Mercury Contamination in Maine Fishes

Reprinted from Maine Sportsman

Mercury in Maine LakesAllagash Lake is one of my favorite places on earth, and each spring I try to get up there for a few days of fishing just after the ice goes out.  Last year, I was lucky, and had a ball catching brook trout and togue with my son Ty, and a group of friends from work.

Imagine my dismay when, upon returning home, I was greeted by a headline in the Bangor Daily News that read, “Mercury Levels in Maine Fish Bring Warning to Limit Consumption”.  The funny thing was, one of the people on that trip was a graduate student from the University of Maine who was responsible for much of the work cited in the article.  He never said much about mercury as we sat around the campfire dining on grilled togue.

Initially, I wasn’t too worried.  But when one of my other kids came home from school a few days later and said the teacher warned the class not to eat fish because they weren’t safe, I decided it was about time I looked into the issue on my own.

I compiled a list of questions related to mercury in Maine fish, and hunted down answers from whatever sources available.  My reason for sharing this information is to help anglers to make better informed decisions about eating fish from Maine waters.

What is mercury?

Mercury is a “heavy metal” that occurs naturally in the environment.  It can exist in several forms, such as elemental mercury (the stuff in thermometers), inorganic (the material whose vapors made felt hat makers in the 19th century “mad”), and organic mercury.  These can change back and forth in the environment, and in high enough doses, they are all toxic.  However, organic mercury (especially methyl mercury) is by far the most dangerous.

How does mercury get into the environment?

Food and Drug Administration toxicologist Mike Bolger estimates that as a result of natural degassing from the earth’s crust and oceans, between 3,000 and 6,000 tons of mercury have been released into the atmosphere each year for centuries.  Recently, however, an additional 2,000 – 3,000 tons have been added as a result of human activities, such as the burning of fossil fuel, and incineration of household and industrial waste.  This increases the total mercury burden on the atmosphere by 30-40 percent; and this is most likely what is responsible for the toxic effects that have been observed.

Where does Maine’s mercury problem originate?

Most of the mercury that pollutes Maine waters arrives via westerly winds from other states.  Maine Department of Environmental Protection biologist Barry Mower says, “Just like with acid rain, most of the hotspots for mercury emissions are along the Eastern seaboard from Washington DC to Boston, and from the Great Lakes through the upper Ohio River Basin.”

Unfortunately, we have no clear idea of exactly how much mercury is actually falling-out over Maine.  The widespread distribution of the problem, however, points directly to these far-away, airborne sources as the principle cause.  In addition, before enactment of the Clean Water Act in 1972, the chemical and paper-making industry here in Maine discharged mercury-contaminated waste directly into our rivers, creating local hotspots of mercury, especially along the lower Penobscot River. That discharged mercury remains in the sediment, where it can be methylated by bacteria, released into the water, enter the food web, and accumulate in the muscle of fish.

Is this a recent problem, or have Maine waters been contaminated with mercury for years?

There isn’t enough data available to answer this question definitively, but based on studies done on the Great Lakes and in Scandinavia, it’s likely that this problem has been around at least since the end of World War II.  The earliest study done on Maine lakes was published by Normandeau Associates in 1978.  It indicated elevated concentrations of mercury in fish taken from a number of Allagash-area lakes.

How does mercury get into the fish?

The majority of mercury found in the environment is in the elemental or inorganic form.  When this material gets into lakes, it can be transformed into the more toxic form (methyl mercury) by bacteria living in bottom sediments.  Almost all of the mercury found in fish tissue is in this highly toxic form and is concentrated in the part of the fish that humans eat.

Fish can absorb methyl mercury directly from water as it passes over their gills, but they receive their highest doses is by eating other organisms that have already been contaminated.  Just like DDT, and a number of other environmental pollutants, mercury tends to get concentrated (bioaccumulated) at the top levels of the food chain.  Thus, large carnivorous species tend to exhibit a greater problem than their smaller, more omnivorous relatives. And, since fish are slow to remove mercury from their muscle, it accumulates over their lifetime, so older fish have much greater mercury levels in their muscle than younger fish.

What are the health effects of mercury on humans?

Methyl mercury is a neurotoxin that has serious negative effects on the development of cells in the central nervous system.  Early signs of methyl mercury poisoning in adults include tingling in the fingers and hands, and blurred vision.  Long term exposure can result in lowered intelligence, impaired hearing and speech, decreased coordination, and kidney damage.

Fetuses and very young children are much more vulnerable because mercury is believed to interfere with enzymes needed to produce nerve cells in their brains.  Mercury is concentrated in fetal blood, so a growing fetus is exposed to greater concentrations of mercury than found in the mother’s blood. As a result, fetuses can develop cerebral palsy, physical deformity and mental retardation from exposure levels that produce no outward symptoms in the mother.  Nursing babies are also at risk since methyl mercury can be transferred via the mother’s milk.

A decrease in IQ is a more subtle effect exhibited in children exposed to high levels of mercury in the womb.  One study of such children in New Zealand showed that they scored consistently lower on standardized tests for measuring intelligence.

Does mercury have any negative effects on the fish themselves?

Laboratory studies have found changes in fish behavior at mercury concentrations as low as 0.5 parts per million (ppm).  But there is no evidence to indicate that current levels of mercury contamination in Maine are affecting the behavior or survival of the fish themselves.  It would be presumptive to link any perceived decline in the quality of Maine’s sport fishery with mercury pollution.

Which species of fish are most contaminated?

Mercury in Maine FishA study by the University of Maine indicated that the highest levels of mercury were found in predaceous warm-water species.  Pickerel, white perch, smallmouth bass and largemouth bass head the list, with levels of mercury that averaged between 0.58 – 0.90 parts per million (ppm).  Mercury levels in lake trout, landlocked salmon and cusk were somewhat lower and ranged between 0.36 – 0.43ppm.   Brook trout, brown trout, yellow perch and black crappie registered the lowest levels of mercury contamination, between 0.18 – 0.29ppm.

How much mercury do fish in Maine contain?

The survey found fish whose mercury levels ranged from less than 0.1 ppm to over 3.4 ppm.  Nearly half the fish tested had mercury levels above 0.4 ppm and 20 of the 117 lakes that were sampled produced fish with mercury levels over the Food and Drug Administration’s safety limit of 1.0 ppm.

The Federal EPA action limit on mercury is 0.3ppm.  And here in Maine, the action level for methyl mercury in fish muscle is 0.2 ppm.  Above this concentration, the most sensitive population – children and women of child-bearing age (who might get pregnant and so expose their fetus to mercury), are told not to eat more than one fish meal per week. It makes sense that if the mercury level in fish is substantially greater than 0.2 ppm, folks in the sensitive population should eat fish less often than once a week.

How does this compare with other areas of the country?

Nearly half of all the health advisories issued in the United States in the past 5 years have involved mercury.  Presently, 30 states (including all those in New England except Rhode Island) have posted fish consumption warnings associated with mercury contamination.  The most acute problems have been identified in Florida, the Great Lakes states and parts of Ontario, Canada.

Officials from different states don’t always agree on what advice should be provided.  This is because states often use different sampling techniques and criteria for evaluating their mercury problem.  This inconsistency sometimes causes confusion for the angling public.  Efforts are being made on the federal level to correct this situation.

Which Maine lakes are most contaminated?

117 lakes scattered throughout the state have been studied.  The amount of mercury each contained was assessed indirectly via the level of mercury contamination in their fish populations.  Generally, lakes in southern and coastal regions produced fish with the highest concentrations of mercury.  But these were also the areas that contained the best habitat for the most heavily contaminated warm-water species such as pickerel and bass.

More study is needed to determine which (if any) Maine lakes have an acute mercury problem.  Studies done in other states, however, indicate that reservoirs that have been fairly recently flooded, and lakes with large watersheds and numerous in-flowing streams, are at greatest risk.

Does mercury contaminate fish in Maine rivers too?

Generally, mercury levels are lower in river-dwelling fish than in lake fish, but there are certain rivers, like the lower Penobscot, that have elevated mercury levels in their fish relative to other rivers. To date, only a limited number of river fish have been tested.

Can you tell if a fish that you catch is contaminated?

You can’t tell the level of mercury contamination by a fish’s outward appearance or behavior.  It must be tested in a laboratory.  Currently there are no state resources available to pay for testing of fish.  However, individuals or groups can arrange for testing of fish privately.

What factors influence the amount of mercury found in fish?

Age and size are the two most important factors in determining the amount of mercury a given species of fish is likely to contain.  That’s because methyl mercury is a relatively long-lived compound that accumulates to its highest levels in the oldest and largest fish.

This point is clearly borne out by several fairly large, very old smallmouth bass taken from Hodgdon Pond in Acadia National Park. One 12-year-old fish which measured 22″ and weighed just under 3 1/4 pounds had a mercury level of 3.4 ppm (nearly 5 times above the state average).  Another 3 3/4 pound, 11-year-old bass taken from the same pond had a mercury concentration of 2.7 ppm.  The longevity of white perch, which turned out to be the oldest fish in study (average age = 8 years old), also probably accounts for the high levels of mercury recorded in this relatively small-bodied species.

Angler preferences also probably have an important indirect influence on fish mercury levels, especially for prized cold-water species such as lake trout and landlocked salmon, which are often removed from the population at a much earlier age than some of the less desirable species.

Generally, the two most important physical parameters associated with the mercury content in fish are:

1.   Water Coloration — Fish living in dark-colored lakes tend to have higher concentrations of mercury than fish living in clearer waters.

2.   Acidity — Fish inhabiting acidic waters tend to have higher levels of mercury than those living at a more neutral pH.

Both of these factors are thought to be related to an increased rate of bacterial methylation of mercury.

Is there any way to reduce the amount of mercury in fish through special cleaning or cooking procedures?

No special cleaning or cooking methods can be used to reduce the mercury level in fish.  That’s because methyl mercury binds tightly to the proteins in fish tissue and accumulates primarily in the muscle (meat), not the fat.  Although the exposure to some contaminants can be limited by removing the fat or skin before cooking, mercury is not one of them.

Has the Federal government set any safety limits regarding human consumption of mercury-tainted fish?

Keep in mind that in our industrialized world, we are exposed to many different toxic substances every day.  Mercury, asbestos, lead, dioxin, PCB’s, DDT, cigarette smoke….. the list goes on and on.  Basically, if you eat food, drink water, or breathe air, it’s likely you take in one harmful substance or another.  You simply can’t avoid them.

Health risks vary greatly from one item to another.  Although cigarettes and nuclear radiation are both “bad” for you, it’s clearly a lot more dangerous to inhale a lung full of radioactive waste than to smoke one cigarette.  The key to leading a healthy life, is to know the level that a given substance causes harmful effects, and doing everything in your power to keep your exposure well below it.

Fortunately, in the United States, there are a number of government agencies (Environmental Protection Agency, Food and Drug Administration, Department of Environmental Protection) that develop safety guidelines for all these items.  The FDA has set the human consumption limit for methyl mercury at 1 ppm.

What does the FDA’s 1 ppm human consumption safety limit mean to the average Maine citizen?

It’s clear that the amount of mercury you ingest is not only related to the concentration of mercury in the fish, but also to the amount of fish you eat.  The 1 ppm FDA guideline adopted by the state of Maine is formulated with the assumption that the typical adult male weighs 170 pounds and eats about 1/2 pound of fish every week.  Many people interpret this to mean that if you consume more than 1/2 pound of fish each week, or eat fish with mercury levels above 1 ppm, you will develop symptoms of mercury poisoning.

Of course, you can’t accurately draw this conclusion because all people are not adult males who weigh 170 pounds and eat precisely 8 ounces of fish once each week.  The key point is that there is tremendous amount of variation, both in the human fish-eating population, and the amount of mercury in each fish that’s eaten.  In order to protect consumers, FDA toxicologists have intentionally built a safety factor into their guidelines.  This is done to protect people from any possible chance of mercury contamination; because once damage is done, it’s essentially irreversible.

What about pregnant women and nursing mothers?

The developing nervous system of a fetus is much more susceptible to the adverse effects of mercury than adults.  Accordingly, the FDA advises children, pregnant women, women who might become pregnant, and nursing mothers, to restrict their consumption of mercury contaminated fish to 1 1/2 ounces per week (or about one meal per month).  There isn’t a tremendous amount of reliable information available on this subject because data are often corrupted by cigarette smoking, alcohol, and drug use.

Considering the grave neurological damage that mercury can do to a fetus, it seems prudent for children and women in at-risk categories to greatly reduce (or eliminate) the consumption of contaminated fish, at least until more scientific information becomes available.

Can people be tested to see if they have a high mercury level?

Testing of hair and blood provide the two best indexes for methyl mercury contamination in adults.  The average concentrations in non-exposed people are 2 parts per billion (ppb) in hair and 8 ppb in blood.  The lowest mercury levels associated with toxic effects are 50 ppb in hair and 200 ppb in blood.  In infants, adverse neurodevelopment effects are possible at much lower levels of mercury.

Hair and blood are excellent tissues to test mercury concentrations in humans. Blood gives the best indication of current exposure, and hair can be cut into sections and analyzed in small segments, and so give the history of mercury exposure over the time it took the hair to grow (hair grows about 1 cm per month).

Are the fish sold in supermarkets completely safe to eat?

With all the controversy swirling around mercury contamination, it might seem strange that your local supermarket continues to sell large quantities of fish without saying a word about it. That’s because most fish sold commercially are saltwater species that are much less susceptible to mercury contamination.

Fish sold commercially are continually monitored by FDA inspectors.  And over the years, there have been a few instances when certain long-lived, highly predaceous species like sharks, swordfish, and very large tuna have exceeded the 1 ppm safety limit.  When this has occurred, warnings were issued and the product was removed from the market.

What can be done to help reduce the mercury problem?

Mercury is contained in a variety of products from batteries and electrical switches, to a popular brand of sneakers that flash each time your foot hits the ground.  It is also used in the production of chlorine and several other industrial chemicals.  Mercury is also a contaminant in the coal burned in Midwest power plants to produce electricity. When the coal is burned, the released mercury goes up the smokestacks, and is carried on the air currents to New England, and beyond.

Mercury gets into the environment during both the manufacturing and disposal of many common household products.  And, there are really only two practical ways to prevent this from happening.

(1)  Reduce the amount of mercury that goes into these products.  This can best be done by forcing the chlorine and caustic soda industries into developing non-mercury alternatives for making their products, and by mandating a conversion to rechargeable and/or recyclable nickel-cadmium batteries.

(2)  Limit the amount of mercury emitted from smokestacks.

Clearly, both these things can be done.  But to date, not much action has been taken.  In fact, the EPA hasn’t established any official guidelines on what it considers acceptable upper limits on how much mercury can be released from smokestacks.

Why doesn’t this problem get fixed?

Primarily because people haven’t spoken out about mercury like they did regarding lead, dioxin and a number of other environmental problems which are now being adequately addressed.  Like it or not, mercury contamination in Maine fish is a political issue that can only be solved at the national level.  Unless Maine people let their voices be heard, it’s unlikely that industries in other parts of the country will voluntarily cost themselves money to clean it up.


During the last gubernatorial campaign, candidates debated for hours about who could do the most to attract business to Maine.  But little time was spent discussing the environment.  And sadly, few people even bothered to ask the candidates environmental questions.

As I watched and listened, I couldn’t help but wonder how Maine will be able to sustain tourism, one of the few thriving industries we have left, when the DEP is forced to issue health advisories on fish during the peak of the spring angling season.

Hopefully, if enough Maine sportsmen get involved, we can begin to get this mercury problem turned around.