Wintering loons in South Korea face an ongoing threat from polycyclic aromatic hydrocarbons: Shifting sources and potential DNA damage.

Diving birds, particularly those sharing coastal habitats with fishing grounds, are at risk from oil pollution. Despite documented cases of bird mortality, the specific role of oil pollution in these death remains unclear. To address this knowledge gap, this study examined polycyclic aromatic hydrocarbon (PAH) contamination, its sources, and its impact on loon health. An analysis of 86 carcasses from three species of loons revealed drowning as the leading cause of death, followed by oil pollution and unknown debilitation. While liver concentrations of 16 PAHs (∑PAHs) showed no significant variation by sex, location, species, or cause of death, it was evident that wintering loons were exposed to PAH pollution along South Korea's eastern coast. The ratio of low (di- and tri-cyclic) to high (tetra-, penta-, and hexa-cyclic) molecular weight PAHs was approximately 3-5 across all three loon species. From 2010 to 2017, the composition of PAHs shifted, with a decline in low molecular weight PAHs (indicative of petrogenic sources) and a concerning increase in high molecular weight PAHs (associated with pyrogenic sources). This trend coincided with a tenfold increase in the toxic equivalency quotient of benzo[a]pyrene (TEQ), despite a decrease in overall ∑PAH concentrations. The detection of benzo[a]pyrene diol epoxide (BPDE)-DNA adducts in some loons further suggests potential genotoxic effects from PAH exposure. These findings underscore the persistent PAH contamination affecting wintering loons. Continued research is crucial to understand the evolving threats posed by PAHs and to support the conservation of these migratory birds along the North America-Asia flyway.