Evidence that Pacific tuna mercury levels are driven by marine methylmercury production and anthropogenic inputs

Abstract

Pacific Ocean tuna is among the most-consumed seafood products but contains relatively high levels of the neurotoxin methylmercury. Limited observations suggest tuna mercury levels vary in space and time, yet the drivers are not well understood. Here, we map mercury concentrations in skipjack tuna across the Pacific Ocean and build generalized additive models to quantify the anthropogenic, ecological, and biogeochemical drivers. Skipjack mercury levels display a fivefold spatial gradient, with maximum concentrations in the northwest near Asia, intermediate values in the east, and the lowest levels in the west, southwest, and central Pacific. Large spatial differences can be explained by the depth of the seawater methylmercury peak near low-oxygen zones, leading to enhanced tuna mercury concentrations in regions where oxygen depletion is shallow. Despite this natural biogeochemical control, the mercury hotspot in tuna caught near Asia is explained by elevated atmospheric mercury concentrations and/or mercury river inputs to the coastal shelf. While we cannot ignore the legacy mercury contribution from other regions to the Pacific Ocean (e.g., North America and Europe), our results suggest that recent anthropogenic mercury release, which is currently largest in Asia, contributes directly to present-day human mercury exposure.

Global change effects on biogeochemical mercury cycling

“…Blanchfield et al ( 2022 ) and Wu et al ( 2021 ) directly link water MeHg concentrations to MeHg in fish. This finding is consistent with surveys and statistical approaches in the marine environment that show that tuna Hg (i.e., MeHg) (Tseng et al 2021 ; Barbosa et al 2022 ; Medieu et al 2022 ) is related to seawater MeHg. Theoretical and modeling approaches also estimate that seawater MeHg levels account for much of the MeHg variability observed in fish (Alava et al 2018 ; Schartup et al 2019 ).…”

Variation Among Species and Populations, and Carry-Over Effects of Winter Exposure on Mercury Accumulation in Small Petrels

“…This result does not agree with the suggestion that the trophic position is the most important factor explaining variation in Hg concentrations in Southern Ocean seabirds (Becker et al, 2002). Likewise, in tunas trophic effects (i.e., geographical changes in foraging ecology) had a limited influence on the spatial variability of tissue Hg concentrations (Médieu et al, 2022).…”

A synthesis of mercury research in the Southern Hemisphere, part 1: Natural processes

“…In ocean waters, most MeHg is likely produced in situ rather than transported offshore from rivers, with MeHg having a short lifetime in the surface ocean (Liu et al 2021 ). As a result, MeHg concentrations in surface ocean waters of the SH are not significantly lower than in the NH, except when compared to specific locations such as the Mediterranean Sea and the northwestern Pacific Ocean, where (compared to other oceans) high MeHg concentrations in the water column and in tuna fish have been recorded (Mason et al 2012 ; Tseng et al 2021 ; Médieu et al 2022 ).…”