Mercury is a naturally occurring metal within the environment. Most of it has been expelled by volcanoes during almost 4 billion years of seismic activity and some even arrived on Earth as Metacinnabar (HgS,) in the chondrites of meteorites and since Cinnabar first appeared some 3 billion years ago, mercury has been present in mineral form in the surface layers of the Earth’s crust [1]. Recently, mercury has more and more man-made sources such as the burning of fossil fuels, mining, the extensive use of chlorine based chemicals and other industrial activities and these have increased the quantity of mercury released into the atmosphere. Once it settles on land, it is then transported by rainwater into lakes and waterways and ultimately into the oceans.
Only since the 1950s has mercury been recognised as being toxic for humans. A violent neurotoxin, it predominantly effects the Central Nervous System causing sensory dysfunction in the form of sight and/or hearing impairment and paresthesia, memory loss and a reduced attention span, and ataxia, in which motor co-ordination is affected. Since it is known to cross the blood/placental barrier, affected children can display learning disorders and, as was found during a study of children in the Faroes Islands where the majority of protein consumed is of marine origin, in very high doses it causes symptoms associated with Parkinson’s Disease. Physically, it is responsible for kidney damage, cardio-vascular diseases and it can cause foetal deformities. Recent studies have also shown that a high level of blood mercury is known to cause infertility in men. All damage it causes either neurologically or physically is irreversible and cannot be undone by removing the mercury from the organism. In extreme cases, it can be fatal, as was observed at Minamata, Japan in the 1950s, at Niigara, Japan in the 1960s and in Iraq in the 1970s [2],[3],[4].
Once inorganic mercury enters an aquatic environment, it becomes metabolised by microrganisms to become organic methylmercury, which facilitates its absorption by biological organisms, and it is many times more toxic in this form than inorganic mercury. In the marine environment, it is absorbed by the smallest, unicellular lifeforms which are in turn consumed by zooplankton, who ingest the methylmercury along with their food item. Small planktivorous fish, molluscs and crustaceans then ingest and absorb the methlyMercury as they feed and being larger animals require a greater number of these organisms, they ingest a greater amount of methylmercury doing so. This biomagnification thus continues all along the marine food chain, each next level mesopredator consuming larger quantities of small prey, stocking larger quantities of methylMercury, before going on to be consumed by even larger predators. Since the biological half-life of mercury is between 57 (+/- 18) and 64 (+/- 22) days, the predators cannot eliminate itand subsequently the quantity increases and is stocked in all tissues, particularly the brain, liver and muscle. These bioaccumulative and biomagnification processes are such that at each meal, a predator takes on even more methylmercury and will do so for as long as it lives. Age-accumulation means that the greatest quantities of methylmercury are found within the tissues of the largest, oldest apex-predators [5].
Since a large percentage of the world’s populations rely on marine resources as a unique source of protein, the Joint FAO/WHO Expert Committee on Food Additives established a Provisional Tolerable Weekly Intake (PTWI) of 1.6µg of methylmercury per Kg of bodyweight and 5µg of Total Mercury per Kg of bodyweight [6]. Many governments and their food agencies have dictated their own guidelines. In Europe, a maximum permissable level of 1mg/Kg of methylmercury in fish, particularly predatory fish, was set by EC regulations, but no limit for individual dietary intake has been set [7].
Studies have shown that ALL seafood contains methylmercury. Over 200 commercial marine species were purchased in markets in Canada and analysed. Oysters (Ostrea edulis) contained an average quantity of 0.011µg/g of methylmercury; Marlin (Makaira nigricanis) contained an average of 1.43µg/g and Swordfish (Xiphias gladius) contained an average of 1.82µg/g. Of all of the predatory fish tested, Albacore tuna (Thunnus alalunga) contained the lowest average quantity of 0.26µg/g and of 5 smaller tuna species, fresh and frozen contained more methylmercury (an average of 0.93µg/g) than canned tuna (0.15µg/g.) There were also variations according to the subspecies [8]. A once monthly consumption of these fish by a woman of childbearing age would provide a dietary intake of methylmercury below the FAO/WHO’s PTWI.
Sharks are long-lived animals at the top of the food chain, apex-predators, so it can be expected that they should accumulte large quantities of methylmercury.The analysis of the flesh of a Blue shark (Prionace glauca) revealed that it contained 1.4µg/g, but the highest value analysed was 4µg/g. This was 208% more than the JEFCA permissable limit [9], [10].
Marine mammals contain extremely high levels of methylmercury. Dolphin meat is still consumed in many cultures, yet studies have shown it to be particularly contaminated. The liver of a Bottlenosed dolphin (Truncata terciops) which had stranded on the Tuscany coast in the Mediterranean contained 13.15µg/dry g [11] and the liver of an individual of the same species bought in Taiji, Japan contained 19.8µg/wet g. The red muscle of a Striped dolphin (Stenella coeruleoalba,) also bought at Taiji, contained 98.9µg/wet g [12].
While quantities may vary from species to species, it has been widely demonstrated that the flesh of marine animals contains large amounts of mercury in its most toxic form, organic methylmercury. This is particularly true of the larger marine predators. Yet in spite of the studies showing to what extent it is present and in spite of various governmental and international guidelines recommending a limited consumption, very little is done to inform the consumers of the potential risks they incur. Whereas cigarettes are labelled on an international scale to warn people of their toxicity, no such labelling can be seen at the fish market. New and bigger fishing vessels are launched every year in the aim of catching even more fish, fishing quotas have shown a marked upward tendance and much hype is made of the benefits of consuming “healthy” seafood. Sea fishing has become an increasingly popular activity, not only as a means to catching food, but as a sport in which the largest individuals of the International Game Fish Association authorised species are targetted. These species are generally predatory fish and sharks whose organic mercury levels have been shown to be among the greatest. Measures to advise and protect consumers must be adopted if new episodes of large scale mercury poisoning such as occurred in Minamata, Niigara and Iraq are to be avoided. The scientific community has done its job, the information now only needs to be shared.
References
[1]. Hazen, RM.; Golden, J.; Downs, RT.; et al. “Mercury (Hg) mineral evolution: A minerological record of supercontinent assembly, changing ocean geochemistry and the emerging terrestrial biosphere.” American Minerologist V:97:7:1013-1042. July 2002
[2]. Ceccabelli, S.; Daré, M.; Moors. “Methylmercury- induced neurotoxicity and apoptosis.” Chem.Biol.Interact, 188 (2010) pp301-308
[3]. Murata, K.; Dakeishi, M. “Neurodevelopmental effects from prenatal exposure to Methylmercury in the Seychellois and Faroes cohorts and the critical concentration: A review.” Japanese Journal of Hygiene V:60:1 pp4-14. Jan 2005
[4]. Leung, TY.; Choy, CMY.; Yim, SF.; Lam, CWK.; Haines,CJ. “Whole blood mercury concentrations in sub-fertile men in HongKong.” Aust NZ Journal of Obstetrics and Gynaecology 41:1:75-77. 2001
[5]. Watanabe, N.; Tayama, M.; Inouye, M.; et al. “Distribution and chemical form of mercury in commercial fish tissues.” Journal of Toxicological Sciences 37:4:853-861
[6]. JEFCA- Summary and Conclusions of the 67th meeting of the Joint FAO/WHO Expert Committee on Food Additives. JEFCA 67/SC (2008.)
ftp://ftpfao.org/ag/agn/jefca/jefca67_final.pdf
[7]. EC, Amending regulations (EC) No 1881/2006 Laying down maximum levels for certain contaminents in foodstuffs. Commission regulation No 629/2008, Oj No 364, 20.12.2006,2008
[8]. Debaka,R.; McKenzie, AD.; Forsyth, DS.; Conacher, HBS. “Survey of total mercury in some edible fish and shellfish species collected in Canada in 2002.” Food Addit.Contam.Part A: Chem.Anal.Control.Expo.Risk Assess.21 (2004) pp 434-440
[9]. Heumann, KG.; Poperechna,N. “Simultaneous multispecies determination of trimethllead, monomethylmercury and three butyltin compounds by species specific isotope dilution GC-ICP-MP in biological samples.” Anal.Bioanal.Chem (2005) 383:153-159
[10]. Kruse,R.; Bartelt,E. “Exposition to methylmercury by fish consumption.” Cuxhaven 2008
[11]. Leonzio, C.; Focardi, S.; Fossi, C. “Heavy metals and seleniumin stranded dolphins of the Northern Tyrrhenian (NW Mediterranean.)” Science of the Total Environment V:119 pp 77-84 June 1992
[12]. Endo,T.; Haroguchi, F.; Cipriano, MP.; Simmonds, Y. et al. “Contamination by mercury and cadmium in cetacean products from Japanese markets.” Chemosphere 54 pp 1653-1662