Study Finds More Mercury in Freshwater Fish American streams are home to mercury-filled fish, but what's the risk to you?

Aug. 21, 2009 -- A study released by the U.S. Geological Survey found mercury in every freshwater fish from nearly 300 streams that were tested, an astonishing result because mercury has usually been associated with large saltwater fish.

"We were surprised to find mercury in every fish. And what surprised us even more is that about a quarter of the fish contain mercury at levels above the EPA safe level," said Donna Myers of the U.S. Geological Survey (USGS).

Click here to ask "Good Morning America" medical contributor Dr. Marie Savard your health questions.

After testing mercury levels in more than 1,000 fish over seven years, the USGS is warning Americans to limit the amount of large, predator freshwater fish they eat, such as large-mouth bass, pickerel, walleye and some catfish.

"People should be more aware of the kinds of fish they are eating, how frequently they are eating them, and to become more educated on where they are getting their fish," said Myers.

In response to the report, the National Fisheries Institute released a statement emphasizing the difference between the fish tested and the fish most Americans are likely to eat.

"It's important to note that this study is not about commercial seafood but recreationally-caught fish; not the kind Americans are likely to find at their grocery stores or in restaurants," the National Fisheries Institute said in a statement.

"If you have a family member that's out there fishing in a stream, beware," "Good Morning America" medical contributor Dr. Marie Savard said. "But that's not what you're buying at the supermarket."

Click Here for a List of Fish that Contain High and Low Amounts of Mercury

Harmful Effects of Mercury

It is a heavy metal and toxic to the brain and tissues in the body.

Mercury then gathers in streams and oceans and forms methylmercury in the water. When fish feed, they absorb the methylmercury, which is the type of mercury that is harmful, according to the EPA.

Too much methylmercury could damage the nervous system and impair mental functioning. It is most harmful to children and pregnant women.

Mercury poisoning has usually been associated with large ocean fish, such as the tuna used in sushi.

But experts say it is the age of the fish that determines its mercury level, because older fish have had more time to absorb the toxin regardless of whether they live in a stream or the sea.

Does Raw Fish Have More Mercury Than Cooked Fish?

There is no method of cooking or cleaning fish that will reduce the amount of mercury in a meal because mercury is tightly bound to proteins in all fish tissue, including muscle.

Young children, pregnant women, women who may become pregnant, or nursing mothers should not eat any fish known to contain high mercury levels, such as shark, swordfish, king mackerel or tilefish, according to the EPA.

Bottom-eating fish, such as sardines, are safer because they eat algae.

The EPA suggest eating no more than 12 ounces, or two average meals, of fish lower in mercury or no more than 6 ounces, or one average meal, of fish higher in mercury each week.

But Savard says one week's consumption of fish does not drastically change the level of methylmercury in the body.

Click Here for a List of Fish that Contain High and Low Amounts of Mercury