Animal reproductionJul 29, 2026

Pacu fish spawning at two times linked to melatonin levels and reproductive success

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗OA ↗

Updated Aug 4, 2026

Abstract

Circulating melatonin levels in pacu females peaked at 7 pm and were significantly associated with reproductive performance.

Melatonin levels increased at 7 pm, remaining stable during the dark phase.
No differences in latency or reproductive performance were found between groups induced at 7 pm and midnight.
Melatonin levels at ovulation were significantly higher in the 7 pm group.
Strong positive correlations were observed between melatonin levels at ovulation and reproductive parameters such as fertility, hatching rates, and fecundity.
PGF2α and DHP peaks at ovulation are associated with successful ovulation in pacu.
Timing of spawning induction influences melatonin levels at ovulation, which are positively correlated with reproductive success.

Simplified

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.

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Pacu fish spawning at two times linked to melatonin levels and reproductive success

Abstract

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