Heavy metal contamination can come from a variety of sources: the paint on boats, zinc in your tires, etc. Heavy metals contaminate the water and settle along the ocean floor where bottom feeders tend to feed. Though only small amounts of metal is consumed, when larger fish that are higher up in the food chain eat the bottom feeders, a higher concentration of the toxin is then present in their body. This is chain reaction is called biomagnification. Since humans are the top on the food chain, humans are also affected by biomagnification, making it a subtle yet dangerous problem.

http://www.safewater.org/PDFS/resourcesknowthefacts/Oil_Spills.pdf