Feature

Mercury in Food

Fish and fish products often contain small amounts of mercury. Just how dangerous is this contamination?

By G. P. Savage

Since the scare over mercury in tuna fish in the US, scientists have been looking more closely at the levels of mercury in foods, particularly fish. They have also paid more attention to the physiological effects of organomercury compounds in humans. The problem of mercury contamination in the environment is a particularly pressing one because there is very little difference between natural concentrations and concentrations which are regarded as dangerous.

Inorganic mercury has been recognised as toxic since early times. The ancient Greeks considered the element too poisonous to be used as a medicine. The expression "as mad as a hatter" is almost certainly derived from the symptoms developed among people who used mercury in the manufacture of felt hats.

The toxic effects of organomercury compounds, which have very different and far more serious consequences, have only been recognised in the last 30 years or so. The significance of organomercury compounds as environmental pollutants was only really understood after two large-scale incidents in Japan. At Minamata between 1953 and 1960, at least 111 people were killed or severely disabled by eating fish and shellfish contaminated with methylmercury compounds originating as effluent from a vinyl chloride and acetaldehyde plant. A similar occurrence in 1965 poisoned at least 26 people in Niigata.

The original effluent was inorganic mercury, which was methylated and then entered the aquatic food chain. Because of the long half-life of methylmercury, and the concentrating effect as it moved up the food chain, very high levels were present in the fish and shellfish consumed by the people of the Minamata area.

Metallic mercury and inorganic mercury compounds usually attack the liver and kidney. Low-level exposures to these compounds do not normally cause problems, as they are not retained in the body long enough to cause damage. Methylmercury compounds are much more dangerous, as they cause irreversible damage to the central nervous system, leading to numbness in the extremities, lips and tongue, followed by deafness, lack of balance, blurring and restriction of vision.

All of these symptoms result from the ability of methylmercury compounds to cross the blood-brain barrier. Methylmercury also has a longer half-life than inorganic mercury, so it has greater potential to accumulate in the body. Unfortunately, most fish contain by far the largest proportion of their mercury in the form of methylmercury.

There appears to be a threshold level below which methylmercury appears to have no effect. This threshold arises because a certain number of brain cells must be damaged before the effect operates. Normally there is a surfeit of brain cells for a particular task, but as brain cells are not regenerated and there is a natural wastage of them throughout life, any cellular damage may only become apparent in old age.

Mercury in Fish

Some New Zealand fish contain higher levels of mercury than found in fish caught overseas. As New Zealand does not appear to have any significant man-made sources of mercury pollution, these high mercury levels must be due to natural contamination. Volcanic and geothermal activity both add mercury to the aquatic environment. Some naturally occurring mercury deposits can also be found. All of these factors tend to add mercury to the seas around our shores.

It is thought that microorganisms in the bottom sediments of the sea convert much of the inorganic mercury in the sea to methylmercury. This form of mercury is more readily taken up by marine life and concentrated in the tissues of fish. Once mercury has entered the aquatic environment it can then move through the natural food chains. As it moves into each trophic level it becomes more concentrated.

After the Minamata disaster, studies in the 1960s in Sweden, Canada and the US showed that high levels of mercury were contaminating the aquatic environment. Regulations were introduced by most countries during this period. A maximum residue limit (MRL) of 0.5 mg mercury/kg of fish was imposed by the US, Australia and New Zealand, as well as many other countries. For human food other than fish, the MRL has been set at 0.03 mg/kg, which can cause problems if fish meal has been used as part of the feed for these animals.

During the initial scare, the regulations closed some fishing industries in the US. As mercury, especially methylmercury, accumulates through food chains, fish which have a long life and/or are at the end of a food chain tend to contain high levels of mercury. Swordfish, snapper and tuna are species which fall into this category. Members of the shark family also have high mercury levels. Fish are also able to absorb mercury directly over their body surfaces, thus adding to the problem.

The mercury levels in marine fish can be thought of as background levels, as the mercury levels in the oceans have remained constant for many hundreds of years. Carnivorous fish, particularly large species such as shark, swordfish, tuna and marlin, contain tissue mercury concentrations which usually range between 0.3-1.0 mg/kg, compared to concentrations of 0.15 mg/kg or less in other marine fish. These carnivorous fish are not only at the top of the food chain, they also live a long time so they have great potential to accumulate mercury during their life.

Very high levels of mercury can be found in Pacific blue marlin and this can be attributed to the area of capture, which is near Hawaii's active volcano, and thus an area of high natural mercury contamination.

On the whole, shellfish appear to contain low levels of mercury, and it also seems that they contain lower levels of methylmercury than fin-fish. The same trends are also apparent with freshwater species, although mercury levels shown here may be partly caused by man-made pollution.

Trout caught in some central North Island lakes and rivers contain moderately high levels of mercury in their flesh. These are due to either geothermal discharge or wastes from pulp and paper mills, but these river discharges do not have a major impact when they reach the sea.

Levels of mercury in snapper (a relatively long-lived species) and other offshore species have moderately high mercury levels, ling reaching values of 2.4 mg/kg mercury and skate up to 2.2 mg/kg. Most of the other species examined have mercury levels below 1.0 mg/kg.

In a 1983 study, the mean concentration of mercury in orange roughy from the Challenger Plateau was found to be 0.47 mg/kg. The levels of mercury in fish caught on the Chatham Rise were significantly higher. These fish were also longer and heavier than those caught on the Challenger Plateau. It is thought that the fish caught on the Chatham Rise were much older and had more time to accumulate mercury over their lifetime.

Effects on Children

The major poisoning incidents that have occurred in Japan have highlighted the dangers of excessive dietary intake of mercury. In New Zealand it has been shown that some of the fish sold in some takeaway bars is locally caught shark which may contain up to 4.4 mg mercury/kg.

Researchers have used the mercury concentration in hair as an indicator of the amount in the body. One study showed that the hair from Polynesian children contained significantly more mercury than that of European children. The Maori and Pacific Island children in the study regularly consumed more fish than European children.

The likelihood of poisoning is low, but the foetus of pregnant women may be at particular risk. The Department of Health has advised against the sale of sharks larger than one metre in length, but it is not clear whether this recommendation is being followed at present. To some extent the public are unaware of the potential problem of the mercury content of some fish, and in many cases they do not realise that they are consuming shark with their chips. This is made worse by the vague use of names for fish in New Zealand.

One of the other sources of mercury in the diet comes via animals fed ground fish meal. Because ground fish meal is concentrated by removing water, mercury is concentrated approximately five times above the level present in the raw material. This means that fish meals produced from fish with even moderate levels of mercury will contain high mercury concentrations. Poultry, pigs and fish fed these meals will also concentrate mercury to undesirable levels if care is not taken.

There are ways to avoid this, such as by withdrawing fish meal from the animals' diet for a period before slaughter. This has been shown to be very effective, particularly in pig feeding. Fish meal is an excellent source of proteins, vitamins and minerals, and it is important that mercury levels be controlled so that its use as a valuable supplement continues.

The Minamata tragedy of the 1960s initiated widespread concern over inorganic and organic mercury pollution. It soon became apparent that fish and fish products were by far the biggest contributors of mercury in the diet and would be important if these products were eaten in significant amounts. The problem of mercury in fish has, however, been overemphasised, as no conclusive evidence of mercury poisoning due to fish consumption in developed countries has ever been established. One reason for this is that fish forms only a small part of the overall diet in many countries. Most countries now realise this, but it has not been possible to set a realistic, safe MRL in fish for human consumption.

Dr Savage is a senior lecturer in biochemistry at Lincoln University.