A study conducted in eight European nations and Israel was designed to find out how mercury levels in the body affect risk for cardiovascular disease and to see if high mercury levels can eliminate the heart-protective effect of eating fish. Researchers measured toenail mercury levels in approximately 700 men who had suffered a heart attack, and another 700 who had not. They also measured levels of the healthy omega-3 acid DHA, provided by fish and fish oil, in these men.
Mercury levels in men who had suffered heart attacks were 15% higher than in men who had not; the odds for a heart attack were twice as high in men with the highest mercury levels as in men with the lowest levels. Conversely, higher DHA levels were linked to a reduced risk for a heart attack, as previous studies have shown.
So, the question regarding fish consumption is, to eat or not to eat? If you are eating fishes high in mercury, you are probably negating the positive effects that could otherwise be obtained from them. Fish to be avoided, in order of higher to lower mercury levels, include tilefish, swordfish, mackerel and shark. Most smaller fishes and shellfish have low mercury concentrations, and are safe to eat.
Guallar E, Sanz-Gallardo MI, et al. Mercury, fish oils, and the risk of myocardial infarction. The New England Journal of Medicine 2002:347(22), pp. 1747-1754.