Tuna. Picture: THINKSTOCK
Tuna. Picture: THINKSTOCK

EATING too much fresh tuna, salmon, swordfish and shellfish risks exposure to high levels of mercury, scientists warn.

Mercury is associated with damage to the central nervous system and exposure during foetal development may cause severe mental retardation, birth defects and foetal death. Vulnerable people include pregnant women, children and women planning pregnancies.

Predatory fish such as tuna and sharks live longer and eat grazer fish — which live on mainly plants containing low levels of mercury — and the heavy metal accumulates in their bodies.

A recent report by the Zero Mercury Working Group, a coalition of 95 nongovernmental organisations including groundWork in South Africa, warned that global mercury contamination of seafood and the health effects from methyl mercury in seafood were above the “safe” level.

The report was submitted to the United Nations climate change talks earlier this month.

“The level of mercury in the Pacific Ocean is projected to rise 50% by 2050 if current pollution trends, such as emissions from Eskom’s coal-fired power plants, continue unabated,” says Rico Euripidou, groundWork environmental health campaigner.

Eskom says it has measures in place that contribute to reducing mercury emissions form its coal-fired power stations. These include fabric filter plants as well as flue gas desulphurisation plants that will be installed at new power stations such as Kusile.

Eskom calculates mercury emissions from its coal-fired power stations based on the mercury content and the amount of coal burned, as well as the installed emissions control technologies installed at the power stations. 

It has undertaken regional air-quality monitoring since the late 1970s as part of its ambient air quality management programme. Currently there are two Eskom ambient monitoring sites that measure mercury — one in Elandsfontein in Mpumalanga and another in Marapong, Limpopo. South Africa’s ambient air quality is affected by emissions from a number of natural, domestic and industrial sources, including Eskom.

Although there is no legislation governing atmospheric mercury emissions in South Africa, Eskom says it will support the development of legislation in South Africa that aims to reduce total mercury emissions.  In addition, Eskom continues to participate and contribute to research activities of the South African Mercury Association.

Mirriam Moswaane, spokeswoman for the National Regulator for Compulsory Specifications, says the watchdog does not consider the public to be at risk from most fish species.

“However, individuals with a very high affinity for shark, swordfish or older specimens of some other types of predatory fish, may be at risk,” she said recently.

The regulator advises general consumers of species with a high mercury risk to limit their consumption to one meal a week, and pregnant women and young children to one meal a month.

“Pregnant women, nursing mothers and young children are also advised to rather eat fish that are lower in mercury and … to eat moderate quantities (of about) 350g per week,” it says.

Mercury occurs naturally in the environment but could also be released into the air through industrial pollution. It accumulates in streams and oceans, where it is turned into methylmercury.

Regulation 500 of the Foodstuffs, Cosmetics and Disinfectants Act sets the maximum regulatory level of mercury in shellfish at 0.5mg/kg, and for methylmercury in predatory fish, including shellfish, at 1mg/kg.

Mercury pollution could come from a number of sources, including waste incinerators, cement production and coal combustion, says the Western Cape environmental affairs department.

The US Food and Drug Administration says methylmercury does leave the body, but it can take up to a year for levels to drop.

A 2009 report by the Council for Scientific and Industrial Research recommended that “subsistence fishermen along the South African coastline should be warned to limit their intake of locally caught fish due to the potential of mercury poisoning”.

James Dabrowski, a principal researcher at the council, says the risks have to be carefully gauged. “There are mercury emissions, but you can’t say that we’re all going to die. It only becomes toxic once it is methylated, and there have to be conditions for that to happen,” he says.

Prof Louw Hoffman of the University of Stellenbosch’s animal sciences faculty says the university is about to begin a study of methylmercury in seafood. “Few labs can measure methylmercury and we’re finalising a technique,” he says.