Sea turtles are essential to marine ecosystem health, maintaining ecological balance by controlling algae and supporting the stability of coral reefs and seagrass beds through grazing. They facilitate nutrient cycling, energy flow and mineral bioaccumulation, serving as critical indicators of ocean health (Lovich et al., 2018; Selmen et al., 2025). Despite their importance, sea turtles are still at risk worldwide. Some species, like the green turtle (Chelonia mydas), have shown signs of recovery and were moved from endangered to least concern on the IUCN Red List in 2025. There have also been more nests in Ogasawara Islands, Japan (Kondo et al., 2017) and Aldabra, Seychelles, Indian Ocean (Hays et al., 2024). However, most marine turtle populations continue to decline. In Southeast Asia, the crisis is acute; for instance, Indonesia’s sea turtle populations dropped by 95% in 2025 due to egg exploitation, climate change and habitat destruction (Selmen et al., 2025).
       
Anthropogenic threats, including coastal development, pollution, marine debris, boat strike and incidental capture in commercial and artisanal fisheries, remain the primary drivers of decline. Studies along India’s coastline for olive ridley (Lepidochelys olivacea), green turtle, hawksbill (Eretmochelys imbricate) and leatherback (Dermochelys coriacea) have highlighted these pressures (Li et al., 2020; Li et al., 2022; Solanki et al., 2024; Williams et al., 2019). Concurrently, climate change exacerbates these risks. Rising global temperatures alter ocean currents, accelerate coastal erosion and directly disrupt reproduction by reducing hatching rates and skewing sex ratios via temperature-dependent sex determination (TSD). Even a few-degree increase in nest temperature can lead to female-biased populations, diminished reproductive success in the next generation and reduced hatching success by affecting embryo growth and survival (Solanki et al., 2024; Yu, 2024).
       
Sea turtles are migratory marine species. Their reproductive cycle involves movement between feeding grounds, offshore mating sites and terrestrial nesting beaches. Successful nesting requires specific habitat attributes, including beach elevation, slope and substrate type (Solanki et al., 2024). Different species exhibit distinct preferences. Olive ridleys and leatherback turtles often choose broad beaches and sandbars at river mouths, while hawksbills and green turtles prefer smaller island beaches. Loggerhead turtles in Florida’s Ten Thousand Islands like wide, flat beaches (Garmestani et al., 2000) and green turtles in Veracruz, Mexico, prefer beaches with a moderate slope (Zavaleta-Lizárraga and Morales-Mávil, 2013). 
       
The leatherback sea turtle, the largest marine turtle species. It stands out for its soft shell, lack of scales and large, deep pits. Leatherbacks are globally distributed across the Pacific, Atlantic and Indian Oceans, such as the Andaman and Nicobar Islands, Mozambique, South America’s coast, southwestern Africa, southeastern Australia to New Zealand and the Andaman Sea coast of Thailand (Piboon et al., 2025; Solanki et al., 2024; Williams et al., 2019). The species is listed as a vulnerable species on the IUCN Red List and is included in CITES Appendix I (Piboon et al., 2025). In the Indian Ocean, populations have declined over the long term; for example, in 2010, nest numbers were 40% lower than in the mid-20^th century (Hays et al., 2024; Piboon et al., 2025). In Malaysia, where they were once abundant, the species have almost disappeared and are virtually extinct (Hays et al., 2024).  In Thailand, leatherbacks are endangered. They are found along both the Andaman Sea and the Gulf of Thailand but nest exclusively along the Andaman Sea coast in Phuket and Phang Nga (Piboon et al., 2025). These nesting sites are under intense pressure from rapid urbanization and the expansion of tourism infrastructure, including beachfront resorts, homes and road construction. Although government-led beach protection and hatchery programs provide support, habitat loss still forces turtles to nest in less suitable places, threatening their reproductive success. Detailed knowledge of what makes a good natural nesting site remains limited, making it hard to focus on conservation efforts.     
       
This study aims to fill this knowledge gap by assessing hatching success and nesting site characteristics of leatherback turtles in Phang Nga Province, a critical natural habitat in Thailand. Our research comprised two parts. First, we examined the physical beach parameters and microenvironmental factors, including beach slope, sediment grain size, sand temperature and humidity, during the 2024 nesting season (October 2024 to March 2025). Second, we analyzed a time series of nesting abundance and hatching success across multiple seasons from 2019 to 2025 to identify shifting trends in reproductive outcomes and site fidelity. We hope our findings will help guide habitat restoration and community conservation efforts to protect this iconic species. 

Study area and survey design
 
The study was conducted along the western coast of Phang Nga Province, Thailand, a critical leatherback nesting area in the Andaman Sea. The Andaman Sea is a marginal sea within the northeastern Indian Ocean, located southeast of the Bay of Bengal and bordered by Myanmar, Thailand, Malaysia, Sumatra and the Andaman and Nicobar Islands (Fig 1). The study covered 239.2 km of coastline across four districts: Takua Pa, Takua Thung, Tai Muang and Khura Buri (9°19′46.50"N 98°23′25.45"E to 8°12′13.91"N 98°16′52.47"E.


       
The leatherback turtle’s nesting season varies by geographic distribution. In the Indian Ocean and Southeast Asia, it typically occurs from November to March (Piboon et al., 2025). We performed daily morning foot patrols (October-March) from 2019 to 2025 to identify nesting evidence (crawl tracks/flipper prints). We conducted this effort in collaboration with Prince of Songkla University, Mangrove Forest Conservation Center No. 10 (Takua Pa), local communities and Department of Marine and Coastal Resources (DMCR) marine rangers. We collected microenvironmental factors during the 2024 nesting season (October 2024 to March 2025) and time-series data on nesting abundance and hatching success across multiple seasons from 2019 to 2025.
 
Monitoring of nesting and microenvironmental factors
 
Once we located a nest, we recorded its coordinates using a handheld GPS device (Garmin eTrex 10). We measured sand temperature and humidity at a depth of 50 cm using a digital probe (Digicon PMS-716), with three replicates per nest between 4 and 6 AM. Surface light intensity was quantified using a digital lux meter (Smart sensor AS803) at each nesting site.
       
Physical parameters included the distance from the nest to the high water line (HWL), defined by the wet/dry boundary and debris marks (Stokes et al., 2024) and the distance to the nearest vegetation line, measured with transect tape. Beach elevation and slope were determined using the Emery (1961), mapping cross-sectional elevation from a fixed inland point to the seaward boundary.
 
Sediment characterization
 
Sediment samples were collected at 50 cm depth using a 5 cm-diameter plastic hand corer. Samples were dry-sieved through 0.063, 0.125, 0.250, 0.500 and 1 mm mesh. We used retained weights to calculate phi-scale percentages and classified grain size according to the Wentworth (1922) scale.
 
Hatching
 
We obtained nesting records (October 2019-March 2025) through morning beach surveys and reports from the local community and DMCR rangers. We recorded the nest’s geographical coordinates using a handheld GPS. Due to animal ethics requirements, we collected data with officers from Mangrove Forest Conservation Center No. 10 (Takua Pa, Phang Nga), who are legally obliged and have permission from the Thai government to manage and operate this activity without interruption while on duty. The nest was excavated and the clutch size was counted. The yolkless eggs were removed and counted and after that, the clutches were transported to the hatchery beach. Following an incubation period of 55-60 days, hatching success was evaluated upon emergence. Hatching outcomes were classified into two main categories: (1) hatching success, defined as the proportion of eggs that successfully hatched, comprising emerged hatchlings and individuals that died in the nest post-hatching; and (2) hatching failure, encompassing yolkless eggs, unfertilized eggs, eggs with early developmental arrest and late dead embryos. We calculated the percentage of hatch success and non-success. 
 
Statistical analysis
 
We analyzed data using Microsoft Excel and RStudio (v4.4.2). We assessed normality using the Shapiro-Wilk test. One-way ANOVA was applied to normally distributed variables (e.g., sand temperature), while the Kruskal-Wallis test with Dunn’s post hoc analysis was used for non-normally distributed data (e.g., humidity and hatching outcomes).

Nesting abundance and spatio-temporal distribution
 
Leatherback nesting in Phang Nga (2019-2024) showed significant inter-annual variation in both location and frequency. Nesting peaked in 2023 (13 nests), primarily in Tai Muang. After a complete absence in 2020 and 2021, nesting shifted in 2024, with all four recorded nests in Khura Buri. Earlier seasons showed higher nest concentrations in Tai Muang and Takua Thung (Fig 2).


 
Microhabitat and environmental characteristics (2024 season)
 
During the 2024 nesting season, all nests (n=4) were on broad, open beaches with slopes of 2.2-3.2°. Nests were positioned above the HWL at a mean distance of 21.8±1.6 m and an average distance of 12.9±14.2 m from the vegetation line (Fig 3). Ambient light intensity was negligible (0-1 lux).


       
Sand temperature at 50 cm depth varied significantly (27.5-31.7°C, p<0.05), whereas sand humidity showed no significant difference (7.1-8.4%, Fig 4). Substrate analysis revealed a predominance of medium sand (0.25-0.50 mm), with one instance of coarse sand (>0.50 mm).


 
Reproductive outcomes and success trends
 
A comparative analysis across four nesting seasons (2019, 2022, 2023 and 2024) showed that while clutch size (averaging 112-121) and incubation time (54-63 days) remained stable across seasons, hatching success declined catastrophically. Success rates fell from 64.3% (2019) and 64.5% (2022) to 46.9% (2023) and a record low of 4.3% in 2024 (p<0.05) (Fig 5). As shown in Table 1, the primary causes of failure were unfertilized eggs (13.6%-56.8%) and yolkless eggs (11.2%-36.8%). Notably, the 2023-2024 seasons saw new failure modes emerge: fertilized but undeveloped embryos (3.7%) and early embryo mortality (4.6%) (Fig 6).






 
Spatio-temporal nesting dynamics and habitat relocation
 
The distribution of leatherback turtle nests along the western coast of Phang Nga Province from 2019 to 2024 reveals a dynamic and precarious nesting pattern. Our findings indicate that while nesting occurs across all four districts, there is a clear and concerning shift in site selection over time. Historically, primary nesting grounds were identified on three beaches in three districts, including Khuk Khak Beach in Takua Pa district, Bor Dan Beach in Tai Muang district and Bang Kwan Beach in Takua Thung district (Piboon et al., 2025). However, our 2024 field surveys found that many of these traditional sites have undergone significant land-use changes, characterized by the rapid expansion of tourism infrastructure, including resorts, cafés and beachfront hotels. This is supported by regional tourism statistics showing a substantial increase in both visitor numbers (from 4.4 million in 2016 to 4.9 million in 2018) and accommodation availability (from 11,903 in 2016 to 14,458 in 2020) in Phang Nga over the past decade (Ministry of Tourism and Sports, 2026).  
       
The relocation of nesting activities to Khura Buri district in 2024 is a significant finding of this study. These new sites are described as “pristine”, situated far from human settlements and roads, requiring significant travel on foot to reach (about 1-2 km). This spatial shift suggests that leatherback turtles are actively seeking areas with minimal anthropogenic disturbance, particularly light pollution, which was recorded at near-zero levels (0-1 lux) in our 2024 survey. While sea turtles generally exhibit nest-site fidelity, returning to their natal beaches, they also exhibit adaptive behavior by randomly selecting nearby alternative sites to optimize offspring survival when traditional habitats become unsuitable due to environmental degradation or human interference (Neeman et al., 2015; Santos et al., 2016). However, as our study demonstrates, the move to undisturbed areas does not inherently guarantee reproductive success.
 
Physical drivers of nest site selection
 
The selection of a nesting site is a non-random process governed by a fundamental niche of environmental cues. In 2024, leatherback turtles in Phang Nga demonstrated consistent preferences for specific beach characteristics. All recorded nests were positioned on open beaches, above the high water line (HWL) at an average distance of 21.8±1.6 m. This behavior is consistent with leatherback populations in other regions, such as Costa Rica (17.4±0.1 m) (Spanier, 2010) and Grenada (27.8±7.7 m) (Charles et al., 2023), highlighting a global evolutionary strategy to avoid seawater inundation. Flooding of the egg chamber can be lethal to developing embryos by inducing osmotic stress from salt and blocking oxygen exchange within the sand substrate (Nasiri et al., 2022; Stokes et al., 2024).
       
Furthermore, the preference for gentle to moderate slope (2.2 to 3.2°) observed in our study aligns with findings for leatherbacks and other sea turtle species globally (Nasiri et al., 2022; Spanier, 2010; Wood and Bjorndal, 2000; Zavaleta-Lizárraga and Morales Mávil, 2013). A moderate slope is critical as it allows the heavy-bodied leatherback to reach a safe elevation efficiently without excessive energy expenditure during the crawl. While steep slopes may be difficult to access (Ficetola, 2007), excessively shallow slopes increase the distance the turtle must travel (Santos et al., 2016), thereby increasing exposure to land-based predators or human disturbance. The substrate in Phang Nga consisted primarily of medium sand, which is known to support better gas exchange and hatching success compared to finer or coarser sediments.
 
Thermal crisis: linking temperature to embryo mortality
 
The most critical finding of this research is the relationship between the nest’s thermal environment and the catastrophic decline in hatching success. Sea turtles exhibit temperature-dependent sex determination (TSD), in which incubation temperature during the middle third of development determines the sex of hatchlings (Charles et al., 2023).  In the 2024 season, we recorded nest temperatures as high as 31.7°C. Temperatures consistently above 30°C for leatherback turtles are known to produce 100% female-biased clutches (Binckley et al., 1998), leading to concerns about the long-term availability of breeding males in the Andaman Sea population.
       
Beyond sex ratios, our data suggest that we are approaching the upper limit of the thermal tolerance range for leatherback embryo development (25-35°C) (Charles et al., 2023). The appearance of “fertilized but undeveloped” eggs (3.7%) and “early embryo mortality” (4.6%) in the 2023 season, which persisted into 2024, is a major “red flag” for this population. Unlike unfertilized eggs, which reflect biological issues with the parent turtles, the death of a fertilized embryo is often a direct result of environmental stress. High temperatures can cause morphological anomalies or total developmental arrest (Calderón Peña and Azanza Ricardo, 2021).
       
Our findings contrast with historical data from Thailand, which reported a much higher hatching rate of 50% (Piboon et al., 2025) to 64% (data in 2019, 2023). The sudden drop to a mere 4.3% success rate in 2024 is statistically significant and clinically alarming. This decline suggests that even on pristine beaches like those in Khura Buri, the microhabitat conditions, specifically sand temperature and moisture, may no longer be sufficient to support successful incubation. The high variability in temperature between nests (27.5 to 31.7°C) further underscores the vulnerability of these clutches to localized microhabitat fluctuations.
 
Biological constraints and reproductive failure
 
While environmental factors are paramount, the high proportion of yolkless and unfertilized eggs cannot be ignored. In the 2024 season, unfertilized eggs accounted for 56.8% of total clutch failure. Yolkless eggs, which are a unique characteristic of leatherback clutches, also remained high at 36.8%. The prevalence of unfertilized eggs suggests a “population bottleneck” where a lack of breeding males (Charles et al., 2023; García–Grajales et al., 2019; Tomillo et al., 2014) or physiological stress in nesting females is preventing successful fertilization. As nesting abundance decreases year by year, the probability of successful mating encounters in offshore waters also diminishes, creating a feedback loop of reproductive failure.
       
This combination of biological constraints (infertility) and environmental stressors (lethal temperatures) creates a synergistic threat that could lead to the localized extinction of leatherback turtles in the Andaman Sea. Similar patterns of decline have been observed in Malaysia, where leatherbacks were once abundant but are now virtually extinct due to a combination of egg exploitation and habitat loss.

Our study highlights the urgent need to protect leatherback turtles in Thailand. Nesting sites have shifted to pristine areas in Phang Nga, but hatching success has still dropped sharply to only 4.3%. This is due to rising embryo deaths and a high rate of unfertilized eggs. This indicates the population now faces serious environmental and biological challenges.
       
To prevent the loss of this iconic species, we propose practical management strategies such as temperature-controlled hatcheries, adding shade to nests and strictly protecting nest sites. Further investigation is needed into the health and genetic diversity of the nesting females to understand the high rates of infertility. Community-based monitoring programs, involving local rangers and citizens, should be expanded to ensure that every nest is identified and protected immediately upon laying.
       
In conclusion, Phang Nga coastline remains the final hope for leatherback turtles in Thailand. However, without immediate, science-based interventions that address both the physical characteristics of the beaches and the biological needs of the embryos, the future of these ancient mariners in Thai waters remains extremely uncertain.

The authors declare that there are no conflicts of interest regarding the publication of this article. No funding or sponsorship influenced the design of the study, data collection, analysis, decision to publish, or preparation of the manuscript.