandrous hermaphrodite gilthead sea bream (Sparus aurata) were fed from early life stages to completion of sex maturation with plant-based diets formulated by BioMar. Total fish meal (FM) was included at 25% in the control diet (D1) and at 5% in the other three diets (D2, D3, D4). Added oil was either fish oil (FO) (D1) or a blend of vegetable oils replacing 58% (D2) and 84% (D3, D4) of FO. A commercial butyrate preparation (BP-70® NOREL) was added to the D4 diet at 0.4%. All fish grew fast through a three-year ongrowing cycle with overall specific growth rates of 1.4%, 0.7% and 0.5% for fish harvest at 300 g, 1 kg and 1.5–1.7 kg, respectively. Overall feed efficiency decreased progressively as fish size increased from 0.99 in 300 g fish to 0.8–0.7 in 1–1.7 kg fish. At the last stage, a clear sexual dimorphism was found for body weight and hepatosomatic index when all sampled fish were considered as a whole. A sexual dimorphism was also found for sex steroids with a peak of estradiol in the females sampled in October–December, whereas the peak of 11-ketotestosterone was delayed in males to December. Plasma levels of testosterone were similar in both sexes. The two first components of principal component analysis (PCA) explained >90% of total variance of plasma levels of sex steroids. The displacement along X-axis clearly separated males and females, whereas the movement along Y-axis was related to sampling time. An androgenic effect in the steroid plasma profile of fish fed plant-based diets was also shown, which was especially evident for the low FM/FO diet formulation (D3). This effect was reversed by butyrate and the female/male ratio of D4 fish (age class +3) did not differ from that of control fish, whereas the proportion of females in D2 + D3 fish was higher (P, This work has been carried out with financial support from the European Union (ARRAINA, FP7-KBBE-2011-5-288925, Advanced Research Initiatives for Nutrition and Aquaculture). Additional funding was obtained from the Spanish MINECO (MI2-Fish, AGL2013-48560; Pubertrait, AGL2016-75400) and from Generalitat Valenciana (PROMETEO FASE II-2014/085 and 051).