The discovery of unacceptably high levels of methyl mercury in shark, merlin and swordfish led UK Food Standards Agency (FSA) toxicology expert Dr Diane Benford to warn pregnant women, children under 16 and couples trying for a baby not to eat them. She also advised against eating tuna. Adults eating just one portion of these fish a week would have almost reached the World Health Organisation safety guideline for the poison. Children, by virtue of their smaller body size but proportionately larger consumption, would exceed the guideline with one portion.

The FSA's Dr Jon Bell explained that it is because the fish are at the top of the food chain, large and long-lived, concentrating mercury from the fish that they eat.

Ed.- Methyl mercury affects the nervous system and, in severe cases, can permanantly damage the brain. It is a developmental toxin so the foetus is more susceptible to its toxicity than adults. Natural sources include volcanoes, natural mercury deposits, and chemical reactions caused by the oceans. The main human-made sources are coal burning, chlorine alkali processing, the burning of wastes, and metal processing.

Scientists estimate that human activities have doubled or tripled the amount of mercury in the atmosphere. The level of this atmospheric pollution increases by about 1.5% per year.

See also Mercury in seaweed