Mercury in Fish Report

Origins of Mercury in Sea Food

Mercury (Hg) is an element naturally occurring in soil, rocks, and the ocean sediment. It could be released into the atmosphere and surface water bodies through various pathways. However, two-thirds of the mercury emitted originates from current and past human activities since industrialization. Mercury compounds are released in higher concentrations as a result of coal power plants operation, as well as from metal mining, chlorine production, and waste incineration (Corbitt et al., 2011; Davis et al. 2012; Fitzgerald and O’Connor, 2001).

Evidence exists that mercury concentrations in the surface ocean have increased fourfold over the past 500 years, with a twofold increase over the last century due to the increasing industrialization and energy production (Streets et al. 2011, Mason et al. 2012). Naturally present in coal, mercury goes into the air in the smoke when burned. Later it is deposited to the Earth’s surface via rain, snow, or gases. It can fall directly on the oceans, or be washed via runoff, and wastewater discharges to downstream coastal waters via river inputs. Atmospheric inputs contribute up to 91% of mercury entering the ocean (Chen et al., 2012).

Mercury in The Various Ocean Regions

Mercury compounds in consumed marine fish continue to exceed the acceptable for humans levels (Chen et al. 2012). More than 75% of the mercury exposure of the seafood eaten is caused by fish from the open oceans (Pirrone and Keating 2010). Even in coastal regions, the most popular seafood is often not local but store-bought shrimp, tuna, or salmon (Mahaffey et al., 2009). The Pacific and the Northeast Atlantic fisheries together supply 63% of the global marine catch (FAO 2011). Mercury pollution affects large areas of the world’s most productive fisheries that serve worldwide consumers (Balcom et al. 2004, 2008, 2010, Chen et al., 2012, Davis et al. 2012, Harris et al. 2012a, 2012b, Kirk et al. 2012, Mason et al. 1999, 2012, Pesch and Wells 2004, Thompson 2010, Sunderland et al. 2012). Mercury inputs to the Arctic Ocean are caused by direct atmospheric deposition (32%), via ocean currents (17%), and river inputs (40%) (Chen et al., 2012). Concentrations of mercury are increasing in the Pacific Ocean and decreasing in the Atlantic Ocean and Mediterranean Sea, reflecting regional shifts in total global mercury emissions (Mason et al. 2012).

Mercury in the Food Chain

The most common source of human exposure to mercury is the consumption of certain fish types in which it is concentrated (Eisler. 2004; Harada, 1995;  Hightower and Moore, 2003; Mahaffey et al. 2004, Stein et al. 2002; Yan et al., 2014). Mercury can build up, or bioaccumulate, in living organisms, causing increasing harm on higher order species. After mercury enters a waterway, the microorganisms transform it to its most toxic form, methyl mercury. This pollution concentrates and moves up the marine food chain. As methylmercury is absorbed by phytoplankton, the food chain continues its way by zooplankton, eaten by small fish, and onwards and upwards as the amount of the toxin increases in ever-accumulating quantities (Levinton and Pochron 2008, Goto and Wallace 2009). Methylmercury biomagnifies, increases its content, through the food chain as larger fish – predators eat other smaller organisms, including fish, each time absorbing and accumulating the methyl-mercury that the smaller food sources contained. The largest long-living predatory fish, like sharks and swordfish, can accumulate in their tissues mercury concentrations that are 100 million times higher than those of their surrounding habitat (Mason et al. 2012).

Health Threats and Symptoms of Mercury Poisoning

People are mainly exposed to mercury via consumption of fish and shellfish containing methylmercury, the most common form of mercury in fish (Carrington et al. 2004; Sunderland 2007). The low and high concentrations of the compound have the potential to impair seriously human health (Bose-O’Reilly et al., 2010; Counter et al., 2004; Crespo-Lópe et al., 2009;  Holmes et al., 2009;  Tchounwou et al., 2003; Zahir et al., 2005).

The risk is to the functions of the brain, kidneys, liver, heart and nervous system. Methylmercury is absorbed into the bloodstream and transmitted throughout the body to multiple locations and could be absorbed by the developing baby through the placenta in pregnant women. The dangerous chemical bio-accumulates in our fat tissue, brain and bones, contributing to the toxic load on our bodies causing loss of vitality and a diverse range of severe disorders. Methylmercury acts mainly as a neurotoxin, the symptoms depending on the magnitude of the dose and the duration of exposure. The developing fetus, babies and young children are the most vulnerable to acute and continuous methylmercury exposure and may be seriously affected at substantially lower doses than in adults. Elevated methylmercury exposure in a baby or young child can cause a decrease in I.Q., delays in walking and talking, lack of coordination, changes in nerve responses, blindness, and seizures. Toxicological impacts like impaired reproduction, fetal and infant growth, particularly – neuro-development in terms of memory, language, attention span and learning ability, in addition to behavioral changes and possible contributions to cardiovascular disease have been extensively documented. Numerous research results have shown that excessive methylmercury can lead to memory loss, impairment of speech, fatigue, personality changes, tremors, impairment in vision, deafness, cardiovascular disease loss of muscle coordination and sensation, intellectual impairment, hair loss, depression, headaches. Symptoms of methylmercury poisoning may also include neuromuscular changes –  weakness, muscle atrophy, twitching as well as emotional changes. In  extreme cases with higher exposures there may be kidney and/or respiratory failure and death (Counter et al., 2004; Crespo-Lópe et al., 2009; Guallar et al., 2002; Holmes et al., 2009;  Tchounwou et al., 2003; Zahir et al., 2005; Grandjean et al. 2005, Mergler et al. 2007, Karagas et al. 2012).

Guidelines for Eating Fish with Higher Mercury Levels


Mercury exposure can be minimized by making informed choices about fish types and consummation frequency. There are no recommended consumption limits for any other types of retail fish in besides fresh/frozen tuna, shark, swordfish, escolar, marlin, orange roughy, and canned albacore (white) tuna (Health Canada, 2008).

According to the Food and Drug Administration and the Environmental Protection Agency it is possible to eat up to 12 ounces a week of any fish and shellfish, except those that should be avoided: shark, swordfish, king mackerel, and tilefish. Wild caught salmon, sardines, cod, tilapia, sole, shrimp, and herring have lower levels of mercury. With the aim of determining the safe mercury content for women of childbearing age some of the most common fish species have been categorized in terms of mercury accumulation characteristics  (FDA, 2004). For example, the highest mercury levels have been found in Mackerel (King), Marlin, Orange Roughy, Shark, Swordfish, Tilefish, Tuna (Bigeye, Ahi), whereas the following species were among those with the least mercury – Anchovies, Butterfish, Catfish, Clam, Crawfish/Crayfish, Haddock (Atlantic), Hake, Herring, Mackerel (N. Atlantic, Chub),  Oyster, Perch (Ocean), Pollock, Salmon, Sardine, Shad (American), Shrimp, Sole (Pacific), Tilapia, Trout (Freshwater), Whitefish, etc.

Frequent consumption and high intake of swordfish, tuna and mackerel are major contributors of mercury exposure in many areas (Hightower and Moore, 2003; Yan et al., 2014). Excessive levels of mercury in albacore (Thunnus alalunga) and bluefin tuna (Thunnus thynnus) caught in the Mediterranean sea have been confirmed (Storelli, 2002), whereas the Pacific troll-caught albacore showed mercury content below the U.S. Food and Drug Administration level and Canadian standards and was positively correlated with length and weight of albacore (Morrissey, 2004).

Levels of Mercury Contamination in Fish

Mercury levels in commercial fish and shellfish
species Mean(ppm) Std dev(ppm) Median(ppm) Comment Trophiclevel Max age(years)
Tilefish (Gulf of Mexico) 1.450 n/a n/a Mid-Atlantic tilefish has lower mercury levels and is considered safe to eat in moderation. 3.6 35
Swordfish 0.995 0.539 0.870 4.5 15
Shark 0.979 0.626 0.811
Mackerel (king) 0.730 n/a n/a 4.5 14
Tuna (bigeye) 0.689 0.341 0.560 Fresh/frozen 4.5 11
Orange roughy 0.571 0.183 0.562 4.3 149
Marlin * 0.485 0.237 0.390 4.5
Mackerel (Spanish) 0.454 n/a n/a Gulf of Mexico 4.5 5
Grouper 0.448 0.278 0.399 All species 4.2
Tuna 0.391 0.266 0.340 All species, fresh/frozen
Bluefish 0.368 0.221 0.305 4.5 9
Sablefish 0.361 0.241 0.265 3.8 114
Tuna (albacore) 0.358 0.138 0.360 Fresh/frozen 4.3 9
Patagonian toothfish 0.354 0.299 0.303 4.0 50+
Tuna (yellowfin) 0.354 0.231 0.311 Fresh/frozen 4.3 9
Tuna (albacore) 0.350 0.128 0.338 Canned 4.3 9
Croaker white 0.287 0.069 0.280 Pacific 3.4
Halibut 0.241 0.225 0.188 4.3
Weakfish 0.235 0.216 0.157 Sea trout 3.8 17
Scorpionfish 0.233 0.139 0.181
Mackerel (Spanish) 0.182 n/a n/a South Atlantic 4.5
Monkfish 0.181 0.075 0.139 4.5 25
Snapper 0.166 0.244 0.113
Bass 0.152 0.201 0.084 Striped, black and sea bass 3.9
Perch 0.150 0.112 0.146 Freshwater 4.0
Tilefish (Atlantic) 0.144 0.122 0.099 3.6 35
Tuna (skipjack) 0.144 0.119 0.150 Fresh/frozen 3.8 12
Buffalofish 0.137 0.094 0.120
Skate 0.137 n/a n/a
Tuna 0.128 0.135 0.078 All species, canned, light
Perch (ocean) * 0.121 0.125 0.102
Cod 0.111 0.152 0.066 3.9 22
Carp 0.110 0.099 0.134
Lobster (American) 0.107 0.076 0.086
Sheephead (California) 0.093 0.059 0.088
Lobster (spiny) 0.093 0.097 0.062
Whitefish 0.089 0.084 0.067
Mackerel (chub) 0.088 n/a n/a Pacific 3.1
Herring 0.084 0.128 0.048 3.2 21
Jacksmelt 0.081 0.103 0.050 3.1
Hake 0.079 0.064 0.067 4.0
Trout 0.071 0.141 0.025 Freshwater
Crab 0.065 0.096 0.050 Blue, king and snow crab
Butterfish 0.058 n/a n/a 3.5
Flatfish * 0.056 0.045 0.050 Flounder, plaice and sole
Haddock 0.055 0.033 0.049 Atlantic
Whiting 0.051 0.030 0.052
Mackerel (Atlantic) 0.050 n/a n/a
Croaker (Atlantic) 0.065 0.050 0.061
Mullet 0.050 0.078 0.014
Shad (American) 0.039 0.045 0.045
Crawfish 0.035 0.033 0.012
Pollock 0.031 0.089 0.003
Catfish 0.025 0.057 0.005 3.9 24
Squid 0.023 0.022 0.016
Salmon * 0.022 0.034 0.015 Fresh/frozen
Anchovies 0.017 0.015 0.014 3.1
Sardine 0.013 0.015 0.010 2.7
Tilapia * 0.013 0.023 0.004
Oyster 0.012 0.035 n/d
Clam * 0.009 0.011 0.002
Salmon * 0.008 0.017 n/d Canned
Scallop 0.003 0.007 n/d
Shrimp * 0.001 0.013 0.009 6.5
* indicates only methylmercury was analyzed (all other results are for total mercury)n/a – data not availablen/d – below detection level (0.01ppm)Source

Mercury in Canned Fish Foods


One of the most popular species, tuna has attracted deep research interest. It is consumed in sandwiches, salads, and for casseroles, mainly in the form of canned “light” tuna and canned “white” tuna, but also as fresh and frozen (Sunderland 2007, Groth 2010). It is assumed that the fish which is canned is younger and smaller, and have less mercury than fresh or frozen tuna. However, Health Canada (2008) has issued a warning of potential mercury risk for children, some groups of women and people with a higher and frequent intake of canned albacore tuna. It is specified that the advice does not apply to canned light tuna, nor does it apply to Canadians outside of the groups in question. Canned albacore tuna is also called canned white tuna, but it is not the same as the canned light tuna. Several other species of tuna such as skipjack, yellowfin, and tongol, which are relatively low in mercury, are accepted to be collectively called “light tuna”. There is data, however, that canned tuna, is the most common source of mercury in Americans’ diet and white (albacore) tuna usually contains far more mercury than light tuna (FDA, 2004).

Store Bought Fish Consumption Advice for Women & Children

Measurements in fish and shellfish samples purchased from supermarket outlets and fish retailers in Canada showed that the predatory fish contained the highest mercury concentrations: swordfish, marlin, shark, and canned, fresh and frozen tuna. The levels of mercury in the fresh and frozen tuna were higher than in the canned tuna. In the canned tuna, mercury concentrations varied with subspecies, the highest average concentrations found in Albacore tuna (Dabeka et al., 2002; Forsyth et al., 2004).

Fresh/frozen tuna (not including canned tuna), shark, swordfish, escolar, marlin, and orange roughy are permitted to be sold as long as they contain less than 1.0 ppm total mercury. These fish are also subject to consumption advice. For all other retail fish (including all canned tuna) a standard of 0.5 ppm total mercury is established (Health Canada, 2008).

Mercury and Pregnancy; Breastfeeding

Canadians are advised to limit consumption of fresh/frozen tuna, shark, swordfish, escolar, marlin, and orange roughy. In general, you can eat up to 150 g per week of these fish species combined. However, women who are or may become pregnant and breastfeeding mothers can eat up to 150 g per month. Children between 5 and 11 years of age can eat up to 125 g per month, while the younger ones (1 – 4 years of age) – no more than 75 g / month. Tuna steak contains higher levels of mercury than canned tuna – the latter is usually younger and smaller and contain less mercury than the larger tuna fish sold as fresh and frozen products. The recommended by the Health Canada, 2011 serving limits of canned tuna per week are as follows:

  • Pregnant women shouldavoid it.
  • Women of childbearing age – 12.5 ounces of light tuna/4 ounces of white tuna.
  • Men and older women – 14.5 ounces of light tuna/5 ounces of white tuna.

Recommendations on canned albacore tuna – children (6 – 12 months)should not consume over40 grams/week, the limit for the aged 5 – 11 yearsis150 grams. Women of childbearing age, including pregnant and breastfeeding women should reduce their intake to 300 grams. No limit is set for men and women after childbearing years (Health Canada, 2011)

The risks from mercury in fish and shellfish depend on the amount of fish and shellfish eaten and the levels of mercury within. Entering a woman’s body, methylmercury is distributed from the maternal blood to the fetus through the placenta. This is the reason for the more restrictive consumption advice for mothers than that for the general population due to the increased susceptibility of the developing fetus and young children. Women who may become pregnant, pregnant women, nursing mothers, and young children are advised to avoid some types of fish and eat only those that are low in mercury.

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Benefits of Fish Consumption

The supporters of the opposite side claim that the risk of mercury accumulation is significantly exaggerated and the benefits of eating fish regularly far outweigh the potential risks, which are neglible. Their arguments are that fish provides lots of nutrients, such as protein, selenium, vitamin D, magnesium and iron. It is also with low content of saturated fat and provides healthy omega-3 fats, favorable for the functions of the brain and heart. They are particularly important for the development of brain gray matter and retina membranes in babies and children in the first years of infancy and have positive impact on atherosclerosis, coronary heart disease, inflammatory disease, and behavioral disorders (Chung et al., 2013;Connor, 2000; Haag, 2003).

Relevant amounts of selenium (Se) can prevent oxidative brain damage and other adverse effects associated with mercury toxicity selenium protects against mercury toxicity, and ocean fish is among the highest dietary sources of selenium. If the fish contains higher ratios of selenium to methylmercury (Se:Hg) it is safer for consumption. When the two elements are found together, they connect, and mercury cannot bind to anything else. It passes through the body leaving un-absorbed (Ralston et al., 2008).

Most fish contain some of the long chain omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), vitamin D, and nutrients such as selenium, iodine, magnesium, iron and copper, which could boost a child’s IQ (EERC, 2009). There are positive associations between maternal DHA levels or fish intake during pregnancy and improved behavioral attention scores, visual recognition and acuity, memory, and language comprehension in infancy, as well as cardiovascular health (Mozaffarian and Rimm 2006; Uauy et al, 2003). The intake of some fatty acids is found to play an important role in reducing autoimmune and inflammatory disorders – atherosclerosis, rheumatoid arthritis, psoriasis, asthma and type 1 diabetes (Kromann, 1980; Weber  and Leaf, 1991).

fish facts

Conclusion

Based on these findings, there are claims that policies advising against eating fish during pregnancy may need to be revised and while those conclusions have received a good dose of media attention, the bulk of researchers continue to urge caution. Addressing the complicated transboundary nature of mercury pollution the United Nations Environment Programme agreed to negotiate a legally binding mercury treaty among 140 nations with the goal to “protect human health and the global environment from the release of mercury and its compounds by minimizing and, where feasible, ultimately eliminating global, anthropogenic mercury releases to air, water, and land”. The “Minamata Convention on Mercury” is named after a city in Japan where severe health damage occurred as a result of mercury pollution. It provides controls and reductions across a range of products, processes and industries where mercury is used, released or emitted (Chen et al., 2012; Lambert et al. 2012, UNEP, 2009).

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