Mercury levels in Pacific yellowfin tuna increasing

Background:
Mercury is a naturally occurring toxic metal found throughout the environment. Mercury enters the environment as the result of the normal breakdown of minerals in rocks and soil from exposure to wind and water, and from volcanic activity. Mercury releases from natural sources have remained relatively constant in recent history. Mercury is also reaching the marine environment where it can be transformed by bacteria to methylmercury which can be bioaccumulated in organisms and biomagnified across the marine food-web. Methylmercury is a potent neurotoxin that once accumulated to high concentration in fish is posing a health risk to all fish eaters.

Human activities since the start of the industrial age (e.g., mining, burning of fossil fuels, waste incineration and cement production) have resulted in additional release of mercury to the environment. Estimates of the total annual mercury releases that result from human activities range from one-third to two-thirds of the total mercury releases. Thus, one might predict that the concentration of mercury in fish should have increased dramatically since the Industrial Revolution. Evidence in support of this hypothesis has been hard to find, however, and some studies have suggested that analyses of fish show no change in mercury concentration over time.

By compiling and re-analyzing three previously published reports on yellowfin tuna caught near Hawaii in 1971, 1998 and 2008, Paul Drevnick from the University of Michigan and two colleagues found that the concentration of mercury in that species increased at least 3.8 percent per year from 1998 to 2008.

In their re-analysis, the researchers included yellowfins between 48 and 167 pounds and used a computer model for the effect of increasing mercury concentration with rising body size (age) of the fish. Total mercury concentration in muscle tissue from 229 fish were analyzed: 111 from 1971, 104 from 1998 and 14 from 2008.

The results indicated constant mercury levels in the yellowfins for the years between the 1971 and 1998 datasets. However, concentrations were higher in 2008 than in either 1971 or 1998. Between 1998 and 2008, the mercury concentration in yellowfins increased at a rate greater than or equal to 3.8 percent a year, according to the new study.

The findings, published online Monday in the journal Environmental Toxicology and Chemistry, add to evidence that anthropogenic mercury emission, particularly from burning coal, is pumping mercury into the ocean food chain, potentially posing a hazard to human health.

The researchers conclude their study with an outlook and call for action: "if current deposition rates are maintained, however, North Pacific Ocean intermediate waters are expected to double in mercury concentration by 2050. Thus, more stringent reductions in emissions are necessary".