Sources and Variations of Mercury in Tuna

Purpose:

These scientists attempt to understand where mercury is entering the seafood chain:

1. in the waters; and/or
2. in the atmosphere.

Bottom Line:

• These scientists believe that methylmercury is formed in the deep waters and/or in the sediments on the bottom of the oceans where manmade mercury emissions have little effect(s).
• These scientists' conclusion about where methylmercury is formed and/or concentrated rests ultimately on the simple idea that if mercury in tuna originated wholly or partly in the air, the increase in mercury in the air should have been reflected to a measurable extent in the methylmercury found in fish caught in 1971 and compared to the same type of fish caught in the same part of the ocean some 27 years later in 1998.
• The fact that the methylmercury concentrations in tuna are identical in 1971 and 27 years later in 1998 either:
  • reflects a remarkable coincidence; or
  • indicates that, regardless of how methylmercury is formed and/or stored, the methylmercury levels in tuna are not responding to the additional mercury pollution being released into the air.

Abstract:

While the bulk of human exposure to mercury is through the consumption of marine fish, most of what we know about mercury methylation and bioaccumulation is from studies of freshwaters. We know little of where and how mercury is methylated in the open oceans, and there is currently a debate whether methylmercury concentrations in marine fish have increased along with global anthropogenic mercury emissions. Measurements of mercury concentrations in Yellowfin tuna caught off Hawaii in 1998 show no increase compared to measurements of the same species caught in the same area in 1971. On the basis of the known increase in the global emissions of mercury over the past century and of a simple model of mercury biogeochemistry in the Equatorial and Subtropical Pacific ocean, we calculate that the methylmercury concentration in these surface waters should have increased between 9 and 26% over this 27 years span if methylation occurred in the mixed layer or in the thermocline. Such an increase is statistically inconsistent with the constant mercury concentrations measured in tuna. We conclude tentatively that mercury methylation in the oceans occurs in deep waters or in sediments.

Find This Paper for Your Review:

Sources and Variations of Mercury in Tuna. Environmental Science and Technology Vol. 37. No. 24. pp. 5551-5558. 2003.