Mercury exposure and health challenges in Rapa Nui green turtles: urging conservation and long-term monitoring in the South Pacific

The endangered green sea turtle (<i>Chelonia mydas;</i> hereafter <i>C. mydas</i>) plays a crucial role in maintaining the balance of marine ecosystems. However, its populations are highly vulnerable to various threats, including marine pollution. Rapa Nui (Easter Island), an isolated location in the southeastern Pacific, provides vital foraging habitats for both morphotypes of Pacific <i>C. mydas</i> (black and yellow). In this study, we examined the demographic structure (morphotype, life stage, sex) and health status (based on blood analytes and mercury-Hg concentration) of <i>C. mydas</i> on Rapa Nui during 2018 and 2023. Turtles from various life stages and sexes were observed, with a predominance of yellow morphotype juveniles, likely recently recruited or emerging from brumation. Haematological analyses revealed low levels of several key analytes (e.g. cholesterol, calcium, phosphorus, total protein, globulins), suggesting poor nutritional status, potentially related to the brumation process, limited food availability or poor food quality in the region. Alterations in both red and white blood cell lines, including anaemia and lymphopenia, indicate ongoing inflammatory states and infections, consistent with clinical observations. Rapa Nui turtles exhibited some of the highest blood Hg concentrations globally. Abnormalities in blood profiles, along with correlations between various analytes and blood Hg concentrations, suggest altered immune function and probable renal and liver dysfunction, likely resulting from both natural and anthropogenic sources of this heavy metal. Additionally, a very high body condition index in turtles with carapace lesions suggests a negative impact from human food subsidies in local bays, particularly from high-trophic-level fish, which may also serve as a pathway for Hg accumulation, both for the turtle aggregation and the human population. Our findings underscore the urgent need for long-term mercury monitoring and turtle movement studies to identify pollution sources, inform effective conservation strategies for this endangered species, and address potential public health concerns on this remote Pacific island.