Spawning of pacu, Piaractus mesopotamicus, at two different times, with a focus on melatonin levels and reproductive...

Melatonin (MTN) plays a direct role in fish ovulation, and we hypothesized that aligning ovulation with an endogenous MTN peak levels could improve reproductive performance. To that, in experiment 1, we assessed circadian variation of plasma MTN in pacu (Piaractus mesopotamicus) females. In experiment 2, pacu females were divided into two groups: one receiving the hypophysation dose at 7 pm (dark onset) and the other at midnight (five hours after dark). Experiment 1 showed that circulating MTN levels increased at 7 pm and remained stable through the dark phase. In experiment 2, no differences in latency or reproductive performance were observed between groups. However, MTN levels at ovulation increased significantly only in the 7 pm group. Strong positive correlations between MTN levels at ovulation and reproductive parameters, such as fertility, hatching rates, and fecundity, were observed. These results confirm that PGF2α and DHP peaks at ovulation are associated with successful ovulation in pacu, consistent with literature that shows that MTN influences ovulation by acting through the Mtnr1a receptor, triggering arachidonic acid release and prostaglandin synthesis. Our findings indicate that the timing of spawning induction influences MTN levels at ovulation, which are positively correlated with reproductive success. This is the first report linking plasma MTN levels at ovulation to successful reproductive performance in fish induced by hypophysation, highlighting the relevance of spawning timing for pacu reproduction.