The study related only to ocean fish of the caliber sold commer- cially in the United States. Freshwater fish caught by recreational fishers may have higher mercury levels. Generally, the larger the fish, the higher the mercury concen- tration.
Scientists therefore warn against eating a lot of whale, shark, swordfish or tuna.
Although the U.S. average and the WHO's recommended limits are lower, research conducted on the Seychelles Islands found that mercury intake could be higher without causing negative effects on neurological development. More often than not, the warnings come because fish is contaminated with mercury. Mercury enters the food chain from natural sources in the atmosphere or from human-produced pollutants. The definition of frequent is the subject of great debate in the scientific community, however, and so is the question of how much of the poisonous metal is safe to consume. Since mercury occurs naturally in foods like spinach and oatmeal, we know our bodies can tolerate modest levels of it.