U.S. regulators recommended on Tuesday that pregnant women, nursing mothers and women who might become pregnant increase the amount of low-mercury fish they eat to between eight and 12 ounces a week as they issued a long-awaited draft update to their advice on mercury levels in seafood.

The draft update by the Food and Drug Administration and the Environmental Protection Agency is the first since 2004 and has been eagerly awaited by scientists and advocacy groups that argue that exposure to mercury may be more dangerous at lower levels than previously thought.

The regulators had previously recommended pregnant women eat as much as 12 ounces of seafood a week but had not suggested a minimum.

The proposed update, which is subject to public comment, retains the recommendation that pregnant women avoid swordfish, shark, king mackerel and tilefish, though they narrowed the warning on tilefish to include only fish from the Gulf of Mexico.

The regulators are seeking public comment on whether to add orange roughy and marlin to the list of fish to avoid.

The agencies continue to recommend lower mercury fish such as salmon, shrimp, pollock, and light canned tuna, and they added tilapia and cod to the list of examples.

The proposed update disappointed some consumer groups that have been seeking to reduce exposure to methylmercury, which can cause prenatal harm. The update was welcomed by the fish industry, which said it focuses less on risk than on the nutritional benefits of fish.

"The FDA is saying pregnant women should eat four times as much fish as they do currently," said Gavin Gibbons, a spokesman for the National Fisheries Institute, which represents the fish industry. "They are changing the narrative about risk."

Critics of the proposed update said regulators failed to adequately address levels of mercury in white, or albacore, tuna. Tuna is the second most popular type of fish eaten in the United States after shrimp and, by dint of the sheer amount eaten, constitutes the greatest risk for mercury exposure, they said.

"Albacore should be on the list of fish to avoid," Edward Groth III, a food safety scientist and adviser to the Mercury Policy Project, said in a statement. "Given the enormous role tuna plays in U.S. mercury exposure, if women are going to eat more fish and also reduce their mercury exposure, they simply have to strictly limit their tuna consumption."

Light tuna accounts for about 70 percent of the market and white tuna for about 30 percent. Regulators reiterated their 2004 advice that pregnant women eat up to six ounces a week of white tuna.

(Reporting by Toni Clarke; Editing by Eric Beech; and Peter Galloway)