Since 2023, the Palau International Coral Reef Center (PICRC) has been collaborating with the Koror State Government (KSG) to conduct research and monitoring aimed at providing essential baseline data to guide the Koror Southern Lagoon Coastal Fisheries Management Plan, a new fisheries management initiative for Koror State. The management plan identified several fish spawning aggregation sites for protection, including the longnose emperor (melangmud, Lethrinus olivaceus) at Mutiaur.
Spawning aggregations occur when fish gather in large numbers to reproduce at specific times and locations, making them particularly vulnerable to overfishing. Palau has many well-known spawning aggregations which often coincide with specific lunar phases, such as groupers at Ngerumekaol channel (Koror) and Ebiil channel (Ngarchelong). However, many other species have been reported to form spawning aggregations, but have never been studied scientifically, posing challenges for effective management.
PICRC conducted surveys of the longnose emperor spawning aggregation at Mutiaur to confirm spawning and document the lunar and seasonal patterns of the aggregation. Diver-operated stereo video surveys, using GoPro cameras, were conducted every month from February 2023 to February 2024, on the days around the new moon. Divers swam across the corner along the same route at 13 m and 20 m to document the size and density of longnose emperors. In addition, researchers collected gonad (reproductive organs) samples from fish at the aggregation site and the JR5 fish market to confirm spawning and assess patterns in spawning activity.
The results confirmed that longnose emperors are spawning at Mutiaur, with strong evidence that aggregations form on the days around the new moon. Peak aggregation months occurred from February to May, with a smaller peak in September and October. High fish densities were also observed outside of the area proposed for protection. These findings indicate that the protection measures proposed in the Koror Southern Lagoon Coastal Fisheries Management Plan should be modified to protect the days around the new moon instead of the full moon and expanded to include the whole corner at Mutiaur, to more effectively protect the longnose emperor spawning aggregation.
PICRC also conducted an acoustic tagging study on the same species at Mutiaur, which will provide more detailed information on the timing, seasonality, and movement patterns of longnose emperor around this site. Results are expected to be published soon.
This project was supported by the Great Barrier Reef Foundation’s Resilient Reefs Initiative and Irish Aid’s Our Shared Ocean Programme. PICRC has presented the report to Koror State Leadership and will continue to work with them to provide the essential technical information on their Management Plan. The full study “Lunar and seasonal patterns of a Longnose Emperor spawning aggregation in Palau” is published in the journal Scientific Reports and is available open access at: https://link.springer.com/article/10.1038/s41598-025-27785-1