In two swordtail species of the genus Xiphophorus, onset of puberty in males and females, fecundity in females, and adult size in males are modulated by sequence polymorphism and gene copy-number variation at the P locus affecting the type 4 melanocortin hormone receptor (Mc4r). The involvement of Mc4r in regulating the onset of puberty outside the genus Xiphophorus remains unclear. In this study we used a transgenic line overexpressing asip1 (asip1-Tg), an endogenous antagonist of both type 1 melanocortin hormone receptor (Mc1r) and Mc4r, to investigate the relevance of the melanocortin system on the onset of puberty and adult reproductive performance in zebrafish (Danio rerio). Comparison of growth, puberty and reproductive performance between wild-type (WT) and asip1-Tg zebrafish revealed that a decreased activity of the melanocortin system did not change the timing of puberty but significantly delayed early growth of transgenic animals. Hatching time was postponed in asip1-Tg fish and they were significantly smaller than their WT siblings at 75 dpf, despite showing enhanced linear growth after having completed puberty. asip1-Tg females produced 1.38 times more eggs but spawned less frequently, and their eggs had showed a 0.89-fold smaller diameter but a 1.04-fold increase in larvae body length at hatching. Therefore, we demonstrate that asip-tg zebrafish do not reach puberty earlier than WT counterparts as it could be expected considering the enhanced length and weight growth during early adulthood. This is so because the effects of transgene on growth are noticeable after a threshold length, when puberty has already been reached. Data show that the inhibition of melanocortin system via asip overexpression is an excellent strategy to promote growth, in absence of obesity, by enhancing food efficiency but without accelerating puberty timing. Data here presented provides a deeper characterization of the phenotype induced by the decreased activity of the melanocortin system in fish thus providing an excellent strategy for future aquaculture especially because U.S. Food and Drug Association has recently approved transgenic fish trading.