51 1. Abstract The loggerhead turtle Caretta caretta is a large marine turtle with a cosmopolitan repartition in warm and temperate waters of the planet. The South Pacific subpopulation is classified as 'Critically Endangered’ on the IUCN Red List, based on the estimated demographic decline. This precarious situation engages an urgent need to monitor nesting populations in order to highlight conservation priorities and to ensure their efficiency over time. New Caledonia encompasses a large number of micro and distant nesting sites, localized on coral islets widely distributed across its large lagoon. Adequately surveying nesting activities on those hard-toreach beaches can prove to be challenging. As a result, important knowledge gaps prevail in those high-potential nesting habitats. For the first time, an innovative monitoring scheme was conducted to assess the intensity of nesting activities, considered as a proxy of the population size, on an exhaustive set of islets located in the ‘Grand Lagon Sud’ area. These data were analyzed using a set of statistical methods specially designed to produce phenology and nesting activity estimates using Bayesian methods. This analysis revealed that this rookery hosts a large nesting colony, with a mean annual estimate of 437 nests (95% Credible Interval=328-582). These numbers exceed that of the previous estimated annual number of loggerhead turtle nests in New Caledonia, highlighting the exceptional nature of this area. Considering the fact that similar high-potential aggregations have been identified in other parts of New Caledonia, but failed to be comprehensively assessed to this day, we recommend carrying out this replicable monitoring scheme to other locations. It could allow a significant reevaluation of the New Caledonian nesting population importance and, ultimately, of its prevailing responsibility for the protection of this patrimonial yet endangered species. 2. Introduction Assessing population abundance and demographic trends is key to develop and evaluate conservation actions for threatened species (Reynolds et al. 2011; Robinson et al. 2018; Ceriani et al. 2019). Marine megafaunal species, such as sea turtles, face major conservation issues worldwide (Wallace et al. 2011; Alves et al. 2022; Grémillet et al. 2022; Herbert-Read et al. 2022). Yet, long-term monitoring programs can prove to be particularly challenging, due to their ocean-scale distributions and migratory nature (Metcalfe et al. 2015; Casale and Ceriani 2020). Onshore quantifications of sea turtle nesting activities have developed into the most common indicator to evaluate the demographic trends, as it is often used as an index of population size (Metcalfe et al. 2015; Omeyer et al. 2022). Several monitoring schemes have been developed to assess long-term population trends according to nesting site conformations and specificities (SWOT Scientific Advisory Board 2011). Monitoring remote and scattered
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