Mercury stable isotopes explore sources of methylmercury in giant Pacific Bluefin tuna
Understanding the accumulation and sources of methylmercury (MeHg) in tuna species for reducing the risk of human exposure through tuna consumption is so important to human well-being and ecosystem health. Professor Tseng’s lab thus collaborated with Professor Reinfelder (Rutgers University) in the US to explore the accumulation and trophic transfer of MeHg in Pacific Bluefin tuna (Thunnus orientalis, PBT) caught in the Northwest Pacific off Taiwan.
A recent study in Limnology and Oceanography Letters: Mercury stable isotopes reveal sources of methylmercury and prey in giant Pacific Bluefin tuna from the western North Pacific Ocean. The team analyzed the concentrations and stable isotopic values of mercury in the muscle tissues of PBT and foraging prey, and then compared them with literature data on Hg stable isotopes in tuna fish in the Central and Eastern Pacific Oceans. This research first found that the values of Hg stable isotopes in δ202Hg and Δ199Hg changed significantly with the age and size of PBT. With the PBT growth, MeHg then increases due to feeding more epipelagic prey from the Kuroshio extension off Eastern Japan which is a more important foraging ground than waters off Taiwan. For all adult PBT, little MeHg is accumulated in the spawning grounds near Taiwan. Ultimately, the Hg isotopic results show different sources or transformations of MeHg prior to accumulation in the food web of the Northwest Pacific than further east and further elevated MeHg levels in PBT apparently related to anthropogenic Hg inputs from East Asians.
In summary, this study identified the dominant sources of Hg in the PBT and their prey from foraging areas in the West Pacific Ocean using Hg stable isotopes. The outcome is useful to examine where Hg sources from that lead to elevated Hg in the top-predator fish, to make us assess the pollution tendency of the marine ecosystem, and to recognize the significance of Hg exposure from seafood.
Spatial differences in Δ199Hg/Δ201Hg slopes of Pacific Bluefin tuna and prey.