93 subpopulation (Bourgogne et al. 2024c): the site of the ‘Roche Percée’ (RP), and the ‘Grand Lagon Sud’ area (GLS). These two distinct nesting areas are roughly 150 km apart, yet show very contrasting habitat geomorphologies. The RP is a coastal, dark-colored substrate beach (Limpus et al. 2005; Barbier et al. 2023), while the GLS is composed of remote and scattered coral-islets with light-colored sand beaches (Bourgogne et al. 2024a). Despite clear geographic discontinuity and habitat discrepancy, a recent study based on molecular evidence shows that females from both areas belong to the same genetic stock while showing some level of site fidelity as indicated by satellite tracking and flipper tags (Bourgogne et al. 2024a). Read et al. (2013) reported a concerning situation in the RP beach, as nest temperatures reached very high levels, suggesting not only a strongly female-skewed hatchling sex ratio in the nests, but also poor hatching success and hatchlings fitness as they approached the upper limit of embryo thermal tolerance. Similar observations have been reported for other nesting areas in eastern Australia (Chu et al. 2008), raising concerns about a rapid collapse of the subpopulation's reproductive success that could lead to its demographic decline. Based on previous studies that documented a significant effect of the substrate color on nest temperatures (Laloë et al. 2014), with dark sand beaches being warmer than light sand beaches, we hypothesize that the substrate of the coral-islets in the GLS area may lead to cooler thermal conditions than in the RP beach, and hence to less extreme female-skewed primary sex ratios (Laloë et al. 2014). This hypothesis was investigated through in situ monitoring of nest temperatures, conducted simultaneously in both areas over the 2019-20, 2021-22 and 2022-23 nesting seasons, from late November to March to fully cover the reproductive phenology of the loggerhead turtle in New Caledonia (Barbier et al. 2023; Bourgogne et al. 2024c). This study provides insight into the benefits of local nesting habitat diversity for sea turtle populations in the face of climate change. 3. Materials & methods 3.1 Study area 3.1.1 The ‘Roche Percée’ nesting area (RP) The RP site is a sandy beach located in the bay of Bourail (S°21.61229, E°165.46461), on the west coast of the mainland (Figure 20-B, Table 8 SI Appendix). It is oriented south-west and is 2.2 km long. The beach is composed of thin and dark arenite-type sand (Allenbach 1998; Limpus et al. 2005b), and the presence of the Nera river mouths at both ends limit the expansion of the coastal vegetation cover. Moreover, due to sand replenishment work at the vegetation margin, a compacted ridge has formed at the rear of the beach, resulting in the
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