While stressing the health benefits of fish, young children and women who are pregnant, nursing or may become pregnant should not eat certain kinds of fish that tend to be high in mercury, the Food and Drug Administration and the Environmental Protection Agency said in their joint recommendation.
"By following this advice, we're confident that women and young children can safely include fish as an important part of a healthy diet," said Lester Crawford, deputy commissioner of the FDA.
While mercury can affect almost any organ in the body, scientists at the FDA are most concerned about its effects on children's brains and nervous systems.
The new guidelines also emphasize that fish and shellfish are a good source of protein and omega-3 compounds that can benefit the heart and "are an important part of a healthy diet."
FDA scientist Dr. David Acheson said fish has many benefits and there is no specific guideline on limiting its consumption in most people's diets. He added that eating fish has very important cardiovascular benefits.
The American Heart Association recommends that people eat a variety of fish at least twice a week, even more for those diagnosed with heart disease.
The agencies say the fish most likely to contain mercury are shark, swordfish, king mackerel and tilefish, and that young children and some women should avoid eating these fish. The way fish is prepared makes no difference as to the amount of mercury included in a serving.
The guidelines also suggest eating up to two meals a week, totaling 12 ounces, of fish known to be low in mercury such as shrimp, canned light tuna, salmon, pollock and catfish.
The agency specified that albacore tuna has more mercury than light tuna, so it should be limited to one meal a week.
The recommendations also state that once mercury has built up in a person's body, it may take over a year before mercury levels in the blood stream drop off significantly.
Several environmental groups were critical of the new recommendations.
The Environmental Working Group told the Associated Press the guidelines favor the interests of the coal and seafood industries at the expense of children by not being stringent enough.
The Natural Resources Defense Council criticized the guidelines as not adequately warning parents of the danger of albacore tuna for children.
Mercury, mostly from coal-fired power plants and other industrial sources, contaminates the water that fish inhabit. The level of a fish's mercury contamination depends on what the fish eats.