Bluefin tuna reveal global patterns of mercury pollution and bioavailability in the world's oceans

Abstract

Bluefin tuna (BFT), highly prized among consumers, accumulate high levels of mercury (Hg) as neurotoxic methylmercury (MeHg). However, how Hg bioaccumulation varies among globally distributed BFT populations is not understood. Here, we show mercury accumulation rates (MARs) in BFT are highest in the Mediterranean Sea and decrease as North Pacific Ocean > Indian Ocean > North Atlantic Ocean. Moreover, MARs increase in proportion to the concentrations of MeHg in regional seawater and zooplankton, linking MeHg accumulation in BFT to MeHg bioavailability at the base of each subbasin's food web. Observed global patterns correspond to levels of Hg in each ocean subbasin; the Mediterranean, North Pacific, and Indian Oceans are subject to geogenic enrichment and anthropogenic contamination, while the North Atlantic Ocean is less so. MAR in BFT as a global pollution index reflects natural and human sources and global thermohaline circulation.

Keywords: bioavailability; bluefin tuna; mercury accumulation rate; mercury bioaccumulation; ocean pollution.

Fig. 1.

Global distributions, spawning areas, migration routes, and average THg concentrations (μg ⋅ g⁻¹ w.w.) of the three BFT species and their subpopulations. Cruise tracks and vertical profile stations where seawater Hg concentrations were measured are indicated by gray lines and crosses. BFT capture locations are indicated by numbers: 1 and 11, this study; 2 and 6, ref. ; 3 refs. , ; 4, ref. ; 5, ref. ; 7, refs. , , ; 8, ref. ; 9, ref. ; and 10, ref. . High density (dark shaded) and probable (light shaded) spatial distributions of BFT in the North Pacific (Thunnus orientalis, Pacific BFT in viridian), North Atlantic (Thunnus thynnus, Atlantic BFT in orange), and Southern Indian Oceans (Thunnus maccoyii, Southern BFT in purple) are shown (IUCN (International Union for Conservation of Nature). 2019, http://www.iucnredlist.org). MARs in BFT (yellow circles) in relation to methylmercury (MeHg) inventories in the world's oceans are also shown (see MARs in BFT Reveal Global Patterns of Pollution and Bioavailability).

Fig. 2.

Relationships between THg concentrations and fish age in ≤15-y-old BFT from the NPO, the MS, the NAO, and the IO. The dashed red line denotes World Health Organization food safety limit of 1 µg ⋅ g⁻¹ w.w. Values are means ± 1 SD. Regression lines (solid lines) with 95% CIs (dashed lines) are plotted for BFT from each basin (SI Appendix, Table S3 for regression statistics).

Fig. 3.

Global comparison of (A) THg concentrations and (B) MARs in BFT, and average concentrations and water column inventories (0 to 1,000 m) of (C) THg and (D) methylmercury (MeHg) in the surface ocean (0 to 150 m) and thermocline layer (150 to 1,000 m) of the NAO, the IO, the eastern PO (EPO), western PO (WPO), NPO, and the MS. Sources for BFT concentrations are in Fig. 2 and SI Appendix, Table S2. Seawater concentrations and water column inventories are from SI Appendix, Table S8. Values are means ± 1 SD.

Fig. 4.

Relationships between MARs in BFT and (A) methylmercury (MeHg) concentrations in surface (0 to 150 m, solid black) and thermocline layers (150 to 1,000 m, open symbols) and (B) water column inventories (0 to 1,000 m) of MeHg across four ocean subbasins, the NAO (square), the IO (triangle), the NPO (circle), and the MS (diamond). Errors are ±1 SD. Regression lines (solid lines) with 95% CIs (dashed lines) are plotted.

Fig. 5.

Comparisons of MARs (average ±1 SD) in BFT with mean (A) concentrations and (B) bioaccumulation factors (BAF) of methylmercury (MeHg) in marine phytoplankton and zooplankton in three ocean basins. BAF is the MeHg concentration (pmol ⋅ kg⁻¹, w.w.) in an organism divided by the dissolved concentration of MeHg (pmol ⋅ L⁻¹) in seawater.