Mercury and fish

Your health, the environment, endangered fish species and the links with Mercury and other heavy metal contamination.

What are the links between mercury and the conservation of fish species and environments?

What is the problem?

"Just as the methyl-mercury bio-accumulates in fish, it bio-accumulates in humans. Our biological systems are not adequately designed to cope with this type of contaminant so it bio-accumulates in our bodies. Over time, our bodies can remove the methyl-mercury. However, it is difficult for the body to remove the methyl-mercury when it is faced with more methyl-mercury entering the body faster than it can remove it."

Studies of the brain development of children whose mothers ate significant amounts of fish with high mercury levels during pregnancy have been carried out in New Zealand, the Faroes and the Seychelles.

The Joint Food and Agriculture Organization (FAO) and World Health Organization (WHO) Expert Committee on Food Additives (JECFA) reviewed these studies in June 2003. These researchers recommended reducing the amount of fish known to contain mercury in the diet, particularly for pregnant women. Australian research shows that mercury levels in some fish, particularly shark, could be even higher than in the areas studied for this research. In fact, it seems that mercury levels in some shark species caught in Victorian waters are particularly high.

Australian guidelines for safe levels are under review
Since the Joint FAO and WHO Expert Committee (JECFA) revised its guidelines on methylmercury, Food Standards Australia New Zealand (FSANZ) has also begun to review its advice on the consumption of fish during pregnancy. When the review is completed by FSANZ, they may give further recommendations.

Should I be worried?

A report in the Medical Journal of Australia has highlighted that small children may be exposed to dangerous levels of mercury if they eat too much of certain fish.

The study examined three cases in Sydney where children aged between 15 months and two years were found to have elevated mercury levels caused by eating up to five times the recommended dietary intake for fish.

Mercury has many known negative affects on humans. It can affect the cardiovascular, immune, and reproductive systems. It becomes a problem for humans because fish are an important part of our diet. Pregnant women are at a greater risk from mercury contamination. Those who consume fish or aquatic beings with high mercury concentrations run the risk of harming the fetus.

In the adult brain, methylmercury, at high levels of exposure, causes a loss of cells in specific areas, most commonly the cerebellum, visual cortex, and other focal areas of the brain. The first effect observed is typically paraesthesia (numbness and tingling in lips, fingers and toes), which frequently appears some months after the exposure first occurred. In severe cases, there is progression to loss of coordination, narrowing of the visual fields, hearing loss and speech impairment.

Mercury and other heavy metals also affect dolphins, whales and even seabirds. Of course for those in Japan and elsewhere who eat dolphin and whale the health risks are huge.

How does the Mercury get into the fish?

Mercury makes its way into the environment from natural and anthropogenic (human) sources that combine to release inorganic mercury in the atmosphere. Inorganic mercury can suspend in the atmosphere up to two years.

From there, mercury ends up washed through wetlands, reservoirs, and lakes. It can be converted by bacteria into a very toxic form of mercury called Methyl mercury. Mercury is usually found in the water in very low concentrations. These concentrations are so low that you can even drink or swim in it with no harm. It goes through biomagnification, a process that means there is a higher concentration the higher up on the food chain.

Why are some fish more dangerous to eat than others?
Mercury occurs naturally in the ocean sediment but can also occur as contamination as a result of human activities. Micro-organisms transform the mercury in methyl-mercury and it bio-accumulates in aquatic organisms. What bio-accumulation actually means is that when a larger fish eats a smaller fish, it accumulates the level of methyl-mercury that the smaller fish contained. When it eats another smaller fish, it accumulates some more methyl mercury. The more small fish it consumes, the more methyl-mercury it accumulates and the level does not drop. Then along comes an even bigger fish and eats the fish that ate the smaller fish and that larger fish accumulates all the mercury of the fish it just ate and so the vicious circle continues.
Fish not only accumulate mercury from consuming smaller fish. All fish absorb methyl-mercury from the water that passes through their gills. The longer the fish lives, the more methyl-mercury it will bio-accumulate. Fish that are not predatory and are short lived are not going to contain as much methyl-mercury as their predatory and long living relatives so these are the fish we want to be including in our diet.

What fish have high mercury concentrations?

Fish species known to be high in mercury include some Tuna, Billfish, Swordfish, Broadbill, Marlin, Mackerel, Shark, Orange Roughy and Deep sea Perch. The recommendation for fish considered high in mercury is to limit to one serve per week (1 serve equals 150g) though there is no guarantee that there will be no negative impacts from eating even this small amount contaminated fish. See the end of this article for a list of species to avoid.

What are the links between mercury and the conservation of fish species and environments?

1. Species life history characteristics and overfishing

The characteristics that concentrate mercury in certain species a) high level in the food chain and b) long life spans or older individuals, are characteristics that make these species also extra vulnerable to overfishing.

Long life history characteristics mean that even seemingly modest catches of these species are not quickly replaced naturally. Experts estimate that large fish species are down to 10% of the natural population levels world wide – these fish are often the long lived or top of the food chain species.

2. Fishing methods

In addition to being toxic for humans, tuna, swordfish, and many other fish are caught in ways that are devastating ocean habitats and fisheries. Longline fishing, the destructive fishing method often used to catch tuna and swordfish, kills thousands of endangered sea turtles per year , kills rare and endangered birdlife and is responsible for massive bycatch (killing and waste of non target fish species). The destructive dredge fishing which damages habitat by scraping the bottom and has massive bycatch issues is also used to catch species such as rays which are high in mercury.

3. Sources of mercury pollution

Each year power plants and chemical facilities create many tons of mercury pollution, which makes its way into our homes and bodies in fish.

Two of the biggest sources of mercury pollution are chlorine chemical plants and coal-fired power plants. Chlorine plants, which use massive quantities of mercury to extract chlorine from salt, "lose" dozens of tons of mercury each year; power plants emit around 50 tons of mercury pollution annually. Facilities that recycle auto scrap are another big source of mercury pollution, pouring 10 to 12 tons of mercury into the air every year.

Coal is naturally contaminated with mercury, and when it is burned to generate electricity, mercury is released into the air through the smokestacks. The bulk of this mercury pollution could be eliminated with the installation of pollution-control devices. Similar devices have proved very successful on municipal incinerators, which were once a significant source of mercury pollution.

The burning of coal is a massive source of greenhouse gasses which cause global warming. So not only are coal plants causing global warming which endangers fish species and polluting local environments but they are also poisoning people and animals (e.g. dolphins) far from the source of the pollution via bioaccumulation.

What is the solution?

What is needed is a reduction in the sources of mercury pollution, less fishing for long lived, older and top of the food chain species such as bluefin tuna, sharks and swordfish and a change in the damaging fishing methods that indiscriminately catch and waste fish as bycatch, while delivering contaminated fish to our plates.

You can help protect your health and the environment by minimizing your catch, and refusing to support the commercial catch, of at risk species.

Species to avoid include:

Bluefin tuna
Spanish Mackerel
Sharks (flake)
Orange Roughy