Palau International Coral Reef Center

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.

ABSTRACTReef‐building corals face continued declines due to climate change‐amplified marine heatwaves. In addition to affecting coral heat tolerance, corals' algal endosymbionts (family Symbiodiniaceae) can reflect their prior heatwave exposure, although understanding is often limited to heatwave‐induced shifts between symbiont genera. Here, we used ITS2 metabarcoding to characterise Symbiodiniaceae assemblages in 293 individuals of the common Indo‐Pacific coral Acropora aff. digitifera in Palau (Western Pacific), between two outer‐reef regions with contrasting heatwave histories. During the strongest recorded heatwaves, southwestern ‘hotspot’ reefs have typically accrued an additional 2°C‐weeks of heat stress compared to thermal ‘refugia’ located 60 km north. In contrast to previous studies that observed declines in symbiont richness following heat stress, we found a greater diversity of symbiont taxa and low‐abundance sequence variants in ‘hotspot’ corals, predominantly within the C40 lineage in genus Cladocopium. Combining these data with experimental heatwave performance from 168 of these corals revealed that approximately 10% of heat tolerance variability at hotspot reefs was associated with hosting different symbiont taxa. Compared to other hotspot corals, those hosting symbionts with the C15h sequence variant suffered bleaching mortality at 0.8°C‐weeks lower heat stress. Despite higher variability in heat tolerance among corals from thermal refugia compared to hotspot reefs, we found no association between heat tolerance and the symbionts hosted by refugium corals. As the world's coral reefs are exposed to intensifying marine heatwaves under accelerating climate change, the low‐abundance variants that characterise symbionts within genera or lineages may become increasingly important indicators of poor heatwave tolerance.

1. Coral seeding devices (CSDs) - tools designed to deliver sexually propagated corals to target locations - offer a promising means to increase coral abundance on degraded reefs. However, evaluating CSD effectiveness for coral reef restoration across wide areas and over many years is limited by the lack of robust and efficient monitoring approaches. Large-area reef imagery offers an attractive potential solution, but manual detection of CSDs within imagery is slow and limits the scalability of CSD monitoring. 2. We investigated whether machine learning classifiers could accurately detect CSDs in reef orthoimages and tested the performance of classifiers created following minimal manual annotation effort. Using freely available software, we first evaluated classifier performance in a single-site experiment using an orthoimage containing 989 CSDs deployed in Palau. We then also evaluated performance in a multi-site experiment using orthoimages containing a different CSD design deployed across seven sites in the central Great Barrier Reef, Australia. 3. In Experiment 1, classifiers trained on just 30 CSDs annotated within 10 minutes achieved mean recall and precision of 96.7% and 97.1% respectively, reducing manual annotation time by 95.6% whilst still detecting 98.8% of the number of devices found manually. Larger training sets yielded less reliable classifiers and required more manual effort. In Experiment 2, classifiers trained on 30 CSD annotations from one site performed excellently across seven orthoimages from multiple reefs, achieving mean recall and precision of 99.2% and 93.3%. 4. We present evidence that CSD classifiers can be highly effective across both single- and multi-site CSD deployments. In using a free and user-friendly software, we also demonstrate their accessibility to reef restoration practitioners. To facilitate wider uptake of CSD monitoring, we provide a step-by-step protocol for implementing CSD classifiers. By improving access to efficient, direct assessment of intervention outcomes, this method can play a vital role in guiding the enhancement of approaches aiming to restore coral reefs.

High mortality of juvenile corals hinder both the natural recovery of populations and the successful implementation of restoration efforts. Grazing is a significant cause of juvenile coral mortality, and grazer exclusion devices have been shown to increase juvenile coral survivorship. However, most experiments have used cages that typically alter water flow and light conditions, making it difficult to separate abiotic effects from those of grazing. Here, we test whether grazing deterrents can increase the survival and growth of six-month-old Acropora digitifera juveniles outplanted to a shallow reef crest, using arrangements of two or four long or short masonry nails designed to physically exclude larger grazers (e.g., parrotfish) while minimising abiotic changes. By the end of our study, colonies with deterrents had significantly larger planar area (almost tenfold for the most effective treatment), more branches, greater height, and higher survival than those without deterrents. A critical period in this study was the first week after outplanting when colonies with deterrents had significantly less tissue area loss from grazing than those without. Lower area loss in the first week was associated with significantly higher survival over the following 14 months, with an almost threefold improvement for the most effective treatment. For heavily grazed systems, our study highlights the importance of incorporating grazing deterrents into outplant devices to counteract the negative impact of large grazers on newly outplanted juvenile corals and boost restoration success.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00338-025-02703-z.

Pocilloporids have short larval dispersal, an important mechanism in creating localized stock-recruitment relationships, whereby the rate of recruitment increases with the size of the adult stock. Therefore, population recovery can occur quickly from even a small source. Here, we explore the existence of a stock-recruitment pattern in brooding species (Pocilloporidae) across 10 sites, separated by about 500 m, within a semicontinuous reef system spanning 4km in the east of Palau, Micronesia. Twenty settlement tile structures were deployed at each site for two consecutive years (2020‐2021), and the nearby benthic community was assessed using photo quadrats. Brooding pocilloporid cover and settlement decreased significantly in 2021, by 0.6-fold and 0.4-fold, respectively ( P < 0.001 for both). A positive stock-recruitment relationship between adult brooder cover and settlement was identified in 2020 (GLMM: P < 0.001), but in 2021 this relationship was no longer significant (GLMM: P = 0.58). In late 2020, the reef was impacted by a modest bleaching event, which led to a 2.5-fold increase in the cover of dead corals in 2021 (GLMM: P < 0.001). We calculated the proportion of newly dead coral in 2021 to obtain a bleaching stress index for every site. Pocilloporid settlement in 2021 was significantly negatively correlated to the stress index ( R = −0.42, P = 0.05). Our results identify a stock-recruitment relationship in brooding corals at the scale of sites and are consistent with studies on thermal stress impacts on coral fecundity and reproductive output. Given that brooders are increasing their dominance on coral reefs, understanding their recovery dynamics will help us identify locations where interventions might be prioritized.

Marine heatwaves are intensifying under climate change, exposing populations of reef-building corals to mass mortality and intense selective pressure. It remains unknown whether adaptation can keep pace with warming and maintain reef functioning. We have developed an eco-evolutionary metapopulation model for Acropora, an ecologically important yet thermally sensitive coral taxon. We found that, although corals have some adaptation capacity, they will suffer severe heatwave-induced declines over the coming decades. For a future in which emissions lead to ~3°C of global warming, natural selection could allow populations to persist, albeit in severely depleted states with elevated extinction risk and potential loss of ecosystem functioning. Yet, for thermally sensitive coral populations to thrive beyond 2050, there must be rapid reductions of greenhouse gas emissions that limit global warming to 2°C.

Abstract A high percentage of island floras are at risk of extinction and have been reduced to relic populations, often in remote hard‐to‐reach areas. Uncrewed aircraft systems (UAS aka drones) are now being utilized to assist in the survey and collection of rare plants in inaccessible areas or vertical cliff habitats. Here, we test the application of this technology for conservation of 23 plant taxa in three oceanic island hotspots: Hawaiʻi, Madeira and the Republic of Palau. We collect high‐resolution imagery using a small UAS to document the distribution and abundance of vascular flowering plants. Location information is then used to map and assess plant populations. Depending on the terrain, collections are completed using either traditional rope techniques or newly developed remote drone‐based collection methods. Over the course of 6 years, we have greatly expanded our knowledge of rare and endangered species, while increasing survey efficiency and staff safety. Most importantly, this work has had a large impact on the conservation of critically endangered plants. Although using drones for botanical conservation comes with limits and challenges, we see great potential in the continued employment of these techniques wherever plants are growing on cliffs or in other hard‐to‐reach areas.

Coral reefs in the Central Indo-Pacific region comprise some of the most diverse and yet threatened marine habitats. While reef monitoring has grown throughout the region in recent years, studies of coral reef benthic cover remain limited in spatial and temporal scales. Here, we analysed 24,365 reef surveys performed over 37 years at 1972 sites throughout East Asia by the Global Coral Reef Monitoring Network using Bayesian approaches. Our results show that overall coral cover at surveyed reefs has not declined as suggested in previous studies and compared to reef regions like the Caribbean. Concurrently, macroalgal cover has not increased, with no indications of phase shifts from coral to macroalgal dominance on reefs. Yet, models incorporating socio-economic and environmental variables reveal negative associations of coral cover with coastal urbanisation and sea surface temperature. The diversity of reef assemblages may have mitigated cover declines thus far, but climate change could threaten reef resilience. We recommend prioritisation of regionally coordinated, locally collaborative long-term studies for better contextualisation of monitoring data and analyses, which are essential for achieving reef conservation goals.

Abstract Bumphead parrotfish (Bolbometopon muricatum) and Napoleon wrasse (Cheilinus undulatus) are large, charismatic species with high economic and cultural value in Palau and are important for ecosystem function.As a result of intense fishing pressure, landings in Palau for both species declined dramatically in the 1990s and, in response, national legislation was enacted in 2006 that imposed a total ban on the extraction, possession, and sale of both species.In 2013 and 2021, fisheries‐independent stock assessments were conducted and found that the spawning potential ratios (SPRs) for bumphead parrotfish increased slightly from 30% to 34%. Conversely, the SPRs for Napoleon wrasse declined from 12% in 2013 to 4% in 2021. Despite the fishery being closed since 2006, these data suggest the likelihood of significant poaching, resulting in low SPR values, particularly for Napoleon wrasse.Prior to the COVID‐19 pandemic, marine tourism played a central role in the economic and social life of Palau. A willingness‐to‐pay survey found that divers would pay $1.27 million (2020 USD) more for strictly managed catches of both species, and nearly $3.00 million more if the fishery remained closed.The total market value for bumphead parrotfish peaked in 1995 at $112,190. The peak total market value for Napoleon wrasse was $66,434 in 2002. The welfare gain/loss for diver tourism is an order of magnitude greater than the fishery market gain/loss, and healthy marine ecosystems are essential for the post‐COVID‐19 economic recovery of Palau. However, because of the cultural importance of these species, there is a desire to manage this fishery for customary use.Opening a general subsistence harvest season is not recommended ecologically; however, the results suggest that if the SPR of bumphead parrotfish can recover to 40%, a limited quota of individuals with a total length of >95 cm could be sustained, which could be allocated via lottery. However, the fishery for Napoleon wrasse should remain closed given the extremely low SPR values and limited number of large adults in the population.