Mackerel
Fish with high levels of mercury include marlin, shark, swordfish and king mackerel, and, to a lesser extent, tuna.
A 2004 study found that mercury levels in a group of men who'd had a heart attack were 15 per cent higher than those who had no heart problems.
A report in the journal Science voiced concerns about levels of harmful chemicals in farmed salmon, particularly those bred in Scotland.
But the Government's Food Standards Agency (FSA) argues that dangers have been exaggerated. Said John Krebs, then head of the FSA, "This study shows that the levels of dioxins and PCBs in salmon are within internationally recognised safety limits."
"There is good evidence that eating oily fish reduces the risk of death from recurrent heart attacks and that there is a similar effect in relation to first heart attacks.
"We advise that the known benefits of eating one portion of oily fish outweigh any possible risks. "
A 2006 survey by the FSA into the levels of metals and other elements in processed fish found that the levels of mercury present didn't raise new safety concerns and supported the current recommendation that people eat no more than one portion of oily fish per week.
Eating fish: weighing up the evidence
The general view is that the benefits of eating fish still outweigh the risks by a very long way.
"It's been proven that the level of mercury in fish isn't dangerous to health,' argues Sarah Jane Smith, spokesperson for the British Heart Foundation. 'We still recommend eating at least two portions of fish a week."