Fish Nationwide Contaminated With Mercury

About a quarter of the fish tested were found to contain mercury at levels exceeding the criterion for the protection of people who consume average amounts of fish, established by the U.S. Environmental Protection Agency.

More than two-thirds of the fish exceeded the U.S. EPA level of concern for fish-eating mammals, according to the study, released on Friday.

"This study shows just how widespread mercury pollution has become in our air, watersheds, and many of our fish in freshwater streams," said Secretary of the Interior Ken Salazar. "This science sends a clear message that our country must continue to confront pollution, restore our nation's waterways, and protect the public from potential health dangers."

The USGS scientists studied mercury contamination in fish, bed sediment and water from 291 streams, sampled from 1998 to 2005.

Atmospheric mercury is the main source to most of these streams - coal-fired power plants are the largest source of mercury emissions in the United States - but 59 of the streams also were affected by gold and mercury mining.

A swamp-fed blackwater stream in northern Florida (Photo by Tim Ross)

Some of the highest levels of mercury in fish were found in the tea-colored or blackwater streams in North and South Carolina, Georgia, Florida and Louisiana - areas associated with relatively undeveloped forested watersheds containing abundant wetlands compared to the rest of the country.

A blackwater stream has a deep, slow-moving channel that flows through forested swamps and wetlands. As vegetation decays in the water, tannins are leached out, resulting in transparent, acidic water that is darkly stained, resembling tea or coffee.

High levels of mercury in fish also were found in relatively undeveloped watersheds in the Northeast and the Upper Midwest.

Elevated levels of mercury also were observed in areas of the Western United States affected by mining.

Mercury, a neurotoxin, is one of the most dangerous contaminants in U.S. waters. The main source of mercury to natural waters is mercury that is emitted into the atmosphere by coal-fired power plants and factories and deposited onto watersheds by precipitation.

But the USGS says atmospheric mercury alone does not explain the contamination found in the fish in U.S. streams. Naturally occurring watershed features, like wetlands and forests, help convert mercury to the toxic form, methylmercury. Methylmercury is taken up by aquatic organisms, resulting in the contamination of fish that eat those organisms.

"This study improves our understanding of where mercury ends up in fish in freshwater streams," said USGS scientist Barbara Scudder. "The findings are critical for decision-makers to effectively manage mercury sources and to better anticipate concentrations of mercury and methylmercury in unstudied streams in comparable environmental settings."

Since USGS studies targeted specific sites and fish species, the findings may not be representative of mercury levels in all types of freshwater environments across the United States, the agency says.

All 50 states have mercury monitoring programs, and 48 states issued fish-consumption advisories for mercury in 2006, the most recent year of national-scale reporting to the EPA.

The EPA regulates mercury emissions to air, land and water. In February, the EPA announced that it intends to control air emissions of mercury from coal-fired power plants by issuing a rule under the Clean Air Act.

Click here to view the USGS report.

Fish and shellfish are an important part of a healthy diet, according to the EPA and nutrition experts. Fish and shellfish contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids.

"Yet, some fish and shellfish contain higher levels of mercury that may harm an unborn baby or young child's developing nervous system," the EPA warns.

By following these three EPA recommendations for selecting and eating fish or shellfish, women and young children will receive the benefits of eating these foods and also reduce their exposure to the harmful effects of mercury.

1. Do not eat shark, swordfish, king mackerel, or tilefish because they contain high levels of mercury. These are larger fish that have lived longer than smaller fish and so have had more time to accumulate methylmercury in their bodies. Safest are fish small enough to fit whole in a pan.


2. Eat up to 12 ounces, or two average meals, a week of a variety of fish and shellfish that are lower in mercury. Five of the most commonly eaten fish that are low in mercury are shrimp, canned light tuna, salmon, pollock, and catfish.
   
   Another commonly eaten fish, albacore tuna has more mercury than canned light tuna. So, when choosing two meals of fish and shellfish, the EPA advises women and children to eat only one average meal of albacore tuna per week.


3. Check local advisories about the safety of fish caught by family and friends in your local lakes, rivers, and coastal areas. If no advice is available, eat up to six ounces per week of fish you catch from local waters, but don't consume any other fish during that week.

For a national listing of fish advisories from the Environmental Protection Agency, click here.

Copyright Environment News Service, ENS, 2009. All rights reserved.