Acoustic telemetry reveals spawning aggregation dynamics and management implications for longnose emperor (Lethrinus olivaceus) in Palau

Abstract

Spawning aggregations are critical reproductive events for many reef fishes around the world, yet they are highly vulnerable to exploitation, making effective management essential for sustaining fisheries and conserving aggregation-forming species. In Palau, a proposed management plan seeks to protect a longnose emperor ( Lethrinus olivaceus ) spawning aggregation during the full moon at an outer reef corner (Mutiaur). We identified migration pathways, examined environmental and biological drivers of aggregation timing, quantified depth use, and evaluated the adequacy of proposed protection measures. An array of 32 acoustic receivers was used to track 16 longnose emperors from 2022 to 2025 across Mutiaur and surrounding habitats. Presence at the aggregation site was significantly influenced by lunar phase, day of year, time of day, tidal height, and seawater temperature. Lunar phase was the strongest predictor, with peak detections from three days before to eight days after the new moon, contradicting the full-moon timing assumed in the management plan. Aggregation presence was highest from January to April and primarily during nighttime hours. Migratory pathways followed the outer reef, with individuals traveling up to 13 km to reach Mutiaur. High detections outside the proposed protected area indicate that current closure boundaries are insufficient. Effective protection will require shifting temporal measures to the new-moon period and expanding the protected area from 0.14 km² to include at least the entire reef corner (0.22 km²). These results highlight the value of species-specific movement data for designing spawning aggregation protections and contribute to broader efforts to improve aggregation-based fisheries management globally. Longnose emperor spawning aggregations peaked around the new moon. Presence at the aggregation site was highest from January to April and at night. Fish migrated up to 13 km along outer reef corridors to the aggregation site. Proposed protection measures do not match observed patterns. Effective protection requires new-moon timing and full reef corner coverage.