Prioritization of legacy and emerging halogenated organic contaminants in humpback dolphins from northern South China Sea

Marine mammals are prone to accumulating elevated levels of halogenated organic contaminants (HOCs) due to their long lifespans, high trophic positions, and substantial lipid stores. However, which HOCs are of greatest concern in these apex predators remain largely unknown. Here, we constructed a toxicological prioritization index (ToxPi) model that integrates detection frequency, persistence, bioaccumulation, and endocrine-disrupting potential to evaluate priority HOCs in Indo-Pacific humpback dolphins (Sousa chinensis, n = 70) from the northern South China Sea (NSCS) between 2008 and 2018. Our findings revealed that legacy HOCs—particularly dichlorodiphenyltrichloroethanes (DDTs) and hexachlorocyclohexanes (HCHs)—ranked as the highest priority HOCs, highlighting their long-lasting health risk to the dolphins despite the global prohibition of these chemicals. Notably, although alternative halogenated flame retardants (AHFRs), as emerging contaminants, constituted the least proportion (0.03 %) among the HOCs analyzed, their ToxPi scores (0.38–0.44) were just below those of DDTs (0.46–0.53) and HCHs (0.48–0.50). This suggests that AHFRs may not be suitable substitutes for traditional flame retardants, and their potential ecological risks deserve further attention. Significant correlations were observed between hormones (thyroid hormones and testosterone) and high-scoring congeners within each HOC category. It is further supported that these priority HOCs may cause endocrine disorders in humpback dolphins through the actual measurement data. Overall, this study identified a priority list of HOCs in the NSCS humpback dolphins, emphasizing the need to monitor both legacy and emerging HOCs in marine mammals, without favoring one over the other.