Tag Archives: fish

Where in the World is Mercury? Part 2: Ocean and Fish

By: Noelle Selin

Our previous posts have addressed mercury in the atmosphere, global reservoirs such as oceans and soils, fish and human hair. Since oceans and fish are so important to global mercury exposure, I thought it would be useful to highlight sources of more information about mercury concentrations there. Two recent major studies have been released looking at the mercury problem in aquatic systems. Both of these are being presented at INC-5.

The Biodiversity Research Institute and IPEN, a non-governmental organization involved in the negotiations, have collected worldwide data on so-called “hotspots” of mercury concentrations in fish and human hair samples. The report, available here, found that mercury contamination is ubiquitous in marine and freshwater systems along the world. The report compares fish mercury concentrations from around the world to U.S. EPA human health advisory guidelines. Depending on the country, between 43 to 100% of fish sampled exceeded guidelines; in Japan and Uruguay, concentrations were so high that no consumption was recommended. These guidelines are for one fish mean per month.

From BRI-IPEN report: % of fish samples above health thresholds

Look for Alice Alpert’s interview with Biodiversity Research Institute’s David Evers, who’s here in Geneva, to be posted soon on our blog.

Another key report came out of the Coastal and Marine Mercury Ecosystem Research Collaborative (C-MERC), brought together by the Toxic Metals Superfund Research Program at Dartmouth College. The report analyzed and synthesized the current science on mercury sources  in seafood, and explored ecosystem responses to potential emissions controls.

The report found that mercury pollution is on the rise. In response to emissions controls, methylmercury in open ocean fish would only begin to decrease within several years to decades, while fish in coastal systems could respond over many decades to centuries. In other words, these effects are very long lasting. An interview with Celia Chen, who co-authored the report, was conducted here at INC-5 by Amanda Giang and is posted below.

Bioamplification, Bioaccumulation and Bioconcentration

By: Julie van der Hoop

The confusion between bioamplification, bioaccumulation and bioconcentration is understandable. Yesterday, delegates asked for a clarification and explanation as to how this happens. These terms are not interchangeable, though they are often used as if they were. This post should clarify the situation.


Bioamplification (or biomagnification, as the picture shows) refers to an increase in the concentration of a substance as you move up the food chain. This often occurs because the pollutant is persistent, meaning that it cannot be, or is very slowly, broken down by natural processes. These persistent pollutants are transferred up the food chain faster than they are broken down or excreted.

In contrast, bioaccumulation occurs within an organism, where a concentration of a substance builds up in the tissues and is absorbed faster than it is removed. Bioaccumulation often occurs in two ways, simultaneously: by eating contaminated food, and by absorption directly from water. This second case is specifically referred to as bioconcentration.

So, what have we learned? Bioconcentration and bioaccumulation happen within an organism, but biomagnification occurs across levels of the food chain. An example: phytoplankton and other microscopic organisms take up methylmercury and then retain it in their tissues. Here, mercury bioaccumulation is occurring: mercury concentrations are higher in the organisms than it is in the surrounding environment. As animals eat these smaller organisms, they receive their prey’s mercury burden. Because of this, animals that are higher in the food chain have higher levels of mercury than they would have due to regular exposure. With increasing trophic level, mercury levels are amplified.

Mercury’s Health Effects

by Alice Alpert, Ellen Czaika, and Amanda Giang

Pathways to exposure

Although these negotiations are explicitly focused on creating an environmental treaty, mercury’s major significance is its toxicity to humans. When you think about mercury, you probably picture a mercury thermometer. In a thermometer, you can literally see the silvery mercury in its bulb – this is liquid, elemental mercury. If you are absent minded and accidentally drop that mercury thermometer on the bathroom floor, the mercury will spills and form into beads. Although it’s not a good idea to touch this mercury, it is also not easily absorbed by the digestive system in this form.

The more pernicious way for this mercury to enter your body is if it vaporizes, which happens to a small amount of the liquid mercury at room temperature. If you inhale the vapor it can easily pass from your lungs into your blood stream and damage tissues. In fact, vacuuming up the spilled mercury can increase its vaporization and therefore the danger.

In truth, most people will not be exposed to mercury in this form. Instead, people working in chlor-alkali production, mercury mining and refining, thermometer production, dentistry, and in the production of mercury-based chemicals are at increased risk. Although measures have been taken to limit occupational exposure to mercury, many workers may continue to be at risk. Similarly, artisanal or small-scale gold miners are routinely exposed to mercury vapor at very high levels, in the process of burning the mercury-gold amalgam used to extract gold from ore. Indeed, miners and their communities often exhibit clear signs of mercury poisoning.

Another important pathway for mercury exposure is through eating seafood. In fact, according to the World Health Organization (WHO) (Section 2.4, paragraph 128), for many people this is the main pathway for human exposure to methylmercury. Exposure happens through the process of bioaccumulation and biomagnification. In brief, mercury is methylated to methylmercury (CH3HgX) by bacteria in the ocean and then accumulates in fish and marine mammals. Long-lived predatory fish at the top of the food-chain, such as swordfish, tilefish, shark, and tuna, can accumulate dangerously high concentrations of mercury. The US EPA lists guidelines for safe consumption of fish. Women who are pregnant or who could become pregnant should be especially careful about eating mercury contaminated fish because the mercury can be harmful to the developing fetus.

In addition, exposure could happen through dental amalgams. Elemental mercury is used in dental amalgam, and it can be ingested or its vapors can be inhaled. This is a contentious issue in the negotiations. The American Dental Association and the US Environmental Protection Agency state that mercury in dental amalgam is safe, while a report by the WHO (p.11) states that dental amalgam is a significant source of mercury exposure in those who have mercury fillings. We encourage you to look into the reports if you are concerned about this issue. For a solid overview of all pathways, see the WHO report on mercury exposure.

How and why does mercury make us so sick?

The most serious effects of elemental mercury vapor concern the nervous system, including tremors, erethism (a neurological disorder characterized by irritability and shyness), insomnia, muscle weakness, and memory loss. At especially high concentrations, the kidneys, thyroid, and pulmonary system can be affected. Similarly to elemental mercury, mercury in its organic form, methylmercury, has serious neurological effects, including neurobehavioral deficits, neuronal loss, loss of muscle movement, hearing loss, paralysis, and death.

Why is mercury so toxic for the nervous system? There are two specific processes: first, elemental mercury and methylmercury can easily cross the blood-brain barrier and once in the brain, can be oxidized to the mercuric ion (Hg2+), which cannot cross back across the barrier. Instead, mercury is trapped in the brain, where the second process begins, neurotoxin by excitotoxicity. What? Okay, we’ll slow down and explain these multi-syllabic words: in studies of rats, Hg2+ inhibits glutamine and glutamate transport, causing receptors for these molecules to become overexcited. This causes a large influx of the calcium ion into the cell, which activates enzymes that can lead to the neuron’s death, and thus the serious neurological effects. This second process is the reason why mercury is so toxic.

Mercury crucially effects developing fetuses. In the same way that methylmercury can cross the blood-brain barrier, it can also pass through the placenta from a mother to her fetus and then to the developing fetus’ brain. As a neurotoxin, methylmercury can also damage its nervous system, and in fact mercury has lasting negative effects when fetuses are exposed to concentrations at levels that are only 10%-20% of toxic levels for adults.

Babies born to women who consumed significant amounts of methylmercury while pregnant display symptoms similar to cerebral palsy, including delayed walking and talking, altered muscle tone and reflexes. Tragically, these impairments are permanent and affected will suffer from these impairments for their entire life. In fact, recently published research estimates that IQ reductions due to chronic, low-level fetal mercury neurotoxicity costs the European Union alone € 8-9 billion euros per year. Clearly, there are significant social and economic impacts from mercury exposure, particularly for the young.

Dinner…with a side of mercury?

by Julie van der Hoop

Worldwide, fish consumption is the main source of human exposure to methylmercury, a highly toxic form of mercury that biomagnifies up the food chain [1]. Accordingly, a fair amount of media attention has focused on mercury content in fish as a potential public health threat [2]. In an effort to protect consumers, US state and federal agencies have set guidelines for limiting consumption of fish that tend to be high in levels of methymercury (these guidelines are particularly important for pregnant women and women of childbearing age, as fetal exposure to methylmercury can create long-term developmental impacts). Additionally, health advisories are available online to help consumers avoid consuming types of fish that are particularly high in mercury and other toxins.

In light of these health concerns, it is no surprise that one of the aims of a global mercury treaty is to, through reducing mercury emissions, ultimately reduce the levels of toxic methylmercury in fish and shellfish.

Should we eat fish?

Fish are often touted for their great nutritional profile: they’re low in fat, with high-quality protein and an added dose of vitamins. The American Heart Association recommends at least two 3-ounce servings of fish per week to obtain beneficial omega-3 fatty acids. However, balancing the risks of mercury consumption and the benefits of fatty acids can be difficult for both consumers and those faced with the task of setting advisories.

Some studies are now urging consumers to weigh the risks and benefits of eating certain types of fish. In terms of what to avoid, shark and swordfish shouldn’t make it to your plate, not even for a single meal per month. The health benefits of fishes such as tilapia, pollack, flounder, shrimp, trout, herring, salmon, canned light tuna, and cod generally outweigh their potential mercury concentration cons [3]. That being said, other contaminants (e.g., persistent organochlorines in farmed salmon) and fishery sustainability (e.g., depletion of many cod stocks) are additional concerns worth considering when planning your meals.

Will fish mercury levels ever decrease?

It’s uncertain when—and whether—you’ll be able to enjoy all-you-can-eat sushi without thinking about how much mercury you’re consuming. However, recent research is helping us better understand how changes in mercury emissions could affect the future methylmercury content in fish.

In a new study, researchers added mercury to an experimental lake to learn how quickly it was incorporated into fish and to track what happened when mercury-addition stopped. Good news: concentrations of methylmercury in lake fish decreased much faster than previously thought. As reported by Susan Bence this week, researchers working on the project believe that reductions in mercury emissions could lead to a fairly rapid decrease in fish mercury levels in certain ecosystems. How fast these levels will decrease, however, will vary depending on where the fish are caught.

Still, mercury cycles through the environment very slowly, and even if we can restrict future emissions to current level, this will not reduce the global mercury burden. A major cut in mercury emissions is needed to reduce methylmercury concentrations in marine fish: as a recent report by Chen et al. shows, cutting mercury emissions by about 40% could lead to a 16% decline in mercury concentrations in fish in the North Atlantic Ocean.

What should you do in the meantime?

To help guide you in your fish dining decisions, check out the EPA’s online guidelines for eating and selecting fish and shellfish, as well as the consumer guides and apps available from groups such as NRDC and the Sierra Club. Still, what might matter most in is not the species of fish, but where it was caught [4]. Informing ourselves as consumers is increasingly important – whether it be for concerns over sustainability or mercury consumption.