Mercury stable isotopes reveal sources of methylmercury and prey in giant Pacific bluefin tuna from the western North Pacific Ocean

Sources of methylmercury (MeHg) in adult Pacific bluefin tuna (Thunnus orientalis, PBT) from the western North Pacific Ocean (WPO) were examined using mercury stable isotopes. Significant increases in delta Hg-202 and & UDelta;Hg-199 values with PBT size and age, along with those of potential prey, indicate a shift in the source of MeHg accumulated by PBT as they age. Among adults from the WPO, this shift likely involves greater accumulation of MeHg from epipelagic prey in the Kuroshio extension in large vs. small and medium-sized PBT. For all adults, little MeHg is accumulated in the spawning grounds near Taiwan. Significantly lower & UDelta;Hg-199/& UDelta;Hg-201 ratios in adult PBT and their prey from the WPO than from the central and eastern North Pacific indicate different sources or transformations of MeHg prior to accumulation in the WPO food web than further east. Our results show that MeHg sources to oceanic food webs vary across the North Pacific Ocean and regionally within the WPO.

Automated summary

The sources of methylmercury (MeHg) in Pacific bluefin tuna (PBT) from the western North Pacific Ocean (WPO) were investigated using mercury stable isotopes. The study found that the source of MeHg accumulated by PBT changes as they age, with larger PBT accumulating more MeHg from epipelagic prey in the Kuroshio extension. In addition, the sources of MeHg in the WPO food web appear to be different from those in the central and eastern North Pacific.