Bluefin tuna reveals mercury pollution in the world’s oceans

Rutuja Borawke, Research fellow at CSIR-IGIB

A recent study published in PNAS, states that an apex predatory, migratory fish named Bluefin tuna (BFT) is a species that has neurotoxic methylmercury (MeHg) in its tissues. Accumulation rates of mercury (Hg) increase with their age and can be used as a global pollution index that may contemplate natural as well as anthropogenic emissions and environmental features. These provide means to compare Hg bioavailability in the thick of geographically distinct populations across oceanic basins, scrutinize trophic dynamics, and improve public health assessment for mercury exposure through seafood.

Why BFT is specifically highlighted?

BFT is long-lived (>30 years) and is the world’s largest tuna (weighing up to 600 kg and 300 cm long). Although the mechanism of distribution of mercury accumulation across the world’s oceans is still unknown, scientists propose tuna species as a potential bio-indicator for changes in global ocean mercury pollution patterns. The species of the BFT include:

• Southern BFT (Thunnus maccoyii)
• Pacific BFT (Thunnus orientalis), and
• Atlantic BFT (Thunnus thynnus)

These organisms have migration routes constrained to southern hemisphere oceans, including the Indian Ocean (IPO), the North Pacific Ocean (NPO), and the North Atlantic Ocean (NAO)/Mediterranean Sea (MS). Although their biological and environmental features are slightly varied, they are closely connected; the three BFT species have identical pelagic migration and feeding behaviors.

What do numbers suggest about global patterns?

Global demand for BFT has increased, leading to overfishing and serious stock shortages. At the same time, increased use of BFT by humans might raise the risk of exposure to highly bio-accumulative pollutants such as Hg. Because BFT accumulates Hg by MeHg’s dietary transfer, the almost whole muscle tissue of Hg (THg) is neurotoxic. Effective trophic transfers result in BFT levels of MeHg up to 108 times greater than in seawater. Thus, tissue concentrations in BFT exceed the thresholds specified by the US Food and Drug Administration and the WHO for the eating of big predatory fish of 1 μg g^-1 wet weight (w.w.). This highlights the need for predictive models to safeguard public health.

Over time, confrontations of the concentrations of mercury in the fish tissue are difficult since the mercury levels in marine fish are affected by age, size, location on the food web, and the type, abundance, which vary according to the local and global environment. The study shows Mercury Accumulation Rates (MARs) in BFT descends from the Mediterranean Sea > North Pacific Ocean> Indian Ocean> North Atlantic Ocean.  The researchers concluded that mercury accumulation rates in Bluefin tuna were higher in MS than others, after a comprehensive assessment of prior studies and mercury analyses in the tissue samples from 1998 to 2019. Along with this, it is observed that MARs proportion increases concerning MeHg bioavailability at the base of each oceanic sub basin’s food web. 

Conclusions and Interpretations

Researchers from Taiwan and Rutgers University are examining connections of Hg buildup in BFT with human sources and global thermohaline circulation based on a thorough collection of overall patterns of Hg cycling and ocean flows. They stated, “Although BFT Hg content might change somewhat over two decades within the same area, this variation does not influence the worldwide patterns of MARs that we found.”

The concentrations and bioaccumulation factors for MeHg in phytoplankton and zooplankton of every ocean basin also had a positive link between the MAR in the BFTs of the MS, NPO, and NAO.

These correlations provide evidence of the connection between dissolved MeHg and accumulation on top of the marine feed networks in apex predators, predicted but not often seen directly because of low sample sizes. It also suggests that the levels of Hg in BFT prey are also different from MeHg in seawater. Scientists cited some distinct anthropogenic factors for the specific buildup of Hg in individual oceans.

These changes in Hg accumulation across marine fish populations in far-ocean basins can be utilized to investigate global Hg pollution, control marine fishing, and protect the health of humans.

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Reference:

1. Tseng, C. M., Ang, S. J., Chen, Y. S., Shiao, J. C., Lamborg, C. H., He, X., & Reinfelder, J. R. (2021). Bluefin tuna reveal global patterns of mercury pollution and bioavailability in the world’s oceans. Proceedings of the National Academy of Sciences of the United States of America, 118(38), e2111205118. https://doi.org/10.1073/pnas.2111205118

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