Most fishermen know that it's risky to eat fish caught in rivers and lakes polluted by industrial wastes. Few realized that it's risky to eat fish caught off one of the most popular fishing spots - a bridge. Auto exhaust fumes and the rubbish people dump in the middle of the night can pollute the waters near a bridge.
Fish caught in coastal water (particularly if near an urban center) are exposed to far more contaminants than deep-water denizens. This is why the flesh of a deep-water, 300-pound tuna is less likely to be contaminated than that of a shore-hugging, 8-ounce porgy.
A large fish (and the people who eat it) may not be as fortunate if it lives near an industrial metropolis. Its flesh will likely have a higher concentration of toxic mercury than that of smaller fishes. Here's why. Bottom-dwelling shellfish ingest mercury waste that settles to the ocean floor. The mercury in these crustaceans is transferred to the fishes that devour them, then transferred in turn to the fishes that eat them. Eventually, some of the mercury reaches the top of the food chain - fishes such as tuna, sharks, and swordfish. These large fishes live longer than smaller fishes and thus eat more flesh in a lifetime. Because mercury is difficult to expel from the body, a large fish living in coastal waters may become a repository of an unsafe quantity of mercury before it's caught.
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