Environmentalists had hoped that the panel would direct the FDA to add tuna to the list because it can contain as much harmful mercury as these other fish, but the panel instead told the agency to study the issue further and in the meantime tell pregnant women to limit the amount of tuna they eat.

The panel did not specify a recommended amount for pregnant women but hopes to formulate one after more research. It said in the interim the FDA should use a pamphlet put out by the state of Wisconsin as a model, which says women should eat no more than two six-ounce (170-gram) cans of tuna per week.

Current FDA guidelines say women should limit their consumption of all types of fish to 12 cooked ounces (340 cooked grams) per week, or roughly two servings, but do not specifically mention tuna, which in fresh or canned form accounts for at least one-quarter of all seafood eaten in the United States.

Panelists said they had concerns that women might replace tuna, which contains healthful omega-3 fatty acids, with other less nutritious dishes.

"We're trying to balance off the positive virtues of fish, including tuna fish, against possibilities of harm, and that's a hard decision to make," panel chair Dr. Sanford Miller told reporters after the meeting. "What do you replace tuna fish with, baloney?"

Mercury enters the environment naturally and through industrial pollution. Nearly all fish contain trace amounts of methyl mercury but longer-lived, larger predator fish like shark or swordfish accumulate the highest amounts of methyl mercury and pose the largest threat to people who eat them regularly.

Environmentalists say the agency bowed to pressure from fishing and packing interests when it left tuna off its do-not-eat list in 2001, but one group said it was somewhat happy with the panel's recommendation.

"We think it's a step forward," said Caroline Smith DeWaal, a food-safety director at the Center for Science in the Public Interest that believes pregnant women should avoid fresh tuna steaks and children under 5 should eat no more than one or two servings of canned tuna per month.

"It's more protective than the advice they have today," De Waal told Reuters.

A food industry spokeswoman said she was pleased with the decision as well because the FDA would ultimately base its decision on hard data.