More info on mercury in seafood

How mercury gets into the food chain: Mercury is a naturally occurring element. High levels are usually caused by air or water pollution from manufacturing that gets into streams and oceans, where it becomes methylmercury. Fish absorb the methylmercury, and larger, more predatory fish that eat millions of smaller fish can develop concentrated levels.

Mercury's impact on your health: The FDA and EPA say that while most people's fish consumption does not cause a health concern, high levels of mercury in the bloodstream of unborn babies and young women may harm the developing nervous system. Other studies suggest that mercury also may cause health problems for some adults, including an increased risk of cardiovascular disease and neurological symptoms.

Who should watch their tuna intake: Nursing mothers, young children, women of childbearing age and other at-risk groups. In 2004, the FDA and EPA advised women and children to eat up to 12 ounces (two average meals) per week of seafood that's lower in mercury and no more than 6 ounces per week of albacore, or "white" tuna, which has more mercury than canned light tuna. The FDA stands by its recommendation, said spokeswoman Stephanie Kwisnek, "though we're always evaluating science and constantly looking at it."

Judy Simon, a clinic dietitian and nutritionist at the University of Washington, said she will continue to advise her clients to eat a variety of seafood to gain its benefits (including omega-3 fats that promote healthy hearts and child development) while minimizing the risk (toxins).

"We're so lucky in Seattle that we have salmon, which is lower in mercury but also one of the best sources of omega fatty acids," Simon said.

Different tuna, different mercury levels: Generally, the bigger and older the tuna species, the higher the level of mercury. Thus, prized and endangered bluefin tuna tends to contain higher levels than skipjack, a smaller species commonly caught for canning. Mercury levels also can hinge on where the tuna lived and how it was caught; bigeye or "ahi" tuna that's troll- or pole-caught, for instance, tends to be younger and have lower levels of mercury than ahi caught by other methods.

"A lot of the tuna we eat for sushi (unless we're millionaires) are yellowfin or bigeye tuna. These are much smaller and shorter lived, and have between a quarter to one-half the mercury concentration. The tuna that ends up in cans is the smaller tuna, with even less mercury," Tim Essington, an associate professor of aquatic and fishery sciences at the University of Washington, wrote via e-mail.

Fish with highest levels: Large, predatory fish such as shark, swordfish, king mackerel or tilefish.

Common seafood with the lowest levels: Shrimp, canned light tuna, salmon, pollock, catfish. Fish sticks and fast-food sandwiches also typically contain fish with low levels of mercury.

This story includes information from The Miami Herald, the FDA and EPA. Seattle Times staff reporter Karen Gaudette contributed to this report.