The biggest can reach 100 pounds and 4 feet long, consuming up to 40 percent of their body weight daily in plankton, the base of the aquatic food chain.

Large, predatory fish - such as tilefish, swordfish and king mackerel - tend to have higher levels of methyl mercury than do smaller fish because they're higher in the aquatic food chain.

Heavy metal ions in water can be concentrated in aquatic organisms, which may threaten human health through the food chain.