Paper: Cheng-Shiuan Lee et al. 2016. Declining Mercury Concentrations in Bluefin Tuna Reflect Reduced Emissions to the North Atlantic Ocean. Environmental Science and Technology. DOI: 10.1021/acs.est.6b04328
You are what you eat
We all know that certain foods, such as french fries, are not as healthy as salads. Some foods, like tuna, are not so straightforward. Tuna has health benefits including omega-3 fatty acids and other nutrients. Unfortunately, it is also high in mercury (chemical symbol Hg) which has serious health consequences in high doses. Young children and women of childbearing age are advised to limit consumption of tuna. There are certainly other species of fish with lower mercury but most are not as easily accessible across America as tuna.
This doesn’t mean you have to stop eating tuna! It is advised that eating tuna in moderation is fine but in the future we may be able to safely eat more. Researchers have found a link between North American reductions in mercury emissions and mercury levels in tuna. This link was found by utilizing long term data. Long term data sets are a useful tool for scientists to study trends over longer periods of time where relationships can be more accurately understood. There is no guarantee of how things will be in the future but long term data sets give scientists more confidence in predicting outcomes.
How did that mercury end up in your tuna in the first place?
Two major mercury sources include coal power plants and mining operations. Mercury is transported long distances in both water and air (Figure 2). Mercury is a versatile chemical because it comes in many forms. The three main forms are Hg(II), Hg(0), and methylmercury (MeHg). Hg(II) is one of the most common forms of mercury found naturally. Hg(0) is elemental mercury and is the dominant form of mercury found in the atmosphere. MeHg is the toxic form that can accumulate in living things (bioaccumulate). Bioaccumulation means that an organism absorbs substances into its body at a faster rate than it can be released. As seen in Figure 2, mercury moves up through the food chain and increases with each level (biomagnification). This is why mercury concentrations are so high in predators, such as tuna. This mercury is consumed by us in our tuna fish sandwiches.
Researchers collected Atlantic bluefin tuna samples (1292 samples) from 2004-2012 through commercial fishing operations. Muscle tissue was sampled and frozen until analysis. Length, age, weight, gender, and year and location of capture were recorded. Age of tuna is estimated from the length using a growth curve which used age-length observations from otoliths (a piece of a fish’s inner ear) and model results. The length-age model works very well but there is a greater uncertainty in knowing the exact age of older fish especially if they are more than ten years old.
Total mercury was measured from the sampled muscle tissue. Since methylmercury is the most common form of mercury found in tuna (>90%), the scientists assumed total mercury equaled methylmercury. Fish of the same age captured in different years from 2004-2012 were compared to study the change in mercury in tuna over time. For example, mercury measured in two-year-old fish collected in 2005 and 2009 would be compared to see how, if at all, mercury in the fish varied with time. This prevents comparing much older fish which would have consumed more methylmercury over time relative to a younger fish. The goal was to detect mercury changes on a yearly timescale.
Older Fish Contain More Mercury
As expected, mercury concentration in tuna increases with age. Larger (and therefore older) fish also had a greater variability in mercury concentration. Older tuna feed on a greater size range and variety of prey over the course of their travels through different ocean regions.
Decreasing Emissions Have a Positive Impact!
Researchers found from 2004 to 2012, mercury declined in each group analyzed. Mercury levels in tuna, regardless of age, decreased by 19%! This means that North America’s efforts to decrease mercury emissions have already positively impacted the environment. Mercury emissions in North America declined by 2.8% per year from 1990-2007. This led to a decrease in wet deposition of mercury (in the form of Hg(II)) and atmospheric and oceanic mercury (in the form of Hg(0)). Oceanic Hg(0) measured from 2001-2009 decreased by 20%. It is important to note that these forms of mercury cannot be taken up into the food web. Unfortunately, there are no long term methylmercury measurements but Hg(0) serves as a good proxy (representative) of mercury in the water column.
Researchers must also separate tuna migratory changes from emission changes. Tuna have a varied diet but researchers found the same dominant prey (herring) over time. This is a good indicator that decreasing mercury in tuna is a result of declining bioavailable mercury and not a change in location and diet.
While regional mercury emissions have decreased, the global oceanic burden of mercury continues to increase. Scientists’ next step is to do the same time series evaluation for other regions (i.e. the Pacific Ocean). It is likely that mercury levels in tuna near increasing emission sources will rise. Hopefully, this study results in further investigation into emission reductions around the world. Tuna certainly are what they eat and if we continue to decrease our mercury emissions it will have a positive impact all the way up the food chain. If you want to learn about safe sustainable seafood options in your area, check out the Seafood Watch guide.