First Preliminary Assessment of Sex-Specific Spawning Migrations in the Honeycomb Grouper (Epinephelus merra) Using Integrated Hydroacoustic and Environmental Assessment

Spawning migrations are critical for the reproduction of many groupers (Family Epinephelidae); however, the environmental conditions experienced during migration remain poorly understood. This study investigated the fine-scale reproductive ecology and environmental dynamics of the honeycomb grouper (Epinephelus merra) on a coral reef in Okinawa, Japan, using passive acoustic telemetry, bottom-water temperature monitoring, and gypsum spheres as relative water-motion indicators. Acoustic telemetry revealed sex-related and monthly variation in migration schedules among the tracked individuals. Females initiated migration from the full-moon day to three days thereafter in May (estimated duration: 6–9 days) and from one day before to the full-moon day in June (4–5 days). The two monitored males departed 15 and 19 days before the May full moon and remained away for 24 and 28 days, whereas June departures occurred 2 and 9 days before the full moon, with shorter or incomplete records. Two females were detected at the spawning ground in both May and June, demonstrating repeated migrations by the same individuals across successive lunar cycles. Environmental monitoring revealed marked gradients between the residential lagoon and reef-edge spawning ground. Lagoon temperatures increased to 30–32 °C during daytime low tides, whereas the spawning ground remained comparatively stable at 24–26 °C, indicating potential short-term thermal buffering. Across five migration events, onset timing was significantly clustered during daylight ebb tides. Gypsum-sphere measurements also indicated greater relative water-motion exposure at the reef-edge spawning ground in May. Together, these findings suggest that diel and tidal rhythms, along with the associated current environment, may influence the timing of spawning migration and spawning ground selection in E. merra.