Quicksilver was once the common name for the shiny liquid metal now more commonly known by its formal name - mercury. Like other heavy metals mercury has the unfortunate property of accumulating in living tissue. Once mercury has entered an organism it stays there. This means that its concentration increases over a lifetime. The type of organism is irrelevant. It also bio magnifies as it passes up a food chain. At every level the predatory species is likely to contain 10 times the mercury concentration of its prey. As there may be three or four levels of predation the concentrations in apex predators will be very high. Fortunately metallic mercury is relatively inert and is not directly taken in by most organisms. To enter the food chain it needs to be metabolised into methyl mercury by bacteria. This occurs most readily in aquatic ecosystems giving rise to our concern about the mercury content of seafood.

Swellnet recently published a link in the forums to a newspaper article revealing that Sydney Water had discharged 40kg of mercury through its sewer system in the previous 12 months. This raised concern about the risks of consuming seafood. Mercury is a nerve poison and its capacity to cause harm was disastrously demonstrated at Minamata in Japan during the 1950s when the Chisso Corporation dumped large amounts of mercury from its industrial plant into the bay. Thousands of local people consuming seafood from the bay suffered permanent disabling nerve damage and some died.

It is estimated that the discharges by the Chisso corporation were in the order of 27 tons. This is orders of magnitude greater than the discharges by Sydney Water. The Minamata discharges also took place in a relatively enclosed water way while the Sydney Water discharges took place through deep water outlets where they were likely to be either dispersed or enter the deep bottom sediments. None of this suggests that there is no risk of the mercury entering the marine food chain, only that the risk of a Minamata type incident in Sydney must be considered as non-existent.

Given that we are aware of these discharges and the probably larger amounts entering the marine environment from the burning of coal and the processing of gold and aluminium, the only way to be sure that mercury levels in seafood are consistently safe is through regular monitoring. This does not appear to be happening. The little data that is available is contained in the National Residue Survey, available here. 

This data mainly deals with export markets but can be taken as a proxy for what we consume. In that regard the clearest conclusions are: 1) That there is insufficient testing done to be sure of the complete safety of our seafood in relation to its mercury content, and  2) That the testing that has been done reveals some areas of concern about the percentage of samples exceeding the recommended limits.

Globally the level of concern about mercury pollution has caused the United Nations to implement the Minamata Convention On Mercury to limit further discharges. Australia is a signatory to the convention but does not yet appear to have ratified it. It appears that there is a report on the issue being prepared by the Department of Health but no information about it is available on their web site.

The NSW Food Authority provides dietary advice based on existing knowledge. This is available here. 

This suggests that pregnant women and young children should limit their seafood intake to minimise the risk. The importance of this advice is emphasised by the occurrence of 3 cases of children with high levels of mercury showing symptoms of nerve damage in Sydney in 2008.

There is clearly a need for more regular and extensive testing of seafood for mercury and this applies across the country as there is practically no data at all for many regions. Having said that public authorities need to assess the risks before they commit funds. There are many more serious issues in our food supply than mercury content and it would be unwise to divert funds from an issue posing greater threat to our safety to conduct regular mercury audits. Until we have better information it would be wise to follow the dietary advice issued by the NSW Food Authority. //blindboy