Tuna and swordfish collected from some California grocery stores and sushi restaurants contained mercury levels as much as three times the threshold that authorizes federal food regulators to pull seafood from shelves, according to a study by an environmental health group.

And despite pervasive concerns about the toxic heavy metal in fish, not one of the restaurants and fewer than half of the grocery stores displayed signs warning consumers about the risks of mercury exposure, according to GotMercury.org, a public health advocacy group in San Francisco.

“They are selling food with high levels of mercury – levels the federal government says are too high for children and pregnant women to eat,” said Buffy Martin Tarbox, lead author of the research. “And consumers have no idea.”

But representatives of the $75 billion-a-year seafood industry say the results are misleading and potentially harmful to U.S. consumers who might shun what is widely considered part of a healthy diet. And the industry’s main trade association also accuses the group of waging a public relations war against seafood in an effort to protect sea turtles that are frequently caught in fishing nets. GotMercury.org is an arm of the Turtle Island Restoration Network, an environmental advocacy group.

“They want to cut down on seafood consumption so the sea turtles don’t end up as bycatch,” said Gavin Gibbons, spokesman for the National Fisheries Institute, the Washington, D.C., seafood trade group. “It’s detrimental to public health and it’s cloaked as helping the public.”