Green Turtles Struggle to Rebound at a Little-Known Borneo Nesting Beach

On the golden beaches of Similajau National Park, roughly 30 kilometers northeast of Bintulu town in Sarawak, Malaysia, green turtles are making a quiet and fragile comeback. A new study published in Frontiers in Amphibian and Reptile Science has compiled more than a decade of scattered records to paint the first detailed picture of nesting activity and reproductive output at this understudied rookery on the South China Sea. The results are sobering: nesting is rare, hatching success is mediocre, and the threats facing every clutch of eggs are relentless. Yet the mere fact that turtles are nesting there at all, after a five-year hiatus that ended in 2015, is being welcomed by researchers as a sign that the site still matters for conservation.

Green turtles, Chelonia mydas, are among the most ecologically important marine megafauna on the planet. Adult turtles graze on seagrass meadows, keeping these underwater pastures healthy, while the animals themselves serve as prey, hosts, competitors and even apex predators within marine food webs. Seven species of sea turtles exist worldwide, and Malaysia hosts nesting by five of them, including the green turtle, known locally as penyu agar or penyu hijau. The species is the most widely distributed sea turtle in the country, with nesting recorded in Sabah, Sarawak, Terengganu, Perak, Pahang and Johor. At the well-known Sabah Turtle Islands, annual nest counts can reach 15,000, and Redang Island records between 700 and 1,500 nests each year.

Similajau National Park tells a very different story. Gazetted in 1976 and spanning 89.96 square kilometers, the park protects golden sandy beaches, mixed dipterocarp forests and kerangas woodlands, along with endemic primates such as the Bornean banded langur and Hose’s surili. Its four main nesting beaches, Chalet Beach, Turtle Beach 1, Turtle Beach 2 and Golden Beach, have never received the scientific attention lavished on Sarawak’s flagship rookeries like Talang-Satang National Park. Records kept by rangers of the Sarawak Forestry Corporation, who patrol the high tide lines looking for flipper tracks, formed the raw material for the new analysis, which covered the years 2015 to 2017 and 2022 to 2024. Years with missing records, including 2018 and the pandemic closure period of 2019 to 2021, were deliberately excluded to avoid mistaking absent data for absent turtles.

The headline numbers are stark. Across the study years, nesting activity was low and inconsistent, averaging just 3.33 nests per year, with annual totals ranging from zero to six. In 2023, despite routine patrols, no green turtle nesting was confirmed at all. The mean hatching success rate across the study period was 45.47 percent, swinging from a low of 24 percent in 2016 to a high of 90 percent in 2022. For comparison, Talang-Satang National Park records more than 2,000 nests annually, Redang Island up to 2,500, and the Sabah Turtle Islands as many as 15,000. Against those benchmarks, Similajau is a footnote in Malaysian sea turtle reproduction, but a footnote that conservationists cannot afford to ignore.

The year 2024 offered the most detailed window into the park’s reproductive biology, because that was the first year clutch-level data were recorded. Four nests on Chalet Beach were discovered and successfully relocated to the park’s roofed, fenced and elevated hatchery. Clutch sizes ranged from 119 to 137 eggs, averaging 129.75 eggs, a figure that sits comfortably within the range reported for green turtles elsewhere in Malaysia and beyond, such as 42 to 150 eggs per clutch in Negeri Sembilan and 74 to 179 in the Chagos Archipelago. Because clutch size generally correlates with female body size, the authors suggest, cautiously, that the females nesting at Similajau may be relatively large, older remigrant individuals, though no tagging or morphometric data were available to confirm this.

The fate of those four nests revealed how precarious reproduction is at the site. Apparent incubation durations ranged from 54 to 62 days, averaging 58.5 days, which is comparable to the roughly 60-day incubation period typical of sea turtles and hints that nest temperatures probably fell within a generally suitable range. But hatching success varied enormously, from a catastrophic 7.69 percent in the first nest to 73.11 percent in the fourth, averaging 41.8 percent with a standard deviation of 26.8. The first nest had been inundated by seawater before it was relocated, and its detection came up to five days after oviposition because of a patrol gap. When researchers excavated the nests after emergence, approximately 95 percent of the unhatched eggs showed no embryonic development at all, a pattern consistent with early mortality caused by flooding, osmotic stress and reduced oxygen availability in waterlogged sand.

Nest relocation itself is a double-edged sword. Moving eggs is standard practice when in-situ nests face near-certain destruction from predators, tidal flooding or human disturbance, and at Similajau the threats are real: ghost crabs, macaques, feral pigs, monitor lizards, ants and fungal infections are all documented in Malaysia and present in the park. But timing matters enormously. Relocation is ideally performed within one to two hours of oviposition, before the yolk sac membrane attaches to the shell, and although it can be done within 12 hours, hatching success declines significantly. At Similajau, remote beaches and irregular patrol coverage mean nests are often found too late for optimal handling, and broken eggs during relocation, six in one nest and nine in another in 2024, further eroded output. The study also notes that a metal pole used to locate eggs within nest pits can break eggs at the top of the clutch.

The 2024 season also exposed the difficulty of simply counting what happens on the beach. Rangers detected 20 nest pits that year, but only four could be confirmed as successful nests with eggs. Five pits were surrounded by scattered predated eggshell remnants, evidence that eggs had been laid and then eaten, while the remaining 11 showed no clear evidence of oviposition and may have been abandoned nesting attempts. Because the pits were clustered close together, it was impossible to determine whether the eggshell fragments represented separate nesting events or the remains of a single clutch dragged apart by wildlife, rain or tides. The authors argue that estimating true nest numbers from eggshell fragments alone is unreliable on beaches inhabited by diverse fauna and continually reshaped by waves and sediment movement.

Why is Similajau so quiet compared with other Malaysian rookeries? The study points to a possible historical legacy: mass mortality events in the 1990s, when roughly 100 sea turtle carcasses were recorded annually along Sarawak’s coasts, largely attributed to illegal trawling in key foraging areas, migratory routes and inter-nesting habitats. Given how far green turtles roam across Sarawak waters, the turtles that once nested at Similajau may have been caught up in those losses. Ongoing pressures compound the problem, from habitat degradation, climate change, egg and meat poaching, fisheries bycatch and ghost nets to unsustainable tourism, marine debris entanglement and vessel strikes. Because green turtles mature late and live long, any recovery in nesting activity will be slow and will require sustained intervention.

The researchers are careful about the limits of their work. The dataset is small and fragmented, patrol coverage varied from daily monitoring on Chalet Beach to monthly visits elsewhere, and records of survey effort over the years were unavailable, so the true nesting frequency could be higher than the counts suggest. No incubation temperatures were recorded, fungal infection of unhatched eggs was not examined, and the emergence success of hatchlings was never measured, though supervised releases mean it probably resembled hatching success. What the study delivers instead is a baseline, the first quantitative characterization of a nesting population that had previously gone unmeasured. The authors recommend intensified patrols during peak nesting periods, faster relocation to minimize embryo mortality, possible in-situ protection with predator exclusion or shading for low-risk nests, expanded surveys of nearby beaches, and community-based monitoring involving local stakeholders, conservation groups and university volunteers. They also call for research into the environmental drivers of reproductive output, including temperature, humidity and microbial infection. The reappearance of nesting at Similajau since 2015 is encouraging, they conclude, but the low and inconsistent numbers make clear that this quiet corner of Borneo will need decades of attention if its green turtles are to endure.

Subject of Research: Green turtle nesting activity and reproductive output at Similajau National Park, Sarawak

Article Title: Nesting activity and reproductive output of green turtles in Similajau National Park, Bintulu, Sarawak

Article References: Ting Jia Lei, A., & Hassan, R. B. (2026). Nesting activity and reproductive output of green turtles in Similajau National Park, Bintulu, Sarawak. Frontiers in Amphibian and Reptile Science, 4, Article 1890215. https://doi.org/10.3389/famrs.2026.1890215

Image Credits: AI Generated

DOI: 10.3389/famrs.2026.1890215

Keywords: green turtle, Chelonia mydas, Similajau National Park, Sarawak, nesting activity, hatching success, nest relocation, embryo mortality, sea turtle conservation, Malaysia, marine megafauna, hatchery management