Lunar and seasonal patterns of a longnose emperor spawning aggregation in Palau

Many species of reef fishes form spawning aggregations during predictable seasons and lunar phases, which are often targeted by fishers, leading to their depletion over time. In Palau, a new fisheries management plan will protect the Longnose Emperor (Lethrinus olivaceus) at an assumed spawning aggregation site around the full moon; however, whether and/or when this species forms spawning aggregations in Palau was unclear. We investigated lunar and seasonal patterns in fish density using diver operated stereo-video surveys 5–8 days around the new moon between February 2023 and February 2024 at the aggregation site. Gonad samples were collected from the aggregation site and local fish market to assess spawning and trends in gonadosomatic index (GSI). Aggregation site new moon fish densities were over 6x greater than full moon densities and other reference sites, and densities peaked during the new moon. The peak aggregation months were February to May, September and October. Hydrated oocytes in gonad samples indicated active spawning at the aggregation site, and GSI was greatest around the new moon. Our results provide multiple lines of evidence that the Longnose Emperor forms spawning aggregations around the new moon in Palau and will be critical to inform fisheries management reforms.