Mercury, food webs, and marine mammals: implications of diet and climate change for human health

Abstract

We modeled the flow of methyl mercury, a toxic global pollutant, in the Faroe Islands marine ecosystem and compared average human methyl mercury exposure from consumption of pilot whale meat and fish (cod, Gadus morhua) with current tolerable weekly intake (TWI) levels. Under present conditions and climate change scenarios, methyl mercury increased in the ecosystem, translating into increased human exposure over time. However, we saw greater changes as a result of changing fishing mortalities. A large portion of the general human population exceed the TWI levels set by the World Health Organization [WHO; 1.6 microg/kg body weight (bw)], and they all exceed the reference dose (RfD) of 0.1 microg/kg bw/day set by the U.S. Environmental Protection Agency (EPA; equivalent to a TWI of 0.7 microg/kg bw). As a result of an independent study documenting that Faroese children exposed prenatally to methyl mercury had reduced cognitive abilities, pregnant women have decreased their intake of whale meat and were below the TWI levels set by the WHO and the U.S. EPA. Cod had approximately 95% lower methyl mercury concentrations than did pilot whale. Thus, the high and harmful levels of methyl mercury in the diet of Faroe Islanders are driven by whale meat consumption, and the increasing impact of climate change is likely to exacerbate this situation. Significantly, base inflow rates of mercury into the environment would need to be reduced by approximately 50% to ensure levels of intake below the WHO TWI levels, given current levels of whale consumption.

Figure 1. (A–C) Simulated decreases of 20% in fishing mortality F: time series of selected taxa showing the proportional increases (A) and the proportional decreases (B) in biomass of main species/functional groups, and the proportional changes in monomethyl mercury (CH₃Hg⁺) concentrations in selected taxa (C). (D–F) Simulated increases of 20% in F: proportional decreases (D) and increases (E) in biomass for the main taxa, and the proportional changes in methyl mercury concentrations in selected taxa (F ). Status quo (no change in fishing mortality rates) represents the same taxa, but the groups do not differentiate graphically.

Figure 2. Effects of climate change and changing fishing mortality (F) on the methyl mercury concentrations in pilot whales (Globicephala melas; A) and cod (Gadus morhua; B). Note the significant difference in scale of the y-axis. Other species showed similar general trends. Increasing F decreased the load relative to the status quo scenario, whereas decreasing F had the opposite effect.

Figure 3. Results of the dietary analysis for the general population and pregnant women compared with the TWI limits set by the WHO (1.6 μg/kg bw) and the U.S. EPA equivalent (RfD of 0.1 μg/kg bw/day expressed as a TWI of 0.7 μg/kg bw). Under all simulations, a substantial portion of the general adult population had dietary intakes greater than the WHO limit, and all members’ consumption was greater than the limit set by the U.S. EPA. The TWI level under a simulated 50% reduction in the base inflow rate (BIR) of mercury into the environment is also shown. Pregnant women were well below the limits set by the WHO and the U.S. EPA under all scenarios. Note that the present-day scenarios for both groups at t₁₀₀ are not presented because they were only marginally different than the status quo at t₂₀₀.