Nearshore use increases propensity to accumulate persistent organic pollutants in two thresher shark species

Predator tissues can accumulate specific signatures of persistent organic pollutants (POPs) based on their feeding ecology. Likewise, stable isotopes, which are also incorporated into tissues via diet, are complementary to POPs as they provide additional geographic and trophic information into the ecology of consumers. The Common Thresher Shark (Alopias vulpinus) and the Bigeye Thresher (Alopias superciliosus) are closely related but have distinct spatial habitat use, with Common Threshers having shallower and more coastal distributions compared to Bigeye Threshers, which occur in deeper offshore waters. This study used two types of chemical markers (stable isotopes and POPs) to understand how ecological differences in nearshore use may influence their propensity to accumulate PCBs and DDXs. Liver samples were obtained from thresher sharks incidentally captured in southern California (a known hotspot for DDX and its metabolites) and analyzed for a suite of organic contaminants and two stable isotopes (δ¹³C and δ¹⁵N). Common Thresher Sharks had significantly higher concentrations for every contaminant group examined compared to Bigeye Threshers, and species were distinguishable based on their contaminant signatures. Contaminant levels significantly declined with length in Common Thresher Sharks, although the dataset was biased towards mature females, but levels significantly increased with length in Bigeye Threshers, which were skewed towards more mature males. Isotopic niche space had little overlap between species, and DDX concentrations positively correlated with δ¹³C (a proxy of nearshore use). The results suggest that proximity to shore may play a key role in hepatic POP accumulation, but intrinsic factors, such as sex, may also be influential.