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The data for both tuna and swordfish lend support to the contention that the mercury levels now being found in wide-ranging ocean fish are not primarily the consequence of man-made pollution but are of natural origin.” Chicago Tribune: Albacore tuna is a big fish and therefore tends to have higher mercury levels. The government has warned young children and pregnant women to limit how much albacore they eat. There are no warnings for light tuna, because most of it is made with skipjack, a relatively small species with lower levels of mercury. But some canned light tuna comes from another species: yellowfin. While the mercury content of yellowfin varies, industry testing found the average to be equal to that of albacore. (December 13, 2005) Response: Canned tuna To anyone even slightly familiar with the issue, it is well known that larger, longer lived species of tuna have relatively higher levels of natural residual mercury. Warm-blooded albacore, yellowfin or even bluefin tuna of matured size, weight and age from certain ocean basins - the Mediterranean for example - have relatively higher mercury levels. (The issues of mercury in canned tuna are further discussed in Sections 16 and 20.) The Real Issue: Natural Sources of Mercury
However, all of this seems a diversion from the real issue: a missing context of mercury source and bioaccumulation in any species of ocean tuna ultimately found in canned tuna products. In other words, how did the mercury in tuna get there in the first place, and can human intervention remove it? (The other key contextual issue of whether present levels are toxic is discussed repeatedly in this review paper.) The general impression with which CT leaves readers is that present mercury levels in tuna are a recent “menace,” with attribution to human activities. However, available fish mercury data challenge the premise that anthropogenic industrial Hg emissions deposited to air and water has led to heightened levels of MeHg in ocean fish over time. In rejecting the hypothesis, evidence strongly suggests that (1) MeHg has always been present in fish, (2) MeHg levels in fish vary naturally over time, and (3) the natural production (and destruction) of MeHg is not limited by the amount of Hg available in aquatic systems, regardless of sources. Hence, theoretical modeling claims of a linear connection between ocean fish MeHg levels and minor (compared to natural emissions, Sec. 5) power plant Hg emissions confirm a serious misunderstanding of or disregard for the emerging science derived from real-world observations.  Pacific Yellowfin tuna data provide but one example. Fig. 15-A shows recent results by Kraepiel et al. (2003) finding no increase in MeHg levels for Yellowfin tuna caught in 1998 relative to a similar cohort caught in 1971. The theoretical expectation (similar to EPA’s) was that methylmercury concentrations “should have increased by 9 to 26%” over the interval “if methylation occurred in the mixed layer or in the thermocline [of the Pacific Ocean].” The theory was not proven. Integral to the Kraepiel findings, Zhang et al. (2002) has estimated that China’s mercury emissions alone from coal combustion are increasing at the rate of 5% per year (from available data from 1978 through 1995), which is consistent with the theoretical expectation of increased amounts of methylmercury in the waters of the Pacific Ocean if the Hg-to-MeHg conversion process in the oceans is sensitive to industrial emissions loading. To the contrary, Kraepiel et al. (2003) clearly concluded that “[s]uch an increase is statistically inconsistent with the constant mercury concentrations measured in tuna.” [Emphasis added] The following year (2004), the authors strongly reconfirmed: Our findings that the concentration of mercury in tuna...has not changed over a period of time during which anthropogenic mercury inputs...have increased supports the idea that the source of methylmercury in tuna is not in surface waters. This provides prima facie evidence that this concentration is not responding to anthropogenic emissions irrespective of the mechanisms by which mercury is methylated in the oceans and accumulated in tuna. The Kraepiel findings are supported by similar research. A study by Barber et al. confirmed that although one can find clear increases of MeHg concentration in western Atlantic blue hake as fish length increases, one can not find significant changes in the fish mercury-size relation between fish samples caught in 1880s and samples caught in the 1970s. The authors concluded: “This result supports the idea that the relatively high concentrations of mercury found in marine fish that inhabit the surface and deep waters of the open ocean result from natural processes, not 20th century industrial pollution.” [Emphasis added]  A third study compared mercury levels in various samples of tuna, including data from canned tuna more recently compiled by FDA and EPA. Dated fish samples (fig. 15-B) from the Smithsonian museum (1878-1909) indicate a relatively elevated mercury level associated with past sources and exposure unrelated to modern power plant mercury emissions. These old samples contain a mean level of about 0.38 ppm, significantly exceeding the mean levels of 0.29 ppm and 0.15 ppm for the 1971-1972 samples and the large FDA (1993) 220-canned tuna samples, respectively. The authors of this important research concluded: “The data for both tuna and swordfish lend support to the contention that the mercury levels now being found in wide-ranging ocean fish are not primarily the consequence of man-made pollution but are of natural origin.” [Emphasis added] Finally, using human-remains mercury levels as proxy for past available fish levels definitively confirm persistent mercury presence in marine life as long ago as the fifth century A.D. (Fig. 15-C). This is not surprising since Hg and MeHg have been present in the marine environment since before fish evolved.  Conclusion
Thus, it seems reasonable to conclude that an appropriate science-based response to the Tribune’s mercury testing and reporting for small samplings of local fish is, “So what?” The above research findings clearly suggest that MeHg concentrations in ocean fish are not likely derived from human activities nor are they likely to be modified by small alterations from inorganic Hg sources (either anthropogenic or natural). This is why claims that EPA’s proposed Clean Air Mercury Rule (CAMR) – or even the stronger regulations being pressed by some states – will lead to a measurable reduction in MeHg accumulated in fish are factually misleading. |
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